HOMING IN MEXICAN GROUND SQUIRRELS A THESIS IN …

HOMING IN MEXICAN GROUND SQUIRRELS

by

TIMOTHY LEWIS TANDY, B.S.

A THESIS

IN

ZOOLOGY

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for

the Degree of

MASTER OF SCIENCE

Approved

Accepted

May, 1978

no. / f n,^f ^? ACKNOWLEDGMENTS

I express my appreciation to Dr. Robert L. Packard for advice,

assistance, and encouragement throughout all phases of the investigation.

I am also indebted to Drs. Robert J. Baker and M. Kent Rylander for

their consultation on my study from its inception and for reading the

manuscript.

Those who assisted in the field work are far too numerous to

mention; however, they all have my gratitude. Among those especially

helpful in this regard were: Eddie Allen, Loretta Eby, Michael Krenz,

John LeVick, and Paul Young. Appreciation is also due to the grounds

crew of the TTU Center at Junction who cooperated in the maintenance

of the study area.

This manuscript is dedicated to my parents, who instilled in me an

appreciation for nature and supported me without reservation when I chose

to make its study my life's work.

11

TABLE OF CONTENTS

ACKNOWLEDGMENTS ii

LIST OF TABLES iv

Chapter

I, INTRODUCTION 1

II. MATERIALS AND METHODS 6

Capture and Processing 6

Homing Trials 8

Analysis of Homing Data 10

III. THE STUDY AREA 11

Soil 11

Vegetation 11

IV. • RESULTS AND INTERPRETATIONS 13

Trap Response 13

Population Structure 15

Home Ranges 15

Analysis of Homing Data 20

Observations During Homing Trials 20

V. SUMMARY 27

LITERATURE CITED 29

111

LIST OF TABLES

Table Page

1. Sex and age of animals live-trapped in the summers of 1976 and 1977 16

2. Mexican ground squirrel 1977 home range sizes estimated by the exclusive boundary strip method 17

3. Results of two-tailed "t" tests applied to 1977 home range size data 18

4. Age, sex, and number of repetitions for animals used in homing experience 21

5. Results of two-tailed "t" tests applied to occupied as compared to unoccupied homing trial times 22

IV

CHAPTER I

INTRODUCTION

Burt (1943) defined home range as " , , . that area traversed by

the individual in its normal activities of food gathering, mating, and

caring for young." The tendency to return to its home range when dis­

placed has been observed in a number of species of rodents. Different

aspects of this "homing behavior" have been studied. Fisler (1966),

Furrer (1973), Murie (1963), and Robinson and Falls C1965) investigated

homing success from varying distances and observed that as distance from

home range increased, homing success decreased. Orientation of dis­

placed animals to the home range has been studied (Bovet, 1968; Murie,

1963), but the mechanisms involved have not been determined, Furrer

(1973), Savidge (1973), and others measured the effect of various

geographical features as barriers to homing.

Most of these studies dealt with homing over considerable distances.

Natural situations requiring long-range homing in rodents are presumably

rather rare. Fire, flooding, or escape from a predator might displace

an animal a short distance from its home range. There may be a number

of extrinsic and intrinsic variables that enter into the total spectrum

of homing behavior. The displaced individual might be required to cross

the home ranges of conspecifics. In doing so, the animal might spend

a considerable amount of time engaging in intraspecific conflicts or

in the avoidance of them. An experimental situation in which displaced

animals were forced to home across the home ranges of conspecifics would

provide insight into the degree of territoriality or behavioral in-

tolerance within a species. The Mexican ground squirrel, Spermophilus

mexicanus, is well-suited for an investigation of this nature for several

reasons. This animal is known to establish a home range and, within

that home range, intolerance of specifics has been reported. Homing

by an individual displaced up to 300 yd has been demonstrated. Further­

more, the short grass habitat of this species facilitates observation

of homing behavior and intraspecific interactions.

The objectives of this study were:

(1) To determine whether the physical presence of conspecifics in­

fluences homing times of displaced animals and, if so, whether

this influence is a function of age and/or sex.

