Many features of animal behaviour are so well suited to their function that it is impossible to imagine that they arose by chance. Echolocation by bats, the nest-building skills of weaver birds (family Ploceidae), and the alarm signals of ground squirrels all serve obvious purposes, and the mechanisms that enable them are remarkably similar to what engineers would design to achieve those ends. However, such adaptive behaviours have no divine designer but instead have arisen through the process of natural selection.

Natural selection is an inherently optimizing process: it favours those versions of an organism’s traits, including behavioral ones, which best enable the organism to propagate copies of its genes into future generations over alternative versions with lower fitness. Creating a formal optimality model is one way to infer the adaptive “design” or function of a behaviour. Using an engineering or economic model to work out the optimal behavioral solution for a given ecological problem is a way of specifying the best design out of a wide range of alternative possibilities. Therefore, if an optimality model embodies an accurate understanding of the function of a behaviour, it can predict the form of the behaviour that is observed in nature.

One of the attractions of using optimality models to test hypotheses about functional design is that these models yield quantitative predictions that can be easily tested. If a model’s predictions regarding the form of a behaviour do not match reality, one knows immediately that the hypothesis expressed in the model is false. For example, foraging honeybees often return to the hive with less than a full load of nectar, and biologists initially assumed this was because a bee maximizes its rate of energy delivery to the hive. The fuller the bee, however, the slower she can fly. As a result, the transportation of a full load was assumed to depress a bee’s rate of nectar collection. On the other hand, when the bees were trained to forage from an array of artificial flowers in which each flower offered a fixed amount of nectar and the time spent flying between flowers was varied to alter the duration and cost of foraging, the size of the bee’s load did not maximize her net rate of energy delivery to the hive. Further analysis revealed that a bee’s decision of when she would return to the hive is based on the maximization of foraging efficiency. Evidently, bees behave so as to achieve the highest foraging efficiency rather than the highest food-delivery rate to the hive.

A classic example of application of the optimality approach to understanding the adaptive design of a behaviour is a study of copulation time in the yellow dung fly (Scatophaga stercoraria) by British evolutionary biologist Geoffrey A. Parker. Shortly after cow excrement is deposited in a meadow, it is invaded by female dung flies that come to lay their eggs on the dung and by males seeking to mate with the females. Competition among the males for females is fierce. Sometimes one male succeeds in kicking a rival off a female during copulation and mounts her himself. Unfortunately for the first male, this means that some of the female’s eggs will be fertilized by the second male. The longer the first male copulates, the more eggs he fertilizes, but the returns for extra copulation time diminish rapidly. How much time should a male spend copulating with a female? Should he copulate for as long as is needed to fertilize all the eggs (about 100 minutes), or should he quit earlier (or permit himself to be displaced) so that he can go search for a new female? Parker hypothesized that a male dung fly chooses a copulation time that maximizes his overall rate of egg fertilizations. He tested his hypothesis using a graphical optimality model.

Before a male dung fly that has just finished copulating with a female can copulate with a new one, he must spend on average 156 minutes searching for her. Once he has found a new female, the proportion of her eggs fertilized by him as a function of copulation time is set by female physiology, and this has been quantified as a curve based on experimentally measured values. The male cannot shorten the time necessary to find a new female or change the fertilization curve, but he can stop copulating at will. The optimal solution, assuming that his decision regarding copulation duration serves to maximize his rate of egg fertilizations, is to copulate for 41 minutes. Because the average observed copulation time, 38 minutes, is quite close to the predicted time of 41 minutes, it is clear that the Darwinian algorithm underlying a male dung fly’s copulation behaviour serves to maximize his rate of egg fertilizations.

A second way of studying the adaptive design of a behaviour is what Darwin called the comparative method, which takes advantage of the thousands of “natural experiments” that have occurred over evolutionary time (that is, throughout the formation of new species and the evolution of their special characteristics). Here again, specific hypotheses regarding how natural selection has shaped a behaviour are tested. Rather than simply examining one species, behavioral researchers collect data from a number of species simultaneously. The idea is to compare the degree to which a particular behaviour occurs in each species with the degree to which the hypothesized selection pressure is part of the ecology of each species.

Australian zoologist Peter Jarman was one of the first to use the comparative method to study the diversity of mating systems, specifically among various species of African antelope. In some species, such as the dik-dik (Madoqua), individuals are solitary and cryptic; however, during mating season, they form conspicuous monogamous pairs. Others, such as the black wildebeest (Connochaetes taurinus), form enormous herds. During the breeding season, only a few males control sexual access to a group of females in a polygynous mating system. When Jarman compared these African ungulates, he found that body size, typical habitat, group size, and mating system were interrelated. Specifically, smaller species with relatively high metabolic rates (such as the dik-dik) need to consume high-quality food—such as fruits and buds in the forests—while concealing themselves from predators. Because of the sparse distribution of food and the need to remain solitary and cryptic to avoid capture by predators, the smaller species are widely dispersed, leaving no opportunity for a single male to monopolize access to many females. Consequently, small-bodied species tend to be monogamous. In contrast, the larger species graze in open plains where food is generally abundant, although seasonally variable in its geographic distribution, and they are highly visible to predators. Thus, species such as the wildebeest live in large herds that migrate with the seasons. Each individual may be hidden within the large number of other animals in the herd; however, group living creates the opportunity for one male to monopolize several females, and polygyny tends to be found in the large-bodied species. This pattern, which holds true for birds and primates as well as ungulates, supports the hypothesis that the mating system of a species is derived from selection pressures associated with food and predation. Selection pressures determine the spatial distribution of females and thus their defensibility by individual males.

Not all comparative analyses of behaviour are so broad. Some focus on just one behaviour or a morphological correlate of behaviour. Consider the case of sexual dimorphism in body size where the males of some species tend to be considerably larger than the females. It had been hypothesized that size is a key advantage in species where males must fight to defend females from rival males. To test the hypothesis that sexual dimorphism was favoured by natural selection, American evolutionary biologist Richard Alexander and his colleagues compared social structure of the breeding group in primates, ungulates, and pinnipeds with their degree of body-size dimorphism. They reported that body size is similar between males and females in species, including humans, where the breeding group typically consists of one male and one female or a few females. Male body size, however, increases compared with female body size in species that breed in groups made up of multiple males and females, and it is highest in species where a single male defends a large group of females. Evidently, male size in primates is an adaptation related to the intensity of male-male physical competition for females.