Bobcat biology in a Mojave Desert community

BLM LIBRARY

88006318

QL
737
.C23
Z63

Bureau of Land Management

BldgV Denver Federal Center
Denver, CO 80225

Department of Interior

BUREAU OF LAND MANAGEMENT

California Desert Planning Program

BOBCAT BIOLOGY IN A M0J.AYE DESERT COMMUNITY 1/

by David 3. Zezulak

and ^ <%

Robert G. Schwab 2/ ^

ABSTRACT

Eight adult bobcats (7 males, 1 female) were trapped in box-traps, radio-
collared and released on Joshua Tree National Monument, Riverside County,
California, during 2634 trap nights between 2 December 1978 and 1 April 1979.
Minimum bobcat density is 0.1 bobcats kn?2 (Q.25/mi2). Home range size of
7 bobcats determined by 1310 radio-locations averaged 25.3 km^ (10.2 mi^).
Male bobcat home ranges were overlapped as greatly as 66* by other male
bobcats. The female bobcat home range was overlapped 75* by one male bobcat,
A crepuscular bobcat activity pattern existed during winter months and a
nocturnal activity pattern existed during the spring months. These bobcat
activity patterns corresponded to the abundance of rodents and rabbits as
determined by roadside census during the study.

1/ Supported by the Bureau of Land Management and the California Department
of Fish and Game; BLM Contract No. CA-060-CT8-76. Final report ('February
1980)

2/ Department of Wildlife and Fisheries Biology, University of California,
Davis, California 95616

Zezulak and Schwab 2.

INTRODUCTION
Biopol itics

Declining populations and concomitant Federal and International
restrictions .on trade of spotted cat pelts has increased the aemand for bobcat
(Felis rufus) an,d lynx (Felis lynx) hides in domestic and foreign fur markets.
Prices of bobcat pelts from California have tripled during the past five
years. Presently, a prime bobcat pelt may have a value exceeding $400. The
increased demand and high market values of bobcat pelts have resulted in a
appreciable increase in trapping pressure.

Historically, the bobcat was an unprotected predator, but was
reclassified as a nongame mammal in California in 1971. Subsequently, the
California Fish and Game Commission established a no-limit se?son which
restricted trapping of bobcats to the months from mid-Novembev through
February. Beginning with the 1979-80 trapping season, bobcats may be taken
between mid-November and the end of January, or until 6000 bobcat pelts have
been tagged for export.

Prompted by an apparent reduction of bobcat population levels in some
areas of the nation and the increased market/trapping pressure imposed on the
bobcat, the Defenders of Wildlife filed a petition on 20 January, 1977 with
the Department of the Interior to place the bobcat on the United States list
of Endangered and Threatened Wildlife. Pursuant to the Endangered Species Act
of 1973, a review of the status of the bobcat was begun July 13, 1977 (Federal
Register - July 1977). The bobcat was subsequently listed as an Appendix II
species by the Convention of International Trade in Endangered Species of Wild
Fauna and Flora and the Endangered Species Scientific Authority banned
international export of bobcat pelts taken after August 29, 1977 (Endangered
Species Technical Bulletin - August 1977). The Endangered Species Scientific
Authority later allowed a quota of 6000 bobcat pelts to be exported from
California during the 1977-1978 trapping season, providing each pelt was
tagged to indicate the origin of the pelt (Endangered Spscies Technical
Bulletin - August 1978). In October, 1978 the Endangered Species Scientific
Authority approved export of bobcat pelts from 34 states, incljding
California, with quotas set for only two states, New Mexico and Wyoming
(Endangered Species Technical Bulletin - October 1978). The California
Department of Fish and Game elected to continue its quota for bobcat pelts
sold in fur markets despite the easing of restrictions.

Federal and state agencies initiated several research contracts in

Zezulak and Schwab 3.

anticipation of the need for detailed data on the biology and status of the
bobcat. This report documents the results of a study on bobcats at Joshua
Tree National Monument in the Mojave Desert of California during the period
from December 1978 to August 1979. The study was contracted by the Bureau of
Land Management and jointly funded by the California Department of Fish and
Game .

Literature

At least oie of four subspecies of bobcat are present in most, if not
all, counties cf California (Grinnell, 1937). The northwestern bobcat (F. r.
fasciatus) occupies the moist, coastal portions of northern California. The
desert bobcat (F. r. baileyi ) is found in arid, desert regions of southeastern
California. Th2 pallid bobcat (F. r. pallescens) is found on the western edge
of the great basin habitat in extreme northeast California. The California
bobcat (F. r. cal ifornicus) is found throughout the balance of the state with
the exception of areas under extensive agriculture. All vegetation types or
life zones are occupied by the bobcat in California.

Bobcats were historically distributed throughout the United States and
most of Mexico and utilized a variety of habitat types. Habitat preference by
bobcats in Minnesota was primarily thick cedar (Thuja occidental is) or spruce
(Picea mariana) swamps (Rollings, 1945). McCord (1974) found that bobcats
preferred cliff areas, spruce plantations, (£. abies) and hemlock (Tsuga
canadensis) -hardwood habitat in Massachusetts. Bailey (1972) classified areas
dominated by sagebrush (Artemesia tridentata) with nearby caves and sagebrush/
juniper (Juniperus osteosperma) areas near volcanic outcroppings as being good
bobcat habitat in Idaho. Most of the bobcat preference for these habitats was
accounted for by prey density and cover availability.

