4

OCCASIONAL PAPERS MUSEUM TEXAS TECH UNIVERSITY

0. Handley, Jr., and Ronald H. Pine, National Museum of Natural History,
for providing information on certain specimens of Vampyrops from
Colombia. Dr. Hugh H. Genoways read the manuscript.

LITERATURE CITED

ALLEN, J. A. 1897. On a small collection of mammals from Peru, with descriptions
of new species. Bull. Amer. Mus. Nat. Hist., 9:115-119.

HERSHKOVITZ, P. 1949. Mammals of northern Colombia. Preliminary report No.

5: Bats (Chiroptera). Proc. U.S. Nat. Mus., 99:429-454.

SANBORN, C. C. 1955. Remarks on the bats of the genus Vampyrops. Chicago
Nat. Hist. Mus., Fieldiana Zool., 37:403-413.

TUTTLE, M. D. 1970. Distribution and zoogeography of Peruvian bats, with com¬
ments on natural history. Univ. Kansas Sci. Bull., 49:45-86.

MUS. COMP. ZOOI_

^ „ LIBRARY

OCCASIONAL PAPERS

JUN 2 3 1972

THE MUSEUM „ARVARD

_ • JNIVERS'TY

TEXAS TECH UNIVERSITY

NUMBER 3

29 MARCH 1972

VARIATION AND ECOLOGY IN A LOCAL POPULATION
OF THE VESPER MOUSE (NYCTOMYS SUMICHRASTI)

HUGH H. GENOWAYS and J. KNOX JONES, JR.

Vesper mice of the genus Nyctomys are relatively rare, arboreal rodents
restricted to Middle America (eastern Panama to southern Mexico). Little
is known concerning the biology of these secretive animals. Studies of vari¬
ation of the one species, Nyctomys sumichrasti, have been limited to de¬
scriptions of new taxa (for example, see Goldman, 1916, 1937, and
Laurie, 1953) and ecological observations have been confined to faunal ac¬
counts of political units (Goodwin, 1934; Hall and Dalquest, 1963) or
were made coincidental to studies of other species (Lawlor, 1969; Flem¬
ing, 1970). Birkenholz and Wirtz (1965) recorded observations on the be¬
havior, reproduction, and early development of young in a laboratory col¬
ony of vesper mice; the male reproductive organs and accessory structures
have been described by Burt (1960), Arata (1964), and Hooper and Musser
(1964).

In the summer of 1964, a local population of Nyctomys sumichrasti
florencei was discovered at San Antonio, Departamento de Chinandega, in
northwestern Nicaragua. This area was visited again in the summers of
1966 and 1967, and in the spring of 1968, and the present paper details
variation in this population and records such ecological information as was
obtained. The first three visits to San Antonio were in the wet season,
whereas the last was in the dry season. Of the 100 specimens of vesper
mice collected there, the majority (55) was taken in 1966. A few mice
were obtained in traps placed on the ground at the bases of trees but most
were shot with pistols (dust shot) as they perched on, or scampered along,
low branches and vines in dense vegetation. It is noteworthy that only 10
N. sumichrasti were obtained elsewhere in Nicaragua in the four years of

our field work.

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OCCASIONAL PAPERS MUSEUM TEXAS TECH UNIVERSITY

All specimens discussed below are deposited in the collections of the
Museum of Natural History at The University of Kansas. All measurements
are in millimeters and weights are recorded in grams.

VARIATION

In the following section, four sources of nongeographic variation — age,
secondary sexual, individual, and seasonal — are analyzed; these are of in¬
terest because they reveal to some extent the genetic variability within the
species. An understanding of them is basic to the study of geographic vari¬
ation.

Variation with Age

All specimens from San Antonio were assigned to one of five age cate¬
gories. These were studied to ascertain whether significant differences re¬
lating to growth occurred between them and to determine which in¬
dividuals had terminated growth, or nearly so, and could be termed
“adults.” Age categories were defined as outlined below — see Fig. 1 for
illustration of first left upper molars typical of each category and Fig. 2
for illustration of crania of typical individuals of each except IV (crania of
which are indistinguishable from those of V).

I. — Exoccipital-basioccipital sutures evident; basioccipital-basisphenoid
suture open; braincase domed; temporal ridges absent or at most slightly
developed; M3 not fully erupted or, if so, unworn.

II. — Exoccipital-basioccipital sutures no longer evident; molars exhibit
only slight wear (the lingual cusps are first to wear and show the most
wear in this group, labial cusps reveal wear only on the tips).

III. — Molars exhibit considerable wear, but cuspidate nature of teeth
still evident; labial cusps worn to a much greater degree than simply on
tips.

IV. — Molars worn so that cuspidate nature of the teeth is no longer
readily evident; occlusal surfaces of teeth consist of a series of dentine
lakes interspersed with enamel re-entrant angles and islands.

V. — All enamel re-entrant angles and islands essentially worn away,
leaving a central lake of dentine surrounded by an enamel rim (especially
on M2), although a small enamel re-entrant angle or island occasionally
may be present.

