Acorn Caching in Tree Squirrels: Teaching Hypothesis Testing in the Park

By McEuen, Amy B.; Steele, Michael A. | The American Biology Teacher, September 2012 | Go to article overview

Acorn Caching in Tree Squirrels: Teaching Hypothesis Testing in the Park


McEuen, Amy B., Steele, Michael A., The American Biology Teacher


Developing effective exercises for ecology courses is challenging, given time and fiscal constraints and scarcity of nearby habitats. Ideally, exercises would develop critical-thinking skills by introducing students to a scientific topic through hypothesis testing, data collection, data analysis, and interpretation. If exercises were linked to existing research, students could explore the topic in more depth using library databases. Here, we describe a laboratory exercise on acorn caching by squirrels that was originally developed for a junior-level general ecology class at the University of Illinois Springfield. Learning objectives are related to hypothesis testing, with students developing predictions based on existing hypotheses regarding squirrel behavior. Our exercise makes use of research that documented differences in tree squirrel (genus Sciurus) foraging in response to acorn characteristics (Steele et al., 2001; Steele & Smallwood, 2002; Steele, 2008).

Not all acorns are alike, and tree squirrels respond to the differences in their feeding and food-storing behavior. Acorns of different oak species (genus Quercus) have different concentrations of fat and tannin and differences in germination schedules (Table 1; Smallwood et al., 2001). Broadly, acorns from the white oak (WO) subsection (including white oak, Quercus alba) have low fat, low tannins, and germinate when acorns ripen in the fall. By contrast, acorns from the red oak (RO) subsection (including northern red oak, Quercus rubra) have high fat, high tannins, and germinate in the spring following fall maturation. Tree squirrels preferentially cache (bury) acorns of the red oak group and eat acorns from the white oak group (Steele et al., 2001). One hypothesis explaining this behavior is the perishability hypothesis, which predicts that squirrels will eat perishable food items and cache hardier food items (Hadj-Chikh et al., 1996). WO acorns are a short-lived food resource because they germinate in the fall and can become seedlings before squirrels can recover the buried acorn. An alternative hypothesis explaining squirrel behavior is the handling-time hypothesis (Jacobs, 1992), which predicts that squirrels will cache acorns with long handling times and eat acorns with short handling times. It is proposed that squirrels are constrained by the time they have to cache acorns in the autumn and should, therefore, bury as many as possible and spend less time eating.

In this exercise, students test these hypotheses in a city park. They present acorns of two species, one from each group, to either fox squirrels (Sciurus niger) or eastern gray squirrels (S. carolinensis), then record squirrel behavior. Damaged and undamaged acorns are also presented to squirrels, allowing students to test multiple predictions of the perishability hypothesis. Specifically, they can test (1) that damaged acorns will be preferentially eaten and undamaged ones cached and (2) that WO will be eaten more than RO. In a 40-student class, enough data are typically generated in two 3-hour lab periods to have a subsequent lab on data analysis and interpretation. Some students also create an oral presentation in which they discuss results in the context of the primary literature. One question that inevitably arises is how such behavior could have evolved. This exercise therefore provides a field and computer experience that could be followed by the role-playing exercise developed by Riechert et al. (2011) on evolution of squirrel caching behavior.

* Classroom Preparation

Before the lab, students receive a lecture on animal behavior that discusses how ecologists develop hypotheses to predict behavior using evolutionary theory (Krebs & Davies, 1991). Animals should behave so as to maximize their fitness (i.e., maximize their survival as well as offspring production and survival). Optimal foraging theory is also reviewed. In its simplest form, this theory predicts that animals should forage to maximize the energy (calories) they ingest while minimizing time, risk, and calories expended (Krebs & Davies, 1991). …

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