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Graduate Student Theses, Dissertations, & Professional Papers Graduate School
1991
Red squirrel (Tamiasciurus hudsonicus) cache sites and cache pattern in Pattee Canyon Montana
Rebecca S. Burton The University of Montana
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Recommended Citation Burton, Rebecca S., "Red squirrel (Tamiasciurus hudsonicus) cache sites and cache pattern in Pattee Canyon Montana" (1991). Graduate Student Theses, Dissertations, & Professional Papers. 7053. https://scholarworks.umt.edu/etd/7053
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. RED SQUIRREL {TAMIASCIURUS HUDSONICUS) CACHE SITES AND CACHE PATTERN IN PATTEE CANYON, MONTANA
By
Rebecca S. Burton
B. S. The Evergreen State College, 1986
Presented in partial fulfillment of the requirements for the degree of
Master of Arts
University of Montana
1991
Approved by :
Chairman, Board of Examiners
Dean, GraduateSeilool f
/ b^ / / Date
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Burton, Rebecca S., M.A., March 1991 Zoology
Red squirrel (Tamiascuirus hudsonicus) Cache sites and cache pattern in Pattee Canyon, Montana. (46 pp.)
Director: Richard L. Hutto
The spatial pattern and physical attributes of seed caches used by red squirrels (Tamiasciurus hudsonicus) were the focus of this study conducted in Pattee Canyon, near Missoula, Montana. Within the 320 X 290-m study site there were 103 active caches. Approximately 58% of the caches were centered around stumps, 18% were centered around logs, 14% were centered around piles of debris, 7% were centered around trees, and 4% were centered around a combination of a stump and a log. Each of the 4 squirrels on the site used at least 10 caches. In comparison with randomly-selected unused sites, cache sites were associated with soft soil. The mean distance from cache to nest was 26.5 m, significantly less than the distance for unused sites. There was a trend towards caches being near large ponderosa pine {Pinus ponderosa) trees and in areas with high tree and log density. Caches had more vegetative cover around the center of the cache than at 3 or 5 m from the center. For unused sites, cover was not significantly different at different distances from the center. Total cover was not significantly different for cache and unused sites. The mean number of cones eaten per cache was 57.4. The mean minimum home range for squirrels on the site was 3600 sq m. There was no evidence of overlapping cache use or theft of cones. These data were used to distinguish among hypotheses about the determination of cache pattern. Results suggest that the number of cache sites may be limited. This population may be acting to reduce the energy and time spent storing cones. Decreasing the time spent storing food may be important because the primary food source used by this population is non-serotinous cones.
11
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS
Page
ABSTRACT...... Ü
LIST OF TABLES...... iv
LIST OF FIGURES...... v
ACKNOWLEDGEMENTS...... vi
INTRODUCTION...... 1
MATERIALS AND METHODS...... 6
Study Site...... 6
Cache sites...... 9
Winter observation...... 11
Behavioral observation...... 13
Analysis...... 16
RESULTS...... 17
Cache sites...... 17
Winter Activity...... 27
Behavior...... 27
DISCUSSION...... 35
Cache pattern...... 35
Theft deterrence...... 36
Predator deterrence...... 36
Site availability...... 39
Energy constraints...... 39
Time constraints...... 41
APPENDIX A ...... 43
LITERATURE CITED...... 44
iii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES
Page
1 Type of sites in five categories. The subset
group was chosen randomly from the group of
all caches...... 12
2 Minimum home range and number of caches for each of
four squirrels...... 18
3 Site variables for cache and unused sites...... 20
4 Cache pattern reported for populations feeding
on serotinous and non-serotinous cones...... 37
5 Caloric value of selected western conifer cones
(after Smith 1970)...... 40
IV
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES
Page
1 Map showing location of the study site...... 8
2 Number of plants in a 50 X 50-cm frame 1 m from the
center of the cache and unused sites...... 22
3 Number of plants in a 50 X 50-cm frame 3 m from the
center of the cache and unused sites...... 24
4 Number of plants in a 50 X 50-cm frame 5 m from the
center of the cache and unused sites...... 26
5 Squirrel movements. Each point signifies one marked
cache. Movement lines were determined by using track
and observation records...... 2 9
6 Proportion of time spent on each type of activity
in relation to month...... 31
7 Proportion of time spent on each type of activity
in relation to ambient temperature...... 34
V
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENT S
I thank the members of my committee; Dr. R. L. Hutto,
Dr. C. B. Henderson, and Dr. B. W. 0^ Gara for their help in
reviewing this manuscript, and for their invaluable advice
on the design of this study.
