How does an animal choose what food to eat?

One might assume that natural selection has influenced the foraging behaviors of animals, and that most animals forage efficiently, spending the least energy to gain the most nutrients. This is the underlying assumption of optimality modeling, a scientific approach to studying foraging behavior. The accompanying animation is based on the results of an experiment on the feeding behavior of bluegill sunfish. Ecologists performed laboratory experiments with these fish to determine their foraging strategies when presented with different sizes of the water flea Daphnia. Before performing the experiment, the investigators made predictions based on the optimality model. They predicted that in an environment stocked with low densities of all three sizes of prey, the bluegills would take every water flea that they encountered, but that in an environment with abundant large water fleas, the fish would ignore the smaller ones. At a high density of prey, the bluegill sunfish becomes the most selective. It eats a diet almost entirely made up of the large, energy-rich prey, and eats very few of the smaller prey. Conclusion: A bluegill sunfish selects prey to maximize its rate of energy intake. At a medium density of prey, the bluegill sunfish becomes selective and preferentially eats the large, energy-rich prey over the smaller ones. By this means, the bluegill maximizes its rate of energy intake; during time spent pursuing, capturing, and eating a small prey, a bluegill could expect to find and eat a large one. At a low density of prey, the A bluegill sunfish maximizes the rate at which it ingests energy by eating small- and medium-sized prey, as well as large individuals. The numbers of small, medium, and large prey that the sunfish eats reflect the effective proportions of these prey in the environment.

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