Means for each measurement were tested for significant differences (sig¬
nificance level .05) among the five age categories using single classification
analysis of variance; if significant differences were found, a Sums of
Squares Simultaneous Test Procedure (SS-STP) was used to determine the

GENOWAYS AND JONES— VARIATION IN NYCTOMYS 3

Fig. 1. — First upper left molars of Nyctomys sumichrasti illustrating tooth wear
with age. From left to right the teeth typify the following age categories: I, KU
106655; II, KU 98059; III, KU 110503; IV, KU 106676; V, KU 106671. The scale
at the right is one millimeter long.

maximally nonsignificant subsets (see Genoways and Jones, 1971, and
Genoways, 1971, for discussion of these procedures). Of the six external
and 10 cranial measurements tested for both sexes, only two (length of ear
for females and length of maxillary toothrow for males) revealed no sig¬
nificant differences between the means of the five age categories (Table 1).
Three nonoverlapping subsets (V-IV, III-II, I) were found for females in
total length, length of head and body, and occipitonasal length, and non¬
overlapping subsets of V-IV and III-II-I resulted in females for rostral
breadth.

All remaining measurements exhibited some overlapping of maximal
nonsignificant subsets. Thirteen of these measurements had means that fell
into only two subsets. One age category is shared between the two subsets

pjg 2. — Dorsal view of the crania of four specimens of Nyctomys sumichrasti
from San Antonio, Nicaragua, illustrating variation with age. The age categories are,
from left to right: I (KU 106655), II (KU 106691), III (KU 110501), and V (KU
106696). Specimens in age category IV, which is not illustrated, have crania that are
essentially the same as those in category V. The scale at the right is five millimeters

long.

4

OCCASIONAL PAPERS MUSEUM TEXAS TECH UNIVERSE! Y

for length of tail for males (V-IIITITV, IV-I) and females (IV-V-III-II, 1I-I),
and depth of cranium in males (IV-V-III-II, II-I). In three measurements
for males (length of ear, breadth of braincase, and length of palatal bridge)
and five for females (interorbital constriction, breadth of braincase, depth
of cranium, length of maxillary toothrow, and length of palatal bridge),
two age categories were shared between two subsets. Three age categories
were shared between subsets for length of hind foot for both males and
females.

Nine measurements exhibited three overlapping subsets. Total length
for males and length of rostrum for females were divided V-IV-III, IV-III-
II, and I; length of head and body for males exhibited the same pattern of
variation but the positions of categories V and IV were reversed. Subsets
of V-IV-III, III-II, and I were found for rostral breadth and length of the
incisive foramen of males. Means of zygomatic breadth for females were
arranged in subsets of V-IV, IV-II, and II-III-I, and length of the incisive
foramen in them revealed the same pattern of variation except the posi¬
tions of categories II and III were reversed. Maximal nonsignificant subsets
for weight of females consisted of V-IV, II-III, and III-I. The three subsets
for interorbital constriction of males were broadly overlapping.

Four measurements of males were divided into four nonsignificant sub¬
sets. Three of these (weight, occipitonasal length, and zygomatic breadth)
exhibited the following pattern of subsets: V-IV, IV-III, III-II, and I. The
fourth measurement (length of rostrum) revealed essentially the same pat¬
tern except that the order of V and IV was reversed.

Based upon this analysis, the most parsimonious grouping of categories
is: I (smallest individuals), II-III (medium-sized mice), and IV-V (largest
individuals). Members of age categories IV or V were found to have the
largest mean for all measurements excepting length of hind foot and length
of maxillary toothrow in males and breadth of braincase in females. For all
measurements except length of maxillary toothrow in females, age cate¬
gory I had the smallest mean. We have termed the three groups mentioned
above as juvenile, subadult, and adult, respectively, and use this termi¬
nology throughout the remainder of this paper.

Secondary Sexual Variation

Adult males were tested against adult females using single classification
analysis of variance to determine if means for the 16 measurements were
significantly different (significance level .05) between them. In only one
measurement, weight, was there a significant difference between the sexes
(Table 2), males averaging nearly four grams heavier. In the remaining 15
measurements, males had the larger mean for seven (total length, length of

GENOWAYS AND JONES— VARIATION IN NYCTOMYS

5

TABLE 1. — Variation with age in six external and 10 cranial measurements of
Nyctomys sumichrasti from San Antonio, China ndega, Nicaragua. Age categories are
listed in decreasing order of means. Groups of means that were found to be signifi¬
cantly different at the .05 level using single classification analysis of variance were
tested with sums of squares-simultaneous testing procedure to find the maximally
nonsignificant subsets. Groups of means that were found to be not significantly dif¬
ferent at the .05 level are marked ns. Age categories are defined in text.

Sex and age
category

N

Mean (Range) ± 2 SE

cv

Results

SS-STP

Total length

Male

V

5

240.2 (231.0-246.0) ± 5.41

2.5

IV

3

228.0 (225.0-231.0) ± 3.46

1.3

III

9

224.0 (215.0-237.0)±4.32

2.9

II

9

218.0 (208.0-233.0) ± 6.73

4.6

I

9

195.0 (167.0-207.0) ± 7.98

6.1

1

Female

V

5

232.6 (230.0-236.0) ± 2.05

1.0

IV

11

229.7 (219.0-242.0) ± 4.50

3.2

III

7

215.7 (206.0-230.0) ± 6.91

4.2

II

6

215.6 (207.0-229.0) ± 6.53

3.7

I

8

196.4 (178.0-207.0) ± 6.36

4.6

1

Length of head and body

Male

IV

9

118.6 (107.0-129.0) ± 5.12

6.5

V

6

118.0 (99.0-125.0) 1 7.85

8.1

III

10

111.0 (104.0-116.0) i 2.51

3.6

II

10

108.3 (94.0-117.0) 1 4.64

6.8

I

9

96.3 (84.0-102.0) 1 3.69

5.8

1

Female

V

8

120.3 (114.0-128.0) i 3.27

3.8

IV

16

116.6 (106.0-124.0) i 2.35

4.0

II

7

106.9 (100.0-113.0) i 3.39

4.2

III

8

106.1 (98.0-115.0) i 4.51

6.0

I

8

96.3 (83.0-103.0) 1 5.30

7.8

1

6

OCCASIONAL PAPERS MUSEUM TEXAS TECH UNIVERSITY

Table 1. — Continued.