Susan Robinson helped with computations, the finding of
marked trees, and the study site map, for which I am most
grateful.
I thank Dave Worthington and Andrew Bosma for all of
their help and advice on computers.
Dr. John R. Burton and Dr, Grace M. Burton gave me a
love for learning and underwrote my undergraduate education
They also provided unfailing love and support throughout
this project, as they have throughout my life.
Part of this study was funded by an American Museum of
Natural History Theodore Roosevelt Memorial Grant,
VI
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION
Animals that cache food have either one central storage
site or several scattered ones. The caching pattern used by
a particular population may be determined by risk of theft,
exposure to predators that feed on the caching animal,
energetic costs of storage and retrieval, site availability,
time availability, type of food stored, or a combination of
these variables (Barry 1976, Stapanian and Smith 1978 and
1984, Kraus 1983, Hurly and Robertson 1987, Elliot 1988,
Waite 1988, Andrusiak and Harestad 1989). The
characteristics of sites used as caches should reflect the
pressures to which a particular population is subject. For
example, such attributes as the location of the cache
relative to food sources, other caches, and to the nest
might be expected to vary depending on what pressures are
most important to the caching animal. I will outline the
predictions related to cache site characteristics for
several hypotheses relating to the determination of cache
pattern.
Large, centralized caches (larder hoards) may be
susceptible to theft because resources are concentrated in a
smaller area around the source and are relatively visible
(Stapanian and Smith 1984, Clarkson, et al. 1986, Waite
1988). If scattering food items reduces theft pressure from
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conspecifics, then one might expect many, scattered storage
sites to be used by animals that are unable to defend their
caches. In contrast, territorial animals would be expected
to use one central cache because there is a no need to
spread the risk among multiple caches. The risk of loss may
even increase as the size of the cache area increases. If
food stores are scattered, then animals might be unable to
detect thieves stealing from distant caches, and fail to
exclude them (Stapanian and Smith 1978 and 1984, Smith and
Reichman 1984).
Cache pattern may also be a response to the risk of
predation on the caching animal itself. Predation risks
increase with distance from the cover provided by a refuge
tree or burrow (Holmes 1984, Lima et al. 1985, Dill and
Houtman 198 9). Therefore, predation pressure might cause an
animal to keep its caches close to its nest in order to
reduce travel distance, and thus lead to the use of a
central cache, or larder hoard. Predation pressure might
also cause the use of sites that have denser vegetation at a
height that would obstruct a predator's view of the prey.
Predatory birds might also be discouraged by vegetation of a
height that would interfere with their ability to maneuver
and attack a prey item.
The energetic costs and benefits of each caching
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pattern have not been quantified, but some general
predictions can be made. Food sources will probably be
distributed throughout an animal's home range. Therefore,
scatter hoarding should decrease the amount of energy needed
to transport food from sources to caches (Clarkson et al.
1986). One would expect the energetic cost of travel to
increase as the size or mass of the food item increases. If
distance from source to cache is an important energetic
consideration, then one would expect to see cache patterns
based on source pattern, with central-place caching in areas
where sources are concentrated, and scatter hoarding in
regions where sources are scattered. Scatter hoarding may
be more pronounced when the cost of carrying stored food
items is high due to the size of the items.
Food items are usually retrieved from caches for
consumption during the winter, a time of low food
availability and harsh climatic conditions. If the cost of
retrieving stored items is more important to the total
energy budget of the squirrelthan the cost of storage, one
might expect to find a single cache placed close to the
nest. If more than one cache is used, then the caches might
be placed close to each other and close to the nest in order
to reduce the amount of travel from the relatively protected
nest (Pauls 1978).