(2) To determine whether significant differences in home range size

exist and, if so, whether these differences are dependent upon

age and/or sex.

The Mexican ground squirrel is somewhat colonial, yet rather asocial

except in the breeding season (Edwards, 1946; Shockley, 1974). Colonies

may result from attraction of squirrels to a particular habitat rather

than to each other (Shockley, 1974). This is also reported in the

closely-related 13-lined ground squirrel, Spermophilus tridecemlineatus

(McCarley, 1966). Edwards (1946) observed the area normally covered by

an individual Mexican ground squirrel was approximately 50 yd in diameter,

although it might range up to 100 yd in diameter, Shockley (1974)

reported much greater home range size for the species. Home ranges

of males were estimated at 2.6 - 5.4 ha (mean =3.9 ha), whereas females

ranged 1,0 ^ 1.5 ha (mean = 1.2 ha). Working with the 13-lined ground

squirrel, Spermophilus tridecemlineatus, McCarley (1966) reported adult

male home ranges of 4.7 ha and adult female home ranges of 1.4 ha.

Both Edwards (1946) and Shockley (1974) reported that the home ranges

of Mexican ground squirrels overlapped; however, the former observed

that territory moved with the animal within its home range, while the

latter did not. Both studies reported ground squirrels engaging in threat

or conflict as a result of one animal entering another animal's terri­

tory. Balph and Stokes (1963) observed intolerance of conspecifics in

close proximity within their home ranges in the Uinta ground squirrel,

S_, armatus.

The breeding season of the Mexican ground squirrel in Central Texas

lasts from April through early May. During this period, males seek out

females and there is considerable behavioral interaction between members

of the same and opposite sex. A male will share a burrow with a female

until she gives birth, at which time he is expelled. After this brief

period of pairing, there is little intersexual behavior (Shockley, 1974).

Mexican ground squirrels construct 3 types of burrows within their

home ranges: adult home burrows, refuge burrows, and home burrows of

young. The first type serves as a home for the adult and, in the case

of females, as a place in which to give birth and raise young. Refuge

burrows, which may be numerous, are shallower excavations. These may

be old or filled-in burrows, or excavations designed specifically for

the purpose of refuge. Home burrows of young are constructed by young

after they leave the maternal burrow. These are often located in the

vicinity of the maternal burrow (Edwards, 1946; Shockley, 1974).

Because Mexican ground squirrels appear attracted to particular habitat

types, this often results in clumped distribution. Neighboring burrows,

therefore, may be relatively close in proximity. Mexican ground squirrels

emit high trills when disturbed or in danger; these are easily heard by

other squirrels in the vicinity (Edwards, 1948; Matocha, 1975; Shockley,

1974). Similar observations have been reported for the 13-lined ground

squirrel (McCarley, 1966; Matocha, 1975). Calls of the Mexican ground

squirrel lack the complexity exhibited by communication in the black-

tailed prairie dog, Cynomys ludovicianus (King, 1955; Matocha, 1975).

The Mexican ground squirrel calls serve more as an early warning system.

Body-rubbing, marking, and other scent-related behavior have been

reported by Steiner (1974) in the Columbian and Arctic ground squirrels

(Spermophilus columbianus and S. undulatus, respectively). Such behavior

has not been reported in the Mexican ground squirrel. The only scent-

related behavior known in the species is anal papillae evertion. This

is known to occur when the animal is frightened or engaged in an intra-

specific encounter (Shockley, 1974). While olfactory cues may be im­

portant once animals have encountered, there is no evidence which

indicates that they prevent encounters. The Mexican ground squirrel

prefers flat areas with low-lying vegetation; therefore, it seems

reasonable that visual cues, rather than olfactory, are of primary

importance. This is supported by the observation that the animal is

often spotted in the field assuming the "picket pin" position, a body

posture facilitating a better view of the surroundings (Edwards, 1946;

Shockley, 1974).

