Final lab reportWEAONE
Running Head: SAMPLE LAB REPORT 1
Sample Lab Report (The Optimal Foraging Theory)
SCI 207 Dependence of Man on the Environment
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Sample Lab Report
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.
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.
This study was conducted at Yates Mill Pond, a research area owned by the North
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Carolina State University, on October 25 th
, 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.
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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).
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.
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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Belovsky, G.E. (1984). Summer diet optimization by beaver. The American Midland Naturalist.
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-
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).
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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.
*Note: This document was modified from the work of Selena Bauer, Miriam Ferzli, and Vanessa