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If cache sites must have certain characteristics, then
animals might be limited by the number of usable sites.
Extremely limited cache site availability could lead to
larder hoarding. Similarly, if cache sites only allow the
storage of a few cones per cache, scatter hoarding will be
necessary (Hurly and Robertson 1990). The pattern of the
caches may simply be a reflection of the distribution of
usable sites.
If time constraints determine spatial caching pattern,
then the time needed to transport and store food should be
minimized by the pattern used. Animals that have a limited
time in which to cache items might be e pected to store food
items close to their source. In addition, cache sites
should allow fast storage of items, and items with a lower
caloric value should not be stored when more valuable items
are available for storage.
Patterns that result from behaviors, such as cache
patterns, are often difficult to interpret because the
pattern observed today may be the result of a behavior that
was genetically fixed under different conditions. An animal
that changes its cache pattern in response to environmental
conditions would, therefore, be ideal for a study of the
determination of cache pattern. One such species is the
American red squirrel (Tamiasciurus hudsonicus), a diurnal
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tree squirrel which is active year-round. During the winter
months it is dependent on food (primarily conifer seeds)
stored during the summer and fall. In several populations
T. hudsonicus larder hoards (Pauls 1978, Rothwell 1979,
Gurnell 1984, Perron et al. 1986, Elliot 1988), However,
red squirrels scatter hoard in other populations (Layne
1954, Hurly and Robertson 1987 and 1990). This behavioral
plasticity maJces the red squirrel an excellent animal for
caching studies. An understanding of the factors that
contribute to cache pattern in red squirrels may help
ecologists to better understand the causes of cache patterns
in other species.
In this study, I examined and mapped T. hudsonicus
caches to determine which cache pattern they used. I then
compared attributes of sites used as caches with attributes
of sites that were not used as caches. Finally, I used
these data to test predictions generated from the
alternative hypotheses listed above about the causes
underlying cache pattern in this population.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. METHODS
Study Site
The study site was a 320 X 290-m plot located near
Crazy Canyon Road in Pattee Canyon (46° 50^ N, 114° 51' W)
in Lolo National Forest, approximately 6 km southeast of
Missoula, Missoula County, Montana (Fig. 1). Douglas-fir
{Pseudotsuga menziesii) and Ponderosa pine {Pinus ponderosa)
were the dominant tree species (Beg 1969). Each of the
trees was marked with a permanent numbered tag, and the
location of each tree was mapped. I marked trees nearest to
caches with blue flagging tape. I recorded the tag number
of this tree and mapped the caches. Caches were defined as
sites where there was evidence of at least five cones having
been stored or eaten in an area 1 m or less in diameter.
Caches were found initially by searching the area
systematically in January of 1989 at the beginning of the
study. I began at the northeast corner of the site and
walked west to the other side. I then walked approximately
10 m south, and walked east to the other side of the site.
I continued in this pattern until I reached the southern
border of the site. Additional caches were found by chance
encounter of caches during subsequent phases of the study.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 1. Map showing location of the study site
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. X M i. St»-hn€l
(icn.c
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cache sites
I Measured cache attributes during September 1990 and
March 1991. For each of 76 cache sites I recorded the
following variables:
1) Distance to large pine — Distance from center of site
to nearest ponderosa pine with a dbh of at least 16 cm.
2) Dbh of nearest large pine — Diameter at breast height
of the nearest ponderosa pine with a dbh of at least 16 cm.
3) Distance to nearest other cache — Distance to nearest
used cache site measured on the ground with a 40-m tape
measure.
4) Distance to nearest nest — Distance to nearest used
nest.
5) Tree biomass — Total dbh of all trees (over 8 cm in
dbh) within 10 m of the center of the site.
6) Tree density — Number of trees over 8 cm dbh within 10
m of the center of the site.
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7) Log density — Number of logs over 8 cm in diameter
within 10 m of the center of the site.
