Contrasting selection pressure on body and weapon size in a polygynous megaherbivore

Sexual size dimorphism (SSD) is a common morphological trait in ungulates,
with polygyny considered the leading driver of larger male body mass and
weapon size. However, not all polygynous species exhibit SSD, while
molecular evidence has revealed a more complex relationship between
paternity and mating system than originally predicted. SSD is, therefore,
likely to be shaped by a range of social, ecological and physiological
factors. We present the first definitive analysis of SSD in the common
hippopotamus (Hippopotamus amphibius ) using a unique morphological
dataset collected from 2994 aged individuals. The results confirm that
hippos exhibit SSD, but the mean body mass differed by only 5% between the
sexes, which is rather limited compared with many other polygynous
ungulates. However, jaw and canine mass are significantly greater in males
than females (44% and 81% heavier, respectively), highlighting the
considerable selection pressure for acquiring larger weapons. A
predominantly aquatic lifestyle coupled with the physiological limitations
of their foregut fermenting morphology likely restricts body size
differences between the sexes. Indeed, hippos appear to be a rare example
among ungulates whereby sexual selection favours increased weapon size
over body mass, underlining the important role that species-specific
ecology and physiology have in shaping SSD.