88053264

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TERRITORIALITY AND NON-RANDOM MATING IN
SAGE GROUSE CENTROCERCUS UROPHASIANUS

•

A thesis submitted to the Faculty of The Rockefeller University
in partial fulfillment of the requirements
for the degree of Doctor of Philosophy

by

&

R1. Haven Wiley', A.B.

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June, 1970

The Rockefeller University
New York, New York

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696

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1973

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iii

PREFACE

During my three seasons in the field my work was aided by the
cooperation and solid advice of four biologists in Wyoming and Montana:
Dr. Kenneth Diem, University of Wyoming, Laramie; Duane Pyrah and
Neil Martin, Montana Department of Fish and Game, Lewistown; and James
June, Wyoming C-ama and Fish Commission, Green River. Without their
help and hospitality my work could not have been done. In 1968 the
Montana Fish and Game Department allowed me to live in their house-
trailer in Winnett. V.N. Bell, Bosler, Wyoming; W.H. Bell, Grass
Range, Montana; and the bureau of Land Management, Rock Springs
District Office, Wyoming granted me permission to work on land which
they owned or administered. Karen P. Wiley helped with most .of the
field work in both 1968 and 1969.

Throughout my study I benefited from the guidance and friendship
of many colleagues at The Rockefeller University in the laboratories of
Drs. Peter Marler, Donald Griffin, and Theodosius Dobzhansky. Peter
Marler especially encouraged me in confronting problems in the evolution
of social systems and animal communication. His perceptive analyses,
wide-ranging interests, and acceptance of me as a colleague stimulated
much of my progress during my years at The Rockefeller University. For
discussions of the genetic implications of mating systems I relied
heavily ca discussions with Dr. Dobzhansky and his colleagues Francisco
Ayala, Lee Ehrman, and Rollin Richmond. Much essential assistance,
including help with many details of organizing field trips, came from
Kathryn Harpham, Cate Dolan, and Ann Lutjen of Dr. Marler' s laboratory.

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usually recur from year to year at the same places within a lek, a
male becomes successful in mating by acquiring a territory at a mating
center.

The behavior of males near a mating center differed in several
respects from that of more peripheral males. For instance, the former
Strutted more persistently and engaged in much briefer and perhaps
slightly more frequent Facing-past Encounters. However, these
differences apparently depended on the proximity of the males to females
rather than on intrinsic differences among the individual males. Most
males two or more years old, whether near a mating center or not,
responded similarly to the presence of females within their territories.
The Strut display shows extreme species-specific stereotypy in its
internal timing. Although individual differences in the Strut display
were detected, these differences were slight and did not correlate with
th3 males' successes in mating.

Since the mating centers tend to remain in the same place from
year to year as well as from day to day, femaleo do not have to dis-
criminate among individual males in order to identify a mating center.
Females might learn the location of a mating center. Naive females
could locate a mating center by observing other females congregated
there or by following experienced females. Only limited guidance could
come from the generally smaller territories of males near a mating
center .

The males' interactions specify how a male eventually occupies
a territory at a mating center. These interactions merge features of
two classical paradigms for social interaction: the territory and the
dominance hierarchy. Reciprocal interactions among males included the
limited scope of neighbors' intrusions into each other's territories
and neighbors' encounters as equals in boundary zones between their
territories. Non-reciprocal, or polarized interactions, among males
resulted from the attraction of males towaxd a mating center. Since
their more peripheral neighbors tended to encroach beyond their
boundary zones toward the females gathered at a mating center, the

vii

To explain the evolution of lek mating systems, at least among
grouse, one must address three problems: (1) a male life cycle with
delayed onset of reproduction; (2) the absence of male parental care;
and (3) the aggregation of displaying males.

ix

Page

IX. INTERACTIONS OF TERRITORIAL MALES 83

A. Territory Size and Exclusiveness 83

B. Nature of the Territorial Boundary 85

C. Initiation of Facing-past Encounters 87

D. Termination of Facing-past Encounters 92

E. Allocation of Time to Strutting and Facing-past

Encounters 93

F. Shifts in the Positions of Mating Centers 97

G. Strutting Away from a Lek 98

H. Reoccupation of Vacancies , gg

I. Mortality ^01

J. Expulsion of Intruding Males 102

K. Arrival at a Lek in Early Spring 104

L. Arrival and Departure in the Morning and Evening . . 106

M. Behavior of Yearling Males 107

X. SUCCESS IN MATING U2

XI. TERRITORIALITY AND POLARITY IN THE INTERACTIONS OF MALES. 116

XII. INDIVIDUALITY IN THE INTERACTIONS OF MALES 122

XIII. LEK MATING SYSTEMS IN PERSPECTIVE 124

A. Mating Systems of Other Grouse 124

B. Leks in Other Birds 130

C. Polygynous Birds with Stable Pair-bonds 130

D. Polygyny in Mammals 131

XIV. PROBLEMS IN THE -EVOLUTION OF LEK MATING SYSTEMS 138

A. Delayed Reproduction Among Males 138

B. Intrasexual Competition 145

C. Evolution of Female Behavior 148

D. Genetic Diversity 149

E. Reduced Male Parental Care , 150

F. Aggregation of Displaying Males 151

XV. LITERATURE CITED . , . 155

XVI. APPENDIX 166

XVII. MAPS 170

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xi

List of Tables
Table Page

1 Two bases for classifying avian mating systems 15

2 Copulations recorded on the Muddy Springs Lek,

East Mating Center, 1967 45

3 Copulations recorded on the Fords Creek Lek, 1968 .... 46

4 Copulations recorded on the Dry Sandy Lek,

Mating Center #3, 1969 47

5 Interrupted copulations, Muddy Springs Lek, 1967 53

6 Interrupted copulations, Fords Creek Lek, 1968 54

7 Interrupted copulations, Dry Sandy Lek, "

Mating Center #3, 1969 55

8 Durations of Strut displays by different males in 1967 . . 66

9 Numbers of Struts per six-mintes in Strutting Posture:

comparison of five males when females were in their
territories 70

10 Relationship between the durations of Facing-past

Encounters and proximity to females 74

11 Durations and frequencies of Facing-past Encounters

by individual males at different distances from females . 75

12 Frequencies with which neighboring pairs of males

initiated Facing-past Encounters . 77

13 Proportion of Facing-past Encounters which included

Wing-fighting 78

14 Initiations of Facing-past Encounters earlier and later

in the morning 91

15 Terminations of Facing-past Encounters between

neighboring males 94

16 Relationships between proximity to females and three

measures of male behavior 95

17 Estimated average annual mortality of Sage Grouse

after their first year 113

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xiii

List of Maps

Map Page

1 East Mating Center, Muddy Springs Lek, 1967 171

2 Territories in 1967 after the disappearance of Male A . . 172

3 Territories of individually identified males at least

two years old, 3-5 April 1958, Fords Creek Lek .... 173

4 The mating center in 1968 174

5a Territorial boundaries and matlngs, 27-29 March 1968 . . 175

5b Facing-past Encounters, 27-29 March 1968 176

6a Territorial boundaries during the morning of 1 April 1968 177

6b Facing-past Encounters, 1 April 1968 178

7 Reoccupation of the territory vacated by Male TM . . . . 179

8a Territorial boundaries and matings, 3-5 April 1968 . . . 180

8b Facing-past Encounters, 3-5 April 1968 181

9a Territorial boundaries and matings, 8-14 April 1968 . . . 182

9b Facing-past Encounters, 8-14 April 1968 183

10a Territorial boundaries', 22-25 April 1968 184

10b Facing-past Encounters, 22-25 April 1968 185

11a Territories of individually recognized males, 8-28

April 1969, Dry Sandy Lek 186

lib Facing-past Encounters, 8-18 April 1969 187

lie Facing-past Encounters, 21-27 April 1969 188

12 Territorial boundaries and matings, 8-28 April 1969 . . . 189

13 Males Strutting in positions along the boundaries of their

territories nearest a group of females 190

14 A yearling male chased persistently by his older neighbors 191

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60
50
40
30
20
10
0

Max. number of males
x — it Max. number of females
Number of molings
observed

T*"—

15

March

Figure l. Seasonal changes in the numbers of males, females, and
matings on the Fords Creek Lek, 1968. The numbers of males
and females are the maxima present at any one time each
morning.

In several other lek-forming species of grouse, the males'
interactions show clear resemblances to classical territoriality:
Black Grouse Lyrurus tetrix (especially Kruijt and Hogan, 1967; and
Koivisto, 1965); Sharp-tailed Grouse Pedioecetes phasianellus (Lumsden,
1965) ; and Prairie Chickens Tympanuchus cupido (Hamerstrom and Hamer-
strom, 1960). Male Black Grouse occupy small, but exclusive territories
on a lek, and neighbors encounter each other along mutual boundaries.
Yet. in all these species successful matings are distributed non-
randomly among the males (Chapter XIII, Section A).

It thus remains unclear whether ordinary conceptions of dominance
hierarchies or territoriality apply to the social interactions of male
grouse on their leks and explain the non-random distribution of matings
among them. Kruijt and Hogan (1967) have begun quantitative observations
on Black Grouse leks, but nothing similar has been attempted for Sage
Grouse. Since Sage Grouse show an extreme form of a polygynous mating
system, a better understanding of their behavioral interactions might
suggest principles that would help understand other species as well.

This paper is particularly concerned with the dominance
hierarchy and the territory as paradigms for social interaction on Sage
Grouse leks. The immediately following section will introduce the
concept of polarity in social interactions as a basic distinction
between classical conceptions of dominance hierarchies and territor-
iality. As a further basis for understanding territorial behavior, a
distinction will be drawn between an aggression gradient and an
exclusion gradient in the expression of territorial behavior. Finally,
the body of this thesis will explore the behavior of male Sage Grouse
on their leks in an attempt to recognize and quantify the interactions
which might regulate the species' mating system. To pursue this goal,
the bases of mate selection by females will also require consideration.

m

II. DOMINANCE HIERARCHIES AND TERRITORIALITY

Differences in social organization ultimately depend on differ-
ences in the patterns of interactions among individuals. Two classical
paradigms for social organization, the dominance hierarchy and
territoriality (see Allee, 1938; Collias, 1944; Howard, 1920; Nice, 1941),
differ in the extent to which individuals' interactions are polarized.
In a dominance hierarchy individuals are ranked by their access to some
resource, such as food or reproductive partners, and often by their
success in agonistic encounters. Hierarchical organization of individ-
uals depends on two sorts of polarity in the social system: (1)
individuals of different rank differ in their behavior; and (2) inter-
actions among individuals of different rank lack reciprocity. To under-
stand a hierarchical social organization one must identify and
characterize the nature of the polarities in individuals' interactions.
In contrast, territorial individuals interact reciprocally, at least in
those species in which individuals usually restrict their activities to
their own territories. Each individual inhabits his own territory, and
neighbors meet as equals at their boundaries.

In many species, however, territorial individuals do not restrict
their movements to their own territories. In these species a bird's
territory emerges as the area in which it dominates all other con-
specific individuals except perhaps its mate. At any time the terri-
torial resident can chase away an intruding individual or supplant the
intruder at a food source within the resident's territory. For example,
the territory of a nesting pair of Bicolored Antbirds GymnoplT:hys bi-
color constitutes that area around their nest where the pair can supplant
all other individuals from the choice feeding spots above swarms of
army ants (Willis, 1967). In many species of birds individuals maintain
territories during the breeding season but during the winter form
flocks or range far beyond their former nesting territories. In a
number of these species an individual ' s rank at a food source depends
on the individual's distance from its previous nesting territory

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Type A

Distance from center of territory

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Figure 2. Representative aggression gradients.

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come from patterns in the reoccupation of vacated territories. In-
dividuals should relinquish their original territorial positions in
order to move into more desirable positions. In several species of
birds when territorial individuals are removed or disappear naturally,
the vacated areas are reoccupied by newcomers , although sometimes only
partially (Beudell and Elliot, 1967; Watson, 1967; Orians, 1961; Delius,
1965; Stewart and Aldrich, 1951; Hensley and Cope, 1951; Holmes, 1966).
Except in the study of Red Grouse Lagopus lagopus scoticus reported by
Watson, in all these studies the previous status of the newcomers was
unknown. Watson reported that most new occupants were birds previously
without territories, although in some cases territorial individuals

i

relinquished their original positions in order to establish new
territories in the vacated areas. Clear evidence for ranking- among
territorial individuals according to their territorial locations is,
therefore, not available for any species. Yet for many species certain
habitats are more favorable for nesting and attract individuals estab-
lishing territories (for instance, Kluyver and Tinbergen, 1953). In
several polygynous species which nest in marshes, differences in the
vegetation in males1 territories correlate with the number of mates
the males can attract (Verner, 1964; Willson, 1966; Kluyver, 1955).
In these species individual males might be expected to relinquish their
original territorial positions when vacancies arise in order to move,
perhaps in the following year, into more desirable positions.

This paper will suggest that polarized territoriality has an
important role in regulating the mating system of Sage Grouse. Fully
adult male Sage Grouse occupy nearly exclusive territories on their
communal display grounds and have Type A aggressive gradients. However,
since females congregate for mating at particular sites (mating centers)
within a lek, the locations of territories differ in the opportunities
they present for mating. Polarity in the territorial behavior of males
involves (1) differences in the behavior of territorial males established
at different distances from a mating center and (2) non-reciprocal
interactions of territorial males. Males move their territories in

§

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11

III. SKETCH OF A MORNING'S ACTIVITY ON A LEK

A brief synthesis of the activity on a Sage Grouse lek during a
typical morning in the mating period will help in reading the detailed
chapters to follow. Many of the points introduced here are based on my
own observations or interpretations and will receive full treatment in
later sections.

On moonlit nights activity may continue all night but usually
reaches a peak about sunrise, when females are most numerous. On a
typical morning during the height of the mating period, activity in-
creases during the hour before sunrise. Males not already present on
their territories arrive at least an hour before sunrise. The males
occupy small, contiguous territories, 30-200 square meters in- area.
Each male occupies the same territory every day. Within his territory
each male assumes the Strutting Posture (Figure 5) , with his tail cocked
and fanned, his head raised, and the white feathers and black plumes on
the sides of his neck erected. Each performs repeatedly at intervals
of 7-10 seconds an elaborate, ctereotyped display, called a Strut. In
this display, by a complex sequence of motions, a male inflates a large
expansion of his esophagous (Figure 4) and then suddenly deflates it to
produce a peculiar sound, coo-oo-poink! In the breeding season the
males' esophageal sac enlarges, and the skin of the chest overlying
this sac becomes edematous and vascularized (Clarke, Rahn, and Martin,
1942) . From time to time two males on neighboring territories interrupt
their Strutting to confront each other at their territorial boundary
in a Facing-past Encounter. In these encounters the males do not face
each other head on j instead they take positions shoulder to shoulder,
so that each faces past his opponent's side (Figure 8). In high-
intensity encounters the males close until they are side-by-side a few
inches apart facing opposite directions. An encounter usually involves
some jockeying backwards and forwards for position. Either male may
dart forward a few inches and force his opponent to retreat. However,
their roles switch quickly, each male retreating in turn as his

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extended. A female usually copulates only once each season and lays
her entire clutch of 6-8 eggs after leaving the lek.

Some females probably travel a number of miles from the lek at
which they mated before they nest (Dalke et al., 1963; Patterson, 1952)
Each female builds her nest on the ground and cares for her eggs and
young with no cooperation at all from a male.

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TABLE 1

Two Bases for Classifying Avian Mating Systems

Association between
mated pairs

Breeding sex ratio

I. Durable pair-bonds
II. Promiscuity

A . Monogamy

B. Polygamy

B'. Polygyny
B" . Polyandry

.SsBt'fe

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Posture, the male twice raises this chest sac as high as possible and
drops it. Each time before lifting the sac the male extends his wings
forward and then, as the sac rises, pulls them backwards across the
stiff feathers on the sides of his chest (Figure 4). Thus, each time
the chest sac rises, the wings produce a swishing sound against the
sides of the chest. As the sac is raised and dropped it fills with air
and extends. As a result, two bare, yellowish olive patches of skin
expand on ths male's chest. After the sac falls for the second time,
it is compressed by the contraction of superficial muscles in the skin
of the chest. Immediately after compression, the air in the sac is
released into the bare patches of skin, which suddenly balloon forward
and then instantly collapse. As the bare skin patches balloon and
collapse, two sharp snaps are produced approximately one tenth of a
second apart. A peculiar resonant quality accompanies the snaps, and
several brief, soft coos precede them. Thus, the entire acoustic output

from the display sounds roughly as follows: swish swish coo-oo-poink.

Following a Strut the male returns to Strutting Posture. The display
itself lasts slightly more than 2 seconds. A male usually Struts at
intervals of 7-10 seconds; intervals of 5-6 seconds are rare. The
sounds of the males Strutting on a lek can usually only be heard within
several hundred yards of the lek. In contrast, the white feathers of
the males' chests, which bob up and down during their displays, are
conspicuous at great distances.

Strutting Posture: a posture assumed by males before and after
bouts of Strutting and between Struts within a bout. The male stands
with his body tilted upward and his head held high (Figure 5) . He cocks
and spreads his tail, erects the white feathers and thin plumes on the
sides of his neck, and expands the yellowish combs over his eyes.

Sleeked, Collapsed, and Semi-collapsed Postures: The postures of
male Sage Grouse grade continuously from the Strutting Posture to the
Sleeked Posture. In the Sleeked Posture the male Strctcicc his neck
upward, closes his tail and holds it slightly below horizontal, and
sleeks his neck feathers. Sleeked Posture occurs during Wing-fighting
and also when the male is alarmed by an approaching predator or a man.

19

Figure 4. Strut. The chest sac is being lifted as the wings scrape
the sides of the chest.

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21

Figure 6. Semi-collapsed Posture.

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Figure 8. Facing-past Encounter.