The density of several bobcat populations has been estimated. For
example, Bailey (1972) believed that the number of adult bobcats in a 3-year
study of the pallid bobcat in Idaho regained constant at 0.05 bobcats per
km2 (0.02/m2). Lembeck (1978) determined densities of 1.27 to 1.52
bobcats per km2 (3.37 to 3.94/mi2) for the California bobcat in the
chaparral of San Diego County, California. Two density estimates are
available for the desert bobcat; Brownlee (1977) estimated bobcat densities
ranging from 2.46 to 5.96 bobcats per km2 (0.95 to 2.3/mi2) in Texas
whereas Jones (1977) estimated that bobcat density in his study area in
Arizona v/as 0.28 bobcats per km2 (0.11/mi2). Brownlee based his estimate

Zezulak and Schwab 4.

on the results of intensive removal trapping while the lower estimates of
Bailey and Jones were based on the number of bobcats captured and released
within their respective study areas.

Several investigations have been made to determine home range parameters
of bobcats (the home range of an animal is generally understood to be the area
it utilizes durrng the course of normal activity). Marston (1942) reported
home range estimates of 47 to 104 km2 (18 to .40 mi2) for bobcats in Maine.
Erickson (1955) estimated bobcat home range size to be 38 to 52 km2 (15 to
20 mi2) in Michigan, and Rollings (1945) estimated that bobcats in Minnesota
have home ranges of 25 to 38 km2 (10 to 15 mi2). Kight (1962) describes
maximum bobcat home ranges of 2.6 km2 (1.0 mi2) in South Carolina, and
Pollack (1949, 1950) reports that bobcats in northeastern United States have
home ranges from 3.9 to 14.2 km2 (1.5 to 5.5 mi2). These data were
obtained by snow- tracking techniques as investigators trailed hunting bobcats
through swamps, spruce, hemlock or cedar thickets. Some estimates were made
of the area utilized by bobcats which left distinctive, identifiable tracks.

As the feasibility of utilizing radio-telemetry equipment in wildlife
investigations increased, home range estimates based on repeated locations of
individual DObcats were obtained. Bailey (1974) reported home ranges of 42.1
km2 (16.3 mi2) for four male, and 19.3 km2 (7.4 mi2) for eight female
bobcats in Idaho. Marshall and Jenkins (1966) reported 4.64 km2 (1.79
mi2), 3.52 km2 (1.36 mi2), and 2.46 km2 (0.95 mi2) home ranges for
an adult female, a juvenile male, and a juvenile female bobcat, respectively,
in South Carolina.

Because the extent of movement is probably closely associated with
survival potential, bobcat movement patterns have been the subject of several
investigations. Bailey (1974) found no significant difference between the
average daily consecutive movements of female (1.2 km or 0.8 mi) and male (1.8
km or 1.1 mi) bobcats in Idaho. These averages were based on movements from
one day to the next and represent the minimum movements in terms of distance
traveled. Marshall and Jenkins (1966) reported average daily movements of
telemetered bobcats in South Carolina to be from 1.88 to 4.78 km (1.17 to 2.97
mi).. Bobcats are reported to move up to 11.7 km (7.3 mi) per day j'n Michigan
(Erickson, 1955) and 4.8 to 11.2 km (3.0 to 7.0 mi) per day in Minnesota
(Rollings, 1945). It should be noted that the data from Michigan and
Minnesota, as well as a 3.2 km (2.0 mi) bobcat movement in Idaho (Bailey,
1974) were obtained via snowtracking; and the effects of snow on the extent of

Zezulak and Schwab D*

bobcat movements has not as yet been determined.

Bobcat sex ratios vary greatly with lows of 0.40 males per female in
Vermont (Foote, 1945) and 0.60 adult males per adult female in Idaho (Bailey,
1972) to a high of 2.1 males per female in San Diego County, California
(Lembeck, 1978). Ratios of male to female bobcats lie between these extremes
in the following* states: 1.04:1 in the northeast United States (Pollock,
1950); 1.11:1 and 1.29:1 in Utah (Gashwiler et aU, 1961); 1.7:1 in Arkansas
(Fritts and Sealander, 1978), 1.26:1 in Arizona (Young, 1958); 1.01:1 in
Wyoming (Crowe, 1975). These sex ratios may not be indicative of actual
population sex ratios since the larger home r: nge and greater movements of
male bobcats may create a differential susceptibility to trapping, thus
resulting in a higher male to female ratio th?.n actually exists. For example,
Robinson and Grand (1958) determined that the mean recapture distance for 22
male and 24 female bobcats was 8.5 km (5.3 mi) and 5.1 km (3.2 mi),
respectively. Bailey (1972) found that the male bobcat home range was 4 to 12
times as large as that of the female,

Crowe (1975) determined that female bobcats are capable of breeding at
one year of age, whereas males are sexually mature at two years of age. Both
sexes remain reproductively active throughout their life which may extend
beyond 14 years in the wild. Fritts and Sealander (1978) reported an average
of 2.5 bobcats per litter in Arkansas. An average litter size of 2.8 bobcats
was determined in studies in Idaho (Bailey, 1972) and in Wyoming (Crowe,
1975). Gashwiler et a]_. (1961), found that litters averaged 3.5 in Utah.
Crowe (1975) and Gashwiler et eH. (1961) report a higher number of corpora
lutea (averages of 3.4 and 4,8, respectively) than the actual number of
bobcats born which indicated the possibility of intrauterine embryo loss.
While the majority of bobcat young are born in April or May in Utah (Gashwiler
et jtU, 1961) and in May and June in Wyoming (Crowe, 1975), both studies
indicate that young may be born during any month between March and October.

