INVESTIGATION OF WISCONSIN SQUIRRELS: USE OF GUD
TO ASSESS BEHAVIORAL CHANGES
CHLOË GULBRONSON, CORINN FRITZ, RHIANNON CORZOLLA, AND COLLIN NUTTING
University of Wisconsin – Stevens Point
INTRODUCTION
The risk-taking behavior of a species is related to our
ability to predict the survival of said species. The
optimal foraging theory states that when an
individual searches for food in its environment, it will
choose the foraging method that has the highest
energy benefit, for the lowest energy cost (Kie,
1999). The most effective foraging mechanism is
passed on through natural selection, driven by social
learning from offspring to parent. This affects an
individual's overall fitness, environmental adaptations
of a species, and, in turn, the individual’s ability to
reproduce. The risk-taking behavior of a species in an
environment can be either beneficial or detrimental to
said species depending on the environment.
With the consideration of the optimal foraging
theory and the survival of a given species, we can
begin to understand the exchange between
populations and behavioral adaptations based upon
ecological effects. The optimal foraging theory is
necessary for survival and, thus, the reproduction of
the species. This is based upon the attainment of food
in each environment. Obtaining food provides an
individual with energy but searching and obtaining
the food requires energy. This exchange is influenced
by risk versus gain ratios due to predatory size, the
quantity of food available, and energy costs (Bowers,
Jefferson, & Kuebler, 1993). In order to quantify the
foraging behavior of a species, the giving up density
is measured. The giving up density is the amount of
food leftover in a designated area. Differing
environments and regions within an environment on
an individual influence the giving up density in a
species.
In this study, we focus on the effects that forest
and college environments have on the foraging
behavior of different species in the family Sciuridae
based upon the measurements of giving-up densities
in these environments. Individuals that were more
willing to approach a tray near a perceived threat (i.e.
humans), displayed risk-taking behavior. Squirrels in
the environments in which the trays had lower giving
up densities were riskier. Additionally, we focused on
the effects that the amount of cover had on the overall
risk-taking behaviors of these individuals. Squirrels
that were more willing to approach a tray that wasn’t
undercover and left them exposed to predation were
considered to display risk-taking behavior. We
focused on measuring the risk-taking behaviors on
the different environments and the presence of cover.
We tested the null hypothesis that environment
(habitat) and location (region) have no effect on the
giving up density of squirrels. Deviations from this
null will be used to make inferences concerning
human and predator impacts on squirrel behavior and
their populations.
METHODS
​To collect data, research was conducted at three
campus locations at UW-Stevens Points and three
off-campus locations at Schmeeckle, a nature reserve
in Stevens Point WI. The on-campus locations where
in heavy used foot traffic areas, near sidewalks and
academic/campus buildings. Off-campus locations
were in wooded areas off the trail and away from
most human contact. In data collection site were
labeled as Campus 1-3 and Schmeeckle 1-3. In order
to collect sample data, trays were set out for eight
days total at each site. For each set of data collected,
two trays were set out, with one tray at the base of a
tree, and the other tray 10 feet away from a tree. The
tree offers an easy escape route for a squirrel, which
is not available in the tray further from a tree. Each
tray contained about 10 grams of shelled sunflower
seeds and 3 liters of sifted sand. Trays stayed out for
about 8 hours, typically being put out around 9:00 am
and picked up around 5:00 pm. After collecting trays,
the sand is sifted, and any remaining shelled
sunflower seeds measured. Using the mass of
sunflower seeds before and after, a giving up density,
or GUD, can be calculated to show the mass of
sunflower seeds eaten during the day. These values
will help us to determine if squirrels prefer
foraging near easy escape areas or are willing to
travel further from a tree to find food.
RESULTS
​Our data base included a total of 6 sites over which
GUDs were measured for 240 8 hour trials. Most of
the trials provided information that grey squirrels
were present in the environment tested with some
level of certainty. Within the 240 trials, 50% (120) of
these were in a campus environment and 50% (120)
of these were in a forest environment. In some cases
were all 10 g removed from the tray in the 8 hour
period.
There are a number of differences between the
GUD among the different environments. The most
noticeable difference was between the mean GUD in
the near and far tray on the campus environment (Fig.
1). The near tray have a mean GUD of 6.86 grams
while the far tray had a mean GUD of 7.99 grams.
ANOVA showed that trays on the campus near cover
oftentimes had squirrels remove more sunflower
seeds than the trays far away from cover
(P=0.029029). As the tray moved closer to cover, the
GUD decreased in the campus trays. In contrast, the
trays in Schmeeckle showed no significant difference
based upon their relative cover (P=0.276513). The
data for the near and far trays at the Schmeeckle sites
were very similar, with their means being 6.12 for far
and 6.47 for near.
The difference in region of the trays also provided
significant differences. When comparing the GUD
between Schmeeckle and campus, GUD in
Schmeeckle is much lower (Fig. 2). This difference is
significant,
based on our t-test analysis
(P=0.00474289). This data shows a significant
difference between the two habitats (deciduous forest
and college campus).
An additional variable we analyzed was the
variation of season over the duration of the tests.
There is a notable difference in the spring and early
fall compared to mid fall (Fig. 3). Plotting this data
on a box-and-whisker plot provided a visual
representation that displays the mean and range
among the three seasons. The difference between
spring and early fall in comparison to mid fall
showed to be significant by ANOVA analysis
(p=0.0000821). As the seasons changed into colder
seasons, the GUD decreased in all trays.