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Post-copulatory Ruffling: After only a second or two of cloacal
contact, the female dashes forth from under the male. The male is left
squatting on the ground, his wings still sprawled to either side. After
a pause of 5-10 seconds he resumes Strutting. The female, as she dashes
forward, begins vigorous shaking movements. She rapidly opens and
closes her tail, fluffs all her feather tracts, and shakes her partly
opened wings up and down alternately. This behavior apparently only
follows completed copulations (Chapter VII, Section B) . In other
situations females occasionally shake themselves briefly, but after a
completed copulation a female will continue Post-copulatory Ruffling
for 10-15 seconds and even sporadically thereafter. As the Ruffling
subsides, the female begins prolonged periods of preening, which often
last fifteen minutes or more.

Quacking Call: a call uttered by flying females. The tonal
quality suggests the quacking of a female Mallard. Unlike a duck,
however, the Quacking Call of a female Sage Grouse is usually single,
rather than rapidly repeated. These calls are particularly frequent
as the females fly over the lek on arrival at dawn. However, as Lumsden
also noticed, they sometimes are heard when females take flight later
in the morning to leave the lek.

28

V. STUDY SITES AND PROCEDURES

A. Study Sites

In order to observe leks with different numbers of attending
males, I studied three leks in three seasons of field work. In 1967
I worked at the Muddy Springs Lek, Albany County, Wyoming (33 km north
of Laramie, Wyoming; 41° 32' 30" N, 105° 34' 40" W; R. 73 W, T. 18 N,
section 9; elevation 7200 feet), the same lek Scott (1942) had observed
in 1940 and 1941. Work continued from 10 April through 25 April. In
1968 I worked at the Fords Creek Lek, Fergus County, Montana (5 km
n6rth-north-west of Grass Range, Montana; 47° 08' 15" N, 108° 53' 05" W;
R. 22 E, T. 16 N, section 24; elevation 3550 feet), the lek that Lumsden
(1968) observed in 1965. Observations here continued from 16 March
through 25 April. I arrived in time to observe the lek briefly before
mating began on 27 March. In 1969 I made an attempt to begin studies
at a large lek well before mating began. However, an unseasonal snow
storm made travel over the prairie impossible until 31 March, once again
shortly before mating began. The lek studied was Dry Sandy Lek No. 1,
Sweetwater County, Wyoming (21 km north-east of Farson, Wyoming; 42° 13'
55" N, 109C 14' 40" f; R. 104 W, T. 26 N, section 17; elevation 6810
feet). After observations began on 31 March, they continued through
28 April.

B. Observation Procedures

Each year on arrival I erected a blind with a clear view of one
mating center. Subsequently, aside from minor adjustments, I did not
move my blind, in hopes of encouraging the grouse to habituate to its
presence. This procedure seemed successful, since in each year both
males and females regularly walked calmly within a few yards of my
blind. However, as I shall describe below, my blinds may have caused
some disturbances soon after they were erected in 1967 and 1968.

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Accurate counts of the numbers of females were even more
difficult to obtain, owing to the large areas covered by the leks in
1967 and 1969 and to the inconspicuousness of the drab females among
the sagebrush. In 1969 I made special efforts to get counts of females
at different times during the morning on several days.

D. Recognition of Individuals and Age-classes

To minimize disturbance of the birds' behavior I did not capture
grouse on their leks to mark. them. Instead, I discovered that I could
recognize individual males and separate yearling and older males
without handling the birds. I was unable to detect individual differ-
ences among the females, though.

Individual differences among males were found in the numbers of
rectrices and the patterns produced by the white tips of the under tail-
coverts. In the Strutting Posture a male cocks his tail vertically and
fans it, so that when he is seen from the rear the white tips to his
under tail-coverts form a bold pattern of white spots. Soon after
beginning field work in 1968 I realized that these patterns remained
stable from day to day. By carefully recording the patterns of spots
I could identify individual males throughout a season. In 1968 and
1969 most males ' under tail-coverts were photographed at intervals of
a week or more throughout the duration of my observations. Figure 9
shows the distinctive patterns of four typical males in 1969. During
my observations in 1967 I had to rely solely on the site-constancy of
the males near my blind to identify them. However, by examining the
films taken at intervals throughout that season, I could confirm by
their under tail-covert patterns the identities of six males around
the mating center.

Major changes in these patterns seemed to occur only when a male lost
under tail-covert feathers, so that a gap appeared in his previous
pattern. In 1969 I observed a Facing-past Encounter in which Male 9
lost about three of his under tail-coverts in a fight. I recorded

30

before and after the encounter the pattern of his white spots. In
1968 Male N*s under tail-covert pattern changed by the deletion of
several feathers. Although I did not see the fight which I presume
was responsible for this deletion, everything indicated that the same
individual was involved before and after the change.

Further confirmation of a male's individual identification came
from the number of his rectrices. Most males had either 10 tail feathers
on each side or 9. A few males had 9 on one side and 10 on the other.
I had difficulty counting the tail feathers of males more than 30 meters
from my blind. I recorded the numbers of rectrices of all closer males
several times during the season.

Occasionally I could identify an individual male by other pecu-
liarities. Male B in 1968 had an unusually white throat. Male TM in
1968 and Males 10 and 12 in 1969 had lost or broken one or more of
their tail feathers.

Maps 1, 3, and 11a present the locations of individually identified
males in 1967, 1968, and 1969. I have also indicated positions regularly
occupied by males which I did not identify individually.

Sage Grouse like other grouse retain their outer two juvenile
primaries during their first year of life, so yearling Sage Grouse can
be reliably identified in the hand (Petrides, 1942; Patterson, 1952;
Eng, 1955) . Other characteristics of yearling males are reviewed by
Patterson (1952: 128, 153-4) and briefly by Eng (1963). They mention
the yearlings' shorter, blunter rectrices, smaller chest sacs, and a
brownish zone across the lower chest which in older males is white.
The first distinction was the easiest for me to observe in the
field. Few males had tail feathers intermediate between the long,
tapering tips of the older males and the blunter tips of the yearling
males. Furthermore, tail feathers of yearling males were often of
irregular length. Other characteristics became less noticeable as
the season advanced, as Eng (1963) also notod. For instance, in late
March and early April occasional males with noticeably smaller chest

•

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32

peripheral males were farther from my blind and thus more difficult
to observe. On the other hand, on days of peak mating the one or two
males in the center of the pack of females often copulated so frequently
that I must have missed some of their matings. It is impossible to
assess how these counteracting biases might have affected my data on
the distribution of matings among different males. My impression is
that, if anything, I underestimated the proportion of matings performed
by the few very successful males.

F. Records of Spatial Relationships

In order to facilitate locating birds on the lek, I placed small
flags on a 50-foot (15.4-meter) grid over the area under study. Each
flag was made of a few short pieces of surveyor's marking tape tied
to the end of a meter length of heavy wire. The birds appeared to
ignore these markers. To aid further in locating positions on the lek
I prepared base maps on which all salient clumps of sagebrush were
indicated. I judged that inaccuracies in locating a bird's position
were probably no more than one-half meter. However, birds over 50
meters from my blind were more difficult to locate, and their positions
probably included greater inaccuracies especially in the radial direction
from my blind.

I frequently mapped the birds' positions and recorded their
movements in field notebooks. However, most of my data on the
positions of matings and Facing-past Encounters come from time-lapse
films. I contrived a method for advancing a Bolex H16 movie camera one
frame at a time by means of a solenoid regulated by a timing module.
On 29 mornings in 1968 and 1969 I recorded birds' activities on time-
lapse film during one to two and a half hours. The field of view
usually included 5-10 territorial males at various distances from a
mating center. In 1968 I used a filming speed of one frame every 3-4
seconds; in 1969 I filmed at about 1 frame/second. On each morning
I determined the precise rate with a stopwatch. Since males Strutted
every 6-10 seconds and remained in one position between Struts, except

P

34

of time-lapse film from 27, 29, and 31 March and 1 and 3 April 1968.
All sequences were separated by at least five minutes. Each male in
erch six-minute period was classified according to his proximity to
females. Three situations were recognized: (1) a female or females
within his territory (Situation T) ; (2) a female or females within 10
meters of the male in a neighboring territory but not in his own
territory (Situation N) ; and (3) no females within 10 meters (Situation
0) during the six-minute period. Thus the behavior of individual males
could be compared under conditions of roughly equivalent proximity to
females. Eight males appeared in eight or more of the 28 six-minute
samples: Male D in 28 samples; Male B in 27; Male X in 22; Males TM
aud UT in 19 each; Male A in 18; Male C in 10; and Male CN in 8 samples.

In each six-minute period I counted how many times each male in
view during the entire period Strutted. In addition, in every third
frame (about every 12 seconds) I recorded whether the male was in
Strutting Posture, in Semi-collapsed Posture, in Collapsed Posture,
in a Facing-past Encounter, mating, or chasing an intruding male (see
Chapter IX, Section J). Thus, for each male in each sample I could
estimate the total amount of time spent in Strutting Posture and in
Facing-past Encounters and the total number of Struts during the period.
For example, the proportion of time spent in Strutting Posture was
estimated by dividing the number of sampled frames in which the male
appeared in Strutting Posture by the total number of sampled frames in
the six-minute period. The proportion of time in Facing-past Encounters
was calculated in the same way.

I also recorded the number of Facing-past Encounters in which
each male participated during each six-minute period. The duration of
each Facing-past Encounter was estimated by dividing the number of
sampled frames in which it appeared by the number of seconds between
sampled frames (about 12 seconds, varying slightly as my filming rate
changed). These estimates are not reliable for very long and very
short Facing-past Encounters. Brief encounters, which did not appear
in a sampled frame (recall that I sampled every third frame) were

«
J

36

mating center or not. For each pausing female I recorded whether or
not she was within 3 meters of a male, whether or not she was within
3 meters of another female, and whether or not she was feeding or
preening. Twenty-three such surveys were completed. However, in two
of the surveys I recorded fewer than 20 females, which probably does
not constitute an adequate sample for estimating females' activities.
On 16 April ten surveys were completed, of which nine included more
than 20 females.

W

T"

38

between rises in the surface of the land at least half a kilometer
away in either direction. The lek I studied in 1968 was also in such
a shallow valley. For a bird like the Sage Grouse, whose eye-level is
no more than half a meter above the ground, a broad valley may offer
advantages in detecting approaching predators (see Chapter IX, Section I).
The bird would have a clear view of the ground as far as the nearest
rises in the prairie, often half a kilometer or more in all directions.
A position on top of a rise, in contrast, would leave the adjacent low-
lying areas obscured by sagebrush near the bird.

Leks also usually occurred in areas where the sagebrush was
sparser and grass or bare soil covered more of the ground. Such areas
probably allowed the males greater freedom of movement.

Even within a lek displaying males preferred areas with certain
features over other areas. Here again these preferences cannot explain
entirely the spatial relationships of the males. In Petroleum and
Fergus Counties, Montana, leks tended to occur in areas where the sod-
forming short grass, Bouteloua gracilis, covered much of the ground.
Territorial males on the Fords Creek Lek preferred to Strut on patches
of this grass and avoided adjacent patches of bare ground a few centimeters
lower in level. A sod-forming short grass also covered much of the
Muddy Springs Lek, which I studied in 1967. Mats of sod were inter-
spersed with patches of a grass which did not form sod and which grew
on slightly lower ground. The displaying grouse preferred to use the
carpet-like mats of sod. In 1969 little grass grew on the lek I
studied. Here the males preferred areas where the sage grew more
sparsely.

Some selection of sites for display thus occurred on both a
large scale and a small scale. On a large scale, the leks appear
selectively in areas where shrubby vegetation is more widely dispersed.
On a small scale, the displaying males chose slightly elevated mats of
sod. Both preferences undoubtedly increased the displaying males'
conspicuousness and freedom of movement. P-wevc*, these preferences

J

40

of my blind (northern site; n, Map 1). However, that evening I
watched Male A twice move about 10 meters south-west and begin to Strut
in an area about 10 meters north-west of my blind (Map 1) . He encoun-
tered a neighboring male at the edge of this area. Also one female
solicited for copulation at this south-west site. The following
morning, 12 April, the females at first congregated even farther from
my blind, 30 meters to the north, but after about 15 minutes they
dispersed and gathered again around Male A, 15 meters north of my blind.
It snowed most -of that afternoon, so the following morning, 13 April,
very few females appeared on the lek. Some of these visited the area
15 meters north and others the area 10 meters north-west of my blind.
On the morning of 14 April, when activity once again reached high
levels, females showed no interest in the area 15 meters north of my
blind, and as many as 42 congregated in the area 10 meters south-west
instead. Male A never left this area all morning and copulated over
30 times. On subsequent mornings females always congregated at this
same place and never again revealed any interest in the site north of
my blind. This south-western site one year later showed clear signs
that the packs of females had gathered there in 1968 also. I believe
that by placing my blind so close to the south-western site I may have
temporarily influenced the females to gather farther away. On this
same lek Scott (1942) observed one mating center in 1941 which moved
temporarily after a snow drift had covered its original location.

The situation in 1968 was more confusing. The Fords Creek Lek
included many fewer males, probably only 16 regularly attending adult
males. Furthermore, the birds at this lek had been trapped with cannon
nets in the immediately preceding years. The small size or human
disturbances may have enhanced the instability of this lek. I began
my observations on 16 March. Females never exceeded 10 nor showed any
consistent localization until 27 March, when females congregated in
the area used by Male D. I saw ma tings for the first time that season
on 27 March. On 29 and 31 March large packs of females assembled in
the same position, but before activity had ended each morning the

41

females had gradually drifted 8 meters south-west (Maps 4, Sab) . On
both mornings Male D accompanied the females as they moved. On 1 April
somewhat fewer females appeared but they also drifted south-west in
the course of the morning. Ten centimeters of snow covered the ground
on 3 April, and only a few females visited the lek. The following two
mornings females were again numerous, but they clustered immediately
at the site where on previous days they had ended the morning, 8 meters
south-west of the area in which they had originally congregated.
Through 5 April I had observed the lek from a blind placed 25 meters
from the pack of females. On the afternoon of 5 April I erected a
second blind (Blind #2, Map 3) on a platform 1.2 meters high in hopes
of obtaining a better vantage for recording the birds' spatial relation-
ships. The new blind was also 25 meters from the site at which females
congregated. On the morning of 8 April I began to film activity on the
lek from this elevated blind, and on that morning for the first time
females began congregating at a site 15 meters north-west of Male D's
territory and farther from both of my blinds (Map 9ab) . Especially when
the females returned after an eagle had flushed the grouse at 05:28, all
the females went to the new site farther from my blinds. After that
date most females tended to congregate in this farther area, although
on most mornings some females did come to Male D's territory. Thus,
the longer move in 1968 appeared correlated with the erection of my
elevated blind. My presence may not have influenced the shorter and
more gradual shift in the position of the mating center. Possibly
mating centers are less well localized in small leks and may shift
positions over distances of 8 meters or so. In 1969 on the large Dry
Sandy Lek, three mating centers which I could clearly observe remained
in the same positions throughout the season. In the mating center which
I studied thoroughly that year virtually all matings occurred in an
area 5x8 meters.

_J

43

IppftBif!}

DRY SANDY LEK, April 1969

® Mating centers
\\v Areas occupied by males
^-^ Drainages

Dirt road

Creek bed

Figure 10. Four mating centers on the Dry Sandy Lek, 1969.

Forty to 100 males displayed around each mating center (hatched
areas) .

44
VII. TERRITORIALITY AND MATING: A STATEMENT OF THE PROBLEM

An examination of some of the features of territoriality in the
Sage Grouse will define the problem of behavioral regulation of their
mating system. After presenting evidence that frequent mating by a
male depends on the coincidence of his territory with a mating center,
subsequent sections will examine how such a coincidence might arise.

A. Stability of the Territorial Boundary

For many males the boundaries of their territories remained
relatively constant over the three to four weeks of my observations
each season. Some changes did occur, as I shall discuss below, especially
when vacancies arose on the lek. In 1968 many changes occurred, largely
because three territorial males disappeared from the lek during my
observations and because the position of the mating center drifted 8
meters from its original location. In 1969, on the other hand, most
of the fully adult males' territorial boundaries remained in essentially
the same locations throughout the period from 8-28 April (Maps 11a, b, c) .

B. Distribution of Matings Among Males

The territorial males varied greatly in their success in mating.
Those established within or near a mating center accomplished virtually
all the matings, while most males never mated during the entire season.
Tables 2, 3 and 4 present data on all the copulations which I observed
during the three seasons of study. A completed copulation satisfied
three criteria: (l) the copulation was not interrupted by another male;

(2) the copulating male lowered his tail below the female's tail; and

(3) the female terminated the copulation by dashing forward with Post-
copulatory Ruffling (see Chapter IV, Section B) . Vigorous Post-copulatory
Ruffling seemed specific to completed copulations. If a male mounted

but did not lower his tail before a neighboring male interrupted the
copulation, the female never showed such vigorous ruffling or persistent
preening. A completed copulation presumably inseminated the female,

J

Hf Table 3 H

Copulations Recorded on the Fords Creek Lek, 1968. For explanation see Table 2. Maximum number , of

■■ males on this lek = 30.

Individual Males Total j

Number of
Unident. Completed
Date DNUT TM F AX ST Periph. B C Copulations

27 March 7 7

1 28 2 2

29 14 14

31 6 1 , 7

1 April 7 (1) (1) 1#1 9

1

4 3 1(1) . 4

5 1 • 2(2) 1 (1) 4

8 4(1) Absent 4

9 3(1) 2

2 Absent 7

10 1 4(3)

1(1)

6

12 4 1

2 1

8

14 10

1 (1)

11

1 15

1

22

0

23 1

1

24

1

1

Total Number
of Completed
| Copulations 41 25 5 3

0

en

3 3 11 1,1,1,1 0 0 87(13)

% 47.2 28.8 5.8 3.E

3.5 3.5 1.2 x 6

|

1

!

47

Table 4

Copulations Recorded on the Dry Sandy Lek, Mating Center #3, 1969
For explanation see Table 3 . Maximum number of males around this
mating center = 52.

