The ultimate function of animals inhabiting and defending a territory is to increase the individual fitness or inclusive fitness of the animals expressing the behaviour. Fitness in this biological sense relates to the ability of an animal to survive and raise young. The proximate functions of territory defense vary. For some animals, the reason for such protective behaviour is to acquire and protect food sources, nesting sites, mating areas, or to attract a mate.
Territories have been classified as six types.Type A: An 'all-purpose territory' in which all activities occur, e.g. courtship, mating, nesting and foraging
Type C: A nesting territory which includes the nest plus a small area around it. Common in colonial waterbirds.
Type D: A pairing and mating territory. The type of territory defended by males in lekking species.
Type E: Roosting territory.
Type F: Winter territory which typically includes foraging areas and roost sites. May be equivalent (in terms of location) to the Type A territory, or for a migratory species, may be on the wintering grounds.
Reports of territory size can be confused by a lack of distinction between home range and the defended territory. The size and shape of a territory can vary according to its purpose, season, the amount and quality of resources it contains, or the geography. The size is usually a compromise of resource needs, defense costs, predation pressure and reproductive needs.
Some species of squirrels may claim as much as 10 hectares of territory. For European badgers, a home range may be as small as 30 hectares in a good rural habitat, but as large as 300 hectares in a poor habitat. On average, a territory may be approximately 50 hectares, with main setts normally at least 500 metres apart. In urban areas, territories can be as small as 5 hectares, if they can obtain enough food from bird tables, food waste or artificial feeding in suburban gardens. Spotted hyenas (Crocuta crocuta) have highly variable territory sizes, ranging from less than 4,000 hectares in the Ngorongoro Crater to over 100,000 hectares in the Kalahari.
In birds, golden eagles (Aquila chrysaetos) have territories of 9,000 hectares, least flycatchers' (Empidonax minimus) territories are about 600 square metres and gulls have territories of only a few square centimetres in the immediate vicinity of the nest.
Territories can be linear. Sanderlings (Calidris alba) forage on beaches and sandflats. When on beaches, they feed either in flocks or individual territories of 10 to 120 metres of shoreline.
The time to develop territories varies between animals. The marine iguana (Amblyrhynchus cris tatus) is a lekking reptile. Males start to establish small display territories two months ahead of the mating season.
Rather than retaining a territory simply by fighting, for some animals this can be a 3-stage process. Many animals create "sign-posts" to advertise their territory. Sometimes these sign-posts are on the boundary thereby demarcating the territory, or, may be scattered throughout the territory. These communicate to other animals that the territory is occupied and may also communicate additional information such as the sex, reproductive status or dominance status of the territory-holder. Sign-posts may communicate information by olfactory, auditory, or visual means, or a combination of these. If an intruder progresses further into the territory beyond the sign-posts and encounters the territory-holder, both animals may begin "ritualised aggression" toward each other. This is a series of stylised postures, vocalisations, displays, etc. which function to solve the territory dispute without actual fighting as this could injure either or both animals. Ritualised aggression often ends by one of the animals fleeing (generally the intruder). If this does not happen, the territory may be defended by actual fighting, although this is generally a last resort.
Scent marking, also known as territorial marking or spraying when this involves urination, is a behaviour used by animals to identify their territory. Most commonly, this is accomplished by depositing strong-smelling substances contained in the urine, faeces, or, from specialised scent glands located on various areas of the body. Often, the scent contains pheromones or carrier proteins such as the major urinary proteins to stabilize the odours and maintain them for longer. The animal sniffing the scent frequently displays a flehmen response to assist in detecting the mark.
Felids such as leopards and jaguars mark by rubbing themselves against vegetation. Prosimians and New World monkeys also use scent marking, including urine washing (self-anointing the body with urine), to communicate. Many ungulates, for example the blue wildebeest, use scent marking from two glands, the preorbital gland and a scent gland in the hoof.