Male

Length of tail

V

5

118.4 (112.0-126.0)

± 5.24

4.9

III

8

112.8 (109.0-122.0)

± 3.06

3.8

II

9

110.7

(99.0-121.0)

± 5.02

6.8

IV

3

109.0 (108.0-110.0)

± 0.58

0.9

I

9

99.2

(83.0-109.0)

± 4.95

7.5

Female

IV

11

113.5 (108.0-124.0)

± 2.96

4.3

V

5

112.2 (102.0-117.0)

± 5.34

5.3

III

7

110.0 (103.0-114.0)

± 2.99

3.6

II

6

107.6 (101.0-117.0)

± 4.31

4.9

I

8

100.1

(95.0-105.0)

± 2.84

4.0

Male

Length of hind foot

III

11

22.9

(22.0-25.0)

± 0.57

4.1

II

10

22.4

(21.0-25.0)

± 0.68

4.8

IV

9

22.2

(21.0-23.0)

± 0.65

4.4

V

6

22.2

(21.0-23.0)

± 0.80

4.4

I

9

21.4

(20.0-23.0)

± 0.68

4.7

Female

V

8

22.6

(22.0-23.0)

± 0.37

2.2

III

8

22.4

(21.0-24.0)

± 0.65

4.1

II

7

22.3

(21.0-24.0)

± 0.72

4.3

IV

16

22.0

(20.0-23.0)

± 0.41

4.1

I

8

21.3

(20.0-22.0)

± 0.63

4.2

Male

Length of ear

V

6

17.0

(16.0-18.0)

± 0.52

3.7

II

10

16.8

(15.0-19.0)

± 0.83

7.8

IV

9

16.7

(15.0-18.0)

± 0.67

6.0

III

11

16.6

(14.0-18.0)

± 0.68

6.7

I

9

15.3

(14.0-17.0)

± 0.67

6.5

Female

IV

16

17.0

(15.0-18.0)

± 0.41

4.1

V

8

16.9

(16.0-18.0)

± 0.40

3.3

III

8

16.8

(15.5-19.0)

± 0.75

6.3

II

7

16.1

(15.0-17.0)

± 0.63

5.2

I

8

16.0

(13.0-18.0)

± 1.13

10.0

ns

GENOWAYS AND JONES— VARIATION IN NYCTOMYS

7

Table 1. — Continued.

Male

Weight

V

6

50.0

(41.7-61.3)

± 5.19

12.7

IV

9

47.5

(39.3-58.5)

± 3.98

12.6

III

11

40.1

(33.3-47.2)

± 2.94

12.2

II

10

36.1

(28.545.5)

± 3.43

15.0

I

9

26.0

(18.1-30.6)

± 2.37

13.7

Female

V

8

46.2

(39.0-51.5)

± 2.55

7.8

IV

16

44.3

(38.2-53.9)

± 2.14

9.6

II

7

36.8

(27.043.6)

± 4.24

15.3

III

8

34.6

(31.643.7)

± 2.78

11.3

I

8

27.7

(16.6-32.4)

± 3.54

18.1

Male

Occipitonasal length

V

6

30.3

(29.7-30.6)

± 0.31

1.2

IV

8

29.4

(28.5-31.2)

± 0.62

3.0

III

11

28.4

(27.3-29.4)

± 0.42

2.5

II

9

27.8

(27.1-29.2)

± 0.48

2.6

I

7

25.9

(24.0-26.8)

± 0.69

3.5

Female

V

8

30.0

(28.8-31.3)

± 0.61

2.8

IV

10

29.4

(28.5-30.3)

± 0.39

2.1

III

7

28.1

(27.3-28.9)

± 0.37

1.8

II

5

28.0

(27.2-28.6)

± 0.56

2.2

I

6

26.4

(25.6-27.1)

± 0.48

2.2

Male

Zygomatic breadth

IV

7

17.1

(16.4-18.0)

± 0.50

3.9

V

5

17.0

(16.6-17.6)

± 0.33

2.2

III

8

16.0

(15.2-16.6)

± 0.34

3.0

II

6

15.6

(15.1-16.3)

± 0.42

3.3

I

5

14.2

(13.1-15.1)

± 0.65

5.1

Female

V

7

17.0

(16.2-17.7)

± 0.42

3.3

IV

11

16.7

(15.8-17.8)

± 0.34

3.4

II

4

15.7

(15.3-16.1)

± 0.35

2.2

III

7

15.5

(15.0-16.2)

± 0.33

2.8

I

6

14.7

(13.4-15.5)

± 0.60

5.0

8

OCCASIONAL PAPERS MUSEUM TEXAS TECH UNIVERSI I Y

Table 1. — Continued.