Various bobcat food habit studies haMe been conducted where the frequency
of occurrence, percent by weight or percent by volume of various prey species
have been determined from bobcat stomach contents. Rodents are the most
frequent bobcat prey item in California (Leach and Frazier, 1954; Grinnell
et al_., 1937; McLean, 1934), Vermont (Hamilton and Hunter, 1939), and in
YiVgi"nia and North Carolina (Progulske, 1955). Rabbits and hares constitute
the next most important prey item in these states (except in Vermont) but are
the most important prey item in Utah and eastern Nevada (Gashwiler et a]_. ,

Zezulak and Schwab 6.

1960) and Minnesota (Rollings, 1945). Deer were the most important prey item
in Massachusetts (McCord, 1974) but ranked second in Utah, Nevada, and in
Minnesota. These three groups of prey are consistent food items of bobcats
and probably vary in importance with their relative abundance within the
habitats studied.

OBJECTIVES

Previous studies document that bobcat sex ratios, reproductive rates,
density, home range size and extent of overlap, movements, and prey items
consumed vary greatly as a function of habitat type and the population of
bobcat studied. The present study investigates certain aspects of the biology
of bobcats in a habitat representative of the California Desert Conservation
Area. The study was conducted in the Mojave Desert well within the boundaries
of the Joshua Tree National Monument. This location reduced the affects of
artificial mortality (hunting and trapping of. bobcats) and minimized man's
influence on the bobcat population.

The objectives of the study include: (1) evaluate bobcat home range and
extent of home range overlap, (2) determine the density of bobcats as
revealed from home range and capture data, (3) analyze the movements of
bobcats within their home range in relation to varying habitat and prey item
abundance, (4) determine the activity patterns of the bobcat on a daily and
seasonal basis and, (5) determine aspects of reproduction, hunting
behavior/success, prey selection, and utilization of denning and resting
sites.

In order to achieve the above objectives we opted to detail the biology
of a small sample of the population rather than attempt a larger scale study
and diffuse our efforts beyond the primary objective of the study. The goal
was to capture all bobcats within an area the size of which was ultimately
defined by the areas utilized by the radio-collared bobcats. Additional
bobcats utilizing at least the fringes of our study area surely existed but
probably did not greatly interact with the radio-tagged bobcats utilizing the
core of the study area. The capture and study of these individuals was beyond
the extent of the telemetry equipment and manpower available. /

We believe that the primary data upon which bobcat management can be
formulated is best obtained by the detailed study of the biology of
interacting animals as opposed to fragmented data from many, non-interacting

7.
Zezulak and Schwab

animals. This seems especially germane when the density of animals, such as
existed during our study, was relatively high. Additionally, because of
extensive tracts of similar habitat and the lack of human disturbance on the
Monument, the data obtained from the bobcats on our study area has a much
wider geographical application.

THE STUDY AREA
The Queen Valley area (R.8E., T.2S.) of Joshua Tree National Monument was
selected as the study area. A considerable degree of protection from human
disturbance occurs on a National Monument which can contribute to stability of
the age/sex structure and density of resident animal populations. The
topography and vegetation of Joshua Tree National Monument, particularly the
creosote bush/shadscale scrub, Joshua tree woodland, and pinyon-juniper
woodland are comparable to adjacent areas. Acknowledging appropriate factors
such as degree of exploitation, data obtained on the Joshua Tree National
Monument may be applied to bobcat populations on adjacent lands of similar

habitat.

Queen Valley (elevation 1351 m, 4435 ft) is a relatively flat pi am with
surrounding mountains which rise to an elevation of 1730 m (5677 ft). The
mountains are uplifted Pinto gneiss with exposed areas of intruded quartz
monzonite. The sand in Queen Valley is predominantly eroded Pinto gnelss and
covers the plains between uplifted areas.

Dominant vegetation of the valley includes creosote bush (Larrea
divaricata), Joshua trees (Yucca brevifolia), Mojave yucca (Y. schidsera), and
antelope bitterbrush (Pnrshia olandulosa). This vegetation continues up the
mountain slopes where Utah juniper (Juni£erus osteosperma) and pinyon pine
(Pinus monophylla) become more common.

The average annual percipitation of the Joshua Tree National Monument
weather station is 10.6 cm (4.2 in). We measured temperature extremes which
varied from -6.7- C. (20* F.) in the winter to 44.4' C. (112° F.l during the
summer months of this study. Temperature changes of 22* C. (40- F.) during a
dial period were not uncommon.

METHODS AND MATERIALS
Bobcats were trapped in box-type live traps (61 x 61 x 122 cm or 2 x 2 x
4 ft.) and were immobilized with ketamine hydrochloride. Drug dosages varied
between 17 and 21 mg/kg (7.6 to 9.7 mg/lb). Heart and respiration rates were

Zezulak and Schwab 8.

monitored while the bobcats ware tractable. Blood samples were taken to be
tested for disease. Standard field measurements and weights were recorded and
each bobcat was tagged with njmbered, size 3 fingerling tags in both ears and
fitted with a collar containing a radio transmitter.

The radio transmitters were manufactured by AVM Instrument Company and
operate at frequencies from 150.85 MHz to 151.15 MHz. Transmitters were
attached to flexible antennas (30.5 cm or 12 in long) and size 660 or 660-3
lithium batteries (Power Conversion Inc.) were used to power the transmitter.
The transmitter package was e nbedded in dental acrylic and laminated between
two layers of neoprene-impregiated nylon belting (2 x 38 mm or 0.06 X 1.5 in).
The radio-collars were fastened around the necks of bobcats and the antenna
was between belting layers of the collar and protruded dorsally approximately
17 cm (6.5 in). The finished transmitter-collar weighed 150 to 165 grams (5.3
to 5.8 oz) and possessed a theoretical life expectancy of 26 to 31 months.

