Abstract.› The theory of optimal foraging and its relation to central foraging was examined by using the beaver as a model› (summarizes the Introduction). 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 (summarizes the Methods).› Beavers avoided certain species of trees and preferred trees that were close to the water.› No preference for tree circumference was noted (summarizes the Results).› These data suggest that beaver food choice concurs with the optimal foraging theory (summarizes the Discussion). = This abstract is a mini-version of the entire paper. It summarizes each section of the report in chronological order.
In this lab, we explore the theory
of optimal foraging and the theory of central place foraging using beavers
as the model animal.›= Identifies
the scientific concept which forms the learning context for the lab.
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).›
This study was conducted at Yates Mill Pond, a research area owned by the North 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.› = A description of procedure used in the lab to gather the data. Includes a description of the study site and measuring techniques.
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. = A
description of the methods used to analyze the data.
Overall, beavers showed a preference
for certain species of trees, and their preference was based on distance
from the central place. = A
statement about the overall findings of the study.
Measurements taken at the study site show that beavers avoided oaks
and musclewood (Fig. 1) and show a significant food preference (x2=447.26,
d.f.=9, P<.05).› 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 (t=3.49, d.f.=268, P<.05) (Fig. 2).› The
tree species that were avoided were not significantly farther from the
water (t=.4277, d.f.=268, P>.05) 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)
(t=1.52, d.f.=268, P>.05) (Fig. 3).› = A
narrative of findings in order of importance with reference to the corresponding
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.›= A statement showing the relationship between the hypothesis and the results. 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. = A comparison of the findings of the lab with other related scientific studies.
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.›= A statement showing the relationship between the hypothesis and the results. As distance from the water increases, a treeŪs net energy yield decreases because food that is farther away is more likely to increase search and retrieval time. This finding is similar to BelovskyŪs finding of an inverse relationship between distance from the water and percentage of plants cut. = A comparison of the findings of the lab with other related scientific studies.
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.›= A statement showing the relationship between the hypothesis and the results. 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. = A comparison of the findings of the lab with other related scientific studies.
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.›= A
statement about the significance of the results within the context of
the scientific concept. 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
statement about other
information that might be useful to the study. 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. = A
statement of possible sources of error.
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 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.›= The conclusion restates the purpose and describes what the students learned.
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.›
Jenkins, S.H., and P.E. Busher.› 1979.› Castor canadensis.› Mammalian Species.›
McGinly, M.A., and 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, and 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., and 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 NCSU graduate students Selena Bauer, Miriam Ferzli, and Vanessa Sorensen.