Individual Males

Date 4

6 April

8

9 3

11 3(2)

14 2,6*

15

16 3(1)

17 6 (1)

18 2(1)
21

22
23
24
26
27
28

Total
Number of
Completed
Copulations 19 17

8

20

12

1
6(1)

12*

1
9
9*

1
1

2*

(1)

(1)

1(1)

2*

3*

43.8 41.4 7.3

1
2.4

1
2.4

1
2.4

Total

Number of

Completed

Copulations

0

0
5

9
2
0

4

9

12

0
0
0
0

0

0
0

42(9)

* Matings with an abnormal female (see text) ; not included among
completed copulations.

4

I

49

problems, so I probably overlooked more matings than I did in previous
years.

On four mornings in 1969 a female with peculiar behavior visited
the lek. On each of these days one female copulated repeatedly with as
many as four males , but she always appeared unable to complete a normal
copulation. She did not raise her tail as normal females do after the
mounted male begins to lower his tail . Males would repeatedly attempt
to effect cloacal contact with this female and finally simply dismount.
Furthermore, the female never performed Post-copulatory Ruffling. Her
pre-copulatory behavior was also abnormal. Although she remained in
one spot while a male Strutted around her, she did not ordinarily
orient obliquely away from the front of the male as normal females do.
On 23 April the males ' repeated treadings actually drew blood from the
abnormal female's back, which stained the white chests of the males
attempting to copulate with her. Since no more than one such female
appeared on any day, and no other female in other years showed similar
behavior, I assume that the 'same individual came all four mornings. I
have not included her attempted copulations in the totals of copulations
completed by each male in 1969. Scott (1942) and Patterson (1952) each
observed one female that copulated repeatedly.

Males successful in mating are those with territories at or
adjacent to a mating center. In each year two or three males, 5-10%
of the males present around each mating center I studied, accomplished
over 75% of all completed copulations there. Figure 11 shows how skewed
the distribution of matings among the individual males in fact was. Data
collected at each of the three mating centers I studied are superimposed.
Males from each year are ranked on the abscissa in decreasing order of
success in mating. Since the total number of males surrounding each
mating center ranged from 30 to 60, most of the males never mated at
all . Matings by peripheral males tended to occur late in the morning
and late in the season when fewer females were present on the lek and
they were more dispersed.

50

Distribution of matjngs among males
at three mating centers

in

en

c

■a

>

a>
w
-D

o

c

O)

o

&

1967
1968
1969

50
40
30
20
10

Number of

matings

observed

Maximum
number of
males

0 — 0

1967 Muddy Springs Lek,
East Mating Center

105

60+5

X X

1968 Fords Creek Lek

87

30

D-a

1969 Dry Sandy Lek,

Mating Center No.3

41

52

males = A 2W C
males = D N F
males- 4 7 3

N 3W
UT TM A
9 8 12

X R ST

Figure ll. The distribution of matings among males at three mating
centers. Individual males are listed on the abscissa in decreasing
order of their successes in mating. The ordinate is the percentage
of the total number of completed copulations observed each year.

•

52

In contrast, male Black Grouse remain within their territories even
when females are on the lek. In the exclusiveness of their territories,
male Sage Grouse seem to resemble Black Grouse more than Prairie Chickens
(see below) . The high frequency of interrupted copulations in Sage
Grouse might instead stem from the small size of the males' territories,
which had a minimum area of 30-50 square meters in contrast with 75-100
square meters in Prairie Chickens and Black Grouse (Chapter XIII,
Section A) . A smaller territory would have a greater fraction of its
area close to the boundary. Hence males might attempt copulations more
frequently near their boundary, where interruptions are likely. Al-
though one in every six or seven attempted copulations was interrupted,
I never felt that a female's chances for copulation were limited.

Do those males more successful in mating tend to interrupt the
matings of their less successful neighbors more than vice versa? The
small numbers of interrupted matings which I observed cannot provide a
conclusive answer to this question (Tables 5, 6, and 7). In some cases,
pairs of neighboring males interrupted each other's matings almost
equally frequently (Males C and N, 1967). In other pairs of males one
performed more interruptions than the other. However, the male that was
interrupted most often sometimes performed more and sometimes fewer
matings than his opponent (compare Males N and F, 1968, and Males A and
C, 1967) . Usually the males less successful in mating seem to have a
greater proportion of their attempted matings interrupted, although
these interrupt ions as usual involve matings attempted near territorial
boundaries. If females congregate within one male's territory, then any
matings attempted by neighboring males are most likely to occur near
the territorial boundary.

Within his territory a copulating male appears immune from his
neighbors' interruptions. Only matings attempted near or within the
zones of Facing-past Encounters, which define territorial boundaries,
risk interruptions by neighboring males. A male achieves success in
mating by occupying a territory that includes a mating center or part
of one, not by directly preventing matings by other males. By contrast,

Table 5

Interrupted Copulations, Muddy Springs Lek, 1967

53

Interrupting
Male

Copulating Male
C N

Totals

Totals

4

11

7

0

22

4

Table 6

Interrupted Copulations, Fords Creek Lek, 1968

Copulating Male

Interrupting
Male

Totals

54

56

•

non-radom mating in captive flocks of junglefowl does depend on one
male's ability to interrupt others' copulations without himself being
interrupted (Lill, 1966).

Thus, one sort of polarity is especially prominent among
territorial male Sage Grouse surrounding a mating center: their differ-
ing successes in mating.

_l

57

VIII. BASES FOR FEMALE SELECTION

How a male's territory comes to coincide with a mating center
depends on behavior of both the males and females. First, I shall
consider the question of how a female choses where to mate. Since
mating centers probably recur from year to year at roughly the same
place within a lek (Chapter VI) , females need not discriminate any
individual differences in the behavior of the males in order to select
a mating partner. Some evidence will indicate that individual differ-
ences in the behavior of males are slight and are not good predictors
of a male's success in mating.

A. Behavior of Females

Before discussing how females might select a place to mate, I
shall describe in more detail the behavior of females that leads to
successful copulation.

Observations on several leks between 16 March and 24 March,
1968, suggest that females first appear on the leks in flocks of 10-30.
Often two or three males in Collapsed Posture accompany these flocks
without performing any displays. These males probably are yearlings,
as I shall discuss in a later section (Chapter IX, Section J). The
loose groups of females wander across the lek without much localization
of their movements. The females in these early groups show little
behavior that suggests an inclination to mate. The females usually
space themselves 1-3 meters apart and drift in unorganized fashion
across the lek. They assume relaxed postures, preen, and feed. At
this stage in the season I never saw females gather in dense packs or
perform any precopulatory activities. On several occasions I saw large
groups of females fly away from the lek together.

In 1969 when I first arrived at the Dry Sandy Lek the females
exhibited similar behavior. On 31 March and 1 April dispersed groups
of females, usually spaced 1-3 meters apart, wandered irregularly
around the lek. Often these groups included males in Collapsed Posture.
The females spent much time' preening and showed no pre-copulatory

59

0500

0600

0700

0800

Sunrise

Time (MST)

Figure 12. Numbers of females during the morning of April 16, 1969.

Dots, number of females at mating centers; open circles, number
of females not at a mating center; crosses, number of flying females.

•I

60

noticed the Quacking Call of female Sage Grouse. He concluded that
after arrival females often made short flights from one place in the
lek to another and uttered the Quacking Calls during these flights.
My observations, however, suggested that once females had alighted at
the periphery of the lek, they usually did not take flight again until
they left the lek later in the morning. Although the Quacking Calls
occur regularly when females are flying into the lek early in the
morning, they are not restricted to this context. Both Lumsden and I
heard females give this call occasionally as they took flight late in
the morning to fly away from the lek.

After alighting at the edge of the lek a female usually paused
for as long as a minute and then began to walk. Early in the morning
the paths of walking females rarely showed signs of aimlessness .
Instead, females usually walked steadily and directly toward the near-
est mating center. In 1968 and 1969 I mapped the paths of a number of
females after they had landed at the edge of the lek. Many of these
females pursued direct or nearly direct paths toward the nearest
mating center. Occasionally a female would pause briefly before
resuming her course or would detour to avoid a male Strutting directly
in her path. Sometimes a female would increase her pace in order to
avoid an approaching male. From my samples of female activity in
1969 I could estimate the fraction of all walking females that were
moving more or less toward (± 30 ) the nearest mating center. Before
and around sunrise the majority of walking females were headed for the
nearest mating center (Figure 13) . When a female came within 15-20
meters of a pack of females congregated at a mating center , she rarely
stopped or deviated before entering the pack.

On a large lek with several mating centers females did occas-
ionally either walk or fly a short distance from the vicinity of one
mating center to another mating center. In 1969 I observed such be-
havior 10 times between 9 and 18 April.

P

62

Once in a pack at a mating center, a female assumed a relaxed
posture with her head drawn onto her shoulders. Individual females
became extremely difficult to follow within these dense packs. All the
females remained in a relaxed posture until motivated to copulate. In
spite of the fact that the females were usually spaced within a half
meter of each other, aggression between females was limited to low-
intensity encounters. Females moved to avoid the path of an approaching
bird and moved away from a zone about one meter in radius around the
front of the Strutting male.. Since the male Strutted back and forth
within the center of this pack, females were continuously shuffled in
position, usually without much aggression among themselves. Occasionally
after the male had approached a female closely or after a female had
briefly assumed the pre-copulatory Solicitation Posture, she would dash
forward a meter or so and chase briefly any female in her path. In 1967
on several mornings I observed one female that exhibited unusually
aggressive behavior toward the other females. This individual spent
most of a morning dashing vigorously after other females , often over
distances of 2-5 meters. If she managed to catch another female, or if
another female adopted the Solicitation Posture near her, she would
attempt to mount. A further abnormality was her tendency to erect the
feathers on the side of her neck, like a male. I presume that the same
individual female with abnormal behavior appeared each of these mornings.
Other observers have noted females which showed elements of male
behavior (Scott, 1942; Patterson, 1952; Lumsden, 1958).

Females usually adopt the Solicitation Posture near the termi-
nation of the male's Strut. The male waits 7-10 seconds after his
previous Strut before mounting. The female may relax before he mounts
and then solicit again following his next Strut. Usually if a female
continues to solicit she progressively increases the duration and
intensity of her Solicitation Posture. Eventually a female may hold
her Solicitation Posture continuously between successive Struts of the
male. Ordinarily a female does not solici . except when she is within
one-half meter of the front of the male, but intensely and persistently

J

•

63

soliciting females sometimes assume their posture more than a meter
from the male. Usually the male will mount any female that will hold
a Solicitation Posture while he steps toward her. Occasionally, though,
especially later in a morning a female may Solicit repeatedly and
intensely after 5 or more Struts before the male will mount. Thus
continued interaction of a male and a female appears necessary to
stimulate both sexes to copulate. Usually, though, a male will respond
to any female that holds a Solicitation Posture. Once a male mounts
copulation is usually completed in 3-5 seconds, unless a neighboring
male interrupts the copulation.

Most females probably mate only once a season. By observing 16
marked females , Lumsden (1968) obtained evidence that each female
visited a lek on several mornings before she was seen to mate. None
of the seven females observed mating was ever seen again. Pyrah (1964)
records a captive hen which laid six fertile eggs after she was removed
from any contact with males. Her last fertile egg was laid at least 17
days after she could possibly have copulated. Thus one copulation may
fertilize a female's entire clutch, which averages 6-8 eggs. On the
other hand, the number of copulations which occur at a mating center
probably exceeds the maximum number of males surrounding the mating
center. At the mating centers which I studied in 1967, 1968, and 1969,
I recorded 1.8, 2.9, and 0.8 times as many copulations as males. Since
I did not observe the leks continuously, I must have missed many
matings, so the ratios of copulations/total males must actually exceed
these figures. If some females copulate more than once in a season,
the number of copulations might well exceed the maximum number of males.
However, ratios greater than one might also arise from an unequal sex
ratio at that season, or from absence of some males from the leks.
Particularly some of the yearling males might not attend the leks
regularly, and thus at any one time a proportion of the males might
not appear on the leks. Only further studies of individually marked
females will clarify the number of times a female mates in one season.
Available evidence suggests that each female visits a lek on several

They usually flew over the lek, landed in the periphery, and then
walked to the mating center. The relatively direct paths of females
walking toward a mating center early in the morning suggests that many
individual females know at least approximately where the mating center
is located.

A female could use five classes of cues in order to locate a
mating center: (1) individual differences among the males which corre-
late with their success in mating; (2) overall patterns in the distri-
bution or activity of males within a lek; (3) the presence of a conspic-
uous pack of females at a mating center; (4) guidance from other females
which had visited the mating center previously; and (5) memory of the
topography of the lek after her first few visits.

B. Individual Differences Among Males

Evidence presented in Chapter VI suggests that mating centers
recur from year to year at roughly or precisely the same sites within
a lek. Thus the mating centers within a lek apparently show a constancy
of location comparable to the traditional locations of the leks them-
selves. This stability in the locations of mating centers could con-
ceivably depend on the presence from year to year of the same individual
males on the lek. However, yearly mortality among fully adult males
two or more years old probably approximates 25-50% (Chapter X), so the
turn-over of individual males on any lek from year to year must be
considerable.

I was unable to find differences among individual males ' Struts
that correlate with differences in mating success. The Strut display
is remarkably stereotyped in its internal organization, as I have
determined from an examination of filmed sequences of different
males' displays. Table 8 presents counts of the numbers of frames
between two distinctive stages in the Struts of different males in
1967. Frames were counted between the time at which the male's chest
sac reached its highest position during its first rising phase and the

66

Table 8

Durations of Strut Displays by Different Males in 1967 .

Figures are numbers of frames between two distinctive

stages in the Strut (see text).

Males, ranked in decreasing order of success in mating

Frames/

sec

(1)

A

C

2W

N

6W

S

SE

W

N

x(2)

2

3

4

5

7

7

7

24

-

3

-

3

-

7

-

-

X

44.3

42.3

45.14

SD

0.58

1.53

1.34

SE

0.33

0.88

0.51

cv(3)

3.0

18

N

12

3

-

14

-

4

5

-

X

30.0

31.7

28.8

31.0

31.4

SD

1.13

0.58

0.80

0.82

1.14

SE

0.33

0.33

0.21

0.41

0.51

cv

3.8

2.8

2.6

3.6

12

N

18

8

14

27

10

-

-

9

X

20.9

20.4

20.9

19.6

19.9

20.0

SD

0.94

1.06

0.92

0.89

0.57

0.87

SE

0.22

0.37

0.24

0.17

0.18

0.29

CV

4.5

5.2

4.4

4.5

2.8

4.3

(1) Approximate, since the camera settings were not calibrated. The
figures in the table suggest that the filming rate at a camera
setting of 24 was slightly greater than twice the rate at a setting
of 12.

(2) Rank in mating success.

(3) Coefficient of variation: 100 x SD/X

6S

40

c 30-

E

in
3

20

en

10

t

E

I

I *

3

P

O ■

]

T N 0 TNO TNO TN0TN0TN0TN0TN0

Males:

D

TM

B

UT

CN

T= Female (s) in the male's territory
N = Female (s) in a neighboring territory
0=No females nearby

Figure 14. Eight individual males compared for differences in their
rates of Strutting in three situations: female (s) within the male's
territory (black histograms, T) ; female (s) within 10 meters in a
neighboring territory (stippled histograms, N) ; and no females nearby
(open histograms, 0). The ordinate is Struts per six-minute period.
In the histograms each small square represents data from one six-
minute period.

69

that this category was heterogenous

Either males responded to females
in neighboring territories with considerable variability, or I failed
to isolate the cues to which the males responded. By comparing the
black histograms (Situation T) for the males sampled, I could establish
only one significant difference in their responses to females in their
territories: Male D, who did most of the mating, tended to Strut less
frequently than his neighbor, Male B, who never mated successfully
(Wilcoxon matched-pairs ranked-sign test, T= 2.5, N = 10, p < .01;
Siegel, 1956). Male D's lower rates of Strutting are not explained by
interruptions for matings or other activities. Instead, Male D in
comparison with his neighbor, Male TM, often Strutted less frequently
per minute in Strutting Posture (Table 9). Fatigue may explain Male D's
slower rate of Strutting. Late in the morning his matings and his rates
of Strutting near females were noticeably slower. In 1967 the most
successful breeder also showed signs of fatigue on mornings when females
were numerous . Late in the morning his matings often became noticeably
slower, ind he was slower to resume Strutting after Facing-past Encounters,
Scott (1942) noticed similar signs of fatigue late in the morning among
males which mated frequently.

The sample in which Male D Strutted least frequently with females
in his territory, 21 Struts/6 minutes, came nearly two hours after sun-
rise, near the end of the morning's activity. In general, though, rates
of Strutting show little correlation with the time of morning, provided
the males' proximities to females are taken into account (Figure 15).
Males with females in their territories Strut at high rates regardless
of the time of morning.

Therefore, rates of Strutting correlate best with a male's
proximity to females, rather than with the male's position on the lek
or with the time of morning. Figure 16, based on the same data,
emphasizes that males Strut at high rates more often when females are in
their territories. Males at or near a mating center must Strut many
more times in a morning than do more peripheral males. Their greater

P

71

• Moles in situation T (/, Mole D)
x ii ii ii N
o ii ii ii D

40-

30

c

E

20-

10-

x t

• • •

t fx

0 •

x°

y

X

X

/•.

8

X •

E

J '

X

X

o
x

30

o

x °

0

O-hSt
60

— r~
90

0

o
o

-°r

Minutes after Sunrise

f

/

o
-Or

120

Figure 15. Rates of Strutting in relation to time of morning and
proximity to females. Black dots: males with females in their
territories. Crosses: males with females within 10 meters in a
neighboring territory. Open circles: males with no females nearby.
Data from 27 March - 1 April 1968. Larger symbols indicate more than
one male with that value. Arrows indicate extreme values for Male D
in Situation T.

J

• Moles in situation T (n = 5l)

x ii ii .1 N (n = 36)

"0 ii n ii 0 (n = 66)

0-9 10-19 20-29 30-39
Struts/6 min

40+

72

Figure 16. Rates of Strutting in relation to proximity to females.

