Dominance hierarchies characterize many species in which individuals live in close proximity to one another. The dominance hierarchy is a social structure within a group of animals in which certain individuals are dominant over others, and are therefore able to claim access to better resources in the form of food, mates, shelter, and other desirable commodities.
The evolution of dominance hierarchies in a species is indicative that there is competition for resources. Members of a dominance hierarchy are aware of how they are positioned within that hierarchy and they behave appropriately. Of particular importance, the establishment of dominance hierarchies allows for the resolution of conflict between individuals without costly fighting that can result in serious injury or even death. In species where organized group living is essential to survival, it also serves to maintain order among pack members.
Establishment of Dominance Hierarchies
Dominance hierarchies are often established through ritualized displays or mild fighting, rather than all-out battle. The loser in a battle for dominance typically moves away from a choice habitat or a disputed mate. Among primates, dominance conflicts frequently involve no more than the display of enlarged canines, sometimes through yawning. Bears, also, will roar or wave their open mouths at social inferiors. Behaviors like these do not require fighting, but do result in the prominent exhibition of potentially formidable fighting weapons. In other cases, as in elephant seals, there actually can be prolonged, often bloody fighting. However, once the hierarchy is established, subsequent fighting is less frequent. In many cases, there is a strong correlation between dominance and large size.
Dominance hierarchies have to be reestablished when certain individuals feel prepared to move up within the hierarchy, or when new individuals are introduced into an area. During such time a series of challenges may occur. This can be a stressful period for all individuals involved.
Dominance and Mate Competition
Mate competition is extremely common in the animal kingdom, and many dominance hierarchies relate to competition either for mates, or for those resources such as admirable territories that will attract them. In most cases males compete for females, although there are also a few instances of females fighting for males.
There are clearly advantages to dominance. Dominant males have been shown in many species to copulate more frequently or to produce more off-spring. In cowbirds, for example, only the dominant male is allowed to sing the songs that are most effective in attracting females. If subordinate males attempt to sing these highly charged songs, they are attacked, often brutally, by more dominant individuals.
Elephant seals are another group in which reproductive success is linked to dominance. Dominance battles in this species involve two males posturing chest to chest and attempting to bite each other, with the loser ultimately retreating. In a few species, such as wolves, the dominant members of a group are the only ones that reproduce.
One tell-tale sign of competition for mates is sexual size dimorphism, which describes a situation where one sex of a given species has much greater body size than the other. In the case of mate competition, it is the males that are larger than females. (There are other species where the females are larger, including, the large majority of frogs. However, in these species the large size of females appears to be associated with increased fertility rather than with the establishment of dominance.)
Sexual size dimorphism is often particularly pronounced in species where it is possible for a single male to monopolize many females, as in elephant seals. In fact, a study across various pinniped species (seals, sea lions, etc.) suggests that the degree of sexual size dimorphism is positively correlated with the size of the harem.
The spotted hyena.
A particularly interesting example of the dominance hierarchy is that of the spotted hyena. It is the largest species of hyena and has also been called the laughing hyena because of the calls that individuals make when they are in danger. Spotted hyenas live in social groups that vary greatly in size, with the largest having as many as eighty members. Each group defends a territory and hunting occurs in packs.
What is unusual about social organization in this species is that females are dominant within the group and at the same time possess reproductive organs that very much resemble those of males. In fact, female genitalia resemble the scrotum and testes of males so closely that it is almost impossible to determine the sex of individuals in the field.
One early hypothesis to explain this male-mimicking anatomy was that females evolved it in order to participate in the hyena greeting ritual, in which members of the same social group sniff each others' erect penises when they meet again after an absence. Because greeting behavior is important to group solidarity, it was argued that females evolved male-like anatomy so they could participate as well.
However, the greeting ritual theory has since been abandoned in favor of an argument based on fighting for dominance within the hierarchy. There are numerous benefits to being the dominant female within a spotted hyena clan. Females who are high in the hierarchy have priority at kills, and obtain more food than subordinate females or males. Dominant females tend to be the largest hyenas of a pack. They also tend to produce dominant off-spring. The production of a dominant male is particularly advantageous because only the dominant male within a pack mates.
Many scientists believe that because aggressive behavior is advantageous in competitions for dominance, female hyenas have evolved high circulating levels of androgens (male sex hormones) such as testosterone, which promote aggression. The curious male-mimicking genitalia are now believed to be a mere side effect of the unusually high testosterone levels. The testosterone circulating in the female's bloodstream while she is pregnant results in the masculinization of the anatomy of both her male and female offspring. It was indeed confirmed that female spotted hyenas do in fact have unusually high testosterone concentrations in their blood. span>
see also Behavior; Social Animals.
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Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton, 1996.
Halliday, Time. Animal Behavior. Norman: University of Oklahoma Press, 1994.
Krebs, John R., and Nicholas B. Davies. Behavioural Ecology: An Evolutionary Approach,4th ed. Cambridge, MA: Blackwell Science, 1997.