McCarley (1966), working with Spermophilus tridecemlineatus and

Shockley (1974), working with S_. mexicanus, found local weather con­

ditions exercise a great influence upon daily behavior of ground

squirrels. Activity is depressed by unseasonably cool temperatures,

high humidity, rain, winds in excess of 15 mph, and overcast skies.

Similarly, extremely high temperatures inhibit above-ground activity.

This last factor is responsible for the bimodal activity pattern

exhibited by ground squirrels on hot summer days.

CHAPTER II

MATERIALS AND METHODS

This study was conducted during the months of June through August,

1976, and June through mid-September, 1977, Work during 1976 dealt with

capturing, marking, and recapturing ground squirrels. These activities

were continued in 1977, with the addition of homing experiments.

Capture and Processing

The first objective of the study was to locate and mark as many

members of the population on the study area as possible. Daily foot

patrols were undertaken, during which all ground squirrel burrows found

on the area were marked with red survey flags. Owing to the camouflaged

nature of burrow entrances, this task was complemented by field obser­

vation. The area was observed with 7 x 35 binoculars and, when an

animal was sighted, the observer moved toward it, forcing it to seek

refuge in one of its burrows. This permitted location of many burrows

overlooked during foot patrols. The alarm trill also provided a means

of locating burrows. The researcher would locate near the study area

and wait until an animal vocalized, then follow the squirrel to the

burrow.

Live-trapping was initiated with livetraps placed near burrow

entrances. Livetraps were of three varieties: (1) wire-mounted on a

wooden base, dimension being 450 x 105 x 103 mm; (2) composed entirely

of wire, with measurements of 305 x 80 x 80 mm; (3) wire gravity door

type with oil can placed over one end, dimensions 390 x 75 x 75 ram.

Initially peanut butter, oats, and combinations of these plus

chicken scratch were used as bait with little success. Sliced apple

proved most successful bait. Traps were covered with dead grass in order

to provide shade for captured animals, and hourly checks were made from

early morning until dusk.

When a ground squirrel was initially captured, it was sexed, classi­

fied as adult or subadult, weighed to the nearest gram, and toe-clipped

for further identification purposes. Due to the relatively large size

of the animal, clipped toes were treated with an antiseptic to minimize

risk of infection (in the two years of study, no recaptured animal showed

evidence of infection due to toe-clipping). Toe-clipping facilitated

identification of a recaptured individual without removal from the trap.

After processing, the animal was released at the capture site and the

red flag was replaced by a blue one. This phase of the study was pri­

marily aimed at effective trapping and marking as many members of the

population as possible. All capture sites were recorded.

In June of 1977, a 1 ha grid was placed on the most active portion

of the 1976 study area. This consisted of 10 x 10 rows of small stakes

spaced 10 m apart. Ground squirrels were live-trapped and processed

in the same fashion as in 1976; however, livetraps were restricted to

the 100 trapping stations (1 trap per station). Trapsite records were

kept for all animals. Traps were shaded by folded paper plates taped to

the traps.

8

Homing Trials

Homing trials were conducted from August through mid-September of

1977. Home ranges of squirrels were determined by the exclusive boundary

strip method (Stickel, 1954) and plotted on a map of the grid. Animals

used in homing trials had been recaptured at a minimum of five different

capture sites. Four classes of ground squirrels were used: adult male

(n = 4), adult female (n = 5), subadult male (n = 5), and subadult female

(n = 5). A release site was chosen for each squirrel 40 to 60 meters

from its trap-determined home range. The chief concern in selecting

a release site was that the displaced animal be required to cross the

home ranges of others upon its return. Two types of trials were run:

