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Simulating the Dilution, Confusion, and Odd Prey Effects

Developed by Theodore E. Burk, Dept. of Biology, Creighton University, Omaha, NE 68178-0103

BACKGROUND:

One of the areas of animal behavior that is most interesting and that has attracted a great deal of attention recently is the study of group living in animals. Behavioral biologists who study the social behavior of animals, sometimes called sociobiologists, have concluded that there are a number of possible benefits of group living that, for some species, outweigh such costs of group living as increased competition among group members and increased risk of disease transmission. Most important among the advantages of group living are foraging advantages and anti-predator benefits.

Living in a group can increase one's safety against predators in several different ways. Among these are what have come to be called the "dilution effect" and the "confusion effect." In the dilution effect, an individual's chance of being captured decreases as group size increases, often because the predator takes only a single or a limited number of prey on each attack. Well-studied examples of animals who benefit from a dilution effect are monarch butterflies, preyed upon by birds in their overwintering roosts, and mud-puddle frogs, preyed upon by frog-eating bats.

In the confusion effect, predators find it difficult to focus their concentration and pursuit on a specific individual prey animal when a large number of prey are escaping from them at the same time, going in different directions, making a lot of noise, etc. A behavioral physiologist might say that the predator finds it difficult to focus on stimuli coming from one prey because of interference from all the stimuli coming from the other escaping prey. This confusion may slow the predator down sufficiently that all of the prey are beyond successful pursuit distance before the predator is able to concentrate its efforts on any one prey animal; at the least, it may lower the risk of capture for each prey individual in the group. The confusion effect has been shown to occur in fish escaping from predators such as other fish or squid, and for ground squirrels escaping from eagles, among other animals. Human quail hunters are also likely to be familiar with this phenomenon.

Another phenomenon that is familiar to students of predators and prey is the "odd prey effect." In this, any prey that differs in a conspicuous way from the others in its group is more likely to draw a predator's notice, leading to an increased chance of its being attacked and captured. The odd prey effect may in some part negate the confusion effect, making it easier for a predator to single out a single escaping prey, the odd one. In the exercise described here, the dilution, confusion, and odd prey effects are examined using a simple simulation involving the class of students as predators and golf balls as prey.

METHOD:

Materials Needed:

Several dozen white golf balls
A dozen or so orange or bright-colored golf balls.

Procedure:

Students should work in pairs. Using the golf balls provided, test for the confusion and dilution effects in the following way. One student should stand in a relaxed manner. The other, standing about eight feet away, will toss a ball(s) at the first student's chest. The ball(s) should be tossed underhanded at a speed that is not impossible to follow but that at the same time requires the first student's concentration to catch. The predator should catch the prey with the hands; "basket catches" using one's chest against which the prey are cradled are not allowed. Repeat, tossing in turn one, two, three, and four balls at once. Record for each alternative how many balls caught by the first student, and how many times at least one ball was caught. Each student should act as tosser and as catcher for a total of 12 balls for each set of prey group sizes (i.e., toss 12 single balls, 6 pairs, 4 trios, and 3 sets of four). Turn in the individual scores to the instructor. The class data will be collated and distributed. From these we can calculate:

1). The probability of capture for each individual prey animal (golf ball) as a function of group size (= percent of prey captured). If this declines as group a dilution effect will have been in operation.

2). The percentage of attacks by the predator that result at least one prey capture, again as a function of group size (= percent of successful hunts). If this declines as group size increases, a confusion effect will have been in operation.

To test the "odd prey effect," carry out procedures exactly as above, except this time, make one of every four golf balls tossed out an orange one. In addition to recording the total number of balls caught, record how many of the captured prey were orange balls. Turn in your data to your instructor. Since 25% of the prey were orange, any significant increase over that in the percentage of prey captured that were orange will indicate that a rare prey effect was in operation.

STATISTICAL ANALYSIS AND DATA PRESENTATION:

Our null hypotheses are: 1) percent prey captured will not decline as prey group size increases; 2) percent successful hunts will not decline as prey group size increases; and 3) orange golf balls will not differ significantly from 25% of prey captures.

By contrast, our experimental hypotheses are: 1) percent prey captured will decline as prey group size increases (the dilution effect); 2) percent successful hunts will decline as prey group size increases (the confusion effect); and 3) orange golf balls will constitute significantly more than 25% of the prey captured (the odd prey effect). To analyze the class data, use the Chi-Square Goodness-of -Fit test, as explained by your instructor. Graph your results, using the Cricketgraph graphics software, following your teacher's instructions.

BACKGROUND READING:

Curio, E. (1976). The Ethology of Predation. New York, NY: Springer-Verlag.

Grier, J.W., & Burk, T. (1992). Biology of Animal Behavior, 2nd Ed. St. Louis, MO: Mosby-Year Book.

NOTES TO TEACHERS:

As you can see, the materials needed for this exercise are few: in addition to some willing predators, you will need several dozen white golf balls and a dozen orange ones. These are readily obtainable for reasonable prices from most discount department stores. Statistical analysis and graphing can be done by hand, or by using one of a number of statistics and graphics software programs. I have my classes do the chi-square tests with pencil, paper, and calculators; for graphing, we use the Cricketgraph software program for Macintosh computers.

Precautions that need to be taken are few: 1) Students should stand about the recommended distance apart; 2) Golf balls should be tossed in such a way as to require concentration to catch them, but not so hard as to endanger the predator!; and 3) The students should perform the exercise away from breakable objects and windows.

It should only take the students about 15 minutes to carry out the simulated predation. Allowing time for explaining the exercise and getting set up, collecting the data, and turning in the results, it should be easily possible to carry out the exercise to the point of the teacher obtaining the results for collating within the time block usually available for a junior high or high school class period. A second period could then be used for statistical analysis and/or graphing the results. All parts of the exercise can easily be accomplished within the 2 to 3-hour time period available for a college course's laboratory session.

I have used this exercise in several different college courses and with junior high students; the results have been very consistent. The dilution effect consistently shows up very strongly. The confusion effect usually manifests itself very weakly, and only with an accumulation of data from several replicates of the exercise may it be statistically significant. The odd prey effect is usually not significant for smaller group sizes, but does usually appear for the group size of four, at least when several replicates' worth of data are compiled. This makes sense: the attention-attracting qualities of the odd prey may only be important for a predator in the more confusing situation of a large group escaping prey.

The data collected in this exercise may be very useful in demonstrating to students the need to conduct statistical analyses in order to arrive at agreed-upon conclusions regarding whether or not one should accept or reject the null or experimental hypotheses. Especially in the cases of the confusion and odd prey effects, slight trends in the expected directions are common, and whether or not they should be accepted as significant is ambiguous until statistical analyses are performed.