A form of defence of food sources, nest-sites, or other resources, against other members of the same species. Many animals defend patches of ground against intruders, usually of the same species. This is sometimes accomplished by outright aggression towards intruders, sometimes by threat displays, by scent marking (in mammals) and by song (in birds).
Territorial defence has both costs and benefits, and animals defend territories only when it is economical to do so. For example, spotted hyenas (Crocuta crocuta) defend territories in the Ngorongoro crater, where their prey is predictable and abundant, but not in the Serengeti plains, where it is very seasonal. The hyenas in the Serengeti wander over a wide home range, but do not defend any territory. Many animals vary their defence strategies depending upon the food supply. When food levels become high intruder pressure increases, and both sunbirds (Nectariniidae) and squirrels (Sciuridae) give up their territories, because the defence costs become too great. At the other extreme, foraging productivity may become so low that, even with territorial defence, the animal is unable to meet its daily energy requirements. Under these conditions the territory is abandoned.
Some animals only defend territories during the breeding season. In some cases the males defend territories, called leks, where the females come for mating. In some cases the territory contains vital resources that the females require, and the males gain access to females by controlling these resources. In the American bullfrog (Rana catesbeiana), for example, some males achieve much better mating success than others. These are the ones that are able to defend a territory containing the best egg-laying sites.