Table 10

Relationship between the durations of males' Facing-past Encounters and proximity to females.
Figures in the table are numbers of encounters. Data from 28 six-minute samples, 27 March -
3 April, 1968.

Durations of Facing-past Encounters

tn
•)■>
a
<u
a
o
a
a.
o

<M

0

31
IB
•H

•H
0

s T~T

S T-N

a

o

■h N-N

o

*> N-O

0-0

T-D

(1)

1-35
sec

36-60
sec

14

5
3
3

ab(2)

be

4
4
2
2
4
1

1.0-1.9
min

2
3

5

2.0-3.9

min

3

2
6

1

4.0-5.9
min

1
1
2

6.0+

min

9
1

Total Number
of Encounters

14

9

11

11

32

3

Number of
Encounters

with
Wing-fightB

2
3
0
0
5
0

(1) "t-T" signifies that both opponents had females within their territories (Situation T) . A male

in Situation N has a female within 10 meters in a neighboring territory. A male in Situation 0

has no female nearby.

2 2

(2) Comparisons: a, X, (Yates correction) = 22.96, p < .001; b, X^ (Yates correction = 11.82,

2

p < .001 j c, Xx (Yates correction) = 0.029, p > .80.

M

*>

*

Table 11
Durations and frequencies of Facing-past Encounters by individual males at different distances

from females.

Proximity
to females

Durations of Encounters

Male

D

TM

B

A
UT

X

c

CN

1-35
sec

15
8

4
1
2

36-60
sec

1-1.9

min

2-3.9

min

4-5.9
min

6.0+
min

Number of Encounters
Total Initiated Terminated

17

8
4

1
4
1
2
0

17

8

4
1
4
1
1

17

8
4

1
4
1
1

No. of

Samples

22
7

10

4
4
1
3
0

Subtotals

36

36

51

N

D

TM
B
A

UT
X
C

CN

3
3

3
2

4

1

2
3

Subtotals

D
TM

B

A
UT

X

c

CN

Subtotals
Totals

2
2
5

1
1
1
1
2

1
2

2
2

3
4
3
2
3
1
1

1
1

4
4
1
1
2
3

0
13
5
0
12
9
0
1

2

8

12

15
9
5
6
9

11
5

10
8

34

2
6
7
9
6
1
2
4

37
107

10

4

11

7

32

0

5
7
9
6
4
4
6

41
109

0
5
8
1
8
13
1
0

36

6
7
9
13
7
9
6
8

65
152

76

•

Frequencies of Facing-past Kncounters might also differ
among individual males. Comparisons among the males are complicated by
two considerations : (1) each encounter involves the interaction of two
individuals, so a male's behavior is influenced both by his own tendencies
and by his neighbor's; and (2) different males have different numbers of
neighbors. In order to minimize the effects of these complications, I
determined the frequencies of Facing-past Encounters for pairs of males
when each male's proximity to females was known (Table 12). Although
the rate of terminating encounters often differed slightly from the
rate of initiations, these differences must be artifacts of my small
samples, since in the long run the rate of terminations must equal the
rate of initiations of Facing-past Encounters. For pairs of males other
than D-TM and D-B, encounters were most frequent in Situations T-T (0.27
initiations/6 minutes) and 0-0 (0.24 initiations/6 minutes) and least
frequent in Situation T-N ( 0.04 initiations/6 minutes). Between Male D
and his neighbors B and TM, encounters are most frequent in Situation
T-T (0.47 initiations/6 minutes), of intermediate frequency in Situation
T-N (0,31 initiations/6 minutes), and least frequent in Situation 0-0
(0.09 initiations/6 minutes) (Table 12). Thus, the most successful
breeder (Male D) encountered his neighbors, B and TM, least frequently
when neither had females nearby (Situation 0-0) , while other pairs of
males encountered each other least frequently in Situation T-N, when
one of them had females in his territory. However, the available data
are too few for such definite conclusions. Encounters were always most
frequent when two neighbors both had females within their territories.
Since in these cases the females usually occurred near the two neighbors '
common boundary, a high frequency of Facing-past Encounters would be
expected.

Do those males successful in mating engage in Wing-fighting more
frequently than others? Table 13 presents the incidence of Wing-fighting
in all Facing-past Encounters recorded for seven pairs of males filmed
on mornings in 1968 when females were numerous (27 March - 4 April,
except 3 April). Encounters that occurred earlier than an hour after

•

•I

to

78

Table 13

Proportion of Facing-past Encounters which Included
Wing-fighting. 27 March - 1 April, 1968

Earlier than one hour
after sunrise

Later than one hour
after sunrise

Opponents

Total

Proportion Total

Proportion

Number

with Number

with

of

Encounters

Wing-fighting of Encounters

Wing-fighting

D-TM

34

.21 13

.15

D-B

15

.20 3

0

D-X

5

.20 3

.33

Subtotals

54

.204 19

.158

TM-UT

11

.27 10

.10

B-A

6

0 5

.20

Subtotals

17

.176 15

.133

A-C

7

.57 9

.11

C-CN

17

.06 6

0

Subtotals

24

.209 15

.071

Grand Total

?

95

. 200 49

.140

Difference

oetween earlier

and later in the grand totals is

just signifi-

cant at .05

le

vel: Xi = 3.

95, .02 < p < .05. No other comparisons show

significant

differences.

79

sunrise are separated from ones that occurred later, when females were

less numerous on the lek, although not absent. The pairs of males are

grouped into three categories depending on their distances from the

mating center in the territory of Male D. In all three categories

Wing-fighting occurred in approximately one-fifth (0.18-0.21) of the

earlier Facing-past Encounters. Later in the morning the frequency of

Wing-fighting declined in all three groups, and the difference is signi-

2
ficant in a X test. Male D's higher incidence of Wing-fighting late in

the morning might depend on the regular presence of a few females in his

territory until late in the morning. These observations again suggest

that differences in the males' behavior relate primarily to differences

in their proximities to females.

In conclusion, these studies of territorial male Sage Grouse at
least two-years old have revealed no clear differences in their behavior
patterns or responsiveness to females which might explain the large
differences in their mating success. Those individuax differences which
I did detect among males, for instance, the durations of their Struts,
do not correlate with their differences in mating success. If the
females use individual differences in the behavior of males to select
a place to mate, they must depend on subtle distinctions. The obvious
differences between males successful in mating and those unsuccessful
stem from the presence of females at the mating center. For instance,
the males ' behavior during Facing-past Encounters and their frequencies
of Strutting depend primarily on their proximities to females and only
secondarily correlate with the males' successes in mating.

This section has only considered males at least two years old.
Yearling males differ from older males in several ways, as I shall
describe in Chapter IX, Section M.

•

81

E. OthBr Females

Does any evidence suggest that females use cues from other

females when they locate a mating center? I attempted to determine in

two ways whether pausing females were attracted to other females. Data

from my samples of female activity in 1969 suggest that females pausing

away from a mating center are associated with other females significantly

2
more often than are walking females (N = 500, X ■ 14.11, 1 df, p < .001) .

Also females paused within 10 feet of a male are near other females signi-
ficantly more often than are females paused farther from males (N = 267,
X =5.37, 1 df, p <,05). Yet females pausing farther than 10 feet

from males associate with other females no more often than do walking

2
females (N = 360, X = 2.88, 1 df, p < .05) . Therefore, females pausing

away from the mating center are attracted either to a Strutting male or

to the combination of a paused female and a Strutting male.

On 17 April 1969 I attempted to use a cluster of taxidermic
models of females to decoy females approaching a mating center. I placed
an artificial pack of five taxidermic mounts of females 45 feet from the
mating center. The decoy failed to attract any live females, although
a number walked directly past on their way to the mating center. The
negative result is inconclusive since most of the females this late in
the season had probably already visited the mating center at least once
on a previous morning. So no direct evidence can confirm that individual
females interact in locating the mating center. However, this possibility
seems so likely that further experiments would be worthwhile.

Possibly a female first identifies the position of a mating
center by seeing the conspicuous pack of females there . Since many
females visit a lek on several mornings before copulating, those
arriving later in the season might use cues from earlier arrivals to
locate the mating centers. This hypothesis would require that females
on their first visit arrive later in the morning than do females that
have been there before. After visiting a mating center several times
in one season, a female might remember at least the approximate location

82

from day to day and possibly from year to year. If yearling females
mate later than older females in the course of each season, they might
learn the position of a mating center by tradition. When trapping
females on leks in Montana, Robert Eng (personal communication) found
indications that yearling females do tend to appear on the leks later
in the season than older females. Dalke et al. (1963: 836) also suggest
that yearling females breed later than older females. The behavior of
yearling and older females requires further study.

Tf some females use cues from other females to locate a mating
center, then females must eventually come to remember the position of a
mating center after they have visited it several times in one season.
In other words , some females must eventually recognize the position of
the mating center independently of stimuli from other females.

F. Summary

A. female probably uses a number of different cues to find the
mating center and possibly different ones on her first visit as a
yearling than on later visits. Since the mating centers apparently
recur from year to year at the same sites within leks, females choosing
a place to mate would not have to rely on individual differences among
the males. Much of the variability in the behavior of different males
on a lek depends on their different proximities to females rather than
on inherent differences between the males . Present evidence suggests
that the density of male territories and the surface features of a lek
do not completely specify the positions of the narrowly localized
mating centers. However, the density of male territories would provide
some orientation for an arriving female. Further cues might come from
other females which remember the position of a mating center from
previous visits.

I^HM^^B^H^^H

84

the positions in which he Strutted when a small group of females lingered
temporarily in the territories of Males 9 and 20 one morning. In the
latter instance Male 8 and Male 13 actually trespassed as much as 2
meters beyond their usual zones of Facing-past Encounters.

Strutting males only occasionally transgressed their boundaries
and very rarely more than Male 8 did on this occasion. Males at least
two years old avoided deep intrusions into each others' territories.
Exceptions occurred occasionally when males were walking to their
positions on arrival at the lek (this Chapter, Section L) or when a
series of males abandoned their positions in order to follow a flock
of females leaving the lek early in the season (this Chapter, Section G) .

i

In the former case, when males which had to cross an occupied territory
in order to reach their own, they always adopted a Collapsed or Sleeked
Posture, detoured around the resident males, and ran or flew if necessary
to avoid them.

When a Strutting male encroached beyond his usual zone of Facing-
past Encounters, as did Males 8 and 13 in the example discussed above,
they usually retreated immediately to the zone of Facing-past Encounters
when the resident male initiated an encounter. Since these encroach-
ments occurred most often when a male had females in his territory, he
would only occasionally interrupt his Strutting around the females to
challenge his encroaching neighbors. In the case discussed above, Male
8 actually encountered Male 9 slightly beyond their usual zone of Facing-
past Encounters, although not so far beyond as Male 8 managed to Strut
in the intervals between his encounters with Male 9. A similar case
occurred on 14 April 1968 when Male N had many females within his
territory (see Chapter VI) . On this morning his neighbors X and UT
transgressed their usual boundaries and encountered Male N 5 meters
beyond their usual zones of Facing-past Encounters (Maps 9ab) . Usually,
however, a Strutting male, after encroaching slightly into a neighboring
territory, retreated to his usual zone of Facing-past Encounters when
challenged.

85

An inhibition against intruding into neighboring territories
became especially evident in two situations. When Male A disappeared
in 1967 and left a vacancy which included part of a mating center, the
vacancy was only gradually reoccupied over a period of several days
(this Chapter, Section H) . Secondly, males did not intrude deeply into
neighbors' territories to interrupt copulations (Chapter VII, Section
C).

B. Nature of the Territorial Boundary

Zones of Facing-past Encounters were usually unrelated to the
occurrence of clumps of sagebrush or other distinctive features on the
ground. In a few cases in which these zones were adjacent to isolated
clumps of sage, the zones were always stable. In 1968 a large clump
of sage lay just east of the area habitually used by A and C for their
Facing-past Encounters (Maps 5b, 8b). In 1969 Males 3 and 6 met each
other just south of an isolated clump of sage (Map lie) . In both these
cases, although individual encounters often did not occur in precisely
the same spots, nevertheless the zones of Facing-past Encounters re-
mained stable over periods of 3-4 weeks. However, other males'
boundaries which did not coincide with physical or vegetative features
of the ground remained just as stable. Presumably the males remember
the locations of sagebrush clumps and other features of the surface in
order to occupy the same positions and encounter their neighbors in the
same zones over several weeks, although conspicuous features of the
surface do not seem necessary for this stability of territorial boun-
daries.

The radial and circumferential extent of the zones of Facing-
past Encounters between different pairs of males showed considerable
variation. The most peculiar territory of any male studied closely
was that of Male 8 in 1969 (Maps llabc) . The area in which he
Strutted was long and narrow, 2.5 x 10 meters. I have already indi-
cated that he chose positions for Strutting, usually near one end or
the other, depending on where he could get closest to females. His

87

The shapes of the zones of Facing-past Encounters between pairs
of neighboring males depended largely on the males' activity patterns
within their territories. If two males habitually Strutted at parti-
cular sites within their territories, most of their encounters occurred
near the intersection of the boundary zone with a line between the two
Strutting males. In a few cases surface features of the lek influenced
the positions of Facing-past Encounters. For instance, in 1968 Male A
met Male C, and later in the season Male CN, at positions between a
large clump of sage to the east and a depression of the ground to the
west (Maps 8ab, 9ab) . Thus, pairs of males showed little evidence of
selecting conventional sites at which to encounter each other.

C. Initiation of Facing-past Encounters

The initiations of Facing-past Encounters offer striking evidence
of non-reciprocal interactions between territorial males. This polarity
results from the attractiveness of the mating center for territorial

males .

When females are present at the mating center , each male tends
to Strut along the side of his territory nearest the mating center.
Periodically his more central neighbor initiates an encounter with him,
since a male Strutting near or within his territorial boundary, as
defined by the zone of Facing-past Encounters, presents a prime stimulus
prompting his nearest neighbor to initiate a Facing-past Encounter.
Consequently, of two neighbors the more peripheral one tends to encroach
into or beyond their usual boundary, and the more central one tends to
initiate more of their Facing-past Encounters.

To quantify this trend I have classified Facing-past Encounters
between different pairs of males according to how they were initiated.
I recognized three kinds of initiations: by one or the other male, and
mutually. A mutual initiation implies that the two opponents started
toward each other so nearly simultaneously that I could not distinguish
which one actually began. In most situations, however, I could easily

'

88

decide which male initiated the encounter. Usually one male dashed
toward his opponent, who was Strutting or, occasionally later in the
morning, sitting in Semi -Collapsed Posture near the zone of Facing-past
Encounters. In mutually initiated encounters both males usually dashed
to meet each other in the zone of Pacing-past Encounters . Especially
later in the morning males often walked, rather than ran, to encounter
their neighbors. Occasionally a male would walk to his boundary even
though his neighbor was not near the boundary. Often the neighbor would
then walk to join the first "male in an encounter. For each pair of
neighbors in Figures 17 and 18 the more central one is starred, and the
number of encounters which he initiated is represented by a black bar.
In 1968 in a series of males progressively farther from the mating center
in Male D's territory, the more central of each successive pair tended
to start more of the pair's encounters; D started more than B; B more
than A; A more than C; and C more than CN.

Each male is, therefore, most active along his boundary nearest
the pack of females at a mating center and yet must maintain his farther
boundary against gradual encroachments by his more peripheral neighbors.

Patterns in the initiations of Facing-past Encounters between
certain pairs of males tended to equalize later in the morning when
females were absent or fewer. Table 14 compares the initiations of
encounters between five pairs of males earlier and later in the morning
on days when females were numerous. Male B initiated proportionately
more of his encounters with his neighbor, MaJe D, later in the morning
than earlier. Thus, in at least one case the tendency for the more
central male to initiate a disproportionate number of a pair's encount-
ers did not persist late in the morning. Encounters between Males D
and TM show the same trend, although in this case the difference is not
quite significant. On the other hand, Males TM and UT showed the
opposite trend: later in the morning the more central male of the pair,
Male TM, initiated proportionately more of their encounters. In con-
clusion, polarity in initiations of Facing-past Encounters seems en-
hanced, at least between some pairs of territorial neighbors, by the

4

90

•

INITIATIONS OF ENCOUNTERS BETWEEN NEIGHBORING MALES

1969

10

in
<r
u

Z)

o
o

z

u 0

u.

o

10

B
Ul

ca

2

i 5

JZL

■331

9" 2m 9 8m 9* 6m 9*20 m 4* 9 m 4* 8 m 7 4m 7"8 m 7*19 m

I_.

JZ3

Q.

n

_□

8" 13 m 3" 6 m 3* 19 m 12 2 m 12 20m 8 20m 20*22m 20 13 m

Figure 18. Initiations of Facing-past Encounters between neighboring
males in 1969. For explanation see Figure 17.

91

Table 14

Initiations of Facing-past Encounters Earlier and Later in
the Morning, 27 March - 14 April 1968 (except 3 April)

How Enc

counters

.ated

icon

iring Males

Ti

me

x?

(Yates
correction)

Betv
Neighbc

Earlier than

One Hour
After Sunrise

Later than

One Hour
After Sunrise

P

1

D->B
B-*D(1)

14
0

2
3

5.97

<

.02

2

TM->UT
UT->TM

5

4

12

0

4.02

<

.05

3

D-*TM
TM->D

27
4

4
4

3.33

>

.05

4

B-»A
A->B

5

1

4

1

0.41

ii

5

A-»C
C->A

3

0

2
3

0.89

I!

6

CN->C

16
0

4
1

0.40

II

3-6

Combined

51
5

14
9

8.23

<

.01

(1) Data for the interaction of Males B and D pertain to the period
before Male TM's disappearance on 8 April.

93

male tended to terminate even those encounters initiated by his more
peripheral neighbor. Table 15 includes the data from encounters between
Males D and TM in 1968 and encounters between Male 9 and his more peri-
pheral neighbors in 19S9. In both cases the more central male, Malu D
or Male 9, terminated significantly more of the encounters initiated by
peripheral neighbors than expected. At least in these two cases the
more central male tended to terminate even more encounters than he
initiated. This tendency for the more central male to terminate most
of a pair's encounters probably stems again from his greater proximity
to females. Males near females appear strongly motivated to resume
Strutting around the females, an interpretation suggested both by their
tendency to terminate their Facing-past Encounters and by their con-
comitant tendency to have brief encounters (Chapter VIII, Section B) .

