BIOLOGY Volume 2 - A Guide to General Biology - 2004

17. ANIMAL COORDINATION AND REGULATION

17.8. Innate Behavior

17.8.6. Territoriality

A territory is a specific area of land or Water occupied by an individual or a population, which is actively defended against other members of the same or different species. Territorial behavior is typical of all vertebrates except amphibians, though it is rare among invertebrates. The Nature and Biological Role of territoriality have been extensively studied in birds and primates. In the latter, territoriality forms a crucial part of social behavior.

While the specific function of territoriality likely varies among animal species, in all cases it ensures that each breeding pair and its offspring have adequate space for feeding and reproduction. This enables species to make optimal use of their natural environment. The size of a territory varies from season to season depending on the Abundance of food resources. Birds of prey and large carnivorous mammals require a territory spanning several square miles to sustain themselves. In contrast, herring gulls and penguins (Fig. 17.61) restrict themselves to just a few square meters, as they use individual territories solely for mating and nesting, while foraging outside of them.

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Fig. 17.61. Territories occupied by penguins (top view). (Aerial photograph.)

Typically, the search for a territory is undertaken by the male prior to the breeding season. Defense of the territory is most intense during the breeding period, with clashes between males of the same species reaching their peak. There is a wide range of behavioral responses associated with the defense of individual plots, including threat displays between neighboring owners. These displays are triggered by specific releasing stimuli, or releasers. For example, Lack demonstrated that an adult male European robin (Erithacus rubecula) attacks a stuffed male of the same species with a red breast, or even a tuft of red feathers, yet ignores a juvenile male whose breast has not yet acquired adult coloration. The closer to the center of the territory, the more aggressive the owner becomes. Male aggressiveness is partly determined by Blood testosterone levels, which can also influence territory size. For instance, administering testosterone to a male red grouse can expand the area it occupies. Figure 17.62 illustrates how the territory sizes of three male grouse changed following testosterone administration. Along the boundaries between neighboring territories, grouse exhibit an equally high level of aggression. Despite the pronounced hostility and conflicts associated with territoriality, actual fights between individuals of the same species—which would be potentially dangerous for the species as a whole—are rare. Matters are usually restricted to threatening sounds, body movements, and distinct postures. Having claimed a territory, many species, especially carnivorous mammals, continuously mark its boundaries by leaving scent trails. Most commonly, they urinate on prominent objects along the territory's perimeter or rub against them with body parts containing scent glands, effectively Setting up boundary markers.

Although, as previously mentioned, territoriality is designed to ensure the optimal distribution of resources among population members, some individuals inevitably fail to secure a territory of their own. In many birds, particularly red grouse, weaker individuals are pushed to the fringes of the habitat, where they cannot find a mate and leave offspring. This appears to be a key aspect of the adaptive significance of territoriality: only the "fittest" reproduce and pass on their genes to subsequent generations. Thus, another function of territoriality is tied to intraspecific competition and can act as a mechanism regulating population size.

Fig. 17.62. Solid lines indicate the boundaries of individual territories occupied by several male red grouse. Dashed lines show the new boundaries established after birds A, X, and Y were administered testosterone. Individuals X and Y previously did not have territories of their own. (After Watson A. (1970). J. Reprod. Fert., Suppl., 11.3.)