The theory of optimal foraging and its relation to central foraging was examined by using

the beaver as a model. Beaver food choice was examined by noting the species of woody

vegetation, status (chewed vs. not-chewed), distance from the water, and circumference of trees

near a beaver pond in North Carolina. Beavers avoided certain species of trees and preferred

trees that were close to the water. No preference for tree circumference was noted. These data

suggest that beaver food choice concurs with the optimal foraging theory.

Introduction

In this lab, we explore the theory of optimal foraging and the theory of central place

foraging using beavers as the model animal. Foraging refers to the mammalian behavior

associated with searching for food. The optimal foraging theory assumes that animals feed in a

way that maximizes their net rate of energy intake per unit time (Pyke et al., 1977). An animal

may either maximize its daily energy intake (energy maximizer) or minimize the time spent

feeding (time minimizer) in order to meet minimum requirements. Herbivores commonly behave

as energy maximizers (Belovsky, 1986) and accomplish this maximizing behavior by choosing

food that is of high quality and has low-search and low-handling time (Pyke et al., 1977).

The central place theory is used to describe animals that collect food and store it in a

fixed location in their home range, the central place (Jenkins, 1980). The factors associated with

the optimal foraging theory also apply to the central place theory. The central place theory

predicts that retrieval costs increase linearly with distance of the resource from the central place

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(Rockwood and Hubbell, 1987). Central place feeders are very selective when choosing food

that is far from the central place since they have to spend time and energy hauling it back to the

storage site (Schoener, 1979).

The main objective of this lab was to determine beaver (Castor canadensis) food selection

based on tree species, size, and distance. Since beavers are energy maximizers (Jenkins, 1980;

Belovsky, 1984) and central place feeders (McGinley & Whitam, 1985), they make an excellent

test animal for the optimal foraging theory. Beavers eat several kinds of herbaceous plants as

well as the leaves, twigs, and bark of most species of woody plants that grow near water (Jenkins

& Busher, 1979). By examining the trees that are chewed or not-chewed in the beavers’ home

range, an accurate assessment of food preferences among tree species may be gained (Jenkins,

1975). The purpose of this lab was to learn about the optimal foraging theory. We wanted to

know if beavers put the optimal foraging theory into action when selecting food.

We hypothesized that the beavers in this study will choose trees that are small in

circumference and closest to the water. Since the energy yield of tree species may vary

significantly, we also hypothesized that beavers will show a preference for some species of trees

over others regardless of circumference size or distance from the central area. The optimal

foraging theory and central place theory lead us to predict that beavers, like most herbivores,

will maximize their net rate of energy intake per unit time. In order to maximize energy, beavers

will choose trees that are closest to their central place (the water) and require the least retrieval

cost. Since beavers are trying to maximize energy, we hypothesized that they will tend to select

some species of trees over others on the basis of nutritional value.

Methods

This study was conducted at Yates Mill Pond, a research area owned by the North

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Carolina State University, on October 25th, 1996. Our research area was located along the edge

of the pond and was approximately 100 m in length and 28 m in width. There was no beaver

activity observed beyond this width. The circumference, the species, status (chewed or not-

chewed), and distance from the water were recorded for each tree in the study area. Due to the

large number of trees sampled, the work was evenly divided among four groups of students

working in quadrants. Each group contributed to the overall data collected.

We conducted a chi-squared test to analyze the data with respect to beaver selection of

certain tree species. We conducted t-tests to determine (1) if avoided trees were significantly

farther from the water than selected trees, and (2) if chewed trees were significantly larger or

smaller than not chewed trees. Mean tree distance from the water and mean tree circumference

were also recorded.

Results

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Overall, beavers showed a preference for certain species of trees, and their preference

was based on distance from the central place. Measurements taken at the study site show that

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beavers avoided oaks and musclewood (Fig. 1) and show a significant food preference. No

avoidance or particular preference was observed for the other tree species. The mean distance of

8.42 m away from the water for not-chewed trees was significantly greater than the mean

distance of 6.13 m for chewed trees (Fig. 2). The tree species that were avoided were not

significantly farther from the water than selected trees. For the selected tree species, no

significant difference in circumference was found between trees that were not chewed

(mean=16.03 cm) and chewed (mean=12.80 cm) (Fig. 3).