Male

Interorbital constriction

IV

9

5.6

(5.4-6. 2)

± 0.18

4.9

V

5

5.6

(5 .5 -5 .9)

± 0.15

3.1

III

10

5.5

(5. 2-5. 8)

± 0.11

3.2

II

10

5.3

(5 .0-5 .7)

± 0.13

3.9

I

9

5.2

(5. 1-5. 3)

± 0.05

1.4

Female

V

8

5.6

(5. 3-6.0)

± 0.19

4.9

IV

15

5.5

(5. 3-5.9)

± 0.10

3.5

III

8

5.4

(5 .0-5.9)

± 0.18

4.8

II

7

5.3

(5. 1-5 .4)

± 0.11

2.8

I

6

5.2

(5. 1-5. 3)

± 0.08

2.0

Male

Breadth of braincase

IV

8

13.2

(12.8-13.6)

± 0.22

2.3

V

6

13.1

(12.8-13.4)

± 0.17

1.6

II

9

13.0

(12.3-13.7)

± 0.31

3.6

III

11

12.9

(12.5-13.3)

± 0.16

2.0

I

7

12.6

(12.3-12.9)

± 0.15

1.6

Female

II

6

13.3

(13.0-13.3)

± 0.12

1.1

V

8

13.2

(12.8-13.6)

± 0.20

2.2

IV

14

13.2

(12.4-13.6)

± 0.20

2.9

III

7

13.1

(12.8-13.5)

± 0.18

1.9

I

5

12.7

(12.3-13.0)

± 0.29

2.5

Male

Rostral breadth

V

6

5.7

(5. 3-5.9)

± 0.19

4.1

IV

9

5.7

(5. 3-6.0)

± 0.16

4.1

III

10

5.4

(5. 1-5. 6)

± 0.12

3.5

II

10

5.3

(5. 1-5.9)

± 0.15

4.5

I

9

4.8

(4.4-5. 1)

± 0.17

5.2

Female

V

8

5.8

(5. 2-6.4)

± 0.27

6.7

IV

14

5.7

(5. 3-6.4)

± 0.16

5.2

III

7

5.2

(5 .0-5. 7)

± 0.19

4.8

II

7

5.2

(4. 9-5. 4)

± 0.13

3.3

I

7

5.0

(4.7-5. 2)

± 0.14

3.6

GENOWAYS AND JONES — VARIATION IN NYCTOMYS

9

Table 1. — Continued.

Male

Length of rostrum

V

6

10.4

(9.7-11.0)

± 0.34

4.0

IV

8

10.1

(9.8-10.7)

± 0.19

2.7

III

11

9.7

(9.2-10.2)

± 0.17

3.0

II

10

9.3

(8.5-10.1)

± 0.29

4.9

I

8

8.5

(7. 7-8. 9)

± 0.27

4.5

Female

V

8

10.3

(9.6-10.7)

± 0.28

3.8

IV

12

10.1

(9.4-10.7)

± 0.28

4.8

III

7

9.6

(9.1-10.3)

± 0.33

4.6

II

6

9.5

(9.0-9 .9)

± 0.25

3.2

I

7

8.6

(7. 7-9.0)

± 0.32

5.0

Male

Depth of cranium

IV

6

11.2

(10.8-11.6)

± 0.26

2.8

V

6

11.0

(10.5-11.6)

± 0.34

3.8

III

11

10.9

(10.2-11.4)

± 0.21

3.2

II

7

10.7

(10.2-11.2)

± 0.24

2.9

I

7

10.2

(9.6-10.8)

± 0.31

4.1

Female

V

8

11.1

(10.2-11.5)

± 0.30

3.9

IV

13

11.0

(10.5-11.5)

± 0.17

2.8

II

6

10.9

(10.6-11.2)

± 0.25

2.8

III

6

10.9

(10.7-11.0)

± 0.09

1.0

I

5

10.4

(10.2-10.6)

± 0.15

1.6

Male

Length of maxillary toothrow

II

5

4.5

(4.44.6)

± 0.07

1.9

V

3

4.4

(4.4)

IV

9

4.4

(4. 3-4. 6)

± 0.06

2.1

III

11

4.4

(4. 1-4.7)

± 0.11

3.9

I

6

4.4

(4.34.6)

± 0.09

2.5

Female

V

5

4.5

(4.34.7)

± 0.13

3.3

IV

14

4.5

(4.24.7)

± 0.08

3.2

III

6

4.4

(4.44.5)

± 0.04

1.2

I

8

4.3

(4.14.5)

± 0.11

3.6

II

4

4.2

(4.14.3)

± 0.10

2.3

ns

10

OCCASIONAL PAPERS MUSEUM TEXAS TECH UNIVERSE! Y

Table 1. — Continued.

Male

Length of incisive foramen

V

6

5.2

(4.9-5. 6)

± 0.20

4.7

IV

8

5.1

(4.8-5 .4)

± 0.15

4.1

III

10

4.9

(4.7-5. 1)

± 0.08

2.5

II

10

4.7

(4.4-5. 0)

± 0.11

3.6

I

7

4.3

(3.94.6)

± 0.17

5.4

Female

V

8

5.2

(4.6-6. 3)

± 0.34

9.3

IV

16

5.0

(4. 3-5.5)

± 0.15

6.1

III

8

4.6

(4.44.9)

± 0.11

3.2

II

7

4.6

(4.2-5 .0)

± 0.22

6.3

I

8

4.4

(4.14.7)

± 0.15

4.9

Male

Length of palatal bridge

V

6

4.4

(4.14.7)

± 0.18

4.9

II

10

4.4

(4.04.6)

± 0.11

3.9

IV

9

4.3

(4.04.6)

± 0.11

3.7

III

10

4.3

(3.74.7)

