Head-to-body size allometry in wasps (Vespidae): does brain housing constrain the evolution of small body sizes?

Species of wasps in the Vespidae family range widely in body size. Vespid wasp species’ mean brain size increases relative to head capsule size in smaller species. In this study, I tested whether head capsule size varied allometrically with overall body size. I compared species ranging in body size from some of the largest to the smallest species. I found that relative head capsule volume and weight showed negative allometry with body size: relative head size was larger in smaller species. This significant negative relationship held after accounting for phylogenetic effects, and the negative allometry held when only social species were analyzed. I suggest that limits on minimum brain size have affected the evolution of body allometry in vespid wasps. Negative head-to-body allometry may be caused by the need to accommodate relatively large brains in smaller bodied species, and brain-driven needs for allometrically enlarged heads could place lower limits on body size evolution in the Vespidae. The relatively large brains of small species could affect head capsule cuticle thickness or muscle mass, with biomechanical implications for species’ behavior and ecology.