Wlf 315 Wildlife Ecology I lab
Fall 2008
Foraging Ecology of Bears:
Effect of Bite Size on Intake Rate
Lab objectives:
1. Introduce concepts of foraging ecology and foraging strategies.
2. Conduct experiments to test hypotheses about how bite size affects intake rate in bears.
3. Learn about the natural history, ecology, and conservation of grizzly bears.
A brief background to foraging ecology:
Nutrition is “the process whereby an animal processes portions of its external chemical
environment for the functioning of internal metabolism” (Robbins 1983). There are 5 main nutrient
categories: energy, protein, water, minerals, and vitamins. Energy is often measured in heat because heat
is given off when energy changes form (e.g. chemical energy in food is transformed into work in the
body). In nutritional studies, energy often is measured in calories (amount of heat needed to raise the
temperature of 1g of water by 1 C). The nutritional value of particular food items is influenced by many
factors, including the animal’s digestive physiology, the amount of water and dry matter in the food,
consumption of other foods, and digestibility of the food.
Animals employ different foraging strategies to meet their nutrient requirements. Many factors
will influence how animals forage including their reproductive state, their energy reserves, body size, and
the quantity and quality of food resources. For herbivores, food intake is generally limited by the amount
of time needed to harvest food. Both the size of an individual bite of food and the rate of bites (number of
bites per minutes) will influence the intake rate of food (grams per minute). The intake rate together with
the nutritional quality of the food will influence how long an animal must forage to meet its nutritional
requirements.
Effect of bite size on intake rate:
When fruits are relatively small (at least smaller than the maximum size that can be eaten in one
bite) and they are scattered around, then the size of each individual fruit equals the bite size because the
fruits are eaten one at a time. In this case, the rate of intake (g/min) will be limited by the time taken to
find and harvest each bite. If it takes the same amount of time to harvest larger and smaller fruits, then
larger fruits will result in greater intake rates. Alternatively, if fruits are clumped together in their
distribution, then an animal can take several pieces of fruit in one bite. In this case, the bite size is
influenced by the distribution of fruits; larger bite sizes will mean greater intake rates. Here’s an analogy:
If humans pick and eat huckleberries one at a time, then our bite size is equal to the berry size. If we eat
berries by the handful (assume that you're digging into a basket of berries), then our bite size is equal to
the handful of berries, and our intake rate would be greater.
Formulating hypotheses about intake rates for bears relative to fruit size and density (& bite size):
An early step in any scientific study is to formulate hypotheses to be tested. Often, results of
previous studies are used to generate hypotheses. In this case, we will use the work of Welch et al. (1997)
to generate hypotheses about the relationships between intake rate and fruit size and density. Look at
Figure 1 of Welch et al. (1997). What happened to intake rate as density of fruit increased? Next,
contrast intake rates of huckleberries (Figure 1a) with intake rates of large grapes (Figure 1d). Which
resulted in higher intake rates at a given density? Lastly, think about the paragraph above, and generate a
hypothesis about intake rates and bite size. Think about these questions and bring answers to class.
Use these questions to form hypotheses about the feeding trials that you will be conducting during
lab.
References:
Robbins, C.T. 1983. Wildlife Feeding and Nutrition. Academic Press, New York, NY.
Welch, C.A., J. Keay, K.C. Kendall, and C.T. Robbins. 1997. Constraints on frugivory by bears.
Ecology, 78:1105-1119.
Wlf 315 Wildlife Ecology I lab
Fall 2008
Lab Exercise: Bite Size and Intake Rate of Grizzly Bears
Reading Assignment:
Welch, C.A., J. Keay, K.C. Kendall, and C.T. Robbins. 1997. Constraints on frugivory by bears.
Ecology, 78:1105-1119.
Exercise Expectations:
1. Participate actively in the experiments. Ask questions.
2. Read assigned article (Welch et al. 1997).
3. Complete data sheets for the experiments.
4. Complete the lab report.
Methods:
1. We will be offering grizzly bears apples cut into two different sized pieces: 1) huckleberry-sized
pieces (smaller pieces), and 2) apples that are quartered (larger pieces).
2. At some point, we will need to calculate the average mass of the small and large apple pieces. To
do this, we will measure 10 pieces of each size and use the average grams in our calculations of
intake rates.
3. We will weigh the total amount of fruit before the trial and weigh left-over fruit after the trial.
The difference will let us know how much was actually consumed by the bears.
4. We will observe feeding by the bears, count number of bites, and estimate intake rates (g/min).
We will conduct the following feeding trials:
a) Observe a bear feeding on small (huckleberry-sized) pieces of apples and count number of
bites and measure the total feeding time.
b) Observe a bear feeding on large pieces of apples (quartered apples) and collect the same
measurements.
c) Observe a bear feeding on clumps of small pieces.
5. Estimate the intake rate using both of the equations below and compare the two results. What
sources of error might contribute to differences between the two estimates?
Intake rate (g/min) = # bites/min x g/bite
Intake rate (g/min) = total g consumed/total time
Assignment:
Compile the data from the feeding trials, use the data to complete the lab report and to answer the
following questions. Your write-up should contain the following sections. See the web-site for tips on
writing each section of a lab report.
Hypothesis and Predictions: You should have formulated an hypothesis about the relationship between
intake rate and bite size. Remember that an hypothesis is an educated guess about a general relationship.
You also should generate specific predictions that follow from the hypothesis. The predictions are what
you actually test. Make sure you are clear about how hypotheses and predictions are different!
Methods: This section should describe the experiments step by step so that someone else could repeat the
experiment exactly as you did.
Results: This section presents the measurements and calculations. You may present data either in the
text, or if appropriate, in a table or graph. Do not present the exact same data in two ways. You should
refer to the tables or graphs in the text. However, you only need to present data in one format. So, if your
Wlf 315 Wildlife Ecology I lab
Fall 2008
data are in a table (notice that the word 'data' is a plural form of 'datum'), then you do not need to list the
data again in the text. However, should reference the table in the text when you stress a particular.
Example: The bear took almost twice as long to consume fruit when it was cut into smaller pieces (Table
1).
So, your results may include some or all of the following:
- text reporting the results
- Table of your measurements
- The formula and calculations for intake rates
- Graph of results (if appropriate).
Discussion: The discussion should be written in paragraph format. Construct clear paragraphs with topic
sentences and a logical order to the discussion. Here are some ideas for points that you might want to
include in your discussion:
- Discuss the data in general based on the tables and/or graphs. Explain your results.
- Explain how your data relate to your hypothesis and predictions. Do the data support or refute your
hypothesis?
- What other factors might affect intake rates in these experiments? In the wild?
- How might foraging by bears in the wild be constrained by feeding on fruits? Why is foraging
efficiency by bears important for reproduction? Use the Welch et al. (1997) paper for background to
discuss this point.