This point was emphasized by Clutton-Brock (2002), who concluded that direct fitness benefits of group augmentation (e.g., improved survival) are common, so that “the direct benefits of helping to the helper’s own fitness have probably been underestimated” and “may be sufficient to maintain cooperative societies.” In contrast to cases in which group augmentation provides direct benefits to helpers, survival might also decrease with increasing group size. Where adult survival increases with increasing group size (e.g., splendid fairy wrens: Rowley and Russell 1990 and dwarf mongooses: Rood 1990), cooperative breeding might be favored through a combination of kin selection or by-product mutualism. These issues are largely due to the difficulty in collecting adequate data on survival it is well recognized that such data are necessary to resolve the relative importance of direct selection and kin selection in the evolution of cooperative breeding. (2014) recently noted that empirical tests of the effect of group size on survival remain relatively rare, and heuristic models of reproductive skew have made widely divergent assumptions about the ways that survival is affected by grouping and helping effort ( Härdling et al. For example, Solomon and French’s (1997), Cooperative Breeding in Mammals has 47 index entries for reproduction and reproductive suppression, but none for survival or mortality. 2004) have examined the effects of group size on reproduction but not survival.
In studies of cooperative breeders, the effects of group size on survival have received considerably less attention than effects on reproduction, and many empirical studies (including our own: Creel et al. Emlen (1997) summarized changes in the focus of this research, first from testing whether nonbreeding helpers truly contribute to the successful reproduction of others, thus creating a positive correlation between group size and the reproductive success of breeders (Skutch 1935 Rowley 1957), to identifying the ecological and demographic conditions that promote nondispersal and helping behavior ( Emlen 1982), to using reproductive skew models to test predictions from inclusive fitness theory about the allocation of reproduction among group members ( Vehrencamp 1980, 1983 Keller and Reeve 1994 Magrath et al. Following Darwin’s (1859) recognition that self-sacrificial behavior presents “a special difficulty, which at first appeared to me insuperable, and actually fatal to the whole theory” of evolution by natural selection, decades of research have considered the manner in which cooperative breeding can evolve through a combination of direct fitness benefits and kin selection ( Hamilton 1963 Wilson 1975). In cooperatively breeding species, adults provide care for the offspring of others by guarding them from predation, grooming them, feeding them, and even nursing them ( Brown 1987 Koenig and Stacey 1990 Creel et al. INTRODUCTION Cooperative breeding, cooperative hunting, and the effects of group size on fitness components More broadly, the relationship of effects of group size on survival and reproduction might be predicted by considering whether cooperation yields benefits that accrue to all group members (e.g., through cooperative vigilance) or benefits that must be apportioned to individuals (e.g., through cooperative hunting). The results also weaken support for the hypothesis that wild dogs are extinction prone due to group-level Allee effects. These results suggest that cooperative breeding in wild dogs cannot be fully explained by mutual direct benefit, thus reinforcing the prior inference that kin selection plays an important role in the evolution of their cooperation. Several tests confirmed that undetected dispersal is unlikely to have produced this pattern. Data from 366 individuals over a period of 6 years showed that the survival of adults decreased with increasing pack size, with a 25% difference between the largest and smallest packs after controlling for the effects of age, sex, social status, year of study, and pack identity. The production and survival of pups are known to increase with increasing pack size, but the effect of pack size on adult survival has not been examined previously.
We tested this hypothesis by examining relationships between group size, survival, and reproduction in African wild dogs ( Lycaon pictus), cooperative hunters with highly cohesive packs within which reproduction is monopolized by the dominant male and female. For cooperative breeders, we hypothesize that the effects of group size on reproduction and survival might run in opposition if the benefits of grouping cannot be shared without cost.
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