(1) homing across occupied home ranges; (2) homing across unoccupied

home ranges. In the former instance, squirrels were trapped and removed

one or two at a time. Care was taken not to remove squirrels from an

area over which another was to home. The subject was placed in a 12 x

16 X 6 inch cardboard release box and allowed to calm down. The top of

the box was remotely removed by a string. Survey flags were posted at

all trap stations where a trial squirrel had been captured. This gave

reference points for determining when the squirrel had entered its home

range, A squirrel was carried in the box to the release site. Approach

was made peripheral to the study area, thus minimizing disturbance of

other squirrels. The box was placed in position and the observer either

took cover behind a tree or bush or lay on the ground (at least 15 m

away). A squirrel was not released until other squirrels were seen or

heard in the vicinity. Usually this was a period of no more than 5 to

10 minutes. Time of homing was recorded with a stopwatch. Timing was

initiated when the box top was removed and ended when the subject entered

a burrow within its trap-determined home range. Preliminary trials

suggested that the squirrel would not linger above ground upon returning

to the home range but, rather, would seek shelter in a known burrow.

Observations of behavior, encounters, and usage of time were recorded.

The second type of homing trial, in which a squirrel was allowed

to home across an unoccupied area, required that other squirrels be

either absent (captured) or underground. During this phase of experi­

mentation, most squirrels were removed by live-trapping. This permitted

a number of experimental trials with different squirrels in a single day.

Since all the squirrels could not be trapped at once, however, plugs

of rolled newspaper and cardboard were utilized in order to keep squirrels

underground during homing trials. All trials and observations were

conducted as in the trials across occupied areas. In most cases, sample

size for each class of trial per individual was 5.

In order to eliminate any effect of possible learning on homing

time differences in the two classes of trials, classes of trials were

intermixed. That is, usually three trials through occupied home ranges

were run, followed by all of the unoccupied trials. The remaining

trials through occupied ranges were conducted. It was felt that any

significant time differences owing to learning would be reflected in

the difference between the early and later trials (occupied).

10

Analysis of Homing Data

A single release site was chosen for each squirrel so that the animal

would be homing the same distance each time. Actual distance from release

site to home range was unknown, since precise boundaries of home range

cannot be determined by recapture data. This, along with between-

squirrel variation, rendered lumped analysis of homing times by classes

meaningless. However, analysis of homing times within squirrels on the

basis of occupied versus unoccupied trials provides an assay of whether

the presence of conspecifics affects homing time. Trial squirrels

possibly entered different burrow within their home ranges during the

series of trials. This probably did not significantly affect the

recorded times, because homing animals with few exceptions proceeded

swiftly to a burrow once they entered their home range. A two-tailed

"t" test was applied to the two classes of homing times for each animal.

If there is no significant difference between a squirrel's homing times

across occupied and unoccupied areas, then there is no indication of an

effect imposed by the presence of conspecifics. If, however, a signifi­

cant difference does exist, then such an effect seems evident.

CHAPTER III

THE STUDY AREA

This study was conducted at the Texas Tech University Center at

Junction, Kimble County, Texas, The TTU Center is located in the

Edwards Plateau region of Central Texas and is typical of the Balconian

Biotic Province (Blair, 1950). Drainage is provided by the South

Llano River. Nearly level to gently sloping outwash plains and level

to nearly level floodplains in valleys between limestone hills comprise

the topography of the area (for a detailed description of the area,

see Packard and Dana, 1978). Elevation of the study area is 1715 feet.

Soil

Soil type on the study area is classified as Nuvalde clay loam.

This deep calcareous soil is typical of outwash plains on the Center

and developed from Edwards Plateau limestone hills alluvium. Perme­

ability of this soil is moderate.

Vegetation

Vegetation on the study area was modified by periodic mowing. In

addition, ground squirrel activity kept grasses and herbs fairly short

near burrow entrances, although denuded areas were rarely found. Because

the area was mowed, vegetation rarely became so high as to make con­

ditions unfavorable for ground squirrel inhabitation. This provided,

in essence, a simulated grazed area. Ground squirrels were rarely

found in comparable areas where the vegetation was left unmowed. Heavy

11

12

rains and rapid vegetation growth during the summer of 1976 necessitated

frequent mowing. The area was only mowed twice in 1977 because of an

extended drought.

