The evolution of head size hypoallometry: Biomechanical implications and brain investment as a possible cause.

Species' mean relative head size decreases with increasing species mean body size in paper wasps, which may have important implications for biomechanics in these flying animals. Here we quantify the allometric relationship (log/log slope) of head size to body size in paper wasps. We sampled species in two genera (Agelaia and Polybia) to test whether head/body allometry was consistent among genera. Head mass/total mass relationships were significantly hypoallometric (log/log slopes ∼0.90) and statistically similar between Agelaia and Polybia. We reanalyzed previously published multi-genus data to calculate the slope of head/body allometry, and to compare two different aspects of head size: the allometry of head mass which could impact weight distribution along the longitudinal axis of the body, and the allometry of head volume which could impact fluid resistance and mobility. The multi-genus data set yielded a similar estimate for the slope of head mass allometry (∼0.90), but the slope of head volume allometry was significantly shallower (∼0.80): relative head volume increases faster than relative head mass as total size decreases. We suggest the demands of brain housing affect the greater investment in head size and head weight in smaller species. Relative brain size is greater for smaller-bodied species within clades (Haller's rule), and brain volume had a significantly lower allometric slope than both head mass and head volume. Relatively large brains may require increased relative head size in smaller-bodied species. Brain housing may represent a basic developmental constraint on head size and head weight, and brain allometry could consequently impact the relationships of body shape and body mass distribution to body size.