Cooperative societies in three- dimensional space: On the origins of aggregations, flocks, and schools, with special reference to dolphins and fish

Abstract

In three-dimensional open space habits, and to a lesser degree open terrestrial habitats, cooperative groupings of animals have repeatedly evolved. These cooperative systems have been observed in a wide variety of animal taxa, ranging from sea urchins to cetaceans. Various attempts have been made to relate the origins of such patterns to kin or altruism theory. An evolutionary stable strategy appears to be involved. We propose a graded series of group structures of increasing complexity by means of which three-dimensional groupings could have evolved without recourse to either group selection or even necessarily kin selection or reciprocal altruism. These structures are asocial and social aggregations, and polarized schools. Social aggregations and polarized schools allow cooperative feeding and avoidance of predation. They confer three predation-related advantages over living alone for animals in open environments: (1) the dilution effect of large prey numbers relative to those of predators, (2) the encounter effect, which provides some protection from searching predators, and (3) the confusion effect by means of which visual tracking by a predator is confounded. We suggest that the gaze stabilization system of the visual system is involved in the most advanced version of the confusion effect. In polarized schools members sense and react to each other, forming a sensory integration system (SIS). This system allows detection and transmission of information across a school, flock, or herd in three dimensions. Because members watch beyond their immediate neighbors the transmission of such group reactions can greatly exceed the reaction speed of individual members, or any predator. Because the confusion effect and the SIS depend upon uniformity of behavior the polarized school is uncommonly difficult and perhaps impossible to cheat against. We perceive this as a key factor in the establishment of the evolutionarily stable strategy of schooling. Polarized schools and aggregations are considered as the extremes of a behavioral continuum. Because in daytime the polarized school is a safer place to be and because the aggregation allows more freedom of movement for such activities as food finding, groups in open space oscillate between the these extremes during varying levels of predation. The social complexity of fish schools seems modest whereas dolphin schools show the complexities of fairly typical mammalian organization. Occupancy of open space by both oceanic dolphins and schooling fish seems to have fostered promiscuous mating. In both open water fish and mammals elements of a cooperative disposition occur, which involves both cooperation and suppression of some aspects of individuality. Such dispositional elements allow the automatic support of a cooperative society. Dolphin schools, which during daytime rest or danger react like fish schools, express typical mammalian organization at other times. Dolphin echolocation has probably allowed the expression of mammalian behavior patterns at sea because it confers a major advantage over shark predators. The expression of mammalian social complexity may have required both kin and reciprocal altruistic patterns in different species.