Territorial behavior is a good subject for cost–benefit analysis. Territoriality is aggressive behavior that actively denies other animals access to a habitat or resource. Optimal habitats and resources may be in short supply, so conspecifics have to compete for them. Many animals—usually males—defend all-purpose territories that provide a nest site, food, and access to mates. The territory holder stakes out his boundaries by engaging in aggressive interactions with neighbors, and must then patrol those boundaries constantly and respond to trespassers. These aggressive interactions usually consist of highly stereotypic, species-specific displays such as birdsong. Through territorial behavior, the male obtains the resources he needs for reproductive success, but he also pays a price.

Territorial displays require considerable expenditure of energy, they make a male more vulnerable to predation, and they detract from the time he has for feeding or engaging in parental behavior. Michael Moore and Catherine Marler at Arizona State University performed an experiment to estimate the costs incurred by male Yarrow’s spiny lizards (Sceloporus jarrovii) when defending a territory. These lizards defend territories that include the home ranges of several females. Their territorial behavior is normally most intense during September and October when circulating testosterone levels of the males are high and the females are most receptive to mating. The researchers varied the intensity of the lizards’ territorial behavior by implanting testosterone capsules in some males in summer, when they are not normally highly territorial (Figure 52.8).

Testosterone-treated males spent more time patrolling their territories, performed more displays, and expended about one-third more energy than control males (energetic cost). They had less time to feed (opportunity cost), captured fewer insects, stored less energy, and had a higher death rate (risk costs). In summer, when females are not normally receptive, these high costs of vigorous territorial defense outweigh the reproductive benefits of territoriality. Thus natural selection has favored seasonal variation in the level of the hormone controlling territorial behavior in this species.

The cost–benefit approach explains the diversity of territorial behaviors seen in different species. Even if a resource is essential to an animal, if it cannot be defended economically, the animal will not engage in territorial behavior. Food is essential for all animals, but if the food is widely distributed in space or fluctuating in availability, there is no benefit to balance the high costs of trying to defend it. For example, the open ocean where seabirds feed cannot be defended. But safe nest sites on islands or rocky cliffs are in short supply, and they can be defended. The territories of seabirds may be no larger than the distance the birds can reach while sitting on their nests (Figure 52.9A).

In some cases the resource that is defended is the female herself. Elephant seals spend most of their lives at sea, but females come to land at traditional beach sites to give birth to their pups. Male elephant seals arrive at these sites ahead of time and stake out territories through vigorous fighting (Figure 52.9B). When the females arrive on the beaches, they have to enter the territories of the males. As long as the male territory holder can fend off challengers, he will be able to mate with all the females using his piece of the beach.

One unusual form of male territorial behavior arises in situations in which neither food, nest sites, nor females are defended. A lek is an area where males gather for the purpose of engaging in intense displays of their territorial prowess aimed at impressing females and winning the opportunity to mate. Even though space is not limited, each male defends a small piece of real estate on which he performs a display (Figure 52.9C). Those territories closest to the center of the lek are the prime sites, and males compete intensely for those locations. The females visit the lek, observe the males, and generally mate with the males holding the prime sites. The benefit of this system to the female is that she is inseminated by a successful competitor, and therefore her offspring will carry the genes that contributed to his success. This is another example of sexual selection (see Key Concept 20.2). The costs of lekking to males are high, as they engage in continuous, intense territorial behavior that precludes eating, drinking, and sleeping until they are displaced. The benefit is the chance to maximize their fitness by mating with many females.