Three grasses constituted the dominant vegetation on the study

area: three-awn (Aristida sp. L,), Texas grama (Bouteloua chondrosoides

Wats.), and Texas needlegrass (Stipa leucotricha Trin. & Rupr.). A few

mesquite trees (Prosopis glandulosa Torr.) were widely spaced, as were

agarito (Berberis trifoliolata Moric), tasajillo (Qpuntia kleiniae D C ) ,

and yucca (Yucca torreyi Shafer.). Small stands of live oak (Quercus

virginiana Mill.) were located along the western and northern borders of

the area, and junipers (Juniperus sp, L.) grew along the northwestern

border. In addition, several kinds of low-growing cacti (Echinocereus

sp. Engelm.) were present in small numbers.

The following plants constituted the major hervaceous component on

the area: yellow wood-sorrel (Oxalis dilleni Jacq.), dandelion

(Taraxacum officinale Wiggers.), henbit (Lamium amplexicaule L.),

rain lilly (Cooperia drummondi Herb.), wild onion (Allium drummondi

Regel.), verbena (Verbena bipinnatifida Nutt.), devil's bouquet

CNyctaginia capitata Choisy.), London rocket (Sisymbrium irio L.), and

silver-leaf nightshade (Solanum elaeagnifolium Cav,).

CHAPTER IV

RESULTS AND INTERPRETATIONS

Trap Response

Presence of livetraps seemingly did not affect daily activity of

the ground squirrels. Adult and subadult squirrels were observed in,

upon, and besides traps. Smaller subadults exhibited the ability to

avoid the release mechanism and steal bait from the larger, wooden-based

traps. All three types of traps were equally successful in capturing

squirrels.

Some squirrels were more frequently trapped than others. Certain

individuals were captured several times a day. Particularly this was

the case in 1977 when almost daily trapping was conducted. The maximum

number of captures for individuals of the four sex/age classes were as

follows: adult males, 5; adult females, 21; subadult males, 15; and

subadult females, 10. The number of captures for individuals of the

adult male class was consistently lower than that of the other classes.

The maximum number of captures (per individual) on a single day was

three adult females caught 4 times each, while the maximum for the other

sex/age classes was 2. These three adult females were lactating. Multi­

ple captures were recorded for each of these three for several days

during a period of 8 to 16 days. This increase in trap response might

reflect an increased demand for nourishment of lactating females.

As noted by Shockley (1974) and McCarley (1966), environmental

conditions greatly influenced ground squirrel activity and, consequently,

13

14

influenced trap success. Chief among these factors were light intensity,

rain, and unusually high wind. Light intensity may be of importance

due to the all-cone structure of the ground squirrel retina (Jacobs and

Yolton, 1971). Low incident light conditions may simply not allow

sufficient visual perception for activity. This may account for reduced

above-ground activity at dawn and dusk. Overcast conditions during any

part of the day inhibited activity. Squirrels were sometimes observed

in reduced incident light, but these seemed trap-shy. Perhaps there is

increased wariness in reduced incident light.

High wind and/or rain caused the greatest activity reduction.

Either element was capable of completely suppressing above-ground

activity. High wind seems disadvantageous in two ways: (1) it inter­

feres with visual ground-predator detection; (2) it reduced auditory

perception of conspecific alarm calls. Because rain usually occurs in

association with reduced light, wind, and lower temperature, correlation

of activity with rain is difficult. Rain interferes with predator de­

tection and auditory perception of alarm calls and, from this standpoint,

inactivity is advantageous to the squirrel. Inactivity during rain

might reflect a physiological response to lowered ambient temperature

and increased humidity. Furthermore, a wet ground squirrel would suffer

considerable heat loss. Observations and trapping data show reduced to

no activity in rain. Above-ground activity was practically nonexistent

during a three-week period of intermittant rains in July of 1976. Rains

ranging in duration from 10 minutes to several hours suppressed activity

in both years.

