Abstract
The goal of this study was to observe the foraging behavior of Eastern Gray Squirrels (Sciurus carolinensis) in urban areas of St Paul/Minneapolis. The study tested whether squirrels in an urban setting followed the optimal foraging theory, or if rarity of a food type played a greater factor in food selection. The study also examined whether urban squirrel behaved the same way as wild squirrels when presented with a food type uncommon to the area, but common in other parts of the area. In five different areas in the twin cities, squirrels were given a number of different food types bought at the store: walnuts (Juglans regia), peanuts (Arachis hypogaea), red oak acorns (Quercus rubra), white oak acorns, and sunflower seeds (Helianthus annuus). Data were collected on which food item was taken and if they chose to eat or cache the item they had chosen. A general pattern of the order of food taken by the squirrels was then made. To see if the squirrels followed the optimal foraging theory, the nutritional content was then taken into consideration. The behavior was also studied to find any correlations with wild squirrels’ behaviors towards food. Chi-square test confirmed the significance of their behavior. It was found that in the face of rare food types, squirrels follow the optimal foraging theory, and are able to estimate the profitability of food items. Urban squirrels do indeed follow the natural behaviors of eastern gray squirrels found in hardwood areas.
Introduction
Eastern gray squirrels(Sciurus carolinensis) are very common tree dwelling mammals found all across the eastern half of North America and . Squirrels have distinct dentition with powerful ever-growing incisors and a diastema where typical omnivores and carnivores have canine teeth. Squirrels are omnivores, consuming many different foods from fruits and nuts, to gnawing on bones and eating small birds and reptiles (Thorington & Ferrell 2006) if food resources become rare. Their natural diets change from season to season depending on what is most plentiful. The autumn and winter diets are mainly composed of seeds and fruits (Gurnell 1987). Most of the time during autumn, squirrels often cache their food items while food is plentiful, and search for their cached items and consume them during the winter. They must cache hundreds of food items during the fall since they are active all throughout the year and do not hibernate during the winter. Gray squirrels cache their food in a method called scatter hoarding. Squirrels will cache each individual item in its own shallow hole in the ground, often relatively near the place where the food item was found. They then rely on memory and a strong sense of smell to relocate the food during the winter and following seasons (Gurnell 1987). This method seems to have pros and cons as food cached by other squirrels who have stored their food in the same area are often found, dug up, and consumed instead. Since eastern gray squirrels must compete against one another for food, the optimal foraging theory should be exhibited by all of them which states that animals will choose the most energetically favorable food based on the energy it takes to obtain it. Research has also been conducted on the foraging behavior of eastern gray squirrels in response to acorns of red and white oak trees. According to Fox (1982), squirrels have different responses when presented with the two different types of acorns. Squirrels tend to cache the red oak and eat the white oak acorns right away. Most researchers have concluded that the squirrels are able to differentiate between the two and that those responses are due to the tannin content of each species of acorn. Red oak acorns contain more tannin than the white oak acorns, and the extra tannin in red oak acorns makes them more bitter tasting. Studies have also shown that white oak acorns germinate faster than red oak acorns. If they germinate, the squirrels will consider the item as perishable, therefore can no longer eat them as the energy would have been lost. That is why squirrels will most likely eat white oak acorns on the spot.
Through this study, the hope is to observe urban squirrel behavior when they encounter foods(white and red oak acorns) commonly found in natural squirrel habitats but largely absent in urban settings, and whether rarity of food types play a greater role than the optimal foraging theory in urban settings. It is hypothesized that urban squirrels will instinctively mimic the foraging behavior of squirrels in locations largely undisturbed by humans, and when presented with rare and common foods, they will be able to forage optimally and choose the food item they will benefit from the most.
Study Sites
Data were collected from five different study sites where squirrel population was mostly seen(Figure 1). These sites were spaced far apart, which ensured that the individual squirrels being tested were from different populations. The areas chosen were near a single red oak tree or stand of red oak trees that were producing mast crops of acorns when the study began. The sites labeled ‘Mounds View’ (MV on the map provided), was located just North of St Paul. The second site labeled ‘North East’ (NE) was located just North East Minneapolis. The third site labeled ‘East’ (ES) is contained east of St Paul. The fourth site labeled ‘Como’ (C) is located by Como ave. The fifth site labeled ‘Battle Creek’ (BC) is located by Lower Afton road.
Figure 1. Map of the St Paul/Minneapolis area with sites used. Sites are marked with a black dot and initials corresponding to each sites. MV = Mounds View, NE = North East Minneapolis, C = Como, ES = East St Paul and BC = Battle Creek.