± 0.19

6.9

I

9

4.1

(3.84.2)

± 0.09

3.2

Female

V

8

4.5

(4.14.7)

± 0.13

4.1

III

8

4.4

(4.34.7)

± 0.09

2.9

II

6

4.4

(4.24.6)

± 0.12

3.4

IV

16

4.4

(3.94.7)

± 0.11

5.3

I

8

4.1

(4.04.4)

± 0.09

3.1

head and body, length of tail, occipitonasal length, zygomatic breadth, in¬
terorbital breadth, and depth of cranium) and females the larger for two
(length of ear and length of maxillary toothrow). In the other six measure¬
ments (length of hind foot, breadth of braincase, rostral breadth, length of
rostrum, length of incisive foramen, and length of palatal bridge), the sexes
had the same mean value. In all nonsignificant measurements in which one
sex averaged larger than the other, the difference was small.

It is evident from the above analysis that there is little or no secondary
sexual dimorphism between males and females in the San Antonio popula¬
tion of Nyctomys sumichrasti. Based on this finding, it would seem justi¬
fiable to combine values for the sexes in any analysis of geographic vari¬
ation.

GENOWAYS AND JONES— VARIATION IN NYCTOMYS

11

Fig. 3. — First upper left molars of three specimens of Nyctomys sumichrasti illus¬
trating variation in development of the mesoloph and mesostyle in the population
trom San Antonio, Nicaragua. Arrows indicate the position of the mesoloph and
mesostyle on each tooth. The specimens illustrated are, from poorly-developed
mesoloph and mesostyle (left) to well-developed (right), KU 98056, KU 106662, and
KU 106689. The scale at the right is one millimeter long.

Individual Variation

Coefficients of variation for dimensions of adult males and adult fe¬
males are given in Table 2. Weight for both sexes has the highest coef¬
ficient of variation of the 16 measurements tested. Only three other mea¬
surements for males (length of head and body, length of tail, and length of
ear) and three for females (rostral breadth, length of incisive foramen, and
length of palatal bridge) have coefficients of variation of 5.0 or more. Co¬
efficients generally agree with those given by Long (1968, 1969) for ro¬
dents of a similar size. Males had the higher coefficient of variation in nine
measurements and females had the higher value in seven.

Most individuals of Nyctomys sumichrasti from San Antonio have a
poorly developed mesoloph and, if present, a poorly developed mesostyle
on Ml (see Hooper, 1957:9, for dental nomenclature). However, in a few
individuals (KU 106687 and 106689, for example) the mesostyle is a large
cusp at the outer edge of the tooth (Fig. 3). Other individuals (KU 98056
and 106662, for example) have a distinct mesostyle that is intermediate in
size between the two extremes.

A subadult male (KU 98061) taken on 9 July 1964 had the rostrum
deflected sharply to the right. This may have resulted from a congenital
defect or an injury early in life, but there was no evidence that the bones
in this area ever had been broken. Two individuals (KU 106667 and
115489) exhibited dental deterioration, possibly resulting from caries.

Seasonal Variation

We were unable to detect variation in color or other seasonal dif¬
ferences, except for active molt, among individuals collected in the dry
season and those taken in the wet season. Fifteen specimens, all taken in
the first 10 days of July, appear to have been molting from one adult pel-
age to another. Seven subadults that were molting were taken at the same

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OCCASIONAL PAPERS MUSEUM TEXAS TECH UNIVERSITY

TABLE 2—1 Secondary sexual variation in six external and 10 cranial measurements
of adult Nyctomys sumichrasti from San Antonio, Chinandega, Nicaragua. Means for
males and females that are significantly different at the .05 level are indicated by an
asterisk ; those that are not significantly different are marked ns.

Sex N Mean (range) ± 2 SE

Total length

Male

8

235.6 (225.0-246.0) ± 5.63

3.4

ns

Female

16

230.1 (219.0-242.0) ± 3.12

Length of head and body

2.8

Male

15

118.3 (99.0-129.0) ± 4.22

6.9

ns

Female

24

117.8 (106.0-128.0) ± 2.00

Length of tail

4.2

Male

8

114.9 (108.0-126.0) ± 4.67

5.7

ns

Female

16

113.1 (102.0-124.0) ± 2.54

Length of hind foot

4.5

Male

15

22.2 (21.0-23.0) ±0.49

4.2

ns

Female

24

22.2 (20.0-23.0) ±0.32

Length of ear

3.5

Male

15

16.8 (15.0-18.0) ±0.45

5.1

ns

Female

24

17.0 (15.0-18.0) ±0.30

Weight

4.3

Male

15

48.5 (39.3-61.3) ±3.11

12.4

*

Female

24

44.9 (38.2-53.9) ±1.67

Occipitonasal length

9.1

Male

14

29.8 (28.5-31.2) ±0.44

2.8

ns

Female

18

29.7 (28.5-31.1) ±0.37

Zygomatic breadth

2.6

Male

12

17.1 (16.4-18.0) ±0.31

3.1

ns

Female

18

16.8 (15.8-17.8) ±0.27

3.4

GENOWAYS AND JONES— VARIATION IN NYCTOMYS

13

Table 2. — Continued.

