Testing Food Preference and Caching Behavior in the Eastern Grey Squirrel

Sarah Heissenberger

Animal Behavior

November 26th, 2016

Abstract

Diet and foraging patterns are an integral aspect of animal survival, and consequently

many animals spend a large portion of the day securing sustenance. In this study, I explored the

foraging and caching behavior of the Eastern grey squirrel, Sciurus carolinensis, as a function of

food type and temperature. Wild subjects were presented with two equally available food

sources, either mixed seed or high-glucose breakfast cereal, and later, mixed acorns. Data was

gathered on the number of individuals that attended either feeder, the date, time, and

temperature, and whether the food was consumed immediately or taken away to be cached in the

area. It was hypothesized that the squirrels would prefer the mixed seed, and that the seed would

be cached more frequently than the cereal. Preliminary observations revealed that the animals

showed almost no interest in the cereal. After adjusting for these findings, another experiment

was proposed that would utilize acorns, a common food source, in place of the cereal. It was

hypothesized that acorns would be cached more frequently than the seed, and that the overall

trend would be towards more frequent caching behavior as temperatures decreased. The results

indicated that the Eastern grey squirrel clearly prefers mixed seed, cached the acorns more often

than the seed (albeit at an exceedingly low rate - only twice), and temperature did not affect the

choice to consume vs. cache food items. It appears that dietary genetic predispositions supported

by evolutionary processes interacting with immediate environmental pressures, such as

competition, drive food preference and caching behavior.

Introduction

All organisms on Earth derive some form of energy from their environment. Energy is

needed to fuel the metabolic and cellular processes that underlie the growth and actions of living

things. The physiology and evolutionary history of an animal largely governs what is consumed

by an animal, and how much is required. Larger animals need to eat much more often, as well as

those with a high metabolic rate, such as birds. A combination of extrinsic (environmental)

factors and intrinsic factors (such as size, reproductive status, evolutionary context) affects food

preference in a species. The variety present in a species’ diet is the cumulative effect of these

interacting factors, as Fleming (1986) describes. Generally, we can separate the feeding

flexibility of species as narrow, or specialist, and as broad, or generalized. Generalized species

tend to flourish in a wide variety of environments, as they make use of a number of different

resources, whereas a specialist species utilizes specific food sources and survives in

environments where particular conditions are met. A more flexible diet as opposed to

consumption of only a single food type can safeguard against starvation in the case that certain

food sources become scarce. However, species that occupy specific ecological niches are better

able to compete with other species when conditions are stable.

Native to central and eastern North America, the versatile Eastern grey squirrel is

currently found in a variety of environments, such as deciduous and coniferous woodland,

suburban neighborhoods, and even urban settings (Goyal, n.d.). Its wide range attests to its

adaptability and ingenuity. Grey squirrels now threaten the existence of the native European red

squirrel overseas and in other regions, and frequently compete with bird species over food

resources; they are known to eat the eggs and nestlings of native bird species as well.

Consequently, the grey squirrel has rapidly found itself placed on the Invasive Species Specialist

Group’s (ISSG) list of the Top 100 Invasive Species in the world (Invasive Species Council of

British Columbia, n.d.). The indiscretion that the grey squirrel demonstrates towards its choice in

food as well as its environmental flexibility may largely help to explain the grey squirrel’s

invasive status in certain regions of the world. The grey squirrel is known to consume a wide

variety of foods, such as fruits, nuts and seeds, plant vegetation and fungi, tree bark, small

insects, reptiles and amphibians, bird eggs, and practically any food left behind by humans. They

are omnivores and will consume discarded cheese, fast food burgers, sandwich meats, and cat or

dog food. They are known to get into trash bins and their no-food-off-limits attitude has earned

them the title of food “opportunists.” The breadth of this organism’s diet categorizes them as

generalists. However, due to constraints in food availability throughout the year, the grey squirrel

adjusts its diet accordingly. Gurnell (1987) finds that grey squirrels consume mainly nuts and

seeds in the autumn and winter months.

Research has shown that grey squirrels primarily cache food items in the autumn season

when food is more abundant. In the winter months, they tend to seek out their caches and live

from those, or otherwise consume food immediately upon attainment. Caching is an interesting

and well-studied behavior in a number of rodent species. Grey squirrels tend to cache their

findings using a strategy known as scatterhoarding. In this approach, the grey squirrel will place

each individual item into a shallow hole in the ground located in the vicinity of where the item

was found (Young, 2013). Furthermore, it has been elucidated that squirrel foraging activity is

inversely related to temperature; foraging activity increases as temperature decreases (Skibiel et

al., 2002).