15

Population Structure

Breakdowns of the population structure for each summer are given

in Table 1. In the summer of 1976, 28 individuals were captured on the

study area, whereas 53 were found on the grid in 1977, Drier conditions

in 1977 and more intensive trapping (rather than actual fluctuations

of numbers) may account for the difference. Male to female ratios for

each summer were approximately 2:3. Adult male to adult female ratios

of 1:2 in 1976 and 1:2.5 in 1977 are similar to the findings of Shockley

(1974), Subadult male to subadult female ratios of approximately 1:1

for each summer also are similar to those of Shockley.

Home Ranges

Because grid trapping was not conducted in 1976, an estimate based

on recaptures in any part of the area aided in plotting home range size.

Squirrels captured a minimum of 3 times were used in this analysis. The

greatest width of trap-determined home range varied from 25 to 100 m.

This corresponds with the observation of Edwards (1946) that the areas

covered by one squirrel ranged from 50 to 100 yd in diameter, the former

being more commonly observed.

Grid trapping in 1977 revealed a more realistic estimate of home

range size. Home ranges estimated by the exclusive boundary strip

method are in Table 2. With the application of 2-tailed "t" tests to

sex and age classes, it was shown (Table 3) that adults had significantly

larger home ranges than subadults. Adult females had significantly larger

home ranges than adult males. Home range sizes of adult males were

not significantly larger than those of subadult males, Subadult male

and subadult female home ranges showed no significant differences.

16

Table 1. Sex and age of animals live-trapped in the summers of 1976 and 1977.

Sex Age 1976 1977

Male Adult 4 7

Subadult 7 14

Female Adult 9 18

Subadult 8 14

17

Table 2. Mexican ground squirrel 1977 home range sizes estimated by the exclusive boundary strip method.

„ . Range of home Mean of home range size (ha) range size (ha)

Male

Female

Adult

Subadult

Adult

Subadult

,075 - ,110

.050 - .125

.140 - .530

.035 - .145

.095

.083

.241

.090

18

Table 3, Results of two-tailed "t" tests applied to 1977 home range size data.

Classes Significance Level

Adult vs. Subadult

Adult Male vs. Adult Female

Subadult Male vs. Subadult Female

Adult Male vs. Subadult Male

Adult Female vs. Subadult Female

.01

.05

NS

NS

.01

19

whereas adult female home ranges were significantly greater than those

of subadult females.

Shockley (1974), working with Spermophilus mexicanus, recorded

observations that differed both qualitatively and quantitatively with

my study, Male home ranges were found to be much greater than those

of females and the sizes of home ranges greater than in the present

study. Shockley estimated male home ranges as 2.6 - 5.4 ha (mean = 3.9

ha) and female home ranges as 1.0 - 1.5 ha (mean = 1.2 ha). Similarly,

in the closely-related 13-lined ground squirrel, . tridecemlineatus,

McCarley (1966) reported adult male home ranges of 4.7 ha and adult

female home ranges of 1.4 ha. These home ranges are larger than estimated

in my study. Adult females had the largest home range (,14 - .53 ha,

mean = ,24 ha). When circular ranges were calculated based on Edwards*

(1946) estimated diameters of 50 - 100 yd, the theoretical areas were

.16 and .66 ha, respectively. This range is similar to that in the

present study (means of classes ranges from .08 to ,24 ha). The home

ranges of Shockley and McCarley were based upon the presence of squirrels

within 200 ft and 100 ft quadrats, respectively. Perhaps this technique

results in overestimation of home range size, Nonetheless, it cannot

account for the considerable difference between their estimates and

mine. The former two studies were conducted on much larger areas with

more variable habitat than the present one. My study area was composed

of uniform suitable habitat.

Adult females may appear to have greater home ranges than adult

males as a result of trap shyness in males. However, home ranges of

20

females (even though based upon small numbers of recaptures) were usually

larger than those of males with equal numbers of recaptures. It is

possible that adult females may require a larger home range because they

care for and feed young. This could account for their larger home ranges.

In an area where food and cover are at a premium, it is of importance

that adult females have access to sufficient food resources. As a

result of competition between sexes for available food and habitat,

adult males may be less favored than adult females. This might account

for the difference in home sizes recorded in the present study.

