Carnivores, but not insectivores, map onto the same social brain relationship as primates

Background It has previously been thought that the social brain hypothesis applies in its full quantitative form only to primates because only primates have bonded social groups. However, previous tests have overlooked the fact that the social brain hypothesis actually consists of a series of grades, not a single homogenous relationship. The presence of grades can cause conventional regression analysis to radically underestimate the slope of a relationship. Here, we test whether the social brain relationship with grades explains the pattern of group sizes observed in carnivores and insectivores. Results Using more appropriate brain indices and less error-prone statistical methods, we show that carnivores do exhibit a quantitative social brain relationship. More importantly, they map directly onto the primate socio-cognitive grades, dividing between loose sociality and bonded sociality paralleling the pattern observed in primates. Although insectivores do not exhibit a full quantitative social brain relationship, their neocortex size does increase with the transition from solitary to a loose form of pairliving in a way similar to that found in other mammals and birds. Conclusions These findings suggest that common trajectories and constraints might underlie social evolution in all mammals. In particular, a transition from a solitary lifestyle to a form of pairliving may have been an important early transition in the evolution of mammalian sociality. These results raise questions as to whether the different carnivore grades exhibit similar kinds of cognition to those found in the corresponding primate grades.