The purpose of this study was to examine the grey squirrel’s foraging behavior in order to

gain an enhanced understanding of this organism’s high adaptability and thus its invasive status.

Given the varied diet of the grey squirrel, it remains to be elucidated whether the grey squirrel

does, in fact, prefer its natural food source (nuts and seeds high in nutritious fats) over one that is

produced and consumed by humans (such as a breakfast cereal that is high in sugar and may

possess an appealing, sweet taste). The optimal foraging theory discussed by Young (2013)

postulates that animals will consistently prefer food items that confer to them the best energy and

nutritional content, subtracting the energy it requires to secure the food item. Optimal foraging

theory assumes that certain factors in the environment are known to the animal, and thus predicts

the animal’s behavior under this assumption (McNamara & Houston, 1985). The importance of

foraging behavior to not just the organism but the perseverance of the species in accord with the

optimal foraging theory is discussed by Pyke (1984), who asserts that an individual’s fitness is

dependent upon its foraging behavior. Furthermore, foraging behavior has heritable links; an

organism who successfully passes on their genes to progeny is more likely than not to have

progeny who forage in a similar way. If these foraging behaviors act to increase the organism’s

fitness, they will increase in frequency in the population. Optimal foraging theory makes yet

another assumption, however: foraging behaviors in animals evolve ahead of changes in the

environment. This being said, it begs the question as to whether squirrels choose to cache food

items that will keep longer and that offer optimal nutrition in a time of scarce resources in

contrast to foods that offer immediate energy and minimal preparation to consume (no removal

of shells). The answer to this may help to shed some light on this hardy animal’s durability and

survival success in harsh conditions, such as the winter months.

I hypothesized that as winter approached and temperatures decreased, grey squirrels

would choose to cache more food items rather than to consume them immediately upon

attainment. The grey squirrels would prefer a natural-based food source that offers a variety of

nuts and seeds to an anthropogenic-based food source, as well as over another food source that is

readily available in the environment.

I also postulated that grey squirrels would choose to cache a natural-based food source

high in fats more often than an anthropogenic-based food source high in starches and sugar. Grey

squirrels would choose to cache the acorns, a natural-based food source high in fats, more often

than a novel food source rich in variety and high in demand by other animals.

Methods

Study Species

The Eastern Grey squirrel, Sciurus carolinensis, is a rodent abundant in Midwestern

North America. It is approximately 25 to 30 cm long and easily recognized by its thick, bushy

tail, which measures 22 to 25 cm long. The grey color of the aptly named grey squirrel is a

combination of white and black hairs, with high white patches on the belly, chest, face, and ears;

patches of golden cinnamon or a rusted red hue may also be apparent (Goyal, n.d.). The grey

squirrel is an excellent field study species as it is abundant, readily identified, and does not

hibernate through the winter months. Personal observation has shown that grey squirrels are

relatively unfazed by humans, often coexisting in close vicinity with them such as in cities.

Furthermore, grey squirrels will readily come to feeders located in close proximity to human

homes (as close as 5 m has been evidenced) and will visit despite the presence of other wildlife

already at the feeder.

Study Site

The study was conducted in a patch of deciduous woodland located in this researcher’s

backyard. Cultivated grass stops about 20 m out from the house; at this point a thick ground

cover of fern begins. Pine, spruce, and oak trees are loosely interspersed about this region, each

about 10-15 m from the other; a large woodpile measuring about 5 m in diameter is located in the

right-hand region of the study site. Approximately 25 m back from the fern region a stone wall

marks the beginning of thick forest.

Procedures

Two squirrel-friendly feeders made of red cedar and measuring 19.5 x 17.8 x 22.9 cm

were mounted on trees of approximately the same diameter 25 m out from the house in a readily

visible, open region. Both feeders were located equidistant from other nearby trees in order to

control for the easy access squirrels may achieve by utilizing trees for travel. Feeders were

erected approximately 2.5 m off the ground and 15 m apart and were identical apart from the

food they offered. One was loaded with common store-bought birdseed, and the other was filled

with a starch cereal high in glucose (Honey Smacks cereal). At the end of each day, both feeders

were refilled to allow for optimal choice at all times between the two food sources. The feeders

were filled in such a way for five days before observations began to allow squirrels to locate the

food source, similar to the study design of Skibiel et al. (2002). During this time, preliminary

observations revealed a complete lack of interest in in the cereal. To adjust for this, and further

investigate food preference, the cereal was swapped out with a variety of acorns, the primary

food source of the Eastern grey squirrel. Therefore, the focus of the study centered on food

preference between a novel food source rich in a variety of nuts and seeds, and the natural food

source of the Eastern grey squirrel that is also readily found in the immediate environment.

