Abstract

The eastern gray squirrel is a commomn site in any forested or urban area of the eastern United States. During the fall, these squirrels begin a long process of eating and storing the new acorn and nut crops with the intention of increasing their body fat and coat and caching away reserves in order to survive the winter. Eastern gray squirrels are notorious troublemakers here in the U.S. and are beginning to cause trouble in their non-native United Kingdom habitat. However, squirrels are extremely important in the regeneration and dispersal of oak trees. For these reasons it is important to understand the behavior of squirrels. In this study, it was hypothesized that in order to increase body fat content and coat thickness, squirrels would increase the amount of time spent foraging as the season progressed toward winter. It was also hypothesized that squirrels would cache more food as winter approached. This study observed squirrels in the autumn months of October and November on the campus of the University of the South, Sewanee, Tennessee. Both foacl samples and scan samples were conductde on each of four days throughout the time period. It was found that foraging behavior did increase as time progressed according to the scan samples, and that caching increased according to both the scan and focal samples.

Introduction

The eastern gray squirrel (Sciurus carolinensis) is one of eight species of tree dwelling squirrels that inhabit the United States, and one of hundreds of species worldwide (4). The eastern gray squirrel can be found as far north as Maine, south into Florida and Texas, and west as far as the Dakotas (4). Normally they grow to be about 18 inches long and weigh between 12 and 26 ounces and both male and female are similar in size and color. The nest consists of leaves, twigs, moss, and sticks and whatever else they may find and are typically about 2 feet wide and located in a tree hollow, branch, or in the fork of a tree (1). The eastern gray squirrel may live 15 to 20 years in captivity, but often only survive one year in the wild (1). Deaths can be attributed to disease, malnutrition, and predation by red-tailed hawks, crows, weasels, foxes, owls, racoons, cats, dogs, cars, and humans (1). In 1994, New York State hunters reportedly killed 577,211 squirrels during a season (1).

The diet of the eastern gray squirrel consists of hickory nuts, beech nuts, acorns, flower shoots, seeds, truffles and other fungi, fruit, insects, tree buds, baby birds, and carrion (1,4,7). Squirrels prefer to eat in the safety of a tree, as they are much more vulneralble on the ground. They most often carry their food to a low branch and eat while holding their food with both hands and keeping an eye on the ground (1). In the fall and winter, however, their diet mainly consists of seeds and nuts, especially acorns from the 32 oak species which grow in the gray squirrel's North American range (7). Oak trees enjoy a special relationship with squirrels. In a large acorn crop year, gray squirrels may cache many acorns in the ground and then fail to recover up to 74% of them (7). This leads to increased regeneration and dispersal of oak trees (7). In fact, squirrels and jays are credited with the spread of the vast oak stands through North America after the glacial retreat 10,000 years ago (7).

In the spring and early summer, when food supplies are relatively plentiful, squirrels spend most of their time in mating behavior (1). In the fall, however, the acorn and nut crops arive and the squirrels are much more concerned with storing food for winter, increasing their fat reserves, and thickening their winter coats (1). Squirrels do not hibernate in the winter, but cache acorns and other nuts in order to have reserves when the crop is depleted, hoarding items in small clumps over a large area (5). This storage of food allows them to reap the harvest of plentiful food with low energy costs in the warmer fall and eat when the food is scarce and energy costs are high in the cold months of winter (3).

The study of foraging behavior is important to understanding the requirements of animals. By understanding the requirements of an animal, it is easier to control/limit populations. Squirrels are particularly difficult to control due to their omnivorous and generalist characteristics. While squirrels rank among the most preferred of urban wildlife due to their loveable faces and antics, they also rank first among troublemakers of urban animals (4). The eastern gray squirrel is also causing trouble in the United Kingdom, where they were taken by Victorian collectors (4). There, the American gray squirrel is responsible for the serious deline in native red squirrels by outcompeting the natives for resources such as acorns (4). As mentioned before, squirrels are also extremely important in the regeneration and dispersal of North American oaks. In Appalachian forests in particular, recent data suggests that oaks are replacing the ecological place of American Chestnuts (Castea dentata) (6). Species which once relied on chestnut crops, such as squirrels and other small mammals, white-tail deer (Odocoileus virginianus), and black beer (Ursus americanus), now rely on oak tree acorns as a major food resource (6). Without the contribution of squirrels, oaks would not be as prevalent and wide-spread as they are presently (7).