Materials and Methods
To avoid confusion and to ensure that existing food in the area did not interfere with the food laid out in the study, a circle about five meters in diameter was swept and cleared with a rake near the base of the tree where squirrels were previously observed to be most active. The food was laid out in separate piles to make it easier to identify by sight, but were only about an inch apart, so as to allow squirrels the opportunity to go amongst the piles and choose the food they wanted to take to cache or feed on. The food piles were made to be about the same size with twenty walnuts (Juglans regia), forty unsalted roasted peanuts (Arachis hypogaea), forty red oak acorns, forty white oak acorns, and eighty unsalted and roasted sunflower seeds (Helianthus annuus). This was done to ensure that initial pile size was not a factor in squirrel food selection. In order to not harm the squirrels, unsalted roasted peanuts and sunflower seeds were used because excess salt is known to be unhealthy for squirrels, and raw peanuts are known to contain a toxic which is documented to cause fur loss and starvation in squirrels. The white and red oak acorns were purchased online and the rest at the store. Acorns with visible acorn weevil holes in them were disposed of and were not used as they were probably hollowed out by the larvae and contained little nutrition. Once the food was placed, a tripod with a camera was set up about 12 meters away to record the squirrels’ behaviors for about 6 hours from 8am to 11am and 1pm to 4pm. Data were recorded on what food item was taken, whether it was cached, which was observed when squirrels left the immediate area, or whether it was eaten, with squirrels usually sitting on the ground or perched in a nearby tree in the area to consume the item. Notes were also made if the same squirrel returned to the site and took more than one food item. Observations were ended if a pile of a certain food type ran out, as data would then be altered as one of the variable food types was gone. Observation also ended when squirrels lost interest and left the area. The data were then compiled, chi square tests were performed, and conclusions were drawn. About forty trials were done from early February to end of May among different sites each. Squirrels were difficult to observed from February to end of March due to unfavorable weather conditions. Winter was very cold and ended early April. Squirrels were most active when the sun rose in the morning as well as in the late afternoon around two p.m.
Results’
The different foods used and their nutritional content (Table 1) may be a factor influencing squirrel food choice. The daily calorie requirements for an adult squirrel are dependant on multiple factors, such as the weight of the squirrel; as well as, any physiological factors such as pregnancy or illness. On the far-right side of the table is the estimated daily intake for the recommended amount of nutrients that domesticated squirrels and those in captivity are required to maintain a healthy life. The amount of nutrients for walnuts, peanuts, and sunflower seeds was not measured directly, but was taken from the nutritional facts supplied by the distributor of the food product. The nutritional content of red oak and white oak acorns was supplied from tables contained in books by Gurnell (1987) and Steele (2001). The table shows that walnuts have the highest caloric content but are also missing key minerals and vitamins that the other foods may contain. For example, there is no significant amount of phosphorus in walnuts, but sunflower seeds have the most, although the energy content is very small. Whereas a lot of salt is not good for squirrels, red and white oak acorns contain a small amount that is healthy for a squirrel to ingest. Squirrels must eat a variety of foods to maintain a healthy life such as vegetables and fruits, and cannot just feed on the most energy-rich items available.
Table 1. Comparison of the different nutrients, minerals, and caloric content contained. Items marked with an asterisk show that the nutrient or mineral was not given, present, or too small to be recorded.
Data were compiled based on observations. Since data were successfully collected at all five sites and squirrel behavior remained the same at each location, the data were analyzed as a whole rather than broken down by each site. Percentages were used (Figure 2), rather than the actual total number of each food item that was consumed, since amounts of each food type offered in each pile differed with the intent to eliminate pile size as a variable in terms of initial squirrel foraging responses
Figure 2. Percentages of each food taken out of the total amount of food that was offered for each type.
Although food was taken, it was not always immediately consumed after the squirrels chose a particular food type. This is important to take into account in analyzing the optimal foraging theory. Figures 3 shows the percentages of each food type that were cached and consumed. Again, the food types are arranged according to caloric content, with walnuts containing the most energy, and sunflower seeds containing the least. It was observed that harder shelled items are cached such as the walnuts and the red oak acorns, while softer shelled items including the white oak acorns, peanuts, and sunflower seeds were more often consumed right away at the site.
Figure 3. Percentages of food items that were cached or eaten after a squirrel chose the item. Harder shelled foods were cached more often than soft shelled items.
Size of each food item was also compared to see if it was a factor in food selection. The food items were lined up and measured in size. Items such as the white oak acorn are clearly bigger in size than red oak acorn, yet contains less nutritional value. This allowed us to check out size as a factor in food selection since the food items with more energy were the ones picked first. Chi square tests were then completed to further test whether observed behavioral patterns were significant (Table 2). It examined the observed results, using the values shown in Figure 2 versus the null hypothesis, that eastern gray squirrels had no preference or did not follow the optimal foraging theory which would mean that chances of squirrels piking any food item are equal. This would mean that the chances of a food type being taken were equal for all items. If the chi square values are greater than 0.05, the behavior exhibited by the eastern gray squirrels are significant. In this research all values calculated were greater than 0.05 which meant that conclusions can be drawn from the observed behaviors.
Chi Square Value (‘)
Walnuts 23.04
Peanuts 2.88
Red Oak Acorns 0
White Oak Acorns 4.5
Sunflower seeds 14.44
Table 2. Chi square values above critical value of 9.488. Null hypothesis can be rejected when alpha is greater than 5%.