8) Soil softness — Depth to which a weighted metal stake
(a right angle shape measuring 1 cm per side, approximately
150 g> penetrated the soil when dropped from a height of 140
cm at 1, 3 and 5 m from the center of the site. Depths were
taken at the four cardinal compass points and averaged.
9) Canopy cover — Percent cover estimated using a cardboard
tube (4 cm in diameter) divided into quadrants with thread.
Estimates were taken at 0, 1, 3, and 5 m from the center of
the site. Estimates were made at the four cardinal compass
points and averaged for each distance from the center.
10) Vegetation density — Vegetation counts were made for
each of 4 height categories. Height category 1 included
plants from 0 to 25 cm, category 2 contained plants between
25 cm and 4 9 cm, category 3 contained plants between 50 cm
and 74 cm, category 4 plants were 75 cm or more in height.
Grass and moss were excluded from consideration. All of the
plants within a 50 X 5 0-cm frame were counted for each
height category at 1, 3, and 5 m from the center of the
site. Counts were made at the four cardinal compass points
and averaged.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 11
I recorded the same information from a control group of
76 unused sites which were selected using a list of tree
numbers provided by a random number generator. The first
potential cache site that was within a 2-m-wide path due
north of the tree was used. A potential site was defined as
any type of site actually used by squirrels in this area.
For instance, because bare ground was never used as a cache
site, it was not considered a potential cache site. All the
caches found contained a stump, log, tree, debris pile, or
some combination of the above. These categories may differ
from each other in various ways. For example, stumps may
tend to be surrounded by soft soil, as compared to trees.
In order to control for such effects, the proportions of
control sites in each of 5 categories were approximately the
same as the used sites (Table 1). Small differences in
proportions were due to rounding error. As the quota of
each type was filled, that type was ignored when searching
for subsequent potential sites.
Winter observation
Winter observation began during the first week of
January, 1989 and continued until April 1989. Winte.r
observations began at 0830. I recorded cloud cover, wind
speed, and precipitation at the beginning of each
observation trip. I noted the tag number of the tree
nearest to caches to allow mapping of caches and recorded
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Table 1. Number of caches in each of 5 categories. The subset group was chosen randomly from the group of all caches.
Type All caches Subset Unused Number % Number % Number %
Stump 60 58 43 57 43 57
Log 18 18 14 18 14 18
Debris 14 14 11 15 11 15
Tree 7 7 5 7 5 7
Stump/Loa 4 4 3 4 3 4
Total 103 100 76 101 76 100
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feeding activity at the cache. I counted and removed new
cone cores at each cache. I recorded which cache was used
and what cone cores or other food items were present. While
collecting cone cores, I noted tracks and recorded the
identification number of trees near the start and end of
tracks. I also noted the number of trees that squirrels
climbed during observations. I examined trees, using
binoculars, to spot nests. Nests were defined as masses of
dead vegetation in trees that had been damaged by mistletoe
where squirrels perched and called, and where tracks started
or stopped. I found the nests of four resident squirrels on
the site.
I used tracks in the snow to determine individual
movements and cache use. The tree where tracks began, trees
passed, and tree where tracks ended were all noted. This
information was noted on a map of the numbered trees. Home
range was determined by fitting the smallest possible convex
polygon around the track records of each squirrel. The area
of each polygon was thenmeasured. Minimum caches per
squirrel were obtained by counting the number of caches
inside the polygon.
Behavioral observation
Behavioral observations of squirrels were made from
July to September, 1989. The first observation block began
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at 0730. There were four blocks per day, each was SO-min
long. The study site was divided into quadrants, which, in
turn, were divided into 12 sections each. Each day a
randomly-selected section from each quadrant was studied
(Appendix A ) . Order of quadrants was random, but each
section was observed once before any were observed again. I
set up a 5-ft step ladder in the middle of each section and
observed behavior with the unaided eye and ear, and with 7 X
35 Bushnell binoculars. The first individual I saw or heard
was the focal individual used for the entire 30-min
observation block. I used a digital stopwatch to obtain
start and stop times for behaviors and recorded these on a
pre-made data sheet. Behaviors were grouped into six
categories :
1) Feeding— Consuming a food item, or preparing food for
immediate consumption. For example, stripping a cone of
scales and eating the seeds in the cone was considered
feeding activity.