4

i

m

E. Allocation of Time to Strutting and Facing-paat Encounters

Owing to their briefer Facing-past Encounters, males with females
in their territories devoted much less time to Facing-past Encounters
than did males with no females nearby (Table 16) . Males with females
in their territories spent 92% of their time in Strutting Posture and
only 5% in Facing-past Encounters. In comparison, when no females were
within 10 meters, males spent only 26% of their time in Strutting
Posture and 38% in Facing-past Encounters. Other activities of males
near females included chasing intruders (this Chapter, Section J) and
mating. Males farther from females also chased intruders, although
this activity never accounted for more than 2% of a male's time. When
not Strutting or engaged in Facing-past Encounters, males with no
females nearby (Situation 0) rested in their territories in Semi-
Collapsed or Collapsed Posture. These two postures therefore occupied
about one-third of their total time.

Male D, the most successful breeder, often had females in his
territory until late in the morning, while other males only occasionally
had females in their territories (Figure 19) . Male D and other males
established near the mating center, therefore, spent far more time

94

Table 15

Terminations of Facing-past Encounters Between Neighboring Males.
In each of the three tables encounters are classified according to
initiating male (rows) and terminating male (columns) . More central
males tend to terminate encounters, even ones initiated by their
more peripheral neighbors.

X2

(Yates correction) P

D
TM

m

D

29

3

4

TM
1
3
3

m
0
1
1

6.81

<.01

More
Central

More
Peri ph.

(2)

More
Central

41

More
Periph.

5
20

7
10

3

0

8.36

<.01

29

Periph.
Neighbor

(3)

c*9
24

3

3

Periph.
Neighbor

0

1

0
0

6.72

<.01

(1) H = the more central neighbor (C) terminates encounters begun by the
more peripheral neighbor (P) in the same proportion that P terminates
encounters begun by C.

(2) This table includes all encounters between Males TM and UT, A and C,
B and A, D and B, D and X, 1968.

(3) Includes Males 2, 6, 8, and 20 in 1969.

XUM

*

%

100

50

0

100

50

0L

I00r

50

Male D

7 4 5 4 4

Male A

4 14 3 3

M

~H

Male C
4 0 3 2

Male

UT

6 3 4 3 2

1

1

//A

mk.

I

$

Male CN
3 13 10

Male

7 4 5

B

4

3

M

1

M

ale

X

5 3 4 4 3

W/t

i

1

1

2 3 4 5

2 3 4 5

12 3 4 5

Figure 19. Percentages of samples in which individual males were at
various distances from females. Abscissa is divided into five half-
hour intervals (I-V) beginning 10 minutes before sunrise. Ordinate
indicates percentage of samples (N is given above each column) in which
the male either had females in his territory (black part of bar) , had
females within 10 meters in a neighboring territory (hatched part) , or
had no females nearby (open part). Data from 27 March - 1 April 1968.
Males near a mating center (D, TM, and B) associate with females more
often than do other males.

m

97

Strutting each morning than did more peripheral males. My data from
time-lapse films, however, probably underestimates the amount of time
that peripheral males spend Strutting, since I usually could not begin
filming until most of the females had arrived and concentrated near the
mating canter. Earlier, the arriving females are more scattered on
the lek, and peripheral males Strut more persistently. However, the
more persistent Strutting by peripheral males around dawn may not stem
entirely from greater chances of having a female nearby. The Quacking
Call of flying females seemed on many occasions to stimulate Strutting.
Nevertheless, as the females congregate near the mating center shortly
after sunrise, peripheral males Strut much less persistently, whereas
males near the mating center Strut steadily as long as females remain,
sometimes more than two hours after sunrise.

F. Shifts in the Positions of Mating Centers

Further indication of the attraction that a pack of females has
for the males comes from two occasions in which the predominant breeder
shifted his boundaries slightly to follow short moves of the mating
center. I have discussed these shifts of mating centers in an earlier
section. In 1968 when the mating center drifted south-west 8 meters,
Male D accompanied it. At first the mating center shifted its place
each morning, and Male D's territory included the entire scope of the
movement (Map 4) . Then on 51 March and 1 April as the female pack
shifted, Male D's encounters with his neighbors TM and B also shifted.
On subsequent mornings the females gathered initially in the area in
which they had ended on previous mornings, and Males D, TM, and B also
encountered each other in the zone which they had used late in the
mornings of 31 March and 1 April. Thus, as the pack of females changed
its behavior, Male D simultaneously surrendered a portion of his
original territory to his neighbors TM and B. He did not displace any
male, because he had no immediate neighbor to the south, where a
depression in the surface of the ground appeared unsuitable for
Strutting males. After the move, just as before, Male D continued
to predominate in mating.

99

Females first appear on leks early in the season in loose,
wandering groups (Chapter VIII , Section A) . The Strutting males
approached as closely as possible to these drifting groups of females,
usually without intruding far into neighboring territories. Males at
the edge of a lek, however, sometimes followed a flock of females as it
wandered away from the lek. On one occasion, after the peripheral
males on the Fords Creek Lek had left their positions to follow a group
of females, more central males also left. Eventually many of the males
had moved away from the lek in order to continue Strutting near the
dispersed group of females. These males spaced themselves 5-20 meters
apart and occasionally encountered each other. Usually, though, the
males near the center of the lek did not follow these loose flocks of
females beyond their territories. Lumsden (1968) watched several males
that followed a group of females beyond the edge of a lek but retained
their approximate positions relative to one another. Strutting around
females away from any lek also occasionally occurs in winter flocks
(Patterson, 1952).

H. Reoccupation of Vacancies

Striking polarity in males' territorial interactions appears
again in the reoccupation of the occasional vacancies which arise when
territorial males disappear. Such vacancies are allocated to more peri-
pheral males, males less successful in mating than the original occupant.
In 1967 one male disappeared: the predominant breeder, Male A. In 1968
a series of three disappearances occurred on the Fords Creek Lek:
Males TM, C, and finally CN. In 1969 only one male disappeared, one
originally near my blind.

In 1967 Male A failed to appear on the morning of 18 April (Maps
1 and 2), but Male N, who was eventually to occupy the entire area
vacated by A, did not immediately move into the vacated territory. On
the first morning Male N only briefly entered A's former area. Late
in the morning Male N once chased an intruding male across the vacated
area. The following morning, 19 April, Male N Strutted for several

_^_j

100

short periods in the center of Male A's former territory but still
centered his activity to the north-west in his own original territory.
On subsequent mornings Male N spent progressively more of his time in
Male A's original area and eventually mated there several times. Male
A originally had three other neighbors, Males C, S, and SE, none of
which extended their territories to include a part of the vacated area.
Male C was also a frequent mater, and the packs of females usually
extended into his territory. Male N succeeded in appropriating the
entire vacated area without "unusual amounts of antagonism with his new
neighbors. He did meet Male S once on the first morning of Male A's
absence, but he did not encounter Male SE in a position at the east

i

edge of Male A's original territory until the following day. Male N
had encountered Male SE in other positions prior to Male A's disappear-
ance, however.

In 1968 a series of three disappearances occurred on the Fords
Creek Lek. The vacant territories were eventually appropriated with
boundaries essentially intact by Males B, CN, and a yearling, respec-
tively. After absences on 6 and 7 April, I returned on 8 April to find
TM gone (Map 7). Male B now encountered one of Male TM's original
neighbors, UT, in roughly the same positions that TM himself had used.
Male B's encounters with Male D, however, did not immediately converge
on the positions where D had previously encountered TM (Maps 9ab) . On
8 and 9 April Male B encountered Male D only rarely. D had extended
his area northward slightly to include a small portion of TM's area.
On 10 April, however, Males B and D encountered each other in positions
near those in which Male D had encountered TM formerly. Male B's
neighbor on the far side, Male A, did not extend his activities as
rapidly as Male B did. Male A confined most of his Strutting to his
original area through 12 April. By 22 April, however, Male A was
encountering Male B 3-6 meters north of their former zone of Facing-
past Encounters (Maps lOab) . So Male B eventually occupied the
territory vacated by TM with boundaries essentially intact, and Male
A, B's more peripheral neighbor, also moved his boundary closer to the

m

102

dismembered bones and feathers of the grouse was one rectrice of an
immature Golden Eagle. In 1967 I found the remains of an old kill
near the periphery of the Muddy Springs Lek. It was also a fully
adult male. The bones and feathers were scattered over several yards,
so presumably some mammal was responsible but no tracks remained. In
each year I walked over much of the lek I studied, but I found no
other kills .

One might expect that peripheral males would be most vulnerable
to predation on a lek. Surprisingly, both of the kills which I did
find were fully adult males with long, tapering tail feathers, although
yearlings are more likely to occupy peripheral positions. On the
other band, the signs of fatigue which the predominant breeder often
showed suggest that these males might become more vulnerable to pre-
dators away from the lek during the mating season. Although the pre-
dominant breeder in 1967 did indeed disappear, all of the males which
mated frequently in 1968 or 1969 remained during my observations.

J. Expulsion of Intruding Males

Scott (1942) described radial differences in the organization of
males around a mating center as follows : a central core composed of a
"master cock" and a "sub-cock", which perform most of the matings;
arrayed around this central core, three to six "guard cocks" which
chase away intruders but perform very few matings. That the "guard
cocks" show altruism is doubtful, but my observations suggest that the
male in a pack of females at a mating center does react differently to
intruding males than do his neighbors. My observations accord with
Patterson's (1952) conclusion that most intruders are yearling males.
I successfully followed a few which moved to the periphery of the lek
and assumed the Strutting Posture. All had the comparatively stubby
tail feathers of yearlings. Scott (1942) apparently mistook these
intruding yearlings for males with hormonal abnormalities.

103

Intruders frequently mingle with the large packs of females
that gather at a mating center early in the mating season. Near the
pack of females they always assume Collapsed or Sleeked Postures and,
therefore, are not so easily distinguished from females as are the
Strutting males. The intruders are larger than females and, when seen
from the front, show the white chest typical of males. Because they
do not erect the feathers on the side of the neck, no white is visible
from the rear and only a little from the side. Thus, an intruder may
escape detection by facing "away from the nearest territorial male. An
intruder within a dense pack of females becomes very inconspicuous
except at close range. Thus, when a male has a dense pack of females
within his territory, he often overlooks intruders among the females
until he approaches with 2-3 meters of them.

Once a territorial male detects an intruder, usually when the
intruder faces him, a vigorous chase ensues. The territorial male
rushes after the intruder, who persistently attempts to circle back
to the pack of females. A chase may weave in and out of the pack of
females two or three times, so that the females scatter considerably.
If an intruder crosses a territorial boundary during a chase, the chase
is continued by the next territorial male. A chase is often propagated
through several territories and involves a succession of territorial
males. An intruder may fly short distances to avoid being overtaken.
Eventually the intruder either regains the cover of the pack of
females or finally crosses into the territory of a male that fails to
notice his presence immediately. In the latter situation the intruder
remains quietly where the chase terminated, usually facing away from
his last pursuer's territory, an orientation that reduces his conspicu-
ousness to his previous pursuers. Although such an orientation
clearly reduced the intruder's conspicuousness, I never saw an intruder
actually turn to face away from an approaching male as if to avoid
detection. If an intruder gains a position , usually in the periphery
of the lek, where he goes unnoticed for several minutes, he may
gradually assume the Semi -Collapsed or rarely the Strutting Posture.

105

•

w

especially late in the morning (around 8:00), without any signs of
localizing their movements. On 31 March, 1, 2, 4, 5, and 10 April I
observed the males in an area 50-100 meters north of Mating Center #1.
Females first showed signs of clustering at Mating Center ffl on 1
April. On these days just preceding and during the first congregations
of females at the mating centers, I could detect nothing unusual in the
intensity or frequency of the males' agonistic encounters. In the area
north of Mating Center #1 some individually identified males occupied
their positions regularly during these early observations. Yet several
other males clearly shifted their zones of Facing-past Encounters.
Beginning 6 April I changed my observations to Mating Center #3, a more
convenient place for observations.

Similarly, in 1968 on the Fords Creek Lek several males appeared
to occupy stable positions during the days when females were first
arriving at the lek. Clustering of females in the territory of Male D
first appeared on 27 March. As early as 19 March, though, Males A, B,
and C had stabilized their zones of Facing-past Encounters within less
than 3 meters of the positions they used until 9 April when Male C
disappeared.

Although evidence remains incomplete, the males apparently
increase their activity and attendance at the lek gradually as the
weather improves in early spring. At least some of the older males,
but probably not all of them, appear to establish stable zones of
Facing-past Encounters before the females begin to congregate at the
mating centers.

Yearling males, as I shall describe presently, establish
positions on a lek later in the season than do older males. Perhaps
a male's tendency to occupy his position on a lek early in the spring
continues to increase with age after his first year, a possibility
not yet investigated. For instance, second-year males might not arrive
on a lek as early in the season, or as persistently during bad weather,
as the third-year males. If the oldest males arrived on a lek earliest

_l

106

-in the spring, or most persistently during bad weather, this behavior
giould predispose them to obtain centrally located territories. At the
lek west of Farson on the first days following their absence during a
period of deep snow, only a few males took their positions. Apparently,
a small proportion of the males has higher motivation to attend the lek.

I have found no mention in the literature that males attend their
leks even sporadically during the autumn, although males of other lek-
forming species of grouse usually do (Chapter XIII, Section A). Autumn
display might allow juvenile males and females to experience a lek for
the first time at ages of 3-5 months, as well as allowing older males
to remain in contact with their leks throughout most of the year.

L. Arrival and Departure in the Morning and Evening

The territorial males arrived each morning singly or in small
groups. Many flew directly to their territories; others apparently
landed some distance away and walked to their territories. In the
evening males first appeared walking from the surrounding sagebrush into
the area occupied by the lek. They proceeded slowly and paused frequently
to feed and preen. After some of the males had taken their positions
and begun to Strut, late comers often flew from the periphery of the
lek to their territories. My observations indicated that neighboring
males usually did not arrive together either in the morning or in the
evening. Lumsden (1968), however, felt that groups of several neighbor-
ing males usually did arrive and depart together each morning.
Occasionally a male walking to his territory would have to cross terri-
tories already occupied by earlier arrivals. In such situations the
trepassing male would adopt Collapsed or Sleeked Posture, detour around
the resident, and run if necessary to avoid him. Arriving males, at
least during the evening, occasionally Strutted in the prairie around
the lek. However, once well within the area occupied by the lek, the
males did not Strut until they reached their territories. I never
recorded an arriving male within the lek that challenged another male
or Strutted except within his own territory.

108

P r

never reaches the average for older males, although yearlings apparently
do produce spermatozoa. They do not normally participate in the mating,
since I never saw one copulate.

At the onset of the mating period yearlings frequently attempt
to associate with the pack of females at a mating center. Probably most
of the intruding males in Collapsed Posture, which often penetrate to
the center of the lek at this stage of the season (this Chapter, Section
J), are yearlings. During this early phase of the mating period yearlings
Strut rarely and then only sporadically for a few minutes at a time in
varying positions around the periphery of the lek. During this period
they are often chased vigorously by older males established nearby.
Map 14 shows where a yearling, Male 25, Strutted in late April 1969 and
the positions in which neighboring adults encountered him. Clearly, his
older neighbors even in late April freely chased him from his favored
positions. As others have also observed (see above), yearlings seem
less attached to a particular site on a lek early in the mating period.
Dalke et al. (1963) present some evidence from marking studies that the
yearling males also show less attachment to a particular lek than do
older males. As the days pass, though, at least some yearling males
slowly consolidate their positions at the edge of a lek, positions un-
occupied prior to their arrival . Maps lOab present the territories of
five yearling males (Males J, Y, Z, K, and I) as they appeared in late
April 1968 on the Fords Creek Lek. These yearlings encountered each
other in stable zones of Facing-past Encounters, and three of them
successfully established stable zones with their adult neighbors. The
yearling Male Z encountered his adult neighbor H frequently at their
stable boundary. Therefore, by late April a lek has acquired an
accretion of yearling males on territories around its periphery.
These additional territorial males increase the number of displaying
males at a lek approximately halfway through the mating period. On the
Fords Creek Lek in 1968 the regular attendance of yearling males in-
creased the number of males on the lek beginning about April 10 (Figure
1) . Other observers have recorded similar increases in the number of

H^^^HB^H

109

males displaying at leks, owing to the increment of yearling males
(Batterson and Morse, 1948; Patterson, 1952; Eng, 1963; Dalke et al . ,
1963) .

The displays of yearling males change appreciably as the season
advances. In late March and early April in Montana in 1968 I occasionally
observed yearling males Strutting at the periphery of the Fords Creek
Lek. These birds had noticeably smaller chest sacs, and the internal
timing of their Struts was quicker. In a few instances I recorded
individuals which lifted their chest sac three times, instead of the
usual two times, before the final, sudden ballooning of the sac.
Furthermore, the sound produced during the final sac throw lacked the
peculiar resonance typical of the displays of older males, as Lurasden
(1968) also noted. In contrast, by 22 April a number of yearling males
had chest sacs essentially indistinguishable from those of older males.
In spite of this, yearlings never produced the full resonant quality
of the older males' Struts. In late April 1969 the yearlings on peri-
pheral territories tended to arrive 10-15 minutes later in the morning
than did the older males around the mating center.

To compare the frequencies of Strutting by yearling and older
males, I used eight six-minute sequences of time-lapse film from the
Fords Creek Lek on several days in late April 1968. Yearling males
tended to Strut somewhat more often when no females were nearby than
did older males in similar circumstances (Figure 20) . Their rates of
Strutting, however, never reached the high rates typical of older males
with females in or near their territories. A yearling male rarely has
a female in or near his territory, so I obtained almost no data for
this situation. Yearlings spent somewhat less of their time in Facing-
past Encounters than did older males with no females nearby (Figure 21) .

