There are many definitions of territory but in general a territory is an area defended by a 'resident' against 'intruders'. Territorial behavior is often intimately related to foraging - when territories are maintained to provide access to resources - but it occurs wherever an animal defends a spatially constrained resource. Territorial behavior is social when a territory has more than one resident, and when residents cooperate to defend this territory against intruders.

Early theoretical work on territorial behavior was based on Brown's idea of economic defendability: whether animals act to defend a territory should depend on whether the benefits gained from the territory outweigh the costs associated in maintaining it. This is difficult to test because the costs of defense and the benefits to be gained from a territory are not easy to measure in the same currency. Consequently, explicit tests of this approach have either been qualitative or based on relatively simple systems (e.g., nectar foragers like hummingbirds, where costs and benefits can both be measured in energy).

Social territories can be transient in the life of an animal, lasting only as long as the bond between mated pairs last or throughout adult life. In a classic study of pied wagtails based around an economic model of territorial defense, Nick Davies and Alasdair Houston demonstrated how changes in cost:benefit ratios might lead to rapid adjustments in the size of social territories. Territories defended by mated individuals are generally maintained throughout the breeding season, and may be defended by single pairs (as in many bird species), pairs with helpers, or complex associations of males and females (e.g., the dunnock, Prunella modularis).

Permanent social territories are maintained across a wide range of taxa. If the possession of a territory determines whether an individual produces offspring and thus gains reproductive fitness, the costs of maintaining a territory are balanced against lifetime reproductive success. Long-term data on many individuals, over lifetimes, are needed to measure this, and studies providing such data are rare.

The ecological effects of territorial behavior depend on how groups interact with each other and with the resources they obtain from the territory. Social territory defense may rely on cooperation among social group members. In ants, territories may be defended through fights, to which individuals are recruited. In lions, group members roar together, presumably as an honest signal of their ability to defend their territory. Of course, not all group members take an equal share in territorial behavior, and the costs of territorial behavior are not spread evenly across the social group. Similarly, the existence of hierarchies or reproductive skew (see the section titled 'Societies') within social groups means that not all individuals benefit equally from group territoriality. Because of these asymmetries, the evolution of social territoriality depends on many more factors than the benefits of resources and costs of defense included in early economic models of territoriality devised for solitary animals. For example, a model developed by Moorcroft and coworkers in 2006 that incorporated individual movement pattern, behavioral interactions, and landscape heterogeneity successfully predicted changes in space use and territories in coyote packs.

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