Analysis of Homing Data

The number of homing trials per subject animal is in Table 4, Re­

sults of two-tailed "t" tests conducted upon homing trial times are

shown in Table 5. Neither adult males or females tested showed any

significant differences in homing times across occupied and unoccupied

areas. Conversely, three of five subadult males and four of five sub­

adult females showed significant differences in homing times. In both

sexes of subadults, the majority of individuals tested exhibited signifi­

cantly shorter homing times across unoccupied areas. This suggests

that with subadults the physical above-ground presence of conspecifics

adversely influenced homing times.

Observations During Homing Trials

Behavior of displaced ground squirrels during homing trials con­

firmed the observations of Shockley (1974) and McCarley (1966). When

released, an animal would typically run a few meters, then become

21

Table 4. Age, sex, and number of repetitions for animals used in homing experiments.

Animal Sex Age Trials across occupied areas

Trials across unoccupied areas

P

Q

R

S

K

L

M

N

0

A

B

C

D

E

F

G

H

I

J

Male

Male

Male

Male

Male

Male

Male

Male

Male

Adult

Adult

Adult

Adult

Female Adult

Female Adult

Female Adult

Female Adult

Female Adult

Subadult

Subadult

Subadult

Subadult

Subadult

Female Subadult

Female Subadult

Female Subadult

Female Subadult

Female Subadult

5

5

5

5

5

5

5

5

5

5

5

5

5

5

5

5

5

5

5

4

5

5

5

5

5

5

4

5

5

5

5

5

5

5

5

5

4

5

22

Table 5. Results of two-tailed "t" tests applied to occupied as compared to unoccupied homing trial times.

Sex Age Number of animals showing significant differences

Number of animals showing nonsignificant differences

Male

Female

Adult

Subadult

Adult

Subadult

0

3

0

4

4

2

5

1

23

stationary, Following a brief period of hesitation, it would then wander

a few meters, often pausing to attain the upright "picket pin" position.

When the squirrel reached a familiar area, especially near its home

range, it would adopt a more bold manner, flick the tail, and scurry

toward a burrow.

Most adults and subadults were successful in quickly orienting them­

selves once they left the release box. During the trials, no animal

failed to home. This suggests that the animals are at least relatively

familiar with areas and objects in the vicinity of their home ranges.

Disoriented movements were usually of only a short duration, followed

by picketing and re-orientation.

Adults were generally more bold than subadults in their movements

during homing. Adults would run up to 20 meters, while subadults tended

to move in a more cautious manner, moving sporadically. This extreme

caution was evidenced whenever subadults homed over either occupied or

unoccupied areas. Caution was most pronounced in movement over occupied

home ranges.

Two basic types of interactions were observed during homing trials.

The most common consisted of a reaction on the part of the displaced

squirrel to a warning-trill vocalization by a conspecific in its

vicinity. This interaction is interesting, because it may be pre­

cipitated by the homing individual, Balph and Stokes (1963) and

Bridgewater (1966) reported that ground squirrels usually flicked their

tails as they moved about above-ground. This was interpreted as a

signal communication to conspecifics that this was a fellow ground

24

squirrel and not a predator. In the present study, displaced animals

rarely flicked their tails when moving outside their home ranges. In

instances where another squirrel vocalized in the vicinity of the

homing individual, a fear or alarm response, rather than aggression,

may have caused vocalization in a resident squirrel in the presence of

a homing squirrel. Squirrels detected movement and, in the absence of

the reassuring tail-flicking, reacted as they would to a predator. Dis­

placed squirrels in seven of eight trials flicked their tails in response

to the calls, then sought cover in a bush or clump of grass. This be­

havior consumed one to four minutes and was coupled with longer homing

time. It was characteristic of subadults, except for one female adult.

She was cautiously homing, approximately 30 m from her trap^-determined

home range, when a nearby squirrel trilled. The displaced individual

stopped, turned away from the home range, crawled 2 m, and pressed her

head and body flat against the ground. This position was maintained for

approximately two minutes, after which time homing was resumed. Similar

hiding behavior was described by McCarley (1966) in S_, tridecemlineatus.

