Strength in numbers: males in a carnivore grow bigger when they associate and hunt cooperatively

Group-living has been recognized as one of the major transitions in evolution. Male sociality along with solitary females is rare in mammals, but it can provide unique insights into the evolution of sociality and cooperation. Because males compete with each other over females, male cooperation in mammals has been explained by joint defense of females against other males. Here, we demonstrate that the benefits of male cooperative hunting can play a major role in shaping sociality. By quantifying differences in morphology, activity, diet, and mating success, we show that in Madagascar’s top predator, the fosa ( Cryptoprocta ferox ), some males associate to jointly hunt large prey, which allows them to grow bigger than both solitary males and females. These associated males’ physical superiority also represents an advantage in contest competition for females, as reflected by higher mating success. Our results demonstrate that enhanced access to food resources by cooperative hunting is a key to physical development and competitiveness in fosas. In contrast to previous findings, we show that male sociality must not be limited to joint defense of territory and females, but that cooperation in food acquisition can favor sociality in sexually dimorphic species.