The two food sources in the feeders were also switched after this preliminary period, so

that the feeder holding the cereal then held the seed, and the feeder holding the seed then offered

the acorns. This was done in order to control for possible bias that may have existed due to the

two different trees the feeders were mounted upon. Animals were given another five days to

acclimate to these changes and locate the new food sources.

Data collection began November 1 and ended on November 22. Observations were

conducted between the morning hours of 7:00 to 8:30 am on Tuesdays, Thursdays, and Fridays.

Late afternoon collection between the hours of 2:30 to 4:00 pm was planned as well, but high

demand of the food by a number of different species resulted in the seed running out in a matter

of hours. These time periods (particularly the morning hours) have been demonstrated by Skibiel

et al. (2002) to be periods of high foraging activity for the grey squirrel.

Data on each individual included the date and time, the current temperature, which feeder

was visited, and whether the food was taken to be cached, which was observed when the squirrel

elected to exit the immediate region with the food item uneaten in its mouth. If the food was

consumed upon attainment, the squirrel was seen sitting at the feeder and eating or may have

chosen to eat the food nearby on the ground or in a neighboring tree (Young, 2013). Only

squirrels sitting directly at the feeders were included as data points; those foraging for scattered

seed on the ground were not included.

Statistical Analysis

Squirrel preference between the two food types was exceeding clear, and can be

optimally displayed using a visual representation in the form of a frequency diagram. The

caching rates between the two food choices was also very straight-forward, and is depicted

through the use of pie charts. As caching occurred only twice, at temperatures almost identical to

which the same food type was also promptly consumed, these results did not require any rigorous

statistical analysis.

Results

It became quickly evident that the Eastern grey squirrel prefers feeder seed over

sweetened, corn-based cereal or a mixed variety of acorns. The animals showed almost a

complete lack of interest in the cereal. A few squirrels were observed on the tree on which the

cereal feeder was mounted during the first few days of the observational period. After this time,

the feeder was almost exclusively ignored, while the feeder loaded with seed attracted more and

more squirrels as the days continued on (a max of ten at once was observed). A number of other

animal species visited this feeder as well, and also demonstrated that they favored this food

source over the breakfast cereal. Once the cereal was replaced with a variety of acorns, and the

feeders were switched with their respective foods, the animals swiftly adjusted to these changes

and continued to visit the seed feeder with just as much fervor as before, confirming that no bias

existed in feeder location. While the squirrels showed more interest in the acorns than they did in

the cereal, the acorn feeder only saw traffic when the seed feeder was extremely busy with a

large number of animals competing with one another for access to the seed. Curiously, if one

squirrel showed interest in the acorn feeder, usually another one or two would follow, also

resulting in some mild squabbling between the animals. This observation merits further

investigation.

The increasing numbers of Eastern grey squirrels as well as a variety of other species led

to some very interesting intra- and inter-species interactions. Dogged chasing behavior across the

ground and along the tree upon which the feeder was mounted was often witnessed, but usually

would not persist further than about fifteen meters away from the food source. Physical

alterations sometimes resulted, with one squirrel lunging at another. These behaviors were

exhibited particularly by larger individuals and increased in frequency as the number of animals

in the vicinity increased. The size of the feeder allowed only one squirrel to perch on the feeder

at a time. This meant all other animals had to forage on the ground below for scattered seeds.

This prime position resulted in a series of scuffles and targeted chasing behaviors to drive off

impending competitors. A single squirrel could sit and hold this position, eating continuously, for

up to an hour. The Eastern grey squirrel utilizes its large, bushy tail very expressively, twitching

and waving it about to demonstrate intimidation, feelings of threat, and as a warning to other

animals that come too close for comfort. They are often observed foraging in groups, as seen in

this study. It was unusual for one solitary squirrel to be evidenced at the feeder; if so, it was at an

early hour, and at least one other squirrel would join shortly. Commonly at least two or three

squirrels could be seen foraging on the ground together.

The Eastern grey squirrels did not cache the feeder seed at all (Fig. 3). Instead, they

elected to consume it immediately, and would continue to do so for as long as the food was

available. Of the few individuals seen visiting the acorn feeder, two individuals chose to cache

the acorns in the ground of the surrounding area (Fig. 2). As caching behavior was only

demonstrated with the acorns, and only twice, it seems quite clear that temperature did not

influence the choice to cache vs. consume in this study (Table 1). Other more immediate and/or

inherent factors appeared to direct this behavior.