Territorial scent marking may involve behaviours specific to this activity. When a wolf marks its territory, it lifts a hind leg and urinates on a scent post (usually an elevated position like a tree, rock, or bush). This raised leg urination is different from normal urination, which is done while squatting. This posture is exclusive to alpha wolves of either sex, although the alpha male does this most often. The alpha female usually urinates on a scent post that her breeding partner has just urinated on, although during the mating season, the female may first urinate on the ground. All other females in the pack, and also young wolves and low-ranking male wolves, urinate while squatting. Males and female ring-tailed lemurs (Lemur catta) scent-mark both vertical and horizontal surfaces at the overlaps in their home ranges using their anogenital scent glands. To do this, they perform a handstand to mark vertical surfaces, grasping the highest point with their feet while applying the scent.
In the Eastern carpenter bee, Xylocopa Virginica, both sexes have glands that evolved for marking the nest. Males, although they have the gland, are unable to produce the marking substance. Female secrete it near the nest site entrance to establish their territory.
Visual sign-posts may be a short-term or long-term mode of advertising a territory. Short-term communication includes the colouration or behaviour of the animal, which can only be communicated when the resident is present. Other animals may use more long-term visual signals such as faecal deposits, or marks on the vegetation or ground. Visual marking of territory is often combined with other modes of animal communication.
Some animals have prominent "badges" or visual displays to advertise their territory, often in combination with scent marking or auditory signals. Male European robins are noted for their highly aggressive territorial behaviour. They attack other males that stray into their territories, and have been observed attacking other small birds without apparent provocation. Such attacks sometimes lead to fatalities, accounting for up to 10% of adult robin deaths in some areas. The red breast of the bird (i.e. badge) is highly visible when it sings (vocal marking) at the boundary of its territory. The ring-tailed lemur (Lemur catta) advertises its territory with urine scent marks. When it is urinating for marking purposes, it holds its extremely distinctive tail high in the air adding a visual component to the advertisement; when it is urinating for eliminative purposes, its tail is only slightly raised.
Rhinoceros have poor vision but may use visual marking. Dominant white rhino bulls mark their territory with faeces and urine (olfactory marking). The dung is laid in well defined piles. There may be 20 to 30 of these piles to alert passing rhinoceroses that it is occupied territory. Other males may deposit dung over the piles of another and subsequently the sign-post grows larger and larger. Such a dung heap can become up to five metres wide and one metre high. After defecating, greater one-horned rhinos scratch their hind feet in the dung. By continuing to walk, they “transport” their own smell around the paths, thus establishing a scent-marked trail. Another method of visually marking their territory is wiping their horns on bushes or the ground and scraping with the feet, although this is likely combined with the smell of the marking animal. The territorial male scrape-marks every 30 m (98 ft) or so around its territory boundary.
After leaving a urination mark, some animals scrape or dig the ground nearby, thereby leaving a visual advertisement of the territory. This includes domestic dogs.
Several species scratch or chew trees leaving a visual mark of their territory. This is sometimes combined with rubbing on the tree which may leave tufts of fur. These include the Canada lynx (Lynx canadensis) and the American black bear (Ursus americanus). Many animals have scent glands in their paws or deposit fur during tree-marking, so tree-marking may be a combination of both visual and olfactory advertising of the territory. The male ring-tailed lemur has a specialised adaptation to assist in leaving visual/olfactory territorial marks. On their inner forearm (antebrachial) is a scent gland which is covered by a spur. In a behaviour called "spur marking", they grasp the substrate, usually a small sapling, and drag the spur over it, cutting into the wood and spreading the gland's secretions. When on the ground, ring-tailed lemurs preferentially mark small saplings and when high in the trees, they usually mark small vertical branches.
European wildcats (Felis silvestris) deposit their faecal marks on plants with high visual conspicuousness that enhances the visual effectiveness of the signal.
Many animals use vocalisations to advertise their territory. These are short-term signals transmitted only when the animal is present, but can travel long distances and over varied habitats. Examples of animals which use auditory signals include birds, frogs and canids.
Wolves advertise their territories to other packs through a combination of scent marking and howling. Under certain conditions, wolf howls can be heard over areas of up to 130 km2 (50 sq mi). When howling together, wolves harmonize rather than chorus on the same note, thus creating the illusion of there being more wolves than there actually are. Wolves from different geographic locations may howl in different fashions: the howls of European wolves are much more protracted and melodious than those of North American wolves, whose howls are louder and have a stronger emphasis on the first syllable.