To recapitulate, yearling males begin displaying regularly on a
lek later in the season than do older males. They apparently do not
mate in normal circumstances, although at least some of them eventually
establish territories around the periphery of a lek. After establishing
a territory their activity patterns resemble those of older males with
no females nearby.

Ill

m

%

100

0)
Q.

O

c

0)

o

u

a.

A A Older moles (N = 3I)

Yearling males (N = 20)

0-10 -20 -30 -40 -50 -60 -70 -80 -90 -100%
Amount of time in facing past-encounters
(% of 6 min)

Figure 21. Allocation of time to Facing-past Encounters by yearling
and older males. 22-25 April 1968.

112

SUCCESS IN MATING

Territoriality among male Sage Grouse includes two dynamic
features: (1) many yearling males establish territories near the edge
of a lek; and (2) the territorial adults during any one season tend to
move their positions centripetally as vacancies arise toward a mating
center. These two observations suggest a mechanism by which a male
might come to occupy a territory at a mating center. If males tend to
return to the same lek in successive years and to occupy positions at
least as near a mating center as in the previous season, then an
individual male in the course of several years might veil reach a
position in or near a mating center. Direct confirmation of this
mechanism would require a marked population of Sage Grouse. In order
to evaluate a negative result, the grouse should not be disturbed on
their leks, since Lumsden (1968) and Dalke et al . (1963) both record
disturbances in the behavior of grouse at a lek after canon-netting.
Nevertheless, observations of marked birds by Dalke and his colleagues
(1963) do suggest that most males return to the same lek in successive
years and that some males return to nearly the same positions on their
leks. Lumsden (1968: 27) observed three banded males that had returned
to approximately the same positions on a lek in two successive ynsrs.

The number of years required for a male to reach a mating center
would depend on the annual mortality of males, which would determine
the frequency of vacancies on a lek. No good estimates of annual
mortality are available for Sage Grouse. Some rough calculations based
on the age distribution of birds trapped during July and August in
western Wyoming (June, 1963) suggest that mortality among males at least
one year old approximates 50% per year (Table 17) . Males in their
second year perhaps have higher mortality than those in their third
and fourth years, but this evidence is not clear (June, 1963). Studies
by Dalke ani his colleagues (1963) suggest mortality rates even higher
than 50% a year, on the basis of the low percentages of banded birds
returning the following year. Dalke did not separate his data by sex and
age-class, however, and it remains uncertain whether trapping and marking

114

birds on their leks might increase mortality or emigration.

Estimates of mortality are available for several other tetraonids
(Table 18). For instance, mortality rates among color-banded Blue Grouse
Dendragapus obscurus range from 25% to 31% per year in unhunted popula-
tions on Vancouver Island (Bendell and Elliot, 1967; Zwickel and Bendell,
1967) , while in a hunted population in Alberta the rate is 56% per year
(Boag, 1966). Bendell and Elliot (1967) found no significant differences
between the mortality rates of yearling and older males or between males
and females.

With a mortality rate of 25-50% per year, transposition of a sur-
viving male into a rating center might require only a few years. This
mechanism implies that the more central males, those most successful
in mating, tend to be the older males. Owing to irregularities in the
organization of the leks and the random nature of most mortality, it
is difficult without long-term marking studies to Judge exactly how
strongly correlated age and mating success in fact are. One result
seems clear, however: yearling males essentailly never copulate, although
yearling females virtually all breed (Chapter VIII, Section A). If my
hypothesis concerning the centripetal movement of males' territories is
correct, two-year-old males should also perform few or no copulations,
although perhaps they are more likely to mate than yearlings are. The
probability that a particular male will occupy a territory in a mating
center might increase steadily with his age. Present evidence cannot
specify whether this probability approaches 1.00 at some age.

To recapitulate, my observations suggest that during their life-
span males migrate centripetally within a lek as vacancies arise toward
the mating center. So polarity in the territorial behavior of the males
probably would correlate at least in a rough way with radial differences
in the males' ages. Attainment of success in mating would then be an
ontogenetic process, which begins when a yearling establishes his
territory at the periphery of a lek. The social organization of a Sage
Grouse lek would depend on polarized interactions between males usually
of different ages.

IWiilUT "

Table 18
Estimates of annual adult mortality rates In grouse and pheasants based on studies of banded populations

Species
(Location)

Annual
Adult
Hunting Mortality

(1)

Mortality in
Second Year
of Life
Compared Sexes
with Later Differ

References

Red

Grouse, L. lagopus

(Scotland)

White-tailed
Ptarmigan, Lagopus
leucurus

(Montana)

65%

29%

Blue Grouse,

Dendragapus

obscorus

(Vancouver I.)

(Alberta)

Ruffed Grouse,
Bonasa umbellus

(Minnesota)
Capercaillie,
Tetrao urogallua

(Finland)

26-31%
56%

(2)

50-60%

40%

Black Grouse
Lyrurus tetrix
(Finland)

80%

Ring-necked
Pheasant
Phasianus sp.
(Wisconsin)

50-80%

Same

No

Jenkins, Watson, Miller, 1967

Territorial
males perhaps
lower =20%

Choate, 1963

Same
Same

No Bendell and Elliott, 1967
No Boag, 1960

Females
Same slightly higher Gullion and Marshall, 1968
than males

? ? Helminen, 1963

Reviewed by Kickey, 1955;
Farner, 1955

(1) Annual mortality of birds after the first year of life.

(2) In hunted areas mortality slightly higher than in protected areas.

117

male intruded more than a short distance into his neighbors' territories
to interrupt their matings. Inhibition against intrusions was particu-
larly evident when a vacancy remained unoccupied for one or two days
after the disappearance of the original occupant.

Encounters between neighboring males apparently function in
establishing the locations of boundary zones between territories rather
than in deciding classical dominance-subordinance relationships between
opponents. Like the boundary encounters of classically territorial
species, the Facing-past Encounters of male Sage Grouse are frequent
but rarely severe. Furthermore, one opponent does not "win" and the
other "lose", since encounters essentially never result in a polarized
outcome. The male initiating an encounter usually terminates it; when
an encounter terminates, both contestants return to their respective
territories. Within the usual boundary zone Facing-past Encounters
virtually never shift position more than a meter. When a Strutting
male transgresses the boundary zone, he usually quickly retreats to the
boundary zone after the resident male initiates an encounter. First-
year males, in contrast, do get chased long distances by neighboring
older males, especially when the yearlings first begin to occupy regular
positions around the periphery of the lek. Eventually, though, many
yearlings succeed in establishing stable boundary zones with their
older neighbors.

Like many other territorial birds, male Sage Grouse occupy
territories separated by narrow, mutually recognized zones, within which
neighbors encounter each other as equals. Within his own territory a
male Sage Grouse is dominant, and the dominance relationship of two
neighbors reverse within a boundary zone which is often only one or
two meters wide. The aggression gradient of a territorial male Sage
Grouse, therefore, resembles a Type A gradient, one in which each male's
capacity to dominate agonistic encounters remains maximal until it
decreases steeply in a narrow boundary zone (Chapter II; Figure 1).
Although a male's territory is largely exclusive with respect to other

118

territorial males, the exclusion gradient does not decrease across
the territorial boundary quite as steeply as does the aggression
gradient. In other words, territorial males occasionally encroach
beyond the boundary zone in which neighbors meet as equals in Facing-
past Encounters. In many respects, therefore, the behavior of male
Sage Grouse conforms to the classical pattern of territoriality.

Nevertheless, territoriality in male Sage Grouse diverges from
the classical conception in having certain polarized, or non-reciprocal,
features, as well as reciprocal features. The Interactions of terri-
torial males nearer a mating center with those farther away lack reci-
procity in the initiation and termination of Facing-past Encounters
between neighbors and in the occupation of vacancies. In all of these
cases, the polarized interactions of radial neighbors apparently origi-
nated in the attractiveness of the mating center for the territorial
males. Territorial males nearer a mating center apparently inhibit
the occupation of these areas by males established in more peripheral
positions. Furthermore, the behavior of males nearer a mating center
differs in several respects from the behavior of those farther away.
Because the more central males have females within or near their
territories for a greater proportion of each morning, they perform most
of the matings, Strut during a greater proportion of the morning, and
engage in briefer Facing-past Encounters . The males nearer a mating
center also usually occupy smaller territories. In effect, the
territories show hierarchical ranking. Present evidence indicates that
occupation of a territory near a mating center should correlate roughly
with increasing age.

Lumsden (1968) has suggested that male Sage Grouse could maintain
a hierarchical ranking throughout the year, away from their lek as well
as on their lek. Like Lumsden I noticed no higher levels of agonistic
activity early in the season than later. However, since I could not
confirm his conclusion that the males arrived and left together each
morning, I feel uncertain that the males around each mating center
remain together when away from the lek.

120

The size of a territory results from the interaction of external
pressure applied by territorial neighbors and internal resistance
exerted by the territorial resident:. Since the more central males
devote less time to agonistic encounters than do their more peripheral
neighbors, the efficiency of their resistance might not equal that of
more peripheral males. Furthermore, external pressure on central
territories might be greater than on more peripheral ones. If the
attraction of the mating center for territorial males decreased at
increasing distances, males established at greater distances from the
mating center might well exert less pressure on their more central
neighbors.

The reoccupation of vacancies on a lek presents an unresolved
problem. Which of two neighbors equally distant from a mating center
will occupy a vacancy arising closer to the mating center? In two cases
a vacancy had two or more neighbors equally distant from the mating
center. When Male TM disappeared in 1968, both Male B and Male UT
appeared equally likely candidates for reoccupation of the vacancy.
In 1967 when the predominant mater, Male A, disappeared and left a
vacancy which included part of a mating center, his three neighbors
Males N, SE, and S seemed equally likely to reoccupy the vacancy. Male
A's fourth neighbor, Male C, already occupied a portion of the area in
which matings occurred frequently. At least in this second case,
following the disappearance of Male A, none of the three neighbors
immediately occupied the vacancy. Not until three days later was Male
N using the vacated area extensively, encountering the other three
neighbors frequently, and mating there. Contrary to what one might
expect, the reoccupation of vacancies was not attended by an increased
frequency or intensity of agonistic encounters in which one male
obtained an advantage. When Male N replaced Male A, agonistic en-
counters occurred infrequently and did not involve Wing-fighting.
Both in this case and in the case of Male TM's disappearance I could
not determine what factors predisposed one of the neighbors to re-
occupy the vacancy. Perhaps differences in the males' ages would give

122

XII. INDIVIDUALITY IN THE INTERACTIONS OF MALES

A male Sage Grouse on a lek has one clear source of individual
identification: his location. The position of a male's small territory
identifies his rank in the hierarchy of territorial males. When males
arrive on a lek in the morning or in the evening, each proceeds to his
own territory. A male does not move his position even if his more
central neighbor arrives later than he does.

Whether or not male Sage Grouse on a lek can recognize each other
individually on the basis of cues other than territorial location, their
interactions reveal little evidence of such recognition. For instance,
a male's behavior changed remarkably little when a former neighbor dis-
appeared and a new neighbor took its place. Usually the zone of Facing-
past Encounters continued in the same locations, as when Male N replaced
Male A in 1967 and when Male CN replaced Male C in 1968. When Male B
replaced Male TM in 1968 certain changes did follow, though. The
boundary with Male D at first shifted, although it finally returned
approximately to its original position. Although the boundary zone
with the other neighbor, Male UT, remained unchanged, the behavior of
Male UT clearly did change. Prior to the replacement, Male TM had
initiated most of the Facing-past Encounters in the boundary zone with
Male UT, while after the replacement Male UT initiated most of the
Facing-past Encounters there (Figure 17; compare encounters between UT
and TM with those between UT and B) . No reversal in the patterns of
initiating Facing-past Encounters occurred when Male CN replaced Male
C in the territory adjacent to Male A. However, in this case Male A
had encountered Male CN previously in other locations.

Other than the strict localization of each territorial male,
Sage Grouse do not have clear sources of individual identification.
Their displays and acoustic signals , as a result of extreme species-
specific stereotypy, show only slight individual differences among
males at least two years old. In contrast, the acoustic signals of
many other birds do contain information which clearly identifies the

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individual (Marler, 1960; Weeden and Falls, 1959; Falls, 1969; Thorpe,
1968) .

Individual recognition is probably not necessary to maintain
hierarchical organization among individuals. Instead, individuals might
discriminate among classes of other individuals. An individual in a
hierarchy might not discriminate among all other individuals in the
hierarchy, but only between individuals likely to have higher rank and
those likely to have lower rank than its own. If rank correlated with
aggressive tendencies, then an individual would have to recognize
individuals with higher and lower aggressive tendencies, for instance,
by, their longer or shorter critical distances for initiating an attack.

These considerations suggest that the hierarchical organization
of territorial male Sage Grouse might depend more on the spatial
localization of individuals than on individual recognition independent
of location. Male Sage Grouse apparently have not evolved any features
of plumage or behavior that would clearly facilitate individual
identification.

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Descriptions of the territorial behavior of males appear in
almost all accounts of lek behavior of these species: Black Grouse
(Selous, 1909-1919; Lack, 1939; Hohn, 1953; Kruijt and Hogan, 1967;
Koivisto, 1965); Capercalllie (Lumsden, 1961); Prairie Chicken (Hamer-
strom, 1939; Hamerstrom and Hamerstrom, 1955, 1960; Schwartz, 1945;
Eobel, 1941); Sharp-tailed Grouse (Lumsden, 1965; Evans, 1969). In all
species the central territories are usually smaller than more peripheral
ones. Territories are smallest in the Sharp-tailed Grouse, the smallest
ones usually about 30 square meters in area (Lumsden, 1965). In Prairie
Chickens the smallest territories are about 75 square meters (Hamer-
strom and Hamerstrom, 1960). In Black Grouse the smallest territories
which Lack (1939) observed were about 35 square meters. However,
Kruijt and Hogan (1967) found none less than 100 square meters in area.
Male Prairie Chickens and Sharp-tailed Grouse frequently leave their
territories to approach females on the lek (Lumsden, 1965; Hamerstrom
and Hamerstrom, 1960; Robel, 1964). In contrast, Black Grouse males
usually remain within their territories when females are on the lek
(Hohn, 1953; Kruijt and Hogan, 1967; Hamerstrom and Hamerstrom, 1960).
Most copulations are performed by males at least two years old estab-
lished near the center of a lek (Robel, 1967; Koivisto, 1965; Kruijt
and Hogan, 1967; Lumsden, 1965). Although Robel (1964, 1966, 1967)
reports that male Prairie Chickens most successful in mating occupy
larger territories than less successful males, his measurements of
territory size are not comparable with those of other observers. He
determined the ranges of the individual males on a lek by plotting
their positions at 15-minute intervals. Other observers have deter-
mined territory sizes from the positions of the agonistic encounters
between neighbors. Apparently the central males, which mate most
often, leave their territories when females visit the lek.

Studies of banded males have shown that male Prairie Chickens,
Sharp-tailed Grouse, and Black Grouse return to the same lek in
successive years (Robel, 1967; Hamerstrom and Hamerstrom, 1960;

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Koivisto, 1965; Evans, 1969) and that males improve their mating success
with age (Robel, 1967; Koivisto, 1965). Evans (1969) confirmed that
four surviving male Sharp-tailed Grouse returned to the same positions
on their lek in the following year. A fifth male moved his territory
8 meters into the center of the lek to occupy a vacancy. This is
exactly the behavior expected on the basis of my observations of Sage
Grouse.

Many studies have mentioned the peripheral positions and less
constant attendance of yearling males (Lumsden, 1961; Lack, 1939;
Koivisto, 1965; Kruijt and Hogan, 1967; Robel, 1967). Yearling male
Black Grouse may penetrate the center of the lek as intruders (Koivisto,
1965; Kruijt and Hogan, 1967). Robel (1965, 1967) presents some
evidence that yearling male Prairie Chickens continue displaying on leks
later in the spring than do older males. Capercaillie yearlings occupy
more peripheral positions and have less developed displays (Klrikov,
1947; Lumsden, 1960). In addition, Kirikov (reviewed by Dementiev and
Gladkov, 1967) found that males in their second spring began displaying
later than the older males. He found that if all the older males were
removed from a lek, the following year two-year-olds began displaying
first but not so early in the season as the older mal<=»s had. If he re-
moved the two-year-olds as well, then yearlings occupied the center of
the lek. Studies of other lek-forming species of grouse apparently
have not compared the seasonal course of display by yearling and older
males, nor the growth of yearling and older males' testes.

Autumn display probably occurs in all lek-forming species of

grouse. Black Grouse in Sweden continue to visit their leks sporadically

throughout the winter (Hjorth, 1968), as do Prairie Chickens in Kansas
and Oklahoma (Jones, 1964).

Although female Sage Grouse are peculiar in the degree to which
they clump together in a small area on the lek, females of other species
of lek-forming grouse do show some tendencies in this direction. Female
Prairie Chickens and Sharp-tailed Grouse tend to concentrate in groups

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and White-tailed Ptarmigan (L. leucurus) also usually form monogamous
pairs, although occasionally two females will settle in one male's
territory. However, the association of a mated pair seems to dissolve
during or shortly after incubation (Choate, 1963; Watson, 1965;
Dementiev and Gladkov, 1967; Weeden, 1965). The pair-bond in Hazel
Grouse seems to resemble that in Rock Ptarmigan. In both species the
bond forms before incubation begins, but males rarely accompany the
female and her brood (Pynnonen, 1954; Fuschlberger , 1956). At least
in the White-tailed Ptarmigan, among these usually monogamous species,
yearling males maintain smaller territories and often do not mate
(Choate, 1963).