Discussion
Although the eastern gray squirrels in the twin cities area live in a more urban setting and constantly come into contact with the human population that shares the same area, they still exhibit the natural behaviors of those in their natural habitat. The data can prove it since a number of different individuals and 5 different populations of squirrel were studied.
Squirrels visit certain areas of their home range regularly during the day. They often use the same paths or tree branches to get to certain areas, so in this case the study was sure to be testing squirrels from the same populations during each feeding session (Connolly 1979). The fact that squirrels have a territory range allowed the observer to recognize individual squirrels and note which squirrel was taking what food. Usually squirrels were recognized by their tails which were different thicknesses with sometimes parts of fur or entire parts of the tail missing. Since there was no human interfering with the squirrels on the areas studied, it was safe to say that the populations remained somewhat constant.
The chi square values show that squirrel preference for more energy-rich items was significant. Since the eastern gray squirrels depleted one food item before going onto another, the null hypothesis can be rejected and therefore not all food item had the same chance of being taken. It was also shown from the study that squirrels will pick the food item with the most caloric content before moving onto the less caloric content.
Squirrels would often be seen picking up a white oak acorn and biting into it as if to test what it tasted like. Many times, they would put the acorn back down and take another food item. In other studies, squirrels are observed performing the same behavior, as biting a small part of the acorn will also tell the squirrel if an acorn is rotten, in which case they will discard the acorn and find another (Steele 2001). From the study the eastern gray squirrels followed the optimal foraging theory entirely, although they still neglected to eat a lot of white oak acorns maybe because food sources are scarcer during the winter or perhaps squirrels were more pressured to eat items that had a higher energy content. Other factors of the optimal foraging theory such as the energy required to obtain the food item and predatory risk were not present when data was taken since all the food was in a small area and the squirrels were not at risk from predators like birds.
Urban areas provide unique habitats where squirrels are somewhat protected from their natural predators, especially aerial predators like eagles and hawks. It was obvious that their predator response instinct are modified in areas with high human traffic, only fleeing when humans approached at close distances, while squirrels in low human activity areas fled when humans approached at longer distances (Cooper 2008). Since humans do not hunt squirrels in urban areas, squirrels seem to let their guard down and are more often found scavenging for food, even those more unnatural food types like anything that humans can throw in the trash. They will forage near human populations, especially risking their lives to feed in areas with less tree cover near roads (Bowers and Breland 1996).
The squirrels could not eat all of the food provided and perhaps they realized it is better to store this new-found food source before it is gone. It is concluded that squirrels will always want to choose the most energetically favorable food, but that the storability of food also plays a part in what they choose to do once they obtain a certain food item. The shells of the peanuts, white oak acorns, and the sunflower seeds are very thin and easy to break, while the walnuts and the red oak acorns have thicker shells. The squirrels do roll the acorns around in their paws and bite them tentatively, as if to test the strength and durability of the shells (Steele 2001). The squirrels seem to determine that if cached, the softer shelled items may spoil. The eastern gray squirrels may also assume that the soft-shelled food items may germinate if cached in the ground, as opposed to the harder shelled items which may last longer, instinctually categorizing these rare food items as related to red and white oak acorns.
Caloric content seems to be the main factor in eastern gray squirrel foraging activity. It is actually common for squirrels to refuse different food types if they feel that it is not energetically favorable, or that it does not offer the nutrients they need. As a result of the eastern gray squirrel’s capacity to estimate the energetic profitability of food items, this may be one of the reasons why they seem to have an easier time adapting to urban settings. Eastern gray squirrels are also observed to have adapted to more urban areas that often have less tree cover than a forest undisturbed by humans (Cooper 2008). With urbanization, trees are cut and buildings and other structures are built, destroying forests and open. With eastern gray squirrels’ abilities to continue to recognize the energetic profitability of certain foods, even in an urban setting with little tree cover, they can easily adapt and become an invasive species.
Conclusion
Eastern gray squirrels in St Paul/Minneapolis come into contact with many different food types. This study concludes that eastern gray squirrels still follow the optimal foraging theory even when presented with natural foods they have never seen before. They are able to estimate the energy contained in food items and that generally provides the driving force in their foraging decisions. Although their decisions seem to be mostly based on caloric content, some foods do contain vitamins and minerals that others do not, so squirrels may choose to feed on different items as well. Shell thickness and the likelihood that items will still be viable once cached underground also seems to play a factor, especially in terms of foraging behaviors. Items with thicker shells were often cached, while softer shelled food items were consumed on the spot. Eastern gray squirrels in urbans areas react to different food items in similar ways to those squirrels that find some of the same naturally occurring food in less populated areas. Specifically, they cached the red oak acorns and chose to eat the white oak acorns. With their ability to adapt to urban settings, and feed on a variety of items, eastern gray squirrels are encroaching on the habitats of other squirrels, endangering the survival of these other species.
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References
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