In this study, we plan to observe the foraging behavior of squirrels in the autumn months of October and early November. We hypothesize that, since squirrels must increase their body fat content and their coat thickness, they will need a large increase in their caloric intake, and will therefore participate in increased foraging behavior as the winter draws near. We also hypothesize that as the winter approaches, the squirrels will cache more food to prepare for the harsh months ahead.

squirrel information

squirrel pictures

Methods

This experiment was conducted in an urban location, on the main campus of the University of the South in Sewanee, Tennessee, during the fall months of October and November. The observations were carried out in the afternoon (between 1p.m. and 5p.m.) of each day. In order to determine if the foraging behavior of squirrels changed as the season progressed, both focal and scan samples were conducted. For the focal sample, the observer located a squirrel, and observed the squirrel for as long as possible (normally 30 minutes to 1 hour). While observing, she recorded the start and end times of each behavior and the behavior exhibited. For the scan sample, the observer walked along a transect through the campus and recorded the time and current behavior of each squirrel that she saw. Foraging behavior was defined as any behavior in which a squirrel had direct contact with food: holding in paws, carrying it in the mouth, burying/caching, or actually eating the food. Other behavior was defined as any behavior in which the squirrel was not in direct contact with food, such as moving around without food, climbing, and interacting with other squirrels. The transect began behind McClurg Dining Hall and proceeded to Snowden, behind the library, through Manigault park, into Abbo's Alley, up South Carolina Avenue, and ending in front of McClurg Dining Hall.

University of the South Campus Map

Results

Using focal samples, squirrels were observed for a total of 2 hours and 4 minutes. On October 9, a squirrel was observed for 10 minutes and 10 seconds. It spent 9 minutes and 58 seconds, or 98% of this time, foraging (Figure 1). On October 17, a squirrel was observed for 56 minutes. It spent 20 minutes and 55 seconds, or 63% of this time, foraging (Figure 2). On October 28, a squirrel was observed for 25 minutes. It spent 8 minutes and 40 seconds, or 65% of this time, foraging (Figure 3). On November 3, a squirrel was observed for 39 minutes. Fourteen minutes, or 64% of that time, was spent foraging (Figure 4). There was no pattern in the percentage of time spent foraging as the season progressed.

Figure 1:Percent of time spent foraging out of 10 min. and 10sec. observed on October 9.

Figure 2:Percent of time spent foraging out of 56 minutes observed on October 17.

Figure 3:Percent of time spent foraging out of 25 minutes observed on October 28.

Figure 4:Percent of time spent foraging out of 39 minutes observed on November 3.

Scan samples were conducted on the same four days as the focal samples. On October 9, a mean of 6 squirrels were observed per hour, of which a mean of 2 squirrels per hour were observed foraging. Percentage-wise, this means 33% of the squirrels were observed foraging. On October 17, a mean of 6.5 squirrels were observed per hour, of which a mean of 2.5 squirrels, or 38%, were foraging. On October 28, a mean of 5 squirrels were observed per hour, of which 2.5 squirrels, or 50%, were foraging. On November 3, a mean of 5.5 squirrels were observed per hour, of which 3.5 squirrels, or 64% were foraging. This data is summed up in Table 1. There is an increasing trend in the percentage of squirrels observed which were foraging as the season progressed (Figure 5).

Table 1: Mean number of squirrels, mean number of squirrels foraging, and the percentage of squirrels foraging per hour.

Figure 5: Comparison of the percentage of squirrels found foraging each day.

Caching, a distinct foraging behavior, was not observed at all during the time observed on the first three days. However, on November 3, two squirrels were observed caching acorns in the focal samples and one was observed caching during the scan sample.

Discussion:

The first part of the hypothesis, that squirrels would participate in increased foraging behavior as winter drew near was supported by the scan sample data, but not the focal sample data. The second part of the hypothesis, that caching will increase as winter approaches, was supported in the data from both focal and scan samples.

The discrepancy between the focal sample data and the scan sample data is most likely due to the variability in the focal sample data. On October 9, the focal sample only lasted about 10 minutes while on the other days, the sample lasted from two to almost four times that long. This inconsistency in the time observed makes comparison between the days difficult and the focal sample data inconclusive.

A more accurate assessment of squirrel foraging behavior would result from an increase in sample size and an extension in the length of the study. If the study was started earlier and finished later in the season, there may have been more conclusive data supporting the hypothesis. It would also be interesting to see if observing at different times of day would have changed the results of the study. Squirrels were only observed in the early afternoon, while literature indicates that squirrels are more active in the early morning and dusk hours (1).