Male

14

Interorbital constriction

5.6 (5.4-6. 2) ±0.13

4.2

ns

Female

23

5.5 (5. 3-6.0) ±0.09

4.0

Male

14

Breadth of braincase

13.2 (12.8-13.6) ±0.15

2.1

ns

Female

22

13.2 (12.4-13.6) ±0.14

2.6

Male

15

Rostral breadth

5.7 (5. 3-6.0) ±0.12

4.0

ns

Female

22

5.7 (5. 2-6 .4) ±0.13

5.7

Male

14

Length of rostrum

10.2 (9.7-11.0) ±0.19

3.4

ns

Female

20

10.2 (9.4-10.7) ± 0.20

4.5

Male

12

Depth of cranium

11.1 (10.5-11.6) ±0.22

3.4

ns

Female

21

11.0 (10.2-11.5) ±0.15

3.2

Male

12

Length of maxillary toothrow

4.4 (4. 3-4. 6) ± 0.06

1.8

ns

Female

19

4.5 (4. 2-4.7) ± 0.06

3.1

Male

14

Length of incisive foramen

5.1 (4. 8-5. 6) ±0.12

4.3

ns

Female

24

5.1 (4. 3-6.3) ±0.15

7.5

Male

15

Length of palatal bridge

4.4 (4.0-4. 7) ± 0.09

4.1

ns

Female

24

4.4 (3.9-4. 7) ± 0.09

5.0

time, and one subadult was in the process of molt on 10 March 1968 and
another on 19 August 1967. Of nine individuals actively molting from ju¬
venile pelage, eight were taken between 5 and 10 July and the other on 9

March.

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OCCASIONAL PAPERS MUSEUM TEXAS TECH UNIVERSITY

Molt from juvenile pelage appears to follow a definite sequence. It be¬
gins ventrally and proceeds over the sides to meet middorsally. From the
middorsal area, molt proceeds anteriorly and posteriorly; the head and
rump molt last. No such pattern was observed in subadult or adult speci¬
mens, in which molt appeared to proceed irregularly over the dorsum.

ECOLOGICAL COMMENTS
Description of the Area

San Antonio lies in the Pacific lowlands of northwestern Nicaragua
about 5 kilometers south-southwest of Chichigalpa and 15 kilometers
southeast of Chinandega (Fig. 4). A series of isolated volcanos, dominated
by Volcan Casita and Volcan San Cristobal, lie approximately 20 kilo¬
meters to the east and north of San Antonio, and the shore of the Pacific
Ocean is about 15 kilometers to the west. Vegetation of this part of Nic¬
aragua was termed “semi-evergreen forest” by Taylor (1963:33) and “low¬
land dry forest” by Stuart (1966:693). Essentially, this forest type is com¬
posed of 30 to 50 per cent deciduous trees of two stories, with the canopy
being formed by the upper story. Unfortunately, most, if not all, of this
original forest has been destroyed and mature secondary forest is un¬
common (see Taylor, 1963:37). Most of this area now is highly agri-
culturalized, with sugar cane and cotton the primary crops. The soils in the
vicinity of San Antonio are generally young soils of volcanic origin
(Taylor, 1963:31 ; Stevens, 1964:308).

San Antonio can be classified as having a “tropical wet-and-dry cli¬
mate” (Vivo Escoto, 1964:212), which is characterized by distinct wet
and dry seasons. The dry season begins in late November and extends into
May, with March and April the hottest months. Maximum rainfall usually
occurs in September and October. Precipitation records from Chichigalpa,
which receives more than 82 inches (2098 millimeters) of rain annually,
are typical of this area. Following is the average monthly rainfall for Chi¬
chigalpa, in millimeters (after Taylor, 1963:30): January, 1; February, 1;
March, 3; April, 25; May, 255; June, 343; July, 193; August, 273; Sep¬
tember, 414; October, 499; November, 84; December, 7.

Habitat and Interspecific Relationships

The area in which vesper mice were taken at San Antonio was a riparian
community along a tributary of the Rio Amalia between San Antonio and
Chichigalpa. Vegetation along the stream was never more than 50 meters
wide on either side and usually was much narrower. Extensive sugar cane
fields were located beyond the riparian community. The lower story of

GENOWAYS AND JONES — VARIATION IN NYCTOMYS

15

area. The insert map at the upper left indicates the position of enlarged area within
Nicaragua.

vegetation was composed of dense second-growth shrubs and small trees,
primarily Cordia diversifolia (family Boraginaceae) and two species of the
family Ribiaceae, in which numerous woody vines were intwined. This
lower story was nearly impenetrable except for a few paths that paralleled
the stream. The upper story of vegetation was composed of figs (Ficus)
and other tall tropical trees, many of which were more than 100 feet in
height. The stream averaged eight to 10 feet wide in the rainy season, but
was much narrower in the dry season.

Other small, nonvolant mammals taken in this area along with vesper
mice were Didelphis marsupialis, D. virginiana, Caluromys derbianus, Mar-
mosa mexicana , Sciunis variegatoides , Liomys salvini, Oryzomys palustris ,

16

OCCASIONAL PAPERS MUSEUM TEXAS TECH UNIVERSTI Y

D C

Fig. 5. — Schematic illustration of spatial distribution of 14 species of small, non¬
volant mammals at San Antonio, Chinandega, Nicaragua. Approximate vertical dis¬
tribution is shown to the left and right as follows: 1 , Nyctomys sumichrasti', 2, Didel¬
phis marsupialis and D. virgin iana', 3, Caluromys derbianus, Sciurus variegatoides, and
Coendou mexicanus', 4, Marmosa mexicana, Reithrodontomys gracilis , and Ototylo-
mys phyllotis. Approximate horizontal distribution of terrestrial taxa is shown be¬
low: A, Liomys salvini and Sigmodon hispidus', B, Oryzomys fulvescens and O. palus-
tris', C, Reithrodontomys gracilis , Peromyscus mexicanus, and Ototylomys phyllotis',
D, Didelphis marsupialis and D. virginiana.

