Evolutionarily informed research has suggested that brains are composed of a number of specialized domain-specific mechanisms. For example, birds use different memory systems and different rules for remembering species song, the taste of poisonous food, and locations of food caches. Many birds learn to sing the song of their species during a brief critical period early in life and then reproduce it perfectly during the next breeding season, without ever having practiced it. On the other hand, birds can learn the characteristics of poisonous foods in a single trial during any time of life. Following yet a different set of rules, locations of food caches are learned, updated, and erased on a daily basis. Using the same decision rules for each of these problems would be highly inefficient, and different memory systems in birds are anatomically distinct. Likewise, humans inherit different memory systems for dealing with different, sometimes conceptually incompatible, tasks, including learning language, learning to avoid poisonous foods, and remembering other people’s faces.

Searching across species for broad theoretical principles

An evolutionary approach to behaviour involves an analysis of particular recurrent problems faced by the members of a given species and a search across species for correlations between common behaviours and common environmental conditions. It can be interesting to catalog unique adaptations (such as the colour bands on coral snakes or the human ability to throw objects over long distances), but evolutionary theorists have a higher goal—to uncover common principles underlying those diverse adaptations. For example, the concept of differential parental investment ties together diverse findings from a wide range of species. Briefly, as animals invest more in their offspring, they become more selective about mating decisions. If an adult fish sprays 1,000 eggs on a rock and then swims away, and can do so every few weeks, the investment in any one offspring is necessarily less than if the reproductive adult guards a nest and protects a smaller number of fry until they are capable of fending for themselves. As each offspring becomes more costly to raise, questions about the fitness of the mate become more important. In most species, the female has a necessarily higher initial investment: eggs are much more nutritionally costly to produce than are sperm. Thus, females have more to lose and are usually more selective about choosing a mate, preferring to mate only with males who are demonstrably more fit than their competitors (as manifested in healthier appearance, more colourful displays, etc.).

Sometimes females choose males who demonstrate a willingness to make their own investment, as in birds where males help build a nest and provide resources before females will mate with them. If one sex is relatively more careful about choosing mates, members of the opposite sex must compete to prove that they are better alternatives. Differential parental investment theory helps explain why male vertebrates are often more competitive, larger, and more colourful—because females generally make a higher investment in offspring (in mammals, e.g., this involves internal gestation and nursing). In some species, such as elephant seals and orangutans, males are much larger than females and considerably more aggressive. In species in which both sexes share in raising offspring, as in swans and penguins, the sexes tend to be less differentiated. The theory explains seeming sex-role reversals, as in phalaropes, birds in which the females are more colourful and more competitive than the males. Male phalaropes actually make the higher parental investment, because they care for the eggs while females go off in search of additional mates. As a consequence, males are relatively more selective in choosing mates, and females are in turn larger and more competitive.

Sexual selection is another broad evolutionary concept closely linked to parental investment. It refers to the process whereby the members of one sex come to have unique characteristics that assist in mating. For example, in many hoofed animals, males have horns and females do not. When features such as horns are found in males, it suggests they are related to mating and are useful in competing with other males or attracting females.

Although human males and females both share in raising offspring, the physical and behavioral differences between them suggest a history of sexual selection. For example, females have deposits of fat on their breasts and hips not found in other primates, which may be there because they advertised fertility to males. Males are taller, have larger upper body muscles, and are more likely to engage in violent competitions with other members of their sex. This suggests that our female ancestors were more likely to mate with males who could physically dominate other males. Modern mate preferences fit with these ideas, though the exact nature and magnitude of the human sex differences forged by sexual selection are still being debated.