Abstract
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. 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. 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. 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. Several tests confirmed that undetected dispersal is unlikely to have produced this pattern. 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. The results also weaken support for the hypothesis that wild dogs are extinction prone due to group-level Allee effects. 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).
Highlights
Cooperative breeding, cooperative hunting, and the effects of group size on fitness components 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 1991; Solomon and French 1997)
This slope represents a 1.4% decline in annual survival per additional group member or a 25% difference in annual survival between the largest and smallest pack sizes observed in this study (Figure 2)
Pup survival was lower than that of adults (Figure 3), but subsequent survival was virtually constant among the adult age classes up to 8 years of age (Figure 3), which comprised more than 95% of the adult population (Creel S and Creel NM 2002; Creel et al 2004)
Summary
Cooperative breeding, cooperative hunting, and the effects of group size on fitness components 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 1991; Solomon and French 1997). 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 2004) have examined the effects of group size on reproduction but not survival. Kingma et al (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 2003; Cant and Field 2005) Solomon and French’s (1997) Cooperative Breeding in Mammals has 47 index entries for reproduction and reproductive suppression, but none for survival or mortality. Kingma et al (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 2003; Cant and Field 2005)
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