These results suggest that the first set of hypotheses were incorrect. Temperature did not

affect the frequency of caching behavior. This being said, the adjusted hypothesis following the

switch to acorns as the second food source was correct, as I predicted that the acorns would be

cached more frequently than the mixed seed. Furthermore, the prediction that the mixed seed

would be preferred over both a high-glucose cereal and mixed acorns received overwhelming

support (Fig. 1).

Discussion

The almost absolute absence of caching behavior was unexpected and puzzling, but

previous studies exist that may help to explain this. Hadj-Chikh et al. (1996) investigated the

choice to cache by Eastern grey squirrels found that individuals regularly ate acorns of high

perishability immediately and chose to cache acorns of low perishability, which would keep

longer. Handling time due to acorn size had no effect on this decision, suggesting that the

perishability of a food item is of primary concern when an individual elects to cache vs.

consume. Perhaps the small seed size and nature of the seeds in the mixed seed food source

(which included kernels of corn, oil sunflower, whole peanuts, and striped sunflower seeds)

conferred to them a high perishability, and may help to explain why the squirrels consistently

consumed the seed straightaway as opposed to caching it. In addition, Smith & Follmer (1972)

found that the thicker the shell size of tree nuts as compared to the kernel, the longer it took a

squirrel to consume a nut and thus receive the according nutritional payoff. The preference

demonstrated toward a variety of different nuts was positively correlated with the nuts’

digestibility. The thin, fine layers of the sunflower seeds and readily available corn kernels of the

mixed seed may therefore have been an energetically more beneficial food choice as stipulated

by the optimal foraging theory.

Additionally, as an increasing number of animals attended to the seed feeder, aggressive

territorial behavior and competition ensued between grey squirrels and also between a variety of

species. Although the Eastern grey squirrel is not considered a territorial species, they will bite

and doggedly pursue other squirrels who are foraging in their area (Melina, 2010). They are

known to establish dominance hierarchies in groups, and behaviors observed in this study

support this finding. This social structure would be an interesting area of further investigation.

Leaver et al. (2006) found that the Eastern grey squirrel adjusts its caching behavior in

the presence of other grey squirrels. An individual seeking to cache a food item will turn its back

toward the onlooker, and will space its caches further apart. However, this finding did not apply

in the presence of other animals, at least not the presence of on-looking corvids. The large

number of Eastern grey squirrels in the immediate area, as well as the occasional American red

squirrel (Tamiasciurus hudsonicus), may thus indicate why the squirrels almost never chose to

cache food items – they may have been wary of pilferage. Furthermore, the choice to cache also

means leaving the feeder and giving up a coveted position. As noted previously, feeder size

meant only one squirrel could perch on the feeder at a time, resulting in aggressive displays and

fights between squirrels vying for this position. Once the squirrel has left the feeder unattended,

another individual would take up this position readily. Energetically speaking, it is wiser for the

squirrel in this position to stay and continually consume the feed.

The diet choice of organisms has a strong genetic component, indicative of an

evolutionary history affected by the types and abundance of foods in past environments of

ancestral species. These inlaid predispositions interact with environmental factors that are

immediately present in the current organism’s surroundings. Studies in Drosophilia tripunctata,

a popular model organism for genetic investigations, found that the genotypes of individuals had

the most powerful influence on food choice, although previous experience and sex had an effect

as well (Jaenike, 1985). However, some species demonstrate greater variety and flexibility in diet

choice; rats, for example, demonstrate marked differences in food preference and diet between

individuals (Cziko, 2000). Moller (1983) studied the diet of the Eastern grey squirrel extensively,

and found that while the grey squirrel lives in hardwood forests and mainly consumes the nuts of

such trees, their diet varies with the seasons, as Gurnell (1987) also found. The grey squirrel eats

buds, young shoots, and flowers throughout the spring, as well as rough plant and animal foods

in summer. Moller notes that both the Eastern grey squirrel and the red squirrel ultimately prefer

seed, and will consume it as long as it is available, only then turning to plant matter and other

food sources. This finding aligns with the results of this study, and also supports the observation

that the feeder filled with acorns was visited when the seed feeder was inaccessible due to the

density of foraging animals. Squirrels were also evidenced at the acorn feeder during times when

the seed feeder had run completely empty.