Radio receivers and null -peak directional antenna systems manufactured by
AVM Instrument Company were used to locate the bobcats. Directional bearings
were taken using the receiver/antenna system mounted in a 4-wheel drive
vehicle from the monitoring sites which provided the most accurate - ...
triangulation of each bobcat's position. These bearings were corrected for
deviation from true north and plotted on plastic overlays of 15 minute U.S.
Geological Survey maps of the study area.' Animal location points were plotted
when two or more intersecting bearings were obtained. Distances between each
plotted point were measured and the minimal home range size for each bobcat
was determined from irregular polygons made by connecting the peripheral
location points with lines.

Activity levels were determined by changes in the signal strength of 10
transmitter pulses mointored at the time each radio-location was taken. Three
pulses monitored prior to the 10-pulse sample established a preliminary
activity level and reduced the bias of when the sample was initiated. Zero
change in the strength of the transmitter pulses indicated a completely
immobile animal whereas ten changes within the ten monitored radio signals
indicated an animal in constant motion (constant motion might include behavior
such as eating, grooming, or constant travel). Intermediate changes of radio
signal strength (1 to 9 chances) indicate various degrees of animal activity.
The validity of the pulse-strength as an indicator of animal activity was
verified by direct observations via spotting scope or binoculars. Weather

Zezulak and Schwab

conditions were noted at the time telemetry readings were taken.

In addition to radio- telemetry techniques, observations of bobcats and
bobcat sign (tracks and feces) were noted. Potential bobcat prey species were
censused while driving between the various mobile monitoring sites. The
number of each genera of prey observed and the time and location of each

observation was' recorded.

Statistical evaluation and comparisons of distance, activity, and home
range data were analyzed using analysis of variance, SS-STP test for
differences of means, Student's t-test and Chi square tests where appropriate
(Sokal and Rohlf, 1969).

RESULTS AND DISCUSSION

Capture of Animals and Bobcat Density

Eight adult bobcats were captured a total of 12 times during 2,634
trap-nights between December 2, 1978 and April 1. 1979 (Table 1). The number
of trap-nights required to capture or to recapture (N-12) bobcats on the study
area averaged 219. However, the initial capture of a bobcat required a -
greater trapping effort in that an average of 329 trap-nights were required,
A total of 21 observations of uncoil ared bobcats were made on the study area.
Seven grey fox (Urocgon cinereoaroenteus) were captured during the study (379
trap-nights/capture). Bobcats and grey fox were the only predators captured.

Cumulative bobcat captures became asymptotic after capture of the sixth
bobcat. Captures of the seventh and eighth bobcats required a two-fold and
three-fold increase of trap-nights, respectively. A decreasing rate of bobcat
captures per unit effort as cumulative bobcat captures increased yielded a
Leslie Method (regression of captures per unit effort, De Lury, 1947)
population estimate of 6 bobcats, although 8 bobcats were captured in the

study area.

The eirht caotured bobcats represent the minimum number of bobcats
utilizing the study area during the trapping phase of the study (2 Dec 78 -
1 Apr 79). The fact that cumulative bobcat captures became asymptotic after 6
captures and that 6 bobcats is the population estimate based on trie rate of
bobcats captured per unit effort versus total bobcat captures does not
necessarily contradict the number of bobcats captured.

Zezulak and Schwab 10.

Table 1. Temporal and biological aspects of adult bobcats (7 male and 1

female) captured on Joshua Tree National Monument, Riverside County,
California (no juveniles were captured).

BOBCAT No.

*

SEX

WEIGHT
kg (lbs)

CAPTURE DATE

1

M

8.1

(17.9)

4 December 1978

2

M

8.9

(19.6)

28 December 1978

3

F

6.2

(13.7)

1 January 1979

4

M

8.3

(18.3)

5 January 1979a

5

M

7.7

(17.0)

12 January 1979

6

M

7.3

(16.1)

23 January 1979

7

M

8.1

(17.9)

7 February 1979

8

M

7.2

(15.9)

3 March 1979b

a Bobcat No. 4 was recaptured on 7 and 10 January and on 9 February 1979.
b Bobcat No. 8 was recaptured on 14 March 1979.

Most population estimators assume no immigration, emigration, natality
or mortality and estimate the population during the concise period of capture,
These assumptions are seldom realized when studying a free-living population.
This bobcat population displayed dynamic con specific interaction based on
analysis of bobcat home ranges. Bobcat home ranges were often severely
altered, presumably as the response of avoidance behavior exhibited when a
solitary mode of exi stance is operant.

Home range Size and Bobcat Density

During this study we obtained sufficient numbers of bobcat locations via
radio- telemetry (1,310) to determine the size of the home ranges occupied by
seven of the eight animals that were captured (Fig. i, Tablj 2). Analysis of
these data indicate considerable variation in the size of the areas occupied
as shown by a range of 4.7 to 53.6 km^ (1.8 to 20.7 mi 2) with a mean of
26.3 km^ (10.3 mi2) and a standard error of 5.75. Bobcat home ranges at
Joshua Tree National Monument were smaller than those reported for the pallid
bobcat in Idaho (Bailey, 1972) and northeastern California (Zezulak, 1978),
but were larger than the home range of the California bobcat studied in

Zczulak and Schwab

i i i t t i • i ■

Zezulak and Schwab

12.