Only two direct conflicts were observed and, in both cases, the

displaced squirrels were subadult females. One squirrel was approxi­

mately 40 m from its home range when it backtracked and, after a

momentary pause, cautiously entered a burrow. Five seconds later,

the squirrel erupted from the burrow entrance with blood spurting from

its nose. Propelling itself with its hind legs, the wounded squirrel

rubbed its head in the grass and soil near the burrow. It then crawled

5 m and pressed its head and body flat against the ground. This posture

25

was maintained for three and one-half minutes, then it continued to

its home range. In the second case, the displaced animal approached

within 15 m of a foraging conspecific. The resident charged the tran­

sient, which fled to the shelter of an agarito bush. The resident

then returned to the general area in which it had been foraging and,

after two minutes, the transient proceeded home.

The most powerful influence of the presence of conspecifics was

the elicitation of avoidance behavior in the case of homing subadults.

This was clearly reflected in significantly shorter homing times across

unoccupied areas. It might appear that adults were not influenced by the

presence of conspecifics; however, lack of influence upon homing times

rather than behavior was demonstrated. Adults, by virtue of age and

experience, are more familiar with the areas surrounding their home

ranges than are subadults. Learning may result from juvenile dispersal

and home range shifts. Doubtlessly size is an important aspect also.

In addition, adults have the advantage of more social experience.

Because of these multiple factors, insignificant differences in homing

times of adults might merely reflect a proficiency at avoiding other

squirrels while they home. In such cases, the behavior of the adults

squirrels would indeed be affected, although not to the extent that

homing times would be significantly different. Another possibility is

that adult behavior is not affected by the presence of conspecifics.

My data lend no support to this, While adults homed more boldly than

subadults, their actions suggested a degree of caution and stealth.

Adults were not observed to pass closely by conspecifics, which would

26

be expected if the presence of conspecifics was unimportant. It is

postulated that the differences in homing times of subadults across

occupied and unoccupied areas reflects inexperience with both the

immediate area and social interaction with conspecifics.

CHAPTER V

SUMMARY

1. The male to female ratio for each year was 2:3. The adult male to

adult female ratios of 1:2 in 1976 and 1:2.5 in 1977, as well as

the subadult sex ratios of 1:1 each year, concur with the findings

of Shockley (1974).

2. Adult females had significantly larger home ranges than did adult

males, subadult males, and subadult females. Between the latter

three classes there were no significant differences. This is in

contrast to data reported by Shockley (1974) and McCarley (1966),

in which adult males had larger home ranges than adult females.

However, the aforementioned studies were conducted on large areas,

while the present study dealt with a small area of suitable habitat.

In the present study, limited food and space resources may have

required adult females to establish larger home ranges in order to

adequately feed and care for young,

3. Home ranges in the present study were smaller than those reported

by Shockley (1974) and McCarley (1966), The small area of suitable

habitat in the present study may have caused compression of home

ranges.

4. There were no significant differences in homing times across occupied

and unoccupied areas by adults of either sex, while three of five

subadult males and four of five subadult females exhibited signifi­

cantly shorter homing times across unoccupied areas. This indicated

27

28

that the homing times of subadults of both sexes were adversely

affected by the physical presence of conspecifics. Observations

confirmed that intraspecific encounters sometimes occurred during

homing trials.

5. Adults of both sexes were observed to home with some degree of

caution across both occupied and unoccupied areas. Even though

adults exhibited no significant differences in homing times across

occupied and unoccupied areas, they may have nevertheless been in­

fluenced by the physical presence of conspecifics. Observation

revealed that displaced adults passed no closer to conspecifics

than did subadults. The lack of significant differences between

the two classes of homing trials in adults may have been a result of

their greater familiarity with the surrounding area and greater

intraspecific contact experience.

LITERATURE CITED

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Bridgewater, D, D, 1966. Laboratory breeding, early growth and behavior of Citellus tridecemlineatus. Southwest. Nat., 11:325-338.

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