Cranial ontogenetic development and sexual dimorphism of the extant water chevrotain Hyemoschus aquaticus

Abstract Tragulids or mouse-deer live in tropical Asia and Africa. They are the sister group of Pecora and have a long fossil record dating back into the Eocene. The extant African species, Hyemoschus aquaticus , is the sister taxon of all other extant tragulids and is therefore often used for comparison with fossil mouse deer. However, although many fossil tragulid skulls were found and studied within the last years, no attempt has been made for a comprehensive and comparative study of the ontogenetic development of cranial sexual dimorphism of H. aquaticus . Based on 39 extant skulls we establish an ontogenetic categorization of this species, comprising nine stages of tooth wear. We show that (I) in H. aquaticus upper canine development remains the main character for distinction of male and female, (II) that the eruption of the permanent upper canines and the development of sagittal crests starts before sexual maturity after the eruption of the second upper molar, and (III) that the development of further cranial crests (nuchal crests, temporal lines) varies greatly within the same age class and among the sexes. Thus cranial ornamentation is not useful as a primary distinction feature between the sexes and of limited importance in identifying ontogenetic age. The newly established categories of wear stages and the observed sexual similarity in cranial crest development can be used in future comparative studies with the other extant mouse-deer and especially with their fossil representatives.