Resources and social organization of ground-dwelling squirrels [Ressources et organisation sociale des écureuils terrestres].

Résumé

(Hoogland 1986). Thus, females probably are forced to accept less closely related kin in order to have assistance in coterie defence. However, coterie space is limited and only a few of the young that are produced can expect to achieve residency. A female should initiate behaviors that will increase the likelihood that her daughters will become residents rather than nieces, granddaughters, cousins, sisters, etc. One strategy is to kill offspring who would compete with one's own offspring for the limited space. Although female prairie dogs defend their burrows against conspecifics, infanticide is common (Hoogland 1985). Competition among female black-tailed prairie dogs is greatest during late pregnancy when the danger of infanticide is high (Hoogland 1986). Infanticide in this context is readily understood as an evolutionary strategy. Furthermore, amicable behavior among females is not distributed according to relatedness but varies inversely with competition (Hoogland 1986). Thus animals do not aid the reproduction of kin, they attempt to gain more by maximizing their direct fitness (see also Rubenstein and Wrangham 1980). Sociality has both cooperative and competitive behaviors. Because social groups are frequently kin groups too much attention has focused on cooperation under the assumption that individuals should assist kin. This assumption implies that indirect fitness is an important component of inclusive fitness. The blend of cooperation and competition suggest that ground-dwelling squirrels attempt to maximize direct fitness (see also Armitage 1986a). The indirect component to fitness is important only as it contributes to an animal's direct fitness. Thus, a female yellow-bellied marmot or a female black-tailed prairie dog tolerates a sister or a daughter, not because of indirect fitness benefits, but because these individuals contribute to her direct fitness. Black-tailed prairie dog are highly agonistic toward non-coterie members, and amicable behavior within the coterie is always greater than amicable behavior with non-coterie members (Hoogland 1986). Obviously, inclusive fitness is greater when sociality involves kin groups rather than unrelated individuals. That is not the issue. What is critical is whether an animal's behavior is directed toward maximizing direct fitness. I suggest that the behavior of ground-dwelling squirrels is directed toward maximizing direct fitness. Social behavior is both cooperative and competitive. In most species, « competitive decisions » are made in the first year of life, dispersal occurs at this time and most species, including white-tailed prairie dogs, do not form matrilineal groups. When the competitive decisions come at age two or later, matrilineal groups are formed. But competition becomes a way of life as each individual plays its game of attempting to maximize direct fitness. I have sketched a scenario. This scenario can be verified or rejected only by focusing on the lifetime reproductive success of individuals of known relatedness. And these studies must include learning the fate of dispersers. Only by documenting the reproductive success of dispersers and resident can we construct a logical story of the relative importance of direct and indirect selection in animal sociality. I predict that direct selection will be by far the most important.