2) Caching— Cutting cones from branches, travelling a
distance of over 1 m while holding food in the mouth,
digging holes for food items, or covering food items with
dirt or debris (Lockner 1968) .
3) Agonism— Threatening, chasing, escaping, fighting.
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drumming, or displacing directed at a conspecific (Wolfe
1966). A threat consisted of sudden movements of the head
or body in the direction of a conspecific. Chasing
consisted of travel in the same direction, and approximately
the speed, as a conspecific. Escaping consisted of being
followed by a conspecific travelling in approximately the
same direction and at approximately the same speed.
Fighting consisted of contact between conspecifics,
excluding mounting or grooming. This behavior sometimes
included biting or scratching.
4) Vocal Agonism— Squeak-Whistle, Squeak, Growl, and
Rolled-R calling (Embry 1970)
5) Grooming— Licking, chewing and scratching of an animal
on its own body surfaces (Ferron 1976, Lockner 1968).
6) Locomotion— Walking or running, and climbing trees,
shrubs, stumps, or other objects (Ferron, 1976).
7) Other— Chee calling (Embry 1970), standing, and
crouching (Lockner 1968, Ferron 1976).
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Analysis
Differences between means of cache and unused sites
were examined using multivariate analysis of variance. The
Hotelling's multivariate test was used to test for
differences between the means of the used and unused groups
using all measured variables. The Roy-Bose method was used
to obtain simultaneous confidence intervals for the
value.
Difference between cone consumption at caches of known
ownership and caches for which the owner was not known was
examined using a T-test.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Results
Caches
Of all cache sites examined/ 57% centered around stumps
and 61% were centered around stumps alone or stumps with
logs (Table 1). Each of the four squirrels in this study
used at least 10 caches (Table 2). Not all used caches
could be identified as belonging to a particular squirrel,
so numbers are probably higher. There were no significant
differences on measured variables between caches used by
different squirrels.
The mean number of ponderosa pine cones eaten at each
cache was 57.4 (n=103 caches, std dev=110.9). Mean rate of
cache use (determined by dividing the total number of cones
eaten at a cache by the number of days over which data were
collected) was 43.0 pine cones per day (n=103 caches, std
dev=75.8). Rate was not associated with other cache
attributes. Caches for which the owner could be determined
were used at an average rate of 72.4 cones per day (n=33,
std dev=102.1), which was significantly greater than the 21
cones per day rate (n=43, std dev=35.3) of the caches for
which ownership could not be determined (P=.00 9). The mean
number of total cones for caches of known owners was 91.8
(std dev=151.6), compared with 31.6 (std dev=55) cones for
caches of unknown owners (P=.036).
17
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Table 2. Minimum home range and number of caches in minimum home range for each of four squirrels.
Minimum Caches within Squirrel Home Range (m^) Min. Home Range
1 1043 11
2 9453 29
3 1899 13
4 2004 10
Mean (std dev) 3600 (3926) 15.8 (8.9)
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The means for all measured variables for the caches and
unused sites considered simultaneously were significantly
different (Hotelling's : F=11.307, p<<.01) . Calculation
of simultaneous confidence intervals showed distance to nest
and soil softness to be significantly different for the
cache and unused sites. Together, these variables explained
approximately forty percent of the variance of the data
(Cannon correlation=.6225) Dbh of nearest pine was not
significantly larger for cache sites than for unused sites,
although there was a trend in that direction (Table 3). Log
and tree density were not significantly different around
cache sites than around unused sites, however there was a
trend toward higher values at cache sites.
Cover was not significantly different for cache and
unused sites. Cache sites had a tendency to have more cover
nearer to the center of the site. There was no difference
between the cover at different distances at unused sites.