Discussion

Although beavers are described as generalized herbivores, the finding in this study

related to species selection suggests that beavers are selective in their food choice. This finding

agrees with our hypothesis that beavers are likely to show a preference for certain tree species.

Although beaver selection of certain species of trees may be related to the nutritional value,

additional information is needed to determine why beavers select some tree species over others.

Other studies suggested that beavers avoid trees that have chemical defenses that make the tree

unpalatable to beavers (Muller-Schawarze et al., 1994). These studies also suggested that

beavers prefer trees with soft wood, which could possibly explain the observed avoidance of

musclewood and oak in our study.

The result that chewed trees were closer to the water accounts for the time and energy

spent gathering and hauling. This is in accordance with the optimal foraging theory and agrees

with our hypothesis that beavers will choose trees that are close to the water. As distance from

the water increases, a tree’s net energy yield decreases because food that is farther away is more

likely to increase search and retrieval time. This finding is similar to Belovskyís finding of an

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inverse relationship between distance from the water and percentage of plants cut.

The lack of any observed difference in mean circumference between chewed and not

chewed trees does not agree with our hypothesis that beavers will prefer smaller trees to larger

ones. Our hypothesis was based on the idea that branches from smaller trees will require less

energy to cut and haul than those from larger trees. Our finding is in accordance with other

studies (Schoener, 1979), which have suggested that the value of all trees should decrease with

distance from the water but that beavers would benefit from choosing large branches from large

trees at all distances. This would explain why there was no significant difference in

circumference between chewed and not-chewed trees.

This lab gave us the opportunity to observe how a specific mammal selects foods that

maximize energy gains in accordance with the optimal foraging theory. Although beavers adhere

to the optimal foraging theory, without additional information on relative nutritional value of

tree species and the time and energy costs of cutting certain tree species, no optimal diet

predictions may be made. Other information is also needed about predatory risk and its role in

food selection. Also, due to the large number of students taking samples in the field, there may

have been errors which may have affected the accuracy and precision of our measurements. In

order to corroborate our findings, we suggest that this study be repeated by others.

Conclusion

The purpose of this lab was to learn about the optimal foraging theory by measuring tree

selection in beavers. We now know that the optimal foraging theory allows us to predict food-

seeking behavior in beavers with respect to distance from their central place and, to a certain

extent, to variations in tree species. We also learned that foraging behaviors and food selection is

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not always straightforward. For instance, beavers selected large branches at any distance from

the water even though cutting large branches may increase energy requirements. There seems to

be a fine line between energy intake and energy expenditure in beavers that is not so easily

predicted by any given theory.

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References

Belovsky, G.E. (1984). Summer diet optimization by beaver. The American Midland Naturalist.

111: 209-222.

Belovsky, G.E. (1986). Optimal foraging and community structure: implications for a guild of

generalist grassland herbivores. Oecologia. 70: 35-52.

Jenkins, S.H. (1975). Food selection by beavers:› a multidimensional contingency table analysis.

Oecologia. 21: 157-173.

Jenkins, S.H. (1980). A size-distance relation in food selection by beavers. Ecology. 61: 740-

746.

Jenkins, S.H., & P.E. Busher. (1979). Castor canadensis. Mammalian Species. 120: 1-8.

McGinly, M.A., & T.G. Whitham. (1985). Central place foraging by beavers (Castor

Canadensis): a test of foraging predictions and the impact of selective feeding on the

growth form of cottonwoods (Populus fremontii). Oecologia. 66: 558-562.

Muller-Schwarze, B.A. Schulte, L. Sun, A. Muller-Schhwarze, & C. Muller-Schwarze. (1994).

Red Maple (Acer rubrum) inhibits feeding behavior by beaver (Castor canadensis).

Journal of Chemical Ecology. 20: 2021-2033.

Pyke, G.H., H.R. Pulliman, E.L. Charnov. (1977). Optimal foraging. The Quarterly Review of

Biology. 52: 137-154.

Rockwood, L.L., & S.P. Hubbell. (1987). Host-plant selection, diet diversity, and optimal

foraging in a tropical leaf-cutting ant. Oecologia. 74: 55-61.

Schoener, T.W. (1979). Generality of the size-distance relation in models of optimal feeding.

The American Naturalist. 114: 902-912.

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*Note: This document was modified from the work of Selena Bauer, Miriam Ferzli, and Vanessa Sorensen, NCSU.