Data on the temperature and weather during the times observed would also be helpful in assessing squirrel behavior. It is possible that squirrels do not prefer to forage in windy weather or extremely cold temperatures, which would account for a dcrease in time spent foraging or the number of squirrels observed foraging on a particular day. A comparison between foraging behavior and temperature or length of photoperiod might also have been more conclusive in determing if foraging behavior increased as the season progressed. In a study on the foraging behavior of the southern flying squirrel (Glaucomys volans), it was found that there was a positive correlation between declining photoperiod and increased foraging behavior as well as a correlation between exposure to extreme cold and foraging behavior (8).

It should also be mentioned that this study (like any study on wildife) could not take into account all of the squirrels (wildlife) in the area. In fact, according to a squirrel expert at the University of Maryland, when scientists attempted to quantify the number of squirrels living in a small area of forest based on how many squirrels were spotted, the number was usually too low by as much as a factor of six (4). Since you do not see everything that is going on, what is seen may not be a good example of reality.

Consideration should also be given to the reality that foraging behavior can not be completely simplified to a correlation with seasonal progression. Squirrels may not increase their time spent foraging, but may increase the time spent foraging for more nutritional food resources or those food resources which are more easily stored. The two major types of oaks, red and white, have acorns which vary considerably in their chemical make-up and in their germination period (7). White oak acorns contain less fat than red oak acorns, making them less appealing as far as nutritional value is concerned, but they contain lower amounts of tannins (a compound which makes acorns less palatable and harder to digest) and therefore may be more appealing as far as taste is concerned (7). It has been shown that squirrels prefer the high-fat, less-platable red oak acorns over white oak acorns (7). Squirrels also show a preferance in which acorns are cached (normally red oak) and which are eaten immediately (white oak) because white oak acorns germinate in the fall, rendering them less edible after being stored than red oak acorns (7). An increase or decrease in time spent foraging might also be due to resource availability. Acorns are a vast but diminishing food resource in the fall as a result of squirrel forage and the foraging of other animals (8). This decrease in the amount of food available may cause either an increase in foraging in order to outcompete other competitors or a decrease in foraging due to the decline in the resource. Territoriality may also play a role in the foraging pattern of squirrels. Less dominant squirrels may have trouble getting to food supplies and therefore decrease their time spent foraging, while dominant squirrels may have a larger available supply and therefore increase their foraging behavior. On the flip side, when one squirrel discovers a large food resource, it may asssist other squirrels in locating the resource. In a study done on bird foraging behavior, it was found that birds tend to travel and to feed where they see other birds foraging (2).

In conclusion, it is apparent that there are many factors which may affect the foraging behavior of the eastern gray squirrel other than the seasonal progression toward winter. Nevertheless, this study has supported the proposal that squirrel foraging behavior does increase as winter approaches, whether that be due to a greater nutritional need, a need for a lasting food supply, decreasing food resource availability, or an increase in competition between squirrels and with other wildlife.

Works Cited:

(1) Ackerman, Diane. "In Praise of Squirrels." National Geographic Nov. 1995: 98-113

(2) Galef, Bennett G. and Luc-Alain Giraldeau. "Social influences on foraging in vertebrates: causal mechanisms and adaptive functions." Animal Behavior Jan. 2001 v.61 no1: 3-15

(3) Kotler, Burt P., Joel S. Brown, and Michael Hickey. "Food storability and the foraging behavior of fox squirrels." The American Midland Naturalist July 1999 v.142 no1: 77-86

(4) Lipske, Michael. "Crazy over Squirrels." National Wildlife Dec. 1997-Jan. 1998 v.36: 40-45

(5) Marchand, Peter J. "Ready Cache." Natural History Oct. 1999. located at http://www.findarticles.com

(6) McShea, William J. "The influence of acorn crops on annual variation in rodent and bird populations." Ecology Jan. 2000. located at http://www.findarticles.com

(7) Steele, Michael and Peter Smallwood. "What Are Squirrels Hiding?" Natural History Oct. 1994: 40-45

(8) Thomas, Richard B. and Peter D. Weigl. "Dynamic foraging behavior in the southern flying squirrel: test of a model." The American Midland Naturalist Oct. 1998 v.140 no2: 264-70