DISCUSSION
Our data reveals several factors affecting squirrel
foraging behavior. Based on our environmental study,
we see a trend that squirrels are more likely to forage
in a deciduous forest environment when compared to
a college campus environment. This is in contrast
with Bowers and Breland (1996), who stated squirrels
in high-density urban areas were more likely to
produce lower GUD than compared to their forest
controls. Our results reject our hypothesis. There are
several explanations that may bring these results. In
the study done by Brown and Breland, their forest
environment was in Sky Meadows State Park, which
does not frequent many visitors daily. Squirrels in
this environment are rarely exposed to human
presence and the implementation of this study may
have brought about the addition of more human
presence. This additional human presence may have
brought the association of an increased risk, leading
to a smaller GUD. With a perceived increased risk in
predation, the squirrels could have avoided the trays
intentionally. Contrastingly, squirrels in an urban
environment may be more accustomed to human
presence and would be more likely to forage the tray,
resulting in a lower GUD. An additional explanation
is the season that the experiment was conducted.
Their trials took place in the summer, beginning in
July and ending in August. During these times, the
vegetation in the forest is in season, which provides
many additional food sources for the squirrels in their
forest control areas. In urban environments, this
additional vegetation may not have been available,
leading the GUD to be lower in comparison to that of
the forest environment.
The data involving the tray distance and
environment reveals a pattern as well. The difference
in the GUD between the near and far trays in the
Schmeeckle environment showed no significant
difference; however, the difference in GUD between
the near and far trays in the campus environment
showed a significant difference. This could be due to
the amount of cover provided in each environment. In
a deciduous forest environment, there are trees
providing cover for most of the forest floor. Although
the far tray was under direct cover of the tree of
which the trays were placed, the tray may have had
additional cover from other trees. Because of this,
there is no difference in the perceived safety or
possible escape routes of the near and far trays based
on cover. In the campus environment, the amount of
trees are limited. The cover provided by the tree
provided safety, as it was a quick escape route from
predators. This led to the lower GUD in the near tray
in the campus. Our results contrast the results of
Nowak, Riahi, and Ward (2018). Their mean GUD
between distances of trays proved to be insignificant
in both a forest and urban environment. This could be
due to the presence of climbing structures in their
urban environment. The presence of more climbing
structures for the squirrels in the urban environment
would lead to the perceived safety despite the
distance of the placed trays. This would cause all
trays to have relatively the same GUD, making the
differences insignificant.
The data involving seasons reveals a pattern
based upon temperature. The difference in GUD was
not significant in the spring or early fall; however, the
difference in GUD in mid fall proved to be
significant. During mid fall, the temperature begins to
decrease. The winter that follows bring a scarcity of
food, causing squirrels to forage for more food. As
this time was approaching, the squirrels sought out
additional food in preparation. This sudden drop in
temperature altered the squirrel’s foraging behavior
to begin foraging more food. Young (2013) shows an
example of this changed behavior. Young showed
that when given a choice between walnuts, peanuts,
acorns, and sunflower seeds in the winter months,
squirrels are more likely to forage walnuts, due to
their high caloric input. When these results were
compared to those of the autumn months, the
percentage of walnuts foraged was significantly
higher in the winter compared to the autumn. This
shows the importance of the consideration of scarcity
of food and how that factor takes a role in the
selection of food versus risk associated. A possible
result of inaccuracies that may occur is the unfamiliar
sunflower seeds, sand, and trays. Measured in this
experiment is the behavioral responses to novel
situations rather than to a common situation. As the
GUD in the forest environment was significantly
lower, it shows that the sunflower seeds seemed to
be quite attractive to the squirrels in this
environment. The fact the GUD did not decrease to as
low of a level in the campus environment does not
necessarily show that the sunflower seeds were less
preferred but rather that more profitable food items
were most likely available in other nearby areas. This
is due to human interaction with this environment,
either with disposed or dropped food products.
Figure 1. ​Comparison of the mean GUD of sunflower seeds in
differing environments and regions. Significant differences were
indicated by different letters. GUD at near and far distances for the
environment in Schmeeckle are not significantly different from one
another.
Figure 2.​ Box-and-whisker plot analysis of GUD in regions of
Schmeeckle and campus. Note the significantly lower GUD in
Schmeeckle.
Nowak, T., Riahi, S., & Ward, M. (2018). Shyness
and boldness in squirrels: risk-taking while
foraging depends on habitat type. ​Eukaryon,
14, 185-186.
Young, Aimee. (2013). Foraging Behavior of Eastern
Gray Squirrels on the University of Maine
Campus. ​Honors College.​ 136.
Figure 3.​ Box-and-whisker analysis of GUD dependent on seasons.
Significant differences were indicated by different letters. GUD during
the spring and early fall were not significantly different from one
another.
Table 1.​ The analysis of variance for the study done comparing spring,
early fall, and midfall.
REFERENCES
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Gray Squirrels on an Urban-Rural Gradient:
Use of the GUD to Assess Anthropogenic
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Bowers, M. A., Jefferson, J., & Kuebler, M. (1993).
Variation in Giving-Up Densities of Foraging
Chipmunks (Tamias striatus) and Squirrels
(Sciurus carolinensis). ​Oikos,​ 66(2), 229-236.
doi:10.2307/3544809
Kie, John G. (1999). Optimal Foraging and Risk of
Predation: Effects on Behavior and Social
Structure in Ungulates. ​Journal of
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