Number of plants 1 m from the center of the site were
not different for cache and unused sites (Fig. 2). Plants
greater than 75 cm high were not different in occurrence at
cache versus unused sites. There was more of the vegetation
less than 75 cm in height at cache sites at 3 and 5 m from
the center of caches (Figs. 3 and 4).
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Table 3. Site variables for used and unused sites.
Cache Sites Unused Sites Variable Mean Std Dev Mean Std Dev
Distance to pine (m) 5.7 5.0 4.7 4.0
Dbh nearest pine (cm) 44 . 6 48.5 31.4 10.5
Distance to cache (m) 9.1 6.9 8.7 16.0
Distance to nest (m) 26.5 23.0 49.9 26.5
Tree biomass (m) 607 .1 176.1 --
Tree density 25.6 8.9 20.9 1.2
Log density 18.2 11.1 12.7 8.2
Soil softness (cm) 7 . 9 1.7 6.0 2.0
Cover at center (%) 54 .2 30,0 50.0 27.5
Cover at 1 m (%) 53.0 20.3 49.4 23.9
Cover at 3 m (%) 47.2 14.0 50.1 24 . 8
Cover at 5 m (%) 46.4 13.5 51.4 24 .0
Total cover (%) 60.4 17. 6 50.2 14.0
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Figure 2. Number of plants in a 50 X 50 cm frame 1 m from
the center of the cache and unused sites. Vegetation is
divided into 4 height categories (see methods). There were
no significant diferences at any height (category 1/ p=.964;
cat. 2, p=,083; cat. 3, p=.898; cat. 4, p=.327).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. M E A 15 N N 1 Meter U M B 10 E R O F P L A N T S 0 to 24 cm 25 to 49 cm 50 to 74 cm over 74 cm HEIGHT CATEGORY
Cache Unused
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Figure 3. Number of plants in a 50 X 50 cm frame 3 m from
the center of the cache and unused sites. Vegetation is
divided into 4 height categories (see methods). The means
in height categories 1, 2, and 3 are significantly different
(p=.008, p=.016, p=.008). Means for category 4 are not
significantly different (p=.153).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. M E A 16 N N 3 Meters U M B 10 E R O F I P L A N T S 0 to 24 cm 25 to 49 cm 60 to 74 cm ovor 74 cm HEIGHT CATEGORY
Cache Unused
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Figure 4. Number of plants in a 50 X 50 cm frame 5 m from
the center of the cache and unused sites. Vegetation is
divided into 4 height categories (see methods). Means are
significantly different for height categories 1 and 2
(p=.001/ P=.009). Means are not significantly different for
categories 3 and 4 (p=.746, p=.773).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. M E A N 5 Meters N U M B E R O F ■ P L A N T S 0 to 24 cm 25 to 49 cm 50 to 74 cm over 74 cm HEIGHT CATEGORY
Cache Unused
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Tree density was an extremely good predictor of tree
biomass (R=.8976, P<.0005), so tree biomass was dropped from
the analysis.
Winter activity
The mean minimum winter home range of the four resident
squirrels on this site was approximately 3,600 m^ (Table 2).
Tracks showed no evidence of theft or overlapping use (Fig.
5). An average of approximately five ponderosa pine cones
were consumed per squirrel per day, while an average of less
than 0.5 douglas-fir cones were consumed per day. A total
of 4660 ponderosa pine cones and 399 douglas-fir cones were
consumed on this site between 8 January and 30 March.
Squirrels visited most of their caches regularly during the
winter months, even when no cones were consumed at the site.
Behavior
Agonistic behavior comprised 61% of squirrel activity
time (Fig. 6). Of this time, 14% was non-vocal agonism.
Caching activity accounted for 26% of the time, and feeding
for 6%.
Proportion of time spent on caching increased from less
than one percent in July to more than 20% in September.
Percent of time spent on vocal agonism decreased from
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Figure 5. Squirrel movements. Each point signifies one
marked cache. Movement lines were determined by using track
and observation records.