O. fulvescens, Ototylomys phyllotis, Reithrodontomys gracilis, Pero¬
myscus mexicanus, Sigmodon hispidus, and Coendou mexicanus. Utili¬
zation of the third dimension (arboreal) of the habitat is the means by
which at least 14 species of small mammals are able to coexist in this re¬
stricted area (Fig. 5). Individuals of Nyctomys sumichrasti usually were
observed in the lower story of vegetation in the wet season, frequently at
heights of 10 to 20 feet. During the dry season, however, many of these
mice were observed much higher, especially in fig trees, where they ap¬
peared to be feeding. This species rarely comes to the ground as evidenced
by poor trapping success in this unusual area of high population density.
Few of the individuals trapped (usually at the bases of trees) or shot but a
few feet above the ground were adults.

The woolly opossum ( Caluromys derbianus), variegated squirrel (Sci¬
urus variegatoides ), and Mexican porcupine ( Coendou mexicanus ) are
three other species that live almost exclusively in tall trees and seldom
come to the ground. The two opossums (Didelphis marsupialis and D. vir-
giniana) probably are the most ubiquitous species in the area, being found
from high in the trees to along the edges of sugar cane fields. The few
mouse opossums (Marmosa mexicana) observed or collected at San Anto¬
nio generally were found in lower vegetation (usually four to six feet
above the ground) than were vesper mice, but none was seen on the
ground. Climbing rats (Ototylomys phyllotis), which also were abundant

GENOWAYS AND JONES— VARIATION IN NYCTOMYS

17

TABLE 3. Sex ratio and age structure of the population of Nyctomys sumichrasti
at San Antonio, Chinandega, Nicaragua, in the wet seasons of 1966 and 1967. The
age groups for the small sample taken in the dry season of 1968 are also shown.

Wet season

Dry season

Age

cate-

1964

1966

1968

Total

Total

Grand total

Total

gories

Male:Female

individuals Male: Female individuals

individuals

individuals

I

4:3

7

3:3

6

13

3

II

6:3

9

6:1

7

16

2

III

4:3

7

6:8

14

21

0

IV

3:3

6

2:11

13

19

3

V

0:2

2

6:6

12

14

0

in this area (see Lawlor, 1969), and slender harvest mice ( Reithro -
dontomys gracilis ) occurred on the lower branches and vines, generally be¬
low the stratum occupied by vesper mice. Both species have been observed
on the ground. Lawlor (1969:37) noted that when climbing rats were pur¬
sued they moved laterally through the vegetation rather than upward.

The remaining five species evidently are terrestrial. The two rice rats
(i Oryzomys palustris and O. fulvescens) and the Mexican deer mouse ( Pero -
myscus mexicanus) seemingly were restricted to riparian vegetation in this
area and usually were found in mesic situations near the stream. Salvin’s
spiny pocket mice ( Liomys salvini), and cotton rats ( Sigmodon hispidus )
were taken in the drier areas on the edge of the riparian vegetation; both
were trapped also at the edge of sugar cane fields.

Sex Ratio and Age Structure

Because most animals in our sample were shot at night from arboreal
vegetation, the bias introduced by trapping (Fisler, 1971) to analyses of
sex ratio and age structure in populations should be reduced. Nevertheless,
those individuals that had not yet left the nest are not represented in our
sample, and those that were mostly active in relatively low vegetation
might predominate in the sample; also, animals with a large home range
might have a greater likelihood of being taken than those with a restricted

home range.

Even with these possible biases in mind, the data presented below are

18

OCCASIONAL PAPERS MUSEUM TEXAS TECH UNIVERSITY

the best estimates of population structure in N. sumichrasti available at
this time. Sex ratios for the various age categories of specimens taken in
1964 and 1966 are shown in Table 3. Because collecting was conducted at
nearly the same time (wet season) in these two years (6-13 July 1964 and
5-10 July 1966), data from each were combined for statistical testing. In
age categories II and IV, there was a significant deviation at the .05 level
(but not the .01 level) from the expected 1:1 ratio of sexes using
Chi-square. In age category II, males outnumbered females 12 to four and
in category IV males were outnumbered five to 14. The latter difference
might be explained as differential survival between males and females, but
this difference was not reflected in the oldest age class (V) in which males
were outnumbered only six to eight by females. When the sex ratio is com¬
pared without regard to age category, the ratio of 40 males to 43 females
does not differ significantly from a 1 : 1 ratio.

As can be seen in Table 3, more individuals representing age groups III
and IV were taken in 1964 and 1966 (data combined) than were mice of
the other three age categories. However, when these differences were
tested (Chi-square) they were found not to differ significantly from the
hypothesis that all age categories are equally represented in the popula¬
tion. We suppose that two factors could combine to form a population in
which all age categories are equally represented. First, the species re¬
produces throughout the year (as our data combined with those of
Fleming, 1970, indicate for Nyctomys), and second, that the population is
relatively free of predatory pressure so that once an individual leaves the
nest there is a reasonable likelihood that it will live to reach the oldest age
category. Vesper mice at San Antonio probably are preyed upon only by
nocturnal raptors; predation, however, is mitigated by the heavy vege¬
tation in which these mice live. Our sample from the dry season, on the
other hand, although small, does favor younger animals. This may result
from sampling bias, however, because during this season many vesper mice
were observed high in trees from which they could not be collected.