Promiscuous Grouse which do not form leks. The remaining four
species of grouse apparently do not form durable pair-bonds, since most
reports suggest that males and females do not remain together at any
stage of the breeding cycle. Movements of female Blue Grouse Dendragapus
obscurus and Ruffed Grouse Bonasa umbel 1 us are not confined to one male ' s
territory (Bendell, 1958; Bendell and Elliot, 1967; Boag, 1966; Brander,
1967). However, Blackford (1963) observed a stable association between
a pair of Blue Grouse, even though the female often left the male's
territory. Displaying male Blue Grouse, Ruffed Grouse (Bump et al.,
1947; Gullion et al., 1962; Gullion, 1967), and Spruce Grouse Canachltes
canadensis (MacDonald, 1968) disperse themselves more or less evenly
throughout the habitat. Blue Grouse males defend an area around their
display site by chasing or fighting intruding males (Bendell and
Elliot, 1967). Many yearling males of this species do not establish
display territories. The yearlings have smaller testes, shorter tails
and weigh less than older males (Bendell, 1955, 1968; Swarth, 1926).

The best evidence for ranking among territorial male Sage Grouse
Is their tendency to shift their territories as vacancies arise in more
advantageous locations. This sort of polarity in the interactions of
males seems absent in Blue Grouse. Once a male has established a dis-
play territory he returns to that territory in successive years as long

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as he survives (Bendell and Elliott, 1966, 1967). Ruffed Grouse males
do often change their display sites if they survive several years, but
the factors which influence the choice of a new site remain unknown
(Gullion, 1967). Bendell and Elliot (1967) investigated the effects of
removing territorial male Blue Grouse during the breeding season. When
they removed males from 31 territories, less than a third of which were
yearlings, only 13 (42%) of these territories were reoccupied that
season. Two-thirds of the new males were yearlings. If displaying
male Ruffed Grouse are removed, only a minority of the display sites
are reoccupied that season and these by yearlings (Dorney and Kabat,
1960). So the removal of displaying males in both of these species
allows some yearling males to establish themselves, bu+ many vacancies
remain unoccupied at least until the following year.

Some display sites are used by a succession of males, so that
these sites have traditional locations. Bendel and Elliot (1967)
present evidence that displaying male Blue Grouse attract yearlings to
their territories. These yearlings might occupy these sites in the
following year if the usual occupant failed to return. Ruffed Grouse
display sites also attract yearling males. A yearling may regularly
frequent the vicinity of a traditional display site, yet rarely display.
On several occasions when the regularly displaying male disappeared,
such a yearling reoccupied the vacated display site (Gullion, 1967).

Nothing is known about the dispersion of matings among males
in these species, owing to the difficulties of observing widely dis-
persed birds in forested habitats. In particular, it remains unknown
whether traditional sites, those occupied by a succession of males,
offer advantages for mating. In both species essentially all yearling
females breed, although apparently many yearling males do not, so
polygyny might result.

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mate (Nero, 1956; Selander and Giller, 1960; Orians, 1961; Wright and
Wright, 1944). The testes of yearling males grow later in the season
and average smaller than older males' (Wright and Wright, 1944). If
the adult territorial males in a nesting marsh are repeatedly collected,
eventually some yearling males establish territories in the areas pre-
viously occupied by older males (Orians, 1961). So the lower breeding
success of yearling males in this species might result both from intrinsic
physiological differences and from social interactions between younger
and older males. Selander and Hauser (1965) have described similar
differences in the patterns of gonadal growth and behavior in yearling
and older male Great-tailed Grackles Quiscalus mexicanus. In general
among polygynous species, males first mate at a later age than do
females and yearling males do not acquire fully adult male plumage
(Selander, 1965; Crook, 1964), yet the changes in male behavior with age
and the interactions among younger and older males have received little
attention. In polygynous species when the opportunities for mating
differ with the locations of the males' territories, polarized inter-
actions might characterize the territorial behavior of the males.

D. Polygyny in Mammals

Delayed breeding among males in comparison with females and lack
of male parental care occur in polygynous mammals as well as birds. In
most mammals other than certain carnivores, the females provide most
of the energy required for fetal growth and parental care. The reduced
contribution of the male to parental care correlates with more frequent
polygyny among mammals than birds (Orians, 1969). A few examples will
illustrate the occurrence of delayed breeding among males in species in
which copulations are not randomly distributed among males.

The polygynous pmnepeds offer some striking parallels with lek-
forming grouse. Species like the elephant seals (Mirounga spp.)
(Bartholomew, 1952; Carrick et al . , 1962a, b) and northern fur seals
Callorhinus ursinus (Bartholomew and Hoel , 1953; Peterson, 1968) gather
for breeding in large colonies on oceanic islands. Here the females

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congregate in dense "harems" around large bulls, which occupy positions
near the shore. Bull northern fur seals defend sharply delineated
territories against intruding males, while bull elephant seals appar-
ently exclude other males from the herds of females but do not defend
fixed boundaries. The females come ashore shortly before parturition.
After giving birth to their single pup they spend several weeks (elephant
seals) or months (fur seals) suckling the pup either with (fur seal)
or without (elephant seals) periodic trips to sea for feeding. Female
northern fur seals enter estrus about six days after parturition just
before leaving for their first excursion to sea, while female elephant
seals copulate in the third week following parturition and a few days
before deserting their well-fed pup. Since the gestation period is
about eight to nine months, parturition and fertilization are brought
into seasonal proximity by delaying implantation of the blastocyst for
several months. Thus although pinneped rookeries represent aggregations
of animals for the purposes of mating, like the leks of grouse, par-
turition and parental care also occur in the rookery. Some polygynous
pinnepeds probably occupy most of the appropriate shoreline in their
small breeding islands, so that the animals appear not to have congre-
gated at particular sites within the suitable habitat. In other species,
though, published accounts definitely suggest that pinneped rookeries do
constitute true social clusters (Callorhinus ur sinus, Kenyon et al . ,
1954; Zalophus, Peterson and Bartholomew, 1952). Polygyny is associated
with the formation of harems, or clusters of females, around breeding
males. The breeding bulls in several species attempt to prevent
females from leaving their harems by herding the females or chasing
them back when they begin to wander away or, rarely in the northern fur
seal, by bodily throwing the female toward the center of the territory.
Early accounts (Allen, 1880) apparently exaggerated the males' influence
on harem formation, though. Several authors have noted that males
cannot prevent females from leaving their harems if the female chooses
to go (Halichoerus, Darling, 1939; Mirounga, Bartholomew, 1952;
Callorhinus , Bartholomew and Hoel , 1953; Peterson, 1968; Arctocephalus ,

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Macquarie respectively) , the average number of females controlled by
breeding bulls is smaller on South Georgia (24 in contrast to 48 on
Macquarie) (Carrick et al. , 1962b) . In response to the heavy, induced
mortality of older males, both males and females breed at younger ages.

Male and female fur seals also differ greatly in the onset of
successful reproduction. In the Pribilof Islands females normally bear
their first young at age five, while in a sample of 199 territorial males
the youngest was one male seven years old (Johnson, 1968) . Most males
do not establish territories until they are 9 to 11 years old (Kenyon
et al . , 1954).

Males of polygynous artiodactyls also reproduce successfully at
later ages than females. For instance, cow Tule Elk Cervus elaphus
nannodes mate for the first time in their first or second year of life,
whereas bulls are clearly older before they manage to defend exclusive
access to a cow herd during the rut (McCullough, 1969) . This author
recognized one bull in two successive years, and this animal improved
his mating success in the second year. Among North American mountain
sheep (Ovis dalli , 0. canadensis) the dominant males, which have the
largest horns, copulate with most of the females (Geist, 1966, 1968).
Instead of dtf ending an entire band of females, as dominant bull elk do
during the rut, a dominant ram instead tends one female at a time as
they come into estrus. Most ewes copulate first when about 2 \ years
old, but rams do not grow full-sized horns, nor normally copulate
successfully, until they are 7 or 8 years old. The older and larger
males of some antelopes (Bovidae) isolate themselves during the rut
in territories from which they exclude other males. In their territories
they court and copulate with females, which wander from territory to
territory in groups. The Uganda kob Adenota kob has developed an
extreme form of this type of social organization. Males occupy small
(15-35 meters in diameter), contiguous territories which cluster around
particular sites two to three kilometers apart in favorable savanna
habitat in Uganda (Buechner, 1961; Buechner and Schloeth, 1965;

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Leuthold, 1966). These clusters of small territories, called Terri-
torial Grounds, resemble the leks of grouse in that almost all mating
activity occurs there. Other male kob maintain much larger (100-200
meters in diameter) territories, called Single Territories, in the zones
between the Territorial Grounds. However, copulations do not normally
occur in these larger territories, since the estrous females usually
visit the Territorial Grounds for mating. Consequently, male kob compete
less intensely for the larger Single Territories. Males established near
the center of a Territorial Ground remain on their small territories
constantly for several days until they must leave to feed, since these
small territories do not include enough food for the males to maintain
themselves indefinitely. Thus the occupants of the territories rotate
every few days as males in poor condition leave to feed elsewhere and
returning males replace them. This rapid turnover of territorial
occupants does not occur on grouse leks, because in these species all
the males leave their lek for at least nine hours each day to feed.
The age-distribution of male kob on Territorial Grounds and on Single
Territories does not differ significantly (modal ages respectively 5
and 4 years) (Leuthold, 1966), although most of the mating occurs on
Territorial Grounds. The relationships between males on Territorial
Grounds and those on Single Territories remain uncertain, however.
Leuthold (1966) reports that, although most males during their absences
from Territorial Grounds join "bachelor" herds, at least some indivi-
dually marked males occupied Single Territories temporarily before
returning to the Territorial Ground. Also most copulations are per-
formed in the central territories on a Territorial Ground, another
resemblance to grouse leks, but Leuthold 's (1966) age determinations
for males collected on Territorial Grounds did not differentiate
central and peripheral males. The behavioral cycles of indivdual male
kob and the interactions between males of different ages require
further study. Females first mate between 1 and 2 years of age, while
males probably do not successfully copulate until they are 3-5 years
old (Buechner et al., 1966).

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find no careful assessment of the effects of age on dominance, or of
the changes in individuals' ranks with age. The rank of an individual's
mother often appears to influence an animal's own rank even more than
age does (Koford, 1963; Sade, 1967; Kawai, 1958; Kawamura, 1958).
Females especially tend to rank just below their mothers in the troop's
dominance hierarchy. Males do not retain such close associations with
their mothers, yet the sons of high-ranking females sometimes reach high rank
among the troop's males when only 5 or 6 years old (Koford, 1963). Thus
the determinants of dominance and mating success among male macaques
seem to involve complexities not found in animals other than primates.
Nevertheless, it seems clear that females begin reproduction regularly
at an earlier age than do virtually all males.

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XIV. PROBLEMS EN THE EVOLUTION OF LEK MATING SYSTEMS

In preceding sections I have drawn attention to several general
features of leks and other polygynous mating systems in birds and
mammals. These features include (1) non-random distribution of matings
among adult males, (2) reduction in or lack of male parental care, and
(3) onset of successful reproduction at a later age in males than in
females; leks are in addition characterized by (4) aggregation for the
purposes of mating. The present section will consider the evolution
of these features of polygynous mating systems. In this discussion
fitness (Dobzhansky ' s (1969) "Darwinian fitness") denotes the rate at
which an individual's genes propagate in subsequent generations,
relative to the rates at which other individuals' genes propagate.
Adaptedness (adjectival form, adaptive) refers to the relation between
an organism and its environment, on which fitness depends. Fecundity
means the number of reproducing offspring.

A. Delayed Reproduction Among Males

Selander (1965) has reviewed evidence that in polygynous species
of Icteridae (blackbirds, grackles and allies) males usually first breed
successfully at a later age than females. He points out that among
Icteridae first-year males do not have fully adult plumage and their
testes do not reach the size of older males', so first-year males in
comparison with older males are handicapped in their chances for mating
by their less developed plumage and their presumably lower androgen
titers. In analogy with Lack's (1954) explanations for delayed repro-
duction by both sexes in certain monogamous species, Selander proposed
that postponed breeding by males in polygynous species can be explained
by combining two considerations: (1) year-old males have less chance
of successfully mating in competition with older, more experienced males,
and (2) year-old males would increase their risks of mortality by
assuming full breeding plumage and competing for territories and matings.
In a more quantitative analysis of Lack's suggestion, Williams (1966)

140

later ages does not alone compensate for the delay in the onset of
reproduction. This is not to imply that delayed reproduction will
inevitably result in decreased fitness. Gadgil and Bossert's (1970)
and W. M. Schaffer's (MS) calculations suggest that the two conditions
which Selander, Williams and Lack originally suggested, when sufficiently
large, could indeed favor delayed reproduction: high risk of mortality
during reproduction and fecundity increasing with age. Stated differ-
ently, if a young male by reproducing so jeopardizes his chances of
survival to the following breeding season and yet gains a sufficiently
small chance that two of his progeny would survive to reproduce, then
by breeding at an early age a male might actually reduce the chances
that his genes would remain in the population.

The arguments so far suggest (1) that in determining a male's
fitness the increase in average fecundity of polygynous males at later
ages will not in itself compensate for their postponement of repro-
duction, but (2) that with sufficiently large risks to survival as a
consequence of reproduction as well as with increasing fecundity at
later ages, males might indeed optimize the rate of spread of their
genes by postponing the onset of breeding.

The proximate regulation of postponed breeding might involve
several factors: (1) direct prevention of younger males' attempts at
mating by older males; (2) physiological immaturity which precludes
mating by younger males even in the absence of older males , owing to
inadequate development of secondary sexual characters and inadequate moti-
vation to attract and elicit oopulations from females; (3) indirect pre-
vention of younger males' matings by social dominance of older males,
which might thereby obtain priority to territories favored by females;
or (4) "psychological castration" (Guhl et al . , 1945) , an interaction
of processes (2) and (3) , by which full physiological development of
gonads and secondary sexual characters is inhibited in subordinant
individuals. To my knowledge, no full evaluation of these four
possibilities is available for any polygynous species. At least in

142

None of these physiological or behavioral processes explains the
adaptedness of the males' delayed breeding in comparison with females
in polygynous species. The theoretical considerations which I have
reviewed implied that delayed reproduction would most likely improve
fitness if potential fecundity increased sufficiently with age and
early reproduction sufficiently jeopardized survival. Epigamic and
parental behavior and morphological specializations might increase
mortality in several ways : CD by reducing the time and energy available
for predator avoidance, maintenance, feeding, resting; (2) by increasing
the conspicuousness or vulnerability of individuals to predators; or
(3) by injuries incurred during intraspeciflc competition for mates.
Thus, intraspecific competition for mates might increase the risks of
mortality either directly through injuries, or indirectly, through the
effects of epigamic behavioral and morphological specializations on
interspecific interactions with predators, food supplies, or competitors.

Although there is no direct evidence that male Sage Grouse improve
their chances of survival by postponing breeding, some indirect evidence
suggests this could conceivably happen. Yearling males certainly spend
less time and energy on a lek than do older males, since yearlings
arrive later in the season than do older males and rarely display as
persistently as older males with females near them. The energy and
time they conserve might increase the yearling males ' chances of sur-
vival, for instance by allowing more time for feeding. Even among the
older males, the more peripheral ones probably expend less energy on a
lek, since they spend less of the morning Strutting. Peripheral males
never showed the occasional signs of fatigue observed in the predominant
breeder late in a morning when females had been numerous. Strutting
must be strenuous exercise for a male. The 7-10 second interval between
Struts might just allow sufficient recovery to permit long-sustained
Strutting. The central male with the most sustained close association
with females occasionally Strutted at unusually low rates late in the
morning, a probable indication of his fatigue. However, satiation,
rather than fatigue, could also explain this lethagric behavior late

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in a morning after much display and mating.

One might expect that yearlings, usually the most peripheral
males on a lek, would incur greater risks of predation, since attacking
predators would first contact the periphery of a lek. Yet, the only
two kills which I found on leks involved males in at least their second
year. The kill by a Golden Eagle on the Muddy Springs Lek in 1968
occurred near the center of the lek, roughly 50 feet from the mating
center. The risks of predation at different positions on a lek need
further investigation.

The direct consequences of competition among males for opportuni-
ties to copulate appear to have little effect on survival. I never saw
a male sustain an injury in a fight, although fighting clearly might
have indirect effects on survival since it represents an appreciable
expenditure of energy. It is not even clear that the males' positions
on a lek were primarily determined by fighting.

Younger males among Sage Grouse and probably other polygynous
species appear, on the basis of incomplete and indirect evidence, to
have reduced some of the indirect effects by which intrasexual com-
petition for mates might reduce survival. My impression is less that
older males prevent the younger from mating than that the younger invest
less effort in competing for matings. In many polygynous species the
reduced "effort" of younger males is manifested not only in less
energetic and persistent behavior but in assuming less conspicuous or
encumbering secondary sexual characters or in not growing to the full
size of older males.

It would be over-simplifying to assert that the younger males
exert less effort merely because they are not likely to succeed against
or might get injured by the older and often larger males. This formu-
lation would leave unanswered the question of why the males did not
develop full reproductive capacity, including full gonadal growth and
secondary sexual characters, at an earlier age, as do females of the
same species. I have tried to suggest that the propagation of a male's

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copulate even without investing as much time and energy as older and
more successful males. Since age at first breeding so strongly in-
fluences the rate of spread of a gene, selection might favor investing
some energy for reproduction at an early age even if the chances of
success were very small. For year-old male Sage Grouse the chances for
copulating seem almost absent. (2) Partial development of adult male
morphology might aid an individual in agonistic interactions other than
competition for mates. I have found no evidence for this possibility in
Sage Grouse. (3) Some time and energy spent at a young age might improve
the male's opportunities for future success. Selander specifically
suggested that experience gained as a yearling eight improve a male's
ability to compete for mates in later years. In Sage Grouse if the
hypothetical centripetal movement of territories proves correct, effort
expended by first-year or older peripheral males might indeed enhance
their future success in at least two ways . In order to attain success
in mating, a male would have to start the process which leads, by the
occupation of successive vacancies, toward a mating center. Further-
more, if leks with more males attract more females and if females tend
to return in successive years to the same lek, then by displaying in a
peripheral position a male might actually increase the number of females
with which he will have a chance to mate in later years. It would then
be adaptive for a yearling male to commit himself to a particular lek
and expend some energy, although presumably not so much as the older
males, in attracting females to the lek.

