THE LIVING WORLD
Unit Eight. The Living Environment
The investigation of animal behavior can be conveniently divided into three sorts of studies: (1) The study of its development. Lorenz’s study of imprinting in geese was a study of this sort. (2) The study of its physiological basis. Analysis of the impact of the fosB gene on maternal behavior in mice was a study of this sort. (3) The study of its function (that is, its evolutionary significance). This third sort of study is addressed by biologists working in the field of behavioral ecology. Behavioral ecology is the study of how natural selection shapes behavior.
Behavioral ecology examines the survival value of behavior. How does an animal’s behavior allow it to stay alive and reproduce, or keep its offspring alive to reproduce? Research in behavioral ecology thus focuses on a behavior’s adaptive significance—that is, on the contribution a behavior makes to an animal’s reproductive success, or fitness.
It is important to remember that all genetic differences in behavior need not have survival value. Many genetic differences in natural populations are the result of random mutations that accidentally become common, a process called genetic drift. It is only by experiment that we can learn if a particular behavior has been favored by natural selection.
Nobel laureate Niko Tinbergen’s pioneering study of seagull nesting provides an excellent example of how a behavioral ecologist investigates the potential evolutionary significance of a behavior. Tinbergen observed that after gull nestlings hatched from their eggs, the parent birds quickly remove the eggshells from the nest. Why? What possible evolutionary advantage would this behavior confer on the birds?
To investigate this, Tinbergen camouflaged chicken eggs by painting them to resemble gulls’ eggs that blend in with the natural background where the gull nests were located (figure 37.7), and distributed them on the ground throughout the nesting area. He placed broken eggshells next to some of the eggs, and, as a control, he left other camouflaged eggs alone without eggshells. He then watched to see which eggs were found more easily by crows. Because the crows could use the white interior of a broken eggshell as a cue, they repeatedly ate the camouflaged eggs that were near broken eggshells, and tended to ignore solitary camouflaged eggs that sat on the ground in plain sight. Tinbergen concluded that eggshell removal behavior is adaptive, that it does confer an evolutionary advantage on birds. Removing broken eggshells from the nest reduces predation of unhatched eggs (and probably of newborn chicks) and thus increases the chance that offspring will survive.
Figure 37.7. The adaptive value of eggshell removal.
Niko Tinbergen painted chicken eggs to resemble the mottled brown camouflage of gull eggs. Mottled eggs look like the rocky ground around a gull nest. The eggs were used to test the hypothesis that camouflaged eggs are more difficult for a predator to find and thus increase the young's chance of survival. He then placed broken eggshells near the camouflaged ones to test the hypothesis that the white interior of broken eggshells attracts predators.
It is not always so easy to learn how an adaptive trait confers its evolutionary advantage. Some behaviors, like eggshell removal, reduce predation. Other behaviors enhance energy intake, allowing an increased number of offspring to be supported. Still others reduce exposure or increase resistance to disease, enhance the ability to acquire a mate, or in some other way increase an individual’s fitness, its ability to contribute offspring to the next generation.
Key Learning Outcome 37.7. Behavioral ecology is the study of how natural selection shapes behavior.