Food Habits

The food of Nyctomys consists primarily of seeds, fruits, and other
vegetative matter. During the wet season at San Antonio, numerous in¬
dividuals were observed feeding on fruits of the borage, Cordia diversifolia,
and madders (Rubiaceae). In the dry season, most individuals that we ob¬
served appeared to be taking wild figs {Ficus). One juvenile was shot as it
fed near the ground on the flower of a composite, a plant abundant along
the edge of the riparian community. In Guatemala, Nyctomys has been
recorded as eating avocados and wild figs (Goodwin, 1934:51).

GENOWAYS AND JONES— VARIATION IN NYCTOMYS

19

Reproduction

Of the 22 adult (age categories IV-V) and 1 1 subadult (III) females
taken at San Antonio in the first two weeks of July, only one was preg¬
nant and one was lactating. The pregnant female contained a single em¬
bryo in the left uterine horn that measured 6 in crown-rump length on 10
July. The lactating female was obtained on 6 July. One of three females
taken on 19 August carried four embryos (two in each horn) that mea¬
sured 2 in crown-rump length; another possessed an enlarged uterus, but
no embryos were visible in gross examination. The only adult female ob¬
tained on 10 March evinced a placental scar in each uterine horn.

Nine adult males taken between 5 and 9 July had testes that averaged
12.2 (1 1-15) in length. Two males taken on 10 March and individuals ob¬
tained on 7 August and 18 August all had testes that measured 12; two
males collected on 19 August had testes 14 and 15 long. We have juveniles
that were obtained on the following dates: 9 March (two) and 10 March; 5
July (two), 6 July (four), 7 July, 8 July, 9 July, and 10 July (three).

Elsewhere in the geographic range of Nyctomys , several females taken
in Colima in August and early September were reported (Walker et al.,
1968:772) as carrying two to four well-developed embryos. In Veracruz,
Hall and Dalquest (1963:295-296) obtained five adult females with small
young clinging to their teats in the last week of March and first weeks of
April. Fleming (1970:481), in Panama, obtained pregnant females on 5
February and 2 June and lactating females on 4 and 5 March. He also re¬
ported finding two subadults with perforate vaginas and a third with two
small embryos, but no dates of capture were given. Birkenholz and Wirtz
(1965:183) raised vesper mice in the laboratory and found the average
number of young born in seven litters was two, with a range of one (two
litters) to three (two litters). One female gave birth to five litters during a
seven-month period, with two litters being born 38 days apart.

These data clearly indicate that vesper mice reproduce during both the
wet and dry seasons and probably are capable of reproduction the year
around. Litter size evidently varies from one to four, with two probably
being the mode.

Ectoparasites

The commonest ectoparasite of Nyctomys sumichrasti at San Antonio
was the laelapid mite, Androlaelaps fahrenholzi (=Haemolaelaps glasgowi),
which was found on 36 of the 85 specimens taken in 1964 and 1966.
Other taxa of laelapids found were a new species of Androlaelaps (on 10
specimens) and Eubrachylaelaps circularis (on one specimen). A trom-
biculid, Fonsecia ( Parasecia ) longicalcar, was found on three specimens

20

OCCASIONAL PAPERS MUSEUM TEXAS TECH UNIVERSITY

and Speleocola secunda was taken from a single specimen. A tick, Or-
nithoderas talaje, was recorded from three animals and an unidentified spe¬
cies of the genus Ornithoderas from another individual.

Elsewhere in Nicaragua, the mites Eutrombicula alfreddugesi and Lis-
trophorus (new species) were found on specimens collected at Santa Rosa
in Departamento de Boaco. Micro trombicula mesoamericana (Webb and
Loomis, 1971:326) was found on specimens taken at Hacienda Bellavista,
Chinandega, and near Diriamba, Carazo. The only other records of ecto¬
parasites from Nyctomys are reports of the laelapid mite, Androlaelaps
fahrenholzi (Tipton et al. , 1966:34) and the flea, Pleochaestis dolens do-
lens (Tipton and Mendez, 1966:31 1), from Panama.

ACKNOWLEDGMENTS

Field work for this study was supported by a contract (DA-49-1 93-
MD-2215) from the U.S. Army Medical Research and Development Com¬
mand. Laboratory phases of the study were supported by a grant from the
National Science Foundation (GB-28957). We acknowledge with pleasure
our field companions, J. R. Choate, C. Estrada R., G. L. Forman, J. D.
Gerber, L. M. Hardy, T. E. Lawlor, C. J. Phillips, D. A. Schlitter, J. D.
Smith, R. W. Turner, and J. E. Ubelaker, who aided in collection of speci¬
mens on one or more of our visits to Nicaragua. We are indebted also to
Sr. Edgard Vargas G. and his staff of Nicaragua Sugar Estates Ltd. for their
hospitality during our visits to San Antonio.

Statistical analyses were performed on the GE-635 computer at The
University of Kansas. Ectoparasites were identified by Russell W. Strandt-
mann (Laelapidae), Richard B. Loomis (Trombiculidae), Eleanor K. Jones
and Glen M. Kohls (Argasidae), and Burruss McDaniel (Listrophoridae).
Mr. Thomas Swearingen prepared the drawings of teeth and crania and
Miss Patricia Allgood made the finished version of Fig. 5.

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