Answers to Review Questions

1. A proximate explanation of a behavior would look within the organism to explain how the behavior occurs, focusing on events that serve as the immediate causes of the behavior.

2. Natural selection is a process in which individuals with certain traits consistently survive and reproduce at higher rates than do individuals with other traits. An animal's behaviors can affect its ability to survive and reproduce. Therefore, natural selection should favor individuals whose behaviors make them efficient at performing such activities as foraging, obtaining mates, and avoiding predators. If the behaviors that confer advantage are heritable, then an animal will pass its advantageous behaviors to its offspring. When this is so, adaptive evolution can occur, in which the frequency of the advantageous behavior in a population increases over time. In cases where we demonstrate that natural selection has favored (or continues to favor) a particular heritable behavior, we can provide an ultimate explanation of the behavior by focusing on the evolutionary and historical reasons for why the behavior occurs.

3. A foraging animal often faces tradeoffs in which its ability to obtain food comes at the expense of other important activities, such as avoiding predators. When this occurs, individuals often alter their foraging decisions. Foragers may, for example, choose areas that provide less food but greater protective cover from predators. Fear of predators can have similar effects. For example, song sparrows exposed to playbacks of sounds made by predators (but no actual predators) fed their young less often, built their nests in denser, thornier vegetation, and spent less time incubating their eggs than did sparrows exposed to playbacks of nonpredators.

4. Sexual selection is a process in which individuals with certain characteristics have a consistent advantage over other members of their sex solely with respect to mating success. Charles Darwin pointed out that when sexual selection occurs, individuals typically use force or charm to gain access to mates. Often, the males compete with one another for the right to mate with females, while the females choose among the competing males; in some cases, the reverse occurs, and females compete for the right to mate with choosy males. Observational, genetic, and experimental evidence indicate that the large size, strength, or special weaponry of the males of many species result from sexual selection; such evidence also indicates that extravagant traits used to charm members of the opposite sex can result from sexual selection. Specific examples mentioned in the chapter include genetic evidence that the large body size and full curl of horns of male bighorn sheep result from sexual selection, along with Malte Andersson's classic experiments showing that sexual selection can explain the extremely long tails of male widowbirds.

5. In one example of how group living has both benefits and costs, goldfinches in a flock consumed more seeds per unit of time than did solitary birds. However, as the size of the flock increased, food supplies were depleted more rapidly, causing the birds to spend more time flying between feeding sites; traveling between feeding sites is energetically expensive and can lead to an increased risk of predation.

6. The greater expenditure of energy required by species B to fly between patches would dictate that it needs to spend longer in each patch in order to meet the assumptions of the marginal value theorem. Because its overall rate of energy gain in the habitat is lower, due to greater amount of energy it expends in traveling between patches, species B should deplete each patch to a greater degree before leaving it than species A.