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In ethology, agonistic behaviour is any social behaviour related to fighting. Thus it is broader than aggressive behaviour because it includes not only actual aggression but also threats, displays, retreats, placating aggressors, and conciliation. The term was coined by Scott and Fredericson in 1951. Agonistic behaviour is seen in many animal species because resources including food, shelter, and mates are often limited.
Some forms of agonistic behaviour are between contestants who are competing for access to the same resources, such as food or mates. Other times, it involves tests of strength or threat display that make animals look large and more physically fit, a display that may allow it to gain the resource before an actual battle takes place. Although agonistic behaviour varies among species, agonistic interaction consists of three kinds of behaviours: threat, aggression, and submission. These three behaviours are functionally and physiologically interrelated with aggressive behaviour yet fall outside the narrow definition of aggressive behaviour. While any one of these divisions of behaviours may be seen alone in an interaction between two animals, they normally occur in sequence from start to end. Depending on the availability and importance of a resource, behaviours can range from a fight to the death or a much safer ritualistic behaviour, though ritualistic or display behaviours are the most common form of agonistic behaviours.
The type of agonistic behaviour observed, whether it be aggressive or submissive, all depend on the likelihood of winning. For instance, size is usually a good predictor of fighting success, and many animals will display to flaunt their size. Animals are better able to assess their next form of agonistic action by judging the opponent's size and if they are likely to win a fight if a physical fight were to occur.
In aggressive behaviour by male stalk-eyed flies the males "square off" by displaying their eyes. Females show a strong preference for mating with males with longer eyestalks. Due to the female preference, males have evolved to compete with each other for mating rights. In the threat display the two flies face each other head-to-head, with their forelegs spread outward and parallel to the eyestalks. This behaviour allows each individual to judge the distance between its competitor's eyes. Eyestalk length increases with body size, and males with shorter eyestalks usually will retreat. A further distance between the eyes conveys a bigger body size, and a better chance of winning.
Physical fighting is actually rare between animals. It would seem that normally the more aggressive an animal is, the more it has to gain. However, in a normal scenario if an animal is too aggressive it might face an unacceptably high cost such as severe injury or death. Unless an animal has a sure indication that they will win without injury, or the resources are valuable enough for the risk of death, animals usually avoid fighting. An animal must weigh the relative costs and benefits of fighting. If the costs are too high, avoiding a fight is preferable.
For animals, display is any behaviour modified by evolution that is used to convey information. Animals display particular signs, which recipients can use to infer something about the mental and physical state of the first animal. To avoid the heavy cost of fighting, animals have evolved sophisticated rituals, which they use to bluff their opponents into backing down or fleeing. The cost-benefit model of display makes three assumptions: (1) type of display varies depending on the cost; (2) the risk of the display increases as the effectiveness of display increases; and (3) the value of resource being disputed over determines the choice of display used. Animals have evolved to use their physical attributes as a display of ability. If contests can be resolved with ritual display, fighting is not needed. Display can be used to dispute for mates, territory, and food through symbolic gestures instead of battles to the death. If an animal can display without fighting that he is more physically fit than his opponent, he will have gained more than he would have if he had fought and in the process possibly been injured.
Male Gray Catbirds fluff their feathers and spread and lower their tails to defend their territory when threatened by another male. The bird that is capable of puffing up and appearing to be the biggest will win the territory.
Male western gorillas display a wide range of both vocal and gestural communications when threatened by an opponent. A silverback (alpha male) will start hooting, throwing, chest pounding, leg kicks, and sideways running when approached by another male. This is done to intimidate the opponent and show physical abilities without actually making any physical contact.
Threat behaviour is any behaviour that signifies hostility or intent to attack another animal. Threat behaviour is meant to cause the opponent to back down and leave. While ritual display can be used for an array of reasons or communicative purposes, threat distinctly is meant for hostility and is the last step before fighting or submission. Threat does not involve physical contact with another animal. Any threat behaviour most often elicits other agonistic behaviour in the recipient. This initiation of threat will result in a display of physical attributes, a fight, or submission; the behaviour or sequence of behaviours depends on what resources are being fought over and each individual’s chance of winning against his opponent. In any animal species, threat always contains components of attack and fleeing, which expresses an animal’s readiness and likelihood of winning. An intimidation display with a means to threat are exhibited through: hair bristling, feather ruffling, raising skin folds and crest, teeth displaying, horn displaying, making sound, etcetera.
Chlamydosaurus kingii, an Australian agamid lizard, uses its frill as a way to display size and aggression to opponents. It is one of the largest and most notable displays seen in the animal kingdom. In comparison to its body size, the frill can flare out to make the lizards head look several times bigger, and it displays bright orange and red scales. Males of C. kinggi fight and display frills often during the mating seasons. The male ritualistic display includes repeated partial erections of the frill, head bobbing, tail lashing, and waving of forelimbs.
Actual fighting in contests is rare because of the risk of injury with the exception of the rooster or fighting cock. It is most likely to occur when individuals are similarly sized, or when reproduction and survival without the contested resource is less likely. Even when agonism escalates to actual fighting, restraint may be used. Fish such as Oreochromis mossambicus often exhibit aggressive displays, but rarely fight to the point of injury or bodily harm. This is the case in fights among some male venomous snakes for females. They wrestle, but refrain from biting.
Agonistic fighting for black mambas involves a wrestling match in which opponents attempt to pin each other’s head repeatedly to the ground. Fights normally last a few minutes but can extend to over an hour or more. The purpose of fighting is to secure mating rights to receptive females nearby during the breeding season.
Submissive behaviour involves an individual indicating by an act or posture that it will not challenge a dominant individual in a social group. Submissive behaviours are part of the maintenance of a dominance hierarchy of cooperating individuals in a social group that have overlapping but not entirely coincident interests.
Communication between animals is often achieved by adding a succession of behavioural responses to a display. Social interactions among bearded dragons (Pogona vitticeps) consist of a unique set of movements that convey visual signals and communication. Waving is one of the most visible signs of submission one lizard can display to another Lizard. The lizard will rest on three of its legs and raise one of the front arms and then slowly wave the arm in a circular motion. This circular motion, along with the dragon puffing up slightly, shows submission. This display is seen between opponents, as well as from adolescents towards adults.