Relationships between individuals create several kinds of organizations within groups of animals.
Coordination between moving animals leads the formation of groups. Examples include swarms of insects, flocks of birds, schools of fish, and herds of mammals. Coordinated group movements, even in very large groups, can be achieved by individuals obeying simple rules, such as 'keep close, but not too close, to your neighbors' and 'head in the same general direction as your neighbors.'
Several mechanisms that channel aggressive behavior create social organization. In social animals, dominance hierarchies reduce the potential costs of conflict over mates and food. Adominance hierarchy emerges when interactions between individuals result in physiological and behavioral changes: for example, 'winning' a contest may elevate testosterone, causing increased dominance behavior, and evoking submissive behavior from individuals who have been less successful in the past. In this way, coherent transitive hierarchies can emerge even when all individuals were initially equal. Similarly, terri-toriality reduces the costs of conflict over resources by partitioning a landscape among a population. Territoriality often generates spatial patterns, such as regular distances between nests in seabird colonies. In this case, the distance between nests is defined by the maximum area that a sitting bird can defend without abandoning her nest. More complex coordinated group behaviors can emerge when individuals take on different tasks and roles within groups. For example, within ant and termite colonies, individuals can develop into a variety of castes, each with distinct roles such as foraging, nest defense, and nursing young. In honeybees, individuals take on different roles at different life stages.
In some cases, upper limits exist on the size that social groups can attain and depend on interactions between the animals. In apes, for instance, where social bonds are maintained by grooming, troop sizes tend to be 30-60 individuals. Larger troops tend to fragment. Among humans, social groups are usually much larger. The anthropologist Robin Dunbar argues that this is a consequence of speech providing more efficient social bonding than grooming, leading to a natural group size of 100-150 individuals.
In most cases, group size may be the outcome of several interacting ecological and social factors. For example, although lions hunt cooperatively, prides and hunting groups are usually larger than is optimal for hunting efficiency. Lionesses cooperate to defend cubs against infanticidal males by forming creches. In addition, hunters are vulnerable to attack by larger groups, and territories are more effectively defended by larger prides.
The origin of cooperation among groups of cells and organisms can also be examined from the perspective of self-organization. The paradox of the evolution of cooperation is that (by definition) selfish individuals outcompete altruists, and therefore in a population of self-replicators, a selfish mutant should always spread at the expense of altruists. Nonetheless, altruism does occur among humans and cooperative behavior is often seen among animals. Such cooperative behavior can self-organize when the network structure that governs interactions among individuals results in the same individuals encountering one another repeatedly (e.g., when individuals are fixed in space, so that their only interactions are with their neighbors), or when their reproductive fate is very closely tied to that of others (as is the case for cells within a multicellular organism). Experimentally, the evolution of cooperation has been induced in bacterial populations by production of adhesive, causing individual cells to clump together. Cooperation can also evolve in marginal environments, where the evolutionary impact of competition between individuals is outweighed by the need to survive. Experimental studies of bacteria in marginal environments show that complex spatial patterns and signaling behaviors can emerge as a result of this selection. In theoretical models, the inclusion of policing behavior (punishing nonconformists) can also enforce high levels of cooperation even when interactions occur at random in large societies.
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