San Diego County, California (Lembeck, 1978). Bobcat home range size is known
to vary among the subspecies studied, but it is difficult to discern the
relationship between bobcat home range size, their population density, habitat
type occupied, and prey availability.

Table 2. Home range size for 7 adult bobcats determined by radio-location techniques at
the Joshua Tree National Monument, Riverside County, California.

BOBCAT NUMBER

KM)

2(M)

3(F)

4(M)

5(M)

6(M)

8(M)

RADIO LOCATIONS

60

145

385

394

51

151

124

PERIOD STUDIED

date captured

12-4-78

12-28-78

1-1-79

1-5-79

1-12-79

1-23-79

3-3-79

date last located

5-6-79

3-29-79

6-28-79

6-29-79

6-6-79

6-28-79

6-28-79

days studied

153

91

178

174

145

156

117

HCME RANGE SIZE

•

kilometers^

25.7

27.8

17.5

34.2

4.7

20.5

53.6

nriles^

9.9

10.8

6.8

13.2

\ 1'6

7.9

20.7

Bobcat 1 displayed a well defined home range, indicated by the southern
and western portion of the home range shown in Figure 1 during the 20 day
period following its capture. Subsequently, bobcat 1 abruptly abandoned this
area and was later located to the northeast and did not return to the
originally documented portion of its home range during the course of the
study. Eleven days later Bobcat 4 was captured in the area previously
occupied by Bobcat 1. During that 11-day period 3 traps within that area were
sprung by an unknown bobcat. Bobcat 4 was captured and recaptured twice in
the same area and radio-locations showed that Bobcat 4 occupied 90% of the
home range previously occupied by Bobcat 1. While not necessarily a causal
relationship, this sequence demonstrates the flux of home range occupancy by
these bobcats. '

The dynamic flux of areas occupied by bobcats is further demonstrated by
Bobcat 8. Bobcat 8 displayed a bi-modal and even tri-modal home range. While
bobcat 8 primarily occupied the Queen Valley portion of its home range, it
left Queen Valley and occupied a secondary area on the east slope of the

Zezulak and Schwab 13.

Little San Bernardino Mountains and a tertiary area in Lost Horse Valley.
Both ancillary areas were 9 kilometers (5.6 mi) from the primary home range.
Bobcat 8 stayed in these secondary or tertiary areas up to 2 weeks before
returning to Queen Valley. The discovery of the secondary and tertiary areas
utilized by Bobcat 8 was fortuitous.

Bobcat 7 was radio-collared, released and subsequently not located again.
Whether this bobcat was a true transient (Bailey 1974) or transmitter failure
occurred is unknown. Specific efforts (672 trap nights) to recapture this
bobcat were unsuccessful. In contrast Bobcit 8 was captured twice in 286 trap
nights and Bobcat 4 was captured 4 times with an average of 182 trap nights
required per capture. Bobcat 2 was not located after 29 March 1979.
Extensive efforts to retrap or locate 4 missing bobcats were only successful
in the cases of Bobcats 1 and 8. Due to thj possibility that some transient
bobcats occur and the dynamic nature of horns range occupancy, the previous
population estimate of 6 bobcats within the core study area (62 km^, 24
mi 2) at any given time may be quite accurate. The simultaneous occupancy of
the core study area by no more than 6 bobcats yields a minimum known density
of 1 bobcat per 10.36 km^ (4 mi'2) as determined by radio- telemetry.
.Translated to numbers of bobcats per square kilometer, 0.1 bobcats per km^
(0.25 bobcats/im*2) represents the minimum known bobcat density determined
during this study.

The density estimate determined by trapping and the density estimate
based on radio- telemetry are in close agreement. The fact that eight bobcats
were captured is ameliorated by the fact that radio- telemetry revealed that
some bobcats were transients and/or left the area. Additionally,
radio- telemetry showed that the average area used by the bobcats was twice the
size predicted by their density. This can only be accounted for by the common
use of space by more than one bobcat.

Home Ranoe Overlap

The home range size and degree of home range overlap among conspecifics
largely determines the density of a partic^ar population. Bobcats have been
described as being territorial in Idaho where home range overlap of pallid
bobcats v/as slight among females and less anong males (Bailey, 1972). - In
northeastern California the pallid bobcat displayed up to 30* overlap among
individual females, and a female could be overlapped by 2 or more other
females (Zezulak, 1978). In the same study only slight overlap existed among

Zezulak and Schwab 14.

male bobcats. Lembeck (1978) reported almost no overlap among female bobcats
but as much as 89% overlap between male California bobcats in San Diego
County, California. Whether these disparate results are due to subspecific
differences or other factors remains unanswered. A parallel between home
range size and home range overlap is suggested. The desert bobcats of this
study displayed 'density, home range sizes and extent of home range overlap
that were intermediary of the other two subspecies and could be catagorized
within this continuum (Table 3).

Table 3. Extent of bobcat home range overlap determined by radio-location
techniques at Joshua Tree National Monument, Riverside County,
California. The percentage of individual bobcat home range (Y)
overlapped by adjacent bobcats (X) is presented below. The extent
that an individual bobcat home range is overlapped by adjacent
bobcats may exceed 100% since individual bobcat home ranges are
often overlapped by several conspecifics.

