Evolution and function of horns and hornlike organs in female ungulates

The evolutionary bases for horns, antlers, and tusks in male ungulate mammals have been extensively investigated, but the reasons for the presence or absence of hornlike organs in female ungulates have not been thoroughly reviewed. Here I examine (1) the taxonomic distribution of horns and hornlike organs in females and evidence on the evolutionary history of the trait; (2) behavioural evidence on the uses of horns and hornlike organs by females and socio-ecological correlations with female hornedness or hornlessness; and (3) evidence on the role of genes and hormones in controlling the expression of female horns. In cervids, antlered females may be rare because of a constraint to regrow antlers yearly, if at all, small social group sizes, and a direct linkage between androgen production and the initiation of antler growth. For non-cervid ungulate females, there appear to be no overriding or unifying aptive factors governing the evolutionary development of horns or hornlike organs, although the trait may be aptive in some cases.