Sexual dimorphism is the difference exhibited between the sexes of a species, often in the form of a larger size, colouration or weaponry in one sex. Commonly associated with polygamous mating systems and greater sociality; many species of ungulates show high levels of sexual dimorphism due to the benefits gained during sexual selection. The hippopotamus (Hippopotamus amphibius) is an African megaherbivore which shows highly competitive and aggressive social behaviours. Their polygamous and social nature predicts high levels of sexual size dimorphism (SSD), with males being substantially larger than females. However, there has been a significant lack of research conducted into the sexual dimorphism of this species. This study suggested there is not a large difference between adult male and female mean body masses that you would expect from this species. However; there was a significant difference found in the mass of the lower jaws, with males having on average 42% heavier lower jaws than females. These results imply hippopotamus do not show a large extend of SSD, regardless of their similarity to species who exhibit large differences in body size between the sexes. Instead it is suggested male hippopotamus invest more into developing larger jaws, possibly to increase success in sexual selection. This is could be related to this species’ semi-aquatic nature as well as their vulnerability to periods of drought. Further study into this species could help examine the drivers involved in the evolution of sexual dimorphism, and the effects niche and environment may have on its presence.