PERCENT INDIVIDUAL BOBCAT HOME RANGE OVERLAPPED BY ADJACENT BOBCATS

4(M) 6(M) 8(M) 3(F)

„ Bobcat (

X)

2(M)

2 (M)

4 (M)

47

6 (M)

1

8 (M)

36

3 (F)

Total Overlap

(%)

83

Home Range

Not

Over! apped

[%)

46

38

Bobcat

(Y)

6(M)

8(M)

1

. 19

18

42
14

28

11

66 37 75

14 25

129 56 100 103

24 63 28 17

While territorial in some instances, it appears that bobcats have the
behavioral flexibility to accomodate denser population levels and concomitant
increased conspecific interaction. The presence of one bobcat within , the home
range boundaries of another bobcat occurred 795 times during this study (Table
4). These data may not reflect a dominance hierarchy but certainly indicate
the extent that some bobcats utilize common areas. An example of differential

Z.ezulak and Schwab is.

use occurs within the area shared by bobcats 4 and 8. Bobcat 4 was located
within this area 206 times compared to the 53 times that bobcat 8 was in their
common area. The ability to interact, primarily through temporal spacing,
permits these bobcats to utilize a larger area than they would be able to
occupy if they were strictly territorial. More investigation and analyses are
required to evaluate the importance of the physical/biotic parameters operant
in the extent of home range overlap displayed by bobcats. However, it is
certain that the amount of overlap of bobcat home ranges has a profound effect
on bobcat density and resource utilization.

Table 4. The occurrence of home range overlap demonstrated by bobcats at the
Joshua Tree National Monument, Riverside County, California. Tne
number of times bobcat "X" v/as located within the home range of
bobcat "Y" is shown.

OCCURRENCE OF OVERLAP BY BOBCAT "X" ON THE RANGE OF BOBCAT "Y"

Bobcat (Y)

Bobcat(X)

2(H)

4(M)

6(M)

8(M)

3(F)

2(M)

42

4(M)

94

14

206

18

6(M)

32

•

43

8(M)

53

8

22

3(F)

145

118

Habitat Utilization

Home range utilization by bobcats in this study was primarily a function
of habitat. Key features such as boulder piles, the desert plain and dry
washes were associated with various types of activity and utilization by
bobcats.

Three relatively small rocky areas that were elevated less than' 30 niters
(100 ft) from the surrounding desert plain v/ere the sites of 6 captures of 4
different bobcats. The fact that these rocky areas v/ere surrounded. by desert
plain for a radius of at least 1 kilometer (0.6 mi) increased their value as a
sanctuary and as den/resting sites. Bobcats were observed to utilize rocky

Zezulak and Schwab 16.

areas during periods of heavy rain and periods of high temperature almost
exclusively. Although bobcats were observed resting under bushes or next to
fallen Joshua trees on the desert plain during the winter months, most
resting/denning activity was associated with rocky areas. Rocky areas were
the key unique resource and as a result were the situations where most bobcat
interaction occurred.

Observations of huntirg behavior were confined to two habitat types. The
undulating terrain at the base of mountains generally supported relatively
lush vegetation and had hicher populations of cottontail rabbits (Sylvilagus
auduboni) . Jackrabbits (Ledus cal ifornicus) were most abundant in the
sparsely vegetated plains or valleys. Bobcats presumably hunted in these
habitats in response to prey density.

Movement

The sequential radio-location of each bobcat indicated whether movement
occurred during the time period since the previous radio-location. The
occurrence of movement (standardized for sample size) was analyzed hourly and
no difference in the time of day bobcats moved v/as found (P < 0.05). The
distance bobcats moved varied on an hourly basis. Movements greater than 2
kilometers occurred during distinct time blocks (80% between 0800 and 1000
hours; 13% between 1400 and 1600 hours; 7% between 1600 and 1800 hours).
While short distance movements (less than 2 km) occurred during all hours of
the day, 30% of these movements occurred between 1400 and 1800 hours. Short
distance movements were generally associated with hunting activity. Long
distance movements were usually between different denning sites and/or in
response to the presence of another bobcat. Conspecific interaction usually
resulted in one or both bobcats moving into the exclusive area of their
respective home range .

Bobcat Activity Patterns

Daily bobcat activity patterns were determined by analyzing variations of
radio-signal strength. Sin:e the daily photoperiod lengthens from winter to
spring, the activity data were analyzed by season. All activity datawere
divided into 2-hour time blocks beginning at midnight. A crepuscular activity
pattern v/as displayed by bobcats during winter months (Fig. 2). The shift to
a nocturnal activity pattern during the spring (Fig. 3) may have been induced
by several factors. Ambient temperature is a prime factor that may affect

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TIME

i IGUR E 2 . Winter bobcat activity pattern determined by radio-tele-
metry techniques at Joshua Tree National Monument, Riverside County,
California. See text for description of technique. The mean activ-
ity level for 2 hour time-blocks is presented as a dot. Bars repre-
sent 2 standard errors on each side of the mean (n=1125). Average
seasonal photoperiod is shown by the solid (hours of darkness) and
open (hours of light) bars at the bottom of the figure.