B. Intrasexual Competition

Although I have suggested that reduced reproductive effort among
younger males should be adaptive for reasons other than competition with
older males for mates, intrasexual competition might still constitute a
critical selective force among the older males. As reproductive success
increases with the males' ages, some age might be reached at which
essentially all males became equally successful breeders; or, con-
versely, large inequities in the distribution of matings might remain

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at all ages. In the first case, the coefficient of vai-iation of
fecundity among individual males would fall to a small value with in-
creasing age, while in the second case it would remain high. The first
case does not differ appreciably from the situation which probably
characterizes most monogamous species in which the majority of males
each obtains one mate. Yet in polygynous species, even if the variance
in fecundities among individual males does approach some small value
with advancing age, there always exists the potential for competition
among males for opportunities to mate.

Intrasexual competition for mates can occur in two guises, as
Darwin (1871), Huxley (1938), and Fisher (1958) describe. On the one
hand, agonistic interactions among males might determine which males
have access to females. The object of competition in this case might
be a territorial location to which females are attracted, a position in
a herd of females, or high rank in a dominance heirarchy. Alternatively,
interactions between males and females might determine which males are
chosen by females as mates. These two forms of intrasexual competition
actually represent two poles of a continuum: males vie with each other
either to attract and stimulate females directly, or to hold positions
that attract females or that increase accessibility to receptive
females.

The mating system of Sage Grouse probably depends on critical
interactions among males, between males and females, and among females.
The context of intrasexual competition for mates is, therefore, not
simple. I have suggested that females might remember at least the
approximate locations of mating centers on leks and return to them on
subsequent visits ; also that on their first visit females might
recognize the mating center partly by the dense aggregation of other
females there. Perhaps some as yet unrecognized male-female inter-
actions might also affect the distribution of matings among the males
near a mating center. Nevertheless, my observations suggest that
interactions among the females could specify the locations of the
mating centers. This would imply that males compete primarily for

_l

•

m

148

as on male-female interactions. Once harems have formed males estab-
lished in peripheral positions tend to expand or move their territories
as much as possible toward the areas favored by females (Kenyon, 1960).

C. Evolution of Female Behavior

My observations failed to reveal evidence that female Sage Grouse
chose a place to copulate by discriminating individual differences in
the behavior of the males. Although undetected influences of individual
differences among males might exist, females appear to base their choice
partly on tradition among the females themselves. In another polygynous
bird, the Long-billed Marsh Wren, females appear to choose a nesting
site on the basis of habitat quality rather than the males1 behavior
(Verner, 1964). Although the proximate mechanisms of female choice do
not necessarily involve discriminations among individual males, these
proximate mechanisms should evolve to maximize a female's changes of
mating with fit males as well as to maximize her fecundity.

A male's fitness, as I have discussed above, consists not only in
his ability to compete successfully with other males for mates, but
also in the optimal allocation of his time and energy throughout his
life cycle to reproduction or to maintenance and growth. Unless a
male life history with delayed reproduction maximized the spread of a
male's genes, a female's genes would propagate faster if she selected
a younger male and left sons that would also tend to breed at an early
age. In other words, females should not restrict their matings with
young males unless early breeding by a male decreases his fitness.
To understand the evolution of females' behavior we must understand
the adaptive significance of the males' life history and behavior.
An assertion that females mate with older males because these have
demonstrated their ability to survive (for instance, Robel, 1967;
Koivisto, 1965) assumes that male fitness consists in survival, an
assumption that is only partially true. Similarly, an assertion that
a male's fitness consists in the number of females with which he mates
also is only partially correct, since reproductive rate depends not

149

only on average fecundity but even more on the age of breeding and
also on the consequences which breeding at a given age have for
expectations of future reproduction.

D. Genetic Diversity

The Sage Grouse exemplifies an extreme form of non-random mating,
in which only a few percent of the males copulate with most of the
females. This distribution of matings might effect genetic diversity
in two ways: by reducing the effective population size, and by producing
strong normalizing selection in favor of those genotypes which could
eventually reproduce successfully. When the two sexes are unequally
represented among breeding individuals, the effective population size
is less than when the sexes are equally represented. The effective
population size was shown by Sewall Wright (see Falconer, 1960) to
equal twice the harmonic mean of the numbers of the two sexes :
1/Ne = 1/41^ + l/4Nf . Letting Nb equal the total number of breeding
individuals and p equal the proportion of males among this total, then
Ne = 4p (1-p) Nb. If males comprise 5% of the total number of breeding
individuals, the effective population size approximates 19% of the total
number of breeding individuals. Thus from the standpoint of random
dispersive processes in their gene pools Sage Grouse populations should
behave like much smaller populations in which equal numbers of both
sexes breed. Exact calculations of the ratio Ne/Nb for Sage Grouse
are not yet possible, since available mathematical treatments do not
take into account overlapping generations or unequal participation by
breeding males in mating. In order to calculate N„ one must also know
the total number of breeding individuals in the population. Although
breeding individuals probably tend to return year after year to the
same lek, there is probably much less correlation between the leks used
by parents and their progeny (this Chapter, Section F) . Thus the total
number of individuals in an interbreeding population of Sage Grouse is
likely to be larger than the number of males and females which mate
at any particular lek.

151

as a function of age. In some circumstances delayed reproduction
might evolve. Armstrong (1955), Snow (1963), and Crook (1965) suggest
that superabundant food supplies might allow females to raise their
broods successfully unaided by a mate, and that reduced male parental
care should then predispose a species to polygyny. However, they
suggest that, once males are relieved of parental duties, polygyny
would evolve solely as a consequence of competition among uiales for
mates, rather than, as I have suggested above, as a consequence of
adaptive readjustments in the expenditure of reproductive effort by
males as a function of age.

•

F. Aggregation of Displaying Males

Aggregation for mating constitutes an important attribute of a
lek mating system, and several authors have considered the adaptive
consequences of the aggregation of displaying males. Most of their
suggestions are not mutually exclusive, so that a number of them might
prove correct. The suggestions fall into four principal categories:
(1) consequences for predation; (2) increased stimulation of females;
(3) population control; and (4) formation of reproductively isolated
denies .

(1) Aggregations of animals have two counteracting effects on
the risks of predation. On the one hand, an aggregation probably is
easier for predators to detect, particularly if it includes individuals
engaging in conspicuous behavior or having conspicuous color patterns.
On a Sage Grouse lek, much of the males' behavior serves in part to
advertise the lek to conspecific males and females. However, pre-
dators must also be attracted. Furthermore, the traditional locations
of leks should render them especially susceptible to a predator that
can remember their positions. Golden Eagles seem particularly likely
to harass Sage Grouse leks, although I could not ascertain whether
individual eagles tended to visit the same lek on different days.
The eagles' behavior clearly indicated that they attempted serious
attacks on Sage Grouse leks (Chapter IX, Section I). On the other

152

band, an aggregation of individuals probably offers advantages in
detecting approaching predators (Koivisto, 1965; Lack, 1968). Sage
Grouse leks seem to have this consequence also. Balancing their
apparent harassment by eagles, leks seem to offer clear advantages in
detecting an eagle's approach. A lone male approached from behind might
fall easy prey, since the eagles attacked at high speed close to the
ground in the dim light of dawn. In a congregation at least one male
might detect the eagle and alert the others by flushing. Koivisto
(1965) described raids by Goshawks Accipiter gentilis against leks of
Black Grouse. He too feels that the aggregation of displaying males
offers them some mutual protection. Raptors killed malo Prairie
Chickens at their leks on th° average less than once every 400 mornings
of observation (Berger et al. , 1963) .

(2) Social stimulation within aggregations of breeding animals
might provide tonic facilitation of successful reproductive behavior.
This possibility, originally formulated by Darling (1938) , has recently
been discussed by Lehrman (1959) and Orians (1961) . The aggregation of
displaying males at leks could exert several sorts of tonic influences
on female breeding behavior. For one thing, larger congregations of
displaying males might attract females disproportionately more than
smaller ones would (Snow, 1963; Braestrup, 1966). Estimates of the
numbers of copulations on Sage Grouse leks in any season ar« difficult
to obtain. Mating activity varies greatly from day to day, and a
majority of the matings occurs on only three or four days of each
season (see Chapter V, Section E) . Two studies of lek-forming species
have attempted to compare the number of copulations per male on leks
with different numbers of males (Hamerstrom and Hamerstrom, 1954;
Koivisto, 1965). The Hamerstroms' extensive observations of Prairie
Chicken leks at the height of the mating season in Wisconsin suggest
that neither the average number of females per morning nor the average
number of copulations per morning increases in proportion to the number
of males on a lek. However, as these authors themselves indicate,
their data might not accurately estimate the frequency of copulations

154

choosing a lek during their first breeding season. Dalke et al. (1963)
present evidence that yearling male Sage Grouse often visit more than
one lek in a season. Female Sage Grouse probably often nest at con-
siderable distances from the lek at which they had mated (Dalke et al. ,
1963; Patterson, 1952). Since broods disperse in the autumn, a female's
progeny would probably not overwhelmingly return to the lek at which
their mother had mated.

155

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«

Ik. I

166

XVI. APPENDIX: RATES OF REPRODUCTIVE INCREASE FOR
MALES OF POLYGYNOUS SPECIES

To calculate the rate of reproductive Increase, r, of males I
used the standard equation (Andrewarths and Birch, 1954),

CO

0 = 1 - Z o-™ W ■ (1)

e
x=o x x

where x is age, and 5 and b are respectively the age-specific survival

and fecundity. If J equals the probability that a male zygote will
m

survive to age one, and s equals the annual survival of males after

m (x-1)

age one, then s for males equals J s .1 assumed that for
■ x mm

polygynous species the fecundity of a male is proportional to the number
of females with which he mates. The average fecundity of breeding males
is then proportional to the ratio, (number of breeding females) /(number
of breeding males). Assuming constant annual recruitment to the popu-
lation this ratio will equal

oo

Jfsf

*-af = B , (2)

z

CO

x=»a

m

m m

]

*

where J. and J are the probabilities of survival to age one for females

and males, a„ and a are the ages at onset of breeding for females and
i m

males, and s. and s are the annual survival rates after age one for
I m

females and males. In addition, I assumed that each male had zero

fecundity until age a and that all males aged a and older shared the

m m

breeding females equally, so In equation (1) b = 0 when x < a , and

x m

b = B when x -^ a . In practice the summations in equations (1) and

, m (x-1)

(2) were carried to the smallest integer value of x at which J s <"

(x-1) m m

.0001 and Jfsf *^ .0001. By setting the production of sons by a

breading male equal to B and J =J =1 , I assumed that each breeding

: _

"n

!
'i

i

!

!

168

Table 19

Rates of reproductive increase of males (r) when adult male and
female annual survival rate;
breeding at age one (af=l) .

female annual survival rates are the same (s =s_) and females begin

m I

•-I

t

in
•h n

s a

S <n
§

to

3 a
■a u

<

.1
.5

.9

Age at first breeding among males, a

m

8

.095
.405
.642

.050
.248
.436

.026
.143
.283

.013
.080
.176

170

XVII. MAPS

a

T
I

b

19-25 April 1967

172

, „ - - -'OO-s

V

N

..A SE

2W « C «
llAlll|l|l»w"f; J

A0

/ V

- OD-< V

' \

I \

I

t

O Blind
10 m |

Map 2. Territories in 1967 after the disappearance of Male A. Male
N occupied the area vacated by A (dotted line) . Other symbols as in
Map 1.

•

173

>-,„,--

0

Blind #1

10 m

N

Blind #2

o

1

i

Map 3, Territories of individually identified males at least two years
old (capital letters), 3-5 April 1968, Fords Creek Lek. Boundaries based
on records of Facing-past Encounters appear as solid lines. Interrupted
lines indicate less accurately known boundaries based on the males ' move-
ments. In reality Facing-past Encounters cluster in zones one to three
meters wide (see Maps 8b, 9b). Stars indicate positions of marker flags
on a 50-foot (15.4-meter) grid. Hatched areas are shallow depressions
in the ground seldom used by displaying males . Several other males
occupied positions south and west of the area mapped.

•

175

I

/
/
/

.'•

I

TM

!• /dsh /

i *r • /
" i •.• • • ,

o □ Matings 27 March 1968

° O " 3/29 0646-0723

• <D " » 0725-0735

• ii n 0737-0807

Male X's limit early on 3/29

Map 5a. Territorial boundaries and matings, 27-29 March 1968. On the
29th Male D's matings shifted southwest during the morning. Larger
symbols indicate two or three matings at the same place. Territorial
boundaries indicated by solid lines are based on Facing-past Encounters
in Map 5b. Boundaries estimated from the males' movements appear as
interrupted lines. Hatched areas are shallow depressions rarely used
by territorial males. Stars indicate the positions of marker flags.

J

177

I

~A

ft

%
i

1

U

%

5

»

UT

?ri

TM „ N

1 April 1968,0545-0630

o 0 Matings
© Interrupted matings

• UT /
V*" TM/" \

x >-< ) B !

J o x X

• *

10 m

1 April 1968, 0630-0810

Map 6a. Territorial boundaries during the morning of 1 April 1968.
The pack of females moved south in Male D's territory (compare the
positions of his matings earlier and later in the morning) . See Map 5a
for explanations of symbols. Territorial boundaries indicated by solid
lines are based on Facing-past Encounters in Map 6b.

V

179

I

\
\

7

UT

,'••- \

/*" \

I
B /
/__

'a"

ti

h

10 m

Boundaries 1-5 April 1968
Male B's territory 9 April 1968

Map 7. Reoccupation of the territory vacated by Male TM. Male TM dis-
appeared after 5 April 1968, and by 9 April Male B had reoccupied the
vacated area with its original boundaiies.

•

180

I

=-
i

i

I

i

f

-

3-5 April 1968

o Matiwjs

® Interrupted niatings

1

9

Map 8a. Territorial boundaries and matings, 3-5 April 1968. Boundaries
indicated by solid lines are based on Facing-past Encounters in Map 8b.
See Map 5a for explanations of symbols.

182

i

8-14 April 1968

10m

— cvj <r

o oQ Matings

« Interrupted matings

Male D's boundary
8 April

Boundaries of Males X and UT
14 April

Map 9a. Territorial boundaries and matings, 8-14 April 1968. Boundaries
indicated by solid lines are based on Facing-past Encounters in Map 8b.
The mating center has now moved to Male N's territory. Males X and UT
extc-.c!?d their boundaries into Male N's territory on 14 April while
Male N was mating frequently. Male TM (Map 8a) has disappeared, and
Male B has occupied the vacancy, although for one day Male D defended
the western portion of TM's former territory. See Map 5a for explana-
tions of other symbols.

184

*~ -v _.•

i T

V i

x F \ N i ut i ST

Y i z ! \ V x \ _ I i ^

) \ \

10 m

»» — s.

22-25 April 1968

Map 10a. Territorial boundaries, 22-25 April 1968. Five yearlings
(Males I, J, Y, Z, and K) occupied stable territories. Boundaries in-
dicated by solid lines are based on Facing-past Encounters in Map 10b.
See Map 5a for explanations of other symbols.

186

I

i I

8-28 April 1969

• Boundaries which dunged alter
18 April

I*

Map 11a. Territories of individually recognized males (numbers), 8-28
April 1969, Dry Sandy Lek. Boundaries based on records of Facing-past
Encounters (see Maps lib and lie) appear as solid lines. Interrupted
lines indicate less accurately known boundaries based on the males'
movements. Some boundary zones are actually far from one-dimensional,
for instance between Males 8 and 13, and between Males 20 and 22.
Stars indicate the positions of marker flags in a 50-foor (15.4-meter)
grid. Many othar males occupied positions in all directions from the
mapped area. .Males 24, 25, 27, and 28 were yearlings.

-±J£l*t,:*--v° - : ■..■■■■■

J

I

187

j

7 +

-I CM V

a A A Males 20-22, 20-8, 2-12. 6-9, 3-4, 3-19, 6-14. 7-8

o O 0Mal« Z2"13' 8"13' 7"4' 3'6' 4'9' 9"2' 12*21' 9*12' 20"12' 8"9
ODD Males 20-13, 9-20, 8-4. 7-19. 6-4, 6-2. 3-11

8-18 April 1969

I

70

*4»

6 ££&

13

o

m 20

D

12

10 m

•

Map lib. Facing-past Encounters, 8-18 April 1969. Numbers identifying
each male are placed near the centers of their territories. Encounters
between each pair of males are assigned one of three symbols. Larger
symbols indicate more than one encounter at the same place. Stars
indicate the positions of marker flags in a 50-foot (15.4-meter) grid.

183

-._'

19

I +

0 O O Matin's

© Interrupted mating:

8-28 April 1969

10 m

Map 12. Territorial boundaries and matings, 8-28 April 1969. Inter-
rupted matings ( h ) occur near or within zones of Facing-past Encounters.
Territorial boundaries represented by solid lines are based on Facing-
past Encounters in Maps lib and lie.

J

190

%

i

•

xjgp 3 females

24 April 1969,05:54-06:12

— CM

a O Males 8 and 9 "] Focing-

A Males 8 and 20 I post

o Males 9 and 20j Encounters

£0 3 fenu'es

23 April 1969,05:19-05:56

9

•

Map 13. Males Strutting in positions (hatched) along the boundaries of
their territories nearest a group of females (strippled area) . Male 8
Strutted at one end of his territory or the other depending on where
females were closest. Males sometimes encroach into a neighbor's territory
to Strut nearer the females. On 23 April Males 8, 13, and 9 Strutted unusu-
ally far beyond their regular boundaries and even met their neighbors in
Facing-past Encounters beyond the usual zone. It was unusual for females
to remain in the territory of Male 20, but they often stayed in the
territories of Males 4 and 7.

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and non-
in sage

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QL 696 .G285 W55 1973

Territoriality and non-
random mating in sage

BLM LIBRARY
BLDG 50, ST-150A
DENVER FEDERAL CBITPR <A ^ A<
J^BOX 23047 R <£&&■

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