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Figure 6. Proportion of time spent on each type of activity
in realtion to month.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. % Time Spent 60
60
40
30
20
10
- f c ji Foraging Caching Agoniam Grooming Other Locom. Voo Agon. Time Via Behavior
July August 1 : i September Total
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approximately 71% in July to 44% in August, and dropped to
26% in September. When ambient temperature was below 4°C,
the only activity observed was vocal agonism (Fig. 7).
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Figure 7. Proportion of time spent on each type of activity
in relation to ambient temperature.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. % Time Spent 100
I
Foraging Caching Qooming Other Locomo. Voc. Agon. Agoniam Time Via. Behavior
-1.0 - 3.9 C 4.0 - 9.4 C CZl 9.6 - 14.9 C 16.0 - 20.4 C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DISCUSSION
Cache Pattern
Information gathered from tracks shows that each of the
four squirrels on the site had at least 10 caches. This
demonstrates that the Pattee Canyon red squirrel population
is scatter hoarding and not using one central cache (larder
hoarding). There were 43 other caches for which ownership
could not be determined. Since these caches were being
used, it seems reasonable to assume that resident squirrels
were using these sites, but that their tracks were not
observed. Therefore, each squirrel probably had more caches
than can be definitely attributed to them. The mean home
range area found in this study is consistent with the
findings of other studies (Smith 1968, Kemp and Keith 1970,
Grodzinski 1971, Rusch and Reeder 1978, Gurnell 1984, Brown
and Batzli 1985). Variability in home range size was high
on this site, but the sample size was small. Most of the
caches were centered around stumps. Logs, debris and trees
were also used as cache sites. All of these objects provide
promontories that could potentially be useful for observing
predators and conspecifics. Stumps, logs, and debris also
offer additional hiding places for cones. Number of cones
consumed at individual stumps was highly variable. It is
possible that the number of cones stored at a site depends
of the number of cones available near the cache, but this
35
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36
was not directly measured in this study. Contrary to the
pattern of larder hoarding predicted, each of the squirrels
on the study site had multiple caches. This pattern has
been described in two additional populations of T.
hudsonicus but differs from the larder hording pattern found
in other populations (Table 4). I now consider each of five
hypotheses about why these squirrels use dispersed caches.
Theft Deterrence
If theft deterrence were the major factor in
determining cache pattern, then one would expect to find
larder hoarding in populations of territorial animals and
scatter hoarding in non-territorial populations (Stapanian
and Smith 1984). In this population, behaviors related to
territoriality, including vocalization such as "Rolled R",
"Squeak-whistle" and "Ghee" calls (Embry 1970), accounted
for a large proportion of squirrel activity time. There was
no evidence of overlap in cache use or theft of cache items,
this is consistent with the findings of others that red
squirrels are effectively territorial (Kemp and Keith 1970,
Kantor 1981, Gurnell 1984).
Predator Deterrence
The predator deterrence hypothesis was not supported by
my findings. If predation pressure determines caching
pattern, then food caches should be concentrated near nests
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Table 4. Cache pattern reported for populations feeding on serotinous and non-serotinous cones.
Study Tree species* Serotiny Cache pattern
Layne 1954 resinosa, No Scattered sylvestris, No and others
Smith 1968 contorta, Yes Central monticola, No ponderosa, No and others
Rusch and banksiana Yes Central Reeder 197 8
Rothwell 197 9 contorta Yes Central with satellites
Gurnell 1984 contorta Yes Central
Hurly and sylvestris, No Scattered Robertson 1987 resinosa No
Hurly and sylvestris No Scattered Robertson 1990
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and should have a greater amount of cover than randomly-
selected sites so that an animal could travel to and from
the cache with less chance of being detected by predators.
Caches were significantly nearer to nests than were unused
sites, suggesting some benefit for keeping food near the
nest. However, the mean distance from nest to cache was
approximately 27 m. This suggests that other pressures are
operating to keep caches from being concentrated near the
nest. Squirrels may have used caches as réfugia, but used
sites were not significantly closer to other caches than
were unused sites.