C3

TIME

FIGURE 3. Spring bobcat activity pattern determined by radio-tele-
metry techniques at Joshua Tree National Monument, Riverside County,
California. See text for description of technique. The mean activ-
ity level for 2 hour time-blocks is presented as a dot. Bars repre-
sent 2 standard errors on each side of the mean (n=1773) . Average
seasonal photoperiod is shown by the solid (hours of darkness) .and
open (hours of light) bars at the bottom of the figure. /

^.ezuidK ana scnwao iy.

bobcat activity levels. Nighttime temperatures were often as low as -6.7° C.
(20° F.) in winter, during which bobcat activity peaked between 0400 and 1000
and again from 1600 to 2200. This pattern allowed at least 3 hours after
sunrise and 2 hours before sunset when higher ambient air temperatures
generally prevailed. The lowest levels of bobcat activity observed during the
winter occurred from 2200 to 0200. During the spring season high amibient air
temperatures (above 26° C, or 78° F.) were associated with reduced bobcat
activity. Bobcats typically sought the cooler temperatures of large boulder
piles when temperatures became excessive. While decreased bobcat activity
appears associated with temperature extremes, it is not necessarily the
proximate factor regulating the level of bobcat activity. The bobcats may
have responded to other factors influenced by or related to air temperature
such as the activity of prey species. Further, non- thermal environmental
factors such as humidity and duration of daily darkness may have influenced
the level of bobcat activity as well as the activity pattern of their prey*

Activity Patterns Of Prey Species

The activity patterns of potential prey species were analysed to
determine a possible relationship to the patterns of bobcat activity. During
winter 1,243 km of roadside census resulted in the observation of 39
jackrabbits, 37 cottontail rabbits, and 16 rodents. A similar census during
spring in which 1,763 km. were assayed resulted in observation of 155
jackrabbits, 31 cottontail rabbits, and 294 rodents of various species. The
level of prey activity, presumably representing the relative abundance, was
considerably less during the winter season than during the spring (Fig. 4).
The increased levels of prey observed during the spring season was largely due
to the increased observations of jackrabbits and rodents.

Peaks of abundance of cottontail rabbits and jackrabbits corresponded to
the onset and termination of bobcat activity during both seasons. The
increase of spring prey activity during the middle portion of the night was
largely due to the abundance of rodents. These differences in prey activity/
abundance n.ay be an important factor contributing to the establishment of
bobcat activity patterns. Maximum efficiency of a predator is attained v/hen
predator and prey activity patterns are in phase. Bobcat activity patterns
are likely the result of various multi-dimensional factors, but dissection of
any single factor obscures its significance.

lezu.lak and Schwab

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H 0500 ' N 1800 M

Time Of The Day

FIGURE 4. Numbers of bobcat prey (rodents and rabbits) observed during
1243 km (winter, 1979) and 1763 km (spring, 1979) of roadside census on the
Joshua Tree National Monument, Riverside County, California. The seasonal
abundance of prey observed is indicated by open circles and broken lines
(winter) and solid circles and lines (spring) relative to the respective
average daily duration of darkness shown by hatched and solid horizontal
rectangles.

. Zezulak and Schwab ci.

Bobcat Population Dynamics

The sex ratio of Joshua Tree bobcats was 7 males per female and no
kittens were observed or captured during this study. The radio-collared
female (Bobcat 3} did not display any signs of- reproduction. She was
recaptured in September and had not lactated nor was she pregnant. Few bobcat
studies have demonstrated a complete absence of reproduction. In Idaho,
Bailey (1972) noticed a decrease in reproductive output during one year of his
three year study and attributed this to a decrease in the rabbit population
during that year. Lembeck (1978) studied a very dense (1.5 bobcats/km^)
population of California bobcats (F. r. californicus) in San Diego County,
California. No reproduction was documented until the last year of his 2 1/2
year study and occurred only after the population experienced a major decline
(13 of 31 bobcats died due to trappers, disease, starvation, and predation).
Thus, bobcat reproduction may be subject to influence by such proximate
factors as prey density or bobcat densities and sex ratios.

An experimental control is, in part, provided for the Joshua Tree
population of desert bobcats. Trappers harvested 110 bobcats in similar
Mojave desert areas. The harvested bobcats had a sex ratio of 1.12 males per
female and an examination of dental cememtum annul i (Crowe, 1972) revealed
that 62% of these bobcats were less than two years old. This is an extreme
contrast to the age and sex ratios of the population studied in the Joshua
Tree National Monument which was effectively buffered by distance from
trapping pressure. ■ •. .

The absence of reproduction by the Joshua Tree bobcats may represent a
physiological response to the high density and degree of conspecific
interaction within this population. One might envision that a sex ratio and
adult/juvenile ratio which is highly skev/ed towards adult male bobcats might
be the result of high intraspecific competition within the population.
Juvenile bobcats may be less able to survive due to inexperience and female
bobcats experience the additional stress of pregnancy and lactation during a
considerable portion of the year. Hence, a population principally dominated
by adult males exhibiting high levels of competition for limited resources may
exemplify the stability/density characteristics established and maintained by
near-pristine conditions on a bobcat population.

Zezulak and Schwab 22.

CONCLUSIONS

The density of several bobcat populations has been estimated on the
basis of trapping results. However, the accuracy of such estimates may be
influenced by the exi stance of transient bobcats. The density estimated by
extrapolation of. home range size and amount of home range overlap evidenced by
a number of bobcats takes variation of these parameters into account and
provides a better estimate of population size. The techniques utilized in
this study provide an estimate of the density and age/sex structure of this
sample population and is probably indicative of that existing in areas of
comparable habitat throughout the monument. Parameters of bobcat biology vary
greatly with respect to density, age/sex structure, prey availability, habitat
and the particular subspecies being studied.