The lack of interest in the cereal raises some questions. Surely the novel and unusual

nature of the food source contributed to its unpopularity, although it had been corn-based, and

the mixed seed contained kernels of corn. Perhaps the high sugar content was unappealing to the

squirrels, as well as to a number of other visiting species. While evolution has played a key role

in the “sweet tooth” of humans, it seems the diet of grey squirrels favors high-fat food items,

such as acorns and other nuts. Evolution and survival demands may not have supported a sugar

preference in the Eastern grey squirrel, and perhaps these animals even lack the appropriate taste

receptors, although this is a question of physiology and evolutionary history that would require

further research and investigation.

The indifference demonstrated by the animals towards the cereal feeder was unexpected

in its totality, as grey squirrels are commonly observed in urban and suburban areas consuming

anthropogenic food sources. This has been personally witnessed and is well-established in the

literature. But it is important to note the difference in environment between squirrels in such

areas and the study site in this investigation, which was in a rural neighborhood. While the

squirrels in this area are likely used to the presence and activities of humans, human-based food

sources are not usually available to them, and consequentially do not make up a large part of

their diet. The alien smell, sight, feel, texture, and taste of the cereal in of itself may have thus

contributed to its unpopularity among this particular population of grey squirrels. This

phenomenon, known as neophobia, is evidenced by a number of intelligent animals, including

corvids. In animal behavior, it refers to the tendency of an animal to avoid or retract from a novel

object, organism, or situation. Such behavior clearly has benefits in terms of self-preservation,

which likely led to its evolution and enduring presence in a number of different species.

Armelagos (2014) discusses what he refers to as the “omnivore’s dilemma”: as omnivores seek

new food items in order to satisfy their diverse diets, they are posed with a problem when they

fear new food items, which pose the risk of poisoning or sickness, and possibly death. Rolls et al.

(1982) states that the varied food choice observed in omnivores is biologically determined.

Consuming a wide number of food types ensures that these foragers receive the proper nutrients

while minimizing the levels of toxins ingested (Remick et al., 2009).

Ultimately, a genetic basis underlying the natural and varied diet habits selected for and

supported by the Eastern grey squirrel’s evolutionary history may be the primary factor that led

to the results observed in this study. But undoubtedly food availability and nutritional benefit,

social cues, and competition played important roles as well. The Eastern grey squirrel

demonstrates clear diet preferences that reflect intrinsic habit and confer optimal nutritional

payoff. Individuals will adjust their foraging behavior accordingly; in the presence of

conspecifics, they are less likely to cache food items (Leaver et al., 2007). Given an optimal food

source such as seed, they will eat as long as it is available (Moller, 1983). It may be that the

animals are forgoing caching in favor of immediate consumption presumably to benefit as much

as possible from a limited food source. Further investigation into the relationship between

foraging behavior and social interaction would be the next topic of study, such as recording the

number of individuals at the feeder in intervals, and also quantitatively documenting territorial

displays, chasing, and fighting behaviors. It would be interesting to further separate the

components of the mixed seed into their individual seed species, and determine which one is

ultimately favored. It may also be worth studying the behavioral differences in foraging activity

and diet between rural-based squirrels and urban squirrels – for example, would the cereal elicit

interest and consumption by urban squirrels acclimated to the food of humans? Perhaps this

study raises many more questions than it answers, but served as a thought-provoking pilot

investigation.

Literature Cited

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the What, How, and Why of Our Behavior. MIT Press, Cambridge MA (2000)

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Retrieved from https://animalspot.net/eastern-gray-squirrel.html

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Facts on File Inc., New York

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Appendix

32

5

Seed Acorns

Frequency Diagram of Preference Between Seed vs. Acorns

Figure 1. Frequency diagram depicting the overall number of individuals that attended each feeder type between Nov. 1 – Nov. 22.

60%

40%

Proportion of Acorns Immediately

Consumed vs. Cached

Eat

Cache100%

Proportion of Seed Immediately

Consumed vs. Cached

Eat

Cache

Figure 2. Pie chart visualizes the

proportion of individuals that chose to

cache vs. consume the mixed acorns.

Figure 3. Pie chart demonstrates that

consistent trend of immediate consumption

observed at the seed feeder.

Individual Cache or Consume Temperature (F)

1 cache 32.0

2 cache 32.2

3 consume 32.0

4 consume 36.5

5 consume 36.6

Table 1. Temperature data gathered from the five individuals who

attended the acorn feeder. Temperatures at which the two behaviors

were evidenced were very similar, with a limited range of only 4.6F.