Cover was not significantly greater for used sites than
for unused ones. Used sites did show a trend toward greater
cover near the center than for areas farther out, but had
the same amount of total cover as unused sites. Cover at
unused sites was not significantly different from cover at
caches at any distance from the nest. Squirrels spend the
majority of their feeding time at the center of the cache,
where they would probably be visible to a predator flying
overhead, so concentrating cover near the center of the site
could be beneficial. The increase in short vegetation
further out from the center of cache sites might make the
detection of ground predators more difficult.
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Site Availability
The large number of caches found on this site suggests
that there were many available sites. This site had been
logged in the past and many stumps, logs, and brush piles
were available. However, differences between cache sites
and unused sites were significant. This indicates that many
sites may not be suitable for cone caching. Thus, the
pattern observed may be due in part to the pattern of
available caches. The need to place cones in suitable sites
to delay cone opening may determine what attributes are
necessary for a cone cache.
Energetic Constraints
Ponderosa pine cones are large and may be energetically
expensive to carry. This may cause animals feeding on this
type of cone to store them closer to source trees. Not only
were cones stored near large pines, they were also stored in
areas with greater tree densities. There is also support
for the prediction that valuable items will be stored
preferentially. Although Douglas-fir trees are much more
abundant than ponderosa pine trees on this site, Douglas-fir
cones were rarely cached. Douglas-fir cones are much less
valuable than ponderosa pine cones (Table 5).
If cost of retrieval were the major factor responsible
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Table 5. Calorie value of selected western conifer cones {after Smith 1970).
Mean Maximum potential
Ponderosa pine 161 24,700
Douglas-fir 57 918
Lodgepole pine 16 2, 560
Western white pine 89 13,400
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for determining cache pattern, then I should have found
caches close to nest trees and close to each other. Caches
were scattered throughout the home ranges of animals, not
clumped. Caches were closer to the nest than were random
sites, but were still fairly far away.
Both retrieval costs and storage costs may have been
reduced by placing caches in areas with a high concentration
of logs, which are used as travel ways by the squirrels.
Log concentrations for cache and unused sites were not
significantly different when examined by multivariate
methods.
Time Constraints
If cache pattern were determined by time constraints
during the seed storage period, such as the need to harvest
and store cones before a conspecific could harvest them,
then one would expect caches to be placed near the source of
the food. In this study, cone crop was not measured
directly. There was a trend toward caches being near large
ponderosa pine trees. Food of lower caloric content was not
cached when higher-quality food was available, as was
demonstrated by the low rate of caching for Douglas-fir
cones. This allows more efficient use of the time available
for seed storage.
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Soil was significantly softer at used sites. This is
probably not a result of use since amount or frequency of
cache use had no correlation with soil softness. Soft soil
could decrease the time needed to bury an item.
One factor that might explain why the pattern observed
in this group of squirrels differs from the larder hoarding
pattern observed in other red squirrels is that time
constraints change with the type of food available.
Populations feeding on non-serotinous cones are probably
time-limited during the caching season. Ponderosa pine
cones are only harvested until they open, then the seeds
fall out easily and the cones are no longer valuable. A
comparison of the available literature on caching by red
squirrels shows that populations feeding on serotinous cones
tend to be larder hoarders while populations feeding on non-
serotinous cones tend to scatter hoard (Table 4). The
hypothesis that serotiny drives this trend requires further
study.
The data obtained in this study suggest that habitat
features such as the species of tree present and the-
presence and distribution of available cache sites are
important in determining cache pattern. Thus, populations
may vary in their cache pattern due to habitat differences.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX A
Four observations were made per day. Each quadrant was
sampled once per day in a random order. A schedule of
observations was made by creating a set of random 4 X 4
Latin squares (a table in which each letter appears only
once in each row or column) . A letter was drawn randomly
once for each line. No letter could appear twice in the
same row or column in a given square. The Latin squares
allowed me to sample each quadrant an equal number of times
in each order, while making the order itself random. This
process was repeated fourteen times. For each letter, I
then drew a number from one to twelve. Thus I sampled each
section the same number of times, and each section was
sampled once in each of the four daily time blocks.
43
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