The desert bobcats occupying the Mojave desert habitat of Joshua Tree
National Monument displayed a relatively high population density and
commensurately small home ranges with a high degree of overlap. If
differential competitive ability exists between bobcats of different ages and
sex, highly skewed age/ sex structures may appear in conjunction with high
population density, as occurred in this study. Indeed, the productivity of
bobcat populations appear to be enhanced by moderate man-induced mortality
(Bailey 1972, Lembeck 1978, Zezulak 1978). This is reflected by a well
stratified age structure and a reproductive level which approaches the maximum
potential . -

Bobcats may live to be 14 years old in the wild. In the absence of
artificially induced mortality some form of natural population regulation must
occur. Aspects of population dynamics, density, and behavior noted in this
population differed from those bobcats in other studies where, at least, some
level of man-induced mortality occurred. Indeed, the possibility that
increased conspecific contact and competition reflected by behavioral changes,
shifts in the age/sex structure and decreased reproduction may be factors
regulating bobcat populations.

/.ezuiaK ana bcnwao 23

ACKNOWLEDGEMENTS

Cooperation and permission to conduct this study at Joshua Tree National
Monument was extended by the Superintendent, R. T. Anderson and the National
Park Service. Field assistance was provided by K. L. Rawson, P. Anderson and
R. Nayyar. Equipment and facility was provided by the Department of Wildlife,
and Fisheries Biology and the Institute of Ecology, University of California
at Davis. Financial support was provided by the Bureau of Land Management and
the California Department of Fish and Game - B.L.M. contract number
CA-060-CT8-76.

Zezulak and Schwab 24.

LITERATURE CITED

Bailey, T. U. 1972. Ecclogy of bobcats with special reference to social

organization. PhD thesis. University of Idaho. 93 pp.
. 1974. Social organization in a bobcat population. J. Wildl. Manage.

38(3): 435-446.
Brownlee, W. C. 1977. Special Report - Status of the bobcat (Lynx rufus) in

Texas. Texas Parks and Wildlife Dept., Austin, mimeo report, 20 p. plus

apend.
Crowe, D. M. 1972. The presence of annul i in bobcat tooth cementum layers,

J. Wildl. Manage., 36:1330-1332.
. 1975. Aspects of ageing, growth, and reproduction of bobcats from

Wyoming. J. Mammal. 56(1): 177-198.
De Lury, D. B. 1947. On the estimation of biological populations.

Biometrics 3: 145-167.
Erickson, A. W. 1955. An ecological study of the bobcat in Michigan. M.S.

Thesis. Mich. State Univ. 133 p.
Foote, L. E. 1945. Sex ratio and weights of Vermont bobcats in autumn and

winter. J. Wildl. Manage. 9(4): 326-327.
Fritts, S. H. and J. A. Sealander. 1978. Reproductive biology and population

characteristics of bobcats in Arkansas. J. Mammal. 59(2): 347-53.
Gashwiler, J. S., W. L. Robinette, and 0. W. Morris. 1961. Breeding habits

of bobcats in Utah. J. Mammal. 42(1): 76-83.
Grinnell, J., J. S. Dixon, and J. M. Linsdale. 1937. Fur-bearing mammals of

California. Berkeley, Univ. Calif. Press, 2: 590-625.
Hamilton, W. J., Jr., and R. P. Hunter. 1939. Fall and winter food habits of

Vermont bobcats. J. Wildl. Manage. 3: 99-103.
Jones, J. H. 1976. Density and seasonal food habits of bobcats on the Three

Bar Wildlife Area, Arizona. M.S. Thesis, Univ. of Arizona. 48 p.
Jones, J. H. and !.'. S. Smith. 1979. Bobcat density and prey selection in

central Arizona. J. Wildl. Manage. 43(3): 666-672.
Kight, J. 1962. An ecological study of the bobcat. Lynx rufus (Schreber),

in West-Central South Carolina. M.S. Thesis. Univ. of Georgi'a. 52 pp.
Leach, H. R. and W. H. Frazier. 1954. A Study on the Possible Extent' of

Predation on Heavy Concentrations of Valley Quail with Special Reference

to the Bobcat. California Fish and Game. 39(4): 527-538.

25.

Zezulak and Schwab

Lembeck, «. 1978. Bobcat study, San Oiego County, Camornia. Final report
„ Cam ^«^J~J? Lments and home ranges of
r/IntJsout* Carina. Proc. An, Con, Southeast Game Co™,.
^Z TT'mZ. Winter Nations of bobcats to whi te-taH ed deer in

Jrr.^™- :r i« . *-. on - — ..

esrv 10 Massachusetts. 0. Mammal. 55(2): «J^'
fcuT^O. "34. Predatory animal studies.

,.,«. zTw. Breeding Mbit, of the bobcat in northeastern

United States. 0. Mammal. 31: 327-330,
. 1951. observations of New England bobcats, a. Mamma!.

32(3): 589-603. ,tlHi zed as food by the bobcat in the

,i no 1055 Game animals utilized as toou ujt

Progulske, 0. R. !»»• t,amc ... ,49-253.

.... i uiirfl Menace, lslti. tH»-t»^«

southern ^f^' ^J"^ c Lparative movements of bobcats and
Robinson, W. B., and E. F. Grand P ^^

coyotes as disclosed by tagging. J. Wildl . Manage
^Z C T. 19«. Habits, foods and parasites of the bobcat

Sokal, Ro R- and F. J. RohlT. i»o».

San Francisco. 776 pp. history life habits,

younc S P 1958. The bobcat of North America: its history n
Young, S. P 19 ^ ^ Qf currfintly recogmzed

economic status and cont • wildl. Mgmt. Inst.,

subspecies. Stackpole Co., Harnsourg,

Washington, D.C. 193 p. f fi -j

ZezuLK, 0. S. 1978. Northeastern California be t study.

report to Calif. Oept. of Fish and Game. 22 pp.

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