Animals use a range of behaviours to intimidate intruders and defend their territories, but without engaging in fights which are expensive in terms of energy and the risk of injury. This is ritualised aggression. Such defense frequently involves a graded series of behaviours or displays that include threatening gestures such as vocalizations, spreading of wings or gill covers, lifting and presentation of claws, head bobbing, tail and body beating, and finally, direct attack. Domestic cats (Felis catus) are very territorial and defend their territories with ritualized body posturing, stalking, staring, spitting, yowling and howling. Spider monkeys (genus Ateles) defend their territory by screams, barks, rattling or dropping branches, and urinating and defeacating on intruders below. Oscar cichlids (Astronotus ocellatus) are able to rapidly alter their colouration, a trait which facilitates ritualised territorial and combat behaviours amongst conspecifics. Individuals of another cichlid species, the blunthead cichlid (Tropheus moorii), defend their feeding territory with a display, quivering the tail and fins to intimidate, or an attack, darting at the intruder and chasing them away. Astatotilapia burtoni cichlids have similar displays of aggressive behaviour if they are territorial, which include threat displays and chasing.
Male ring-tailed lemurs have scent glands on their wrists, chests, and in the genital area. During encounters with rival males they may perform ritualised aggression by having a "stink fight". The males anoint their tails by rubbing the ends of their tails on the inside of their wrists and on their chests. They then arch their tails over their bodies and wave them at their opponent. The male toward which this is directed either responds with a display of his own, physical aggression, or flees. "Stink fights" can last from 10 minutes to one hour.
Ritualised aggression can sometimes include actual fights. The creek chub (Semotilus atromaculatus) engages in ritualized aggression when others of the species invade its territory. Engaging in parallel swimming, the fish widens its fins and mouth and swims at a caudal fin beat. Intimidating opponent fish throughout these rituals, the forward fish stops and directs blows to the head of the other fish to ensure territory dominance.
Territories may be held by an individual, a mated or unmated pair, or a group. Territoriality is not always a fixed behavioural characteristic of a species. For example, red foxes (Vulpes vulpes) either establish stable home ranges within particular areas or are itinerant with no fixed abode. Territories may vary with time (season), for example, European robins defend territories as pairs during the breeding season but as individuals during the winter. Resource availability may cause changes in territoriality, for example, some nectarivores defend territories only during the mornings when plants are richest in nectar. In species that do not form pair bonds, male and female territories are often independent, i.e. males defend territories only against other males and females only against other females. In this case, if the species is polygynous, one male territory probably contains several female territories, while in some polyandrous species such as the northern jacana, this situation is reversed.
Animals may use several strategies to defend their territories.
The first game theory model of fighting is known as the hawk-dove game. This model pits a hawk strategy (always try to injure your opponent and only withdraw from the contest if an injury is received) against a dove strategy (always use a non-injurious display if the rival is another dove and always withdraw if the rival is a hawk).
Another strategy used in territory defence is the war of attrition. In this model of aggression, two contestants compete for a resource by persisting while constantly accumulating costs over the time that the contest lasts. Strategically, the game is an auction in which the prize goes to the player with the highest bid, and each player pays the loser's low bid.
Some animals use a strategy termed the dear enemy effect in which two neighbouring territorial animals become less aggressive toward one another once territorial borders are well-established and they are familiar to each other, but aggression toward unfamiliar animals remains unaffected. The converse of this is the nasty neighbour effect in which a territory-holder shows heightened aggression toward neighbouring territory-holders but unaffected aggression to unfamiliar animals or distant territory-holders. These contrasting strategies depend on which intruder (familiar or unfamiliar) poses the greatest threat to the resident territory-holder.
In territory defence by groups of animals, reciprocal altruism can operate whereby the cost to the benefactor in helping defend the territory is less than the gains to the beneficiary.
Many species demonstrate polyterritoriality, referring to the act of claiming or defending more than one territory. In the European pied flycatcher (Ficedula hypoleuca), researchers assert that males exhibit polyterritoriality to deceive females of the species into entering into polygynous relationships. This hypothesis, named the deception hypothesis, claims that males have territories at distances sufficiently great that females are unable to discern already-mated males. The observation that males travelled long distances, ranging from 200m to 3.5 km, to find a second mate supports this the argument. The debate about polyterritoriality in this species may initiate research about the evolution and reasons for polyterritoriality in other unrelated species.