VIRILIZED SEXUAL GENITALIA AS ADAPTATIONS OF FEMALE SPOTTED HYAENAS

The study of traits as adaptations is reviewed using virilized female sexual genitalia of spotted hyaenas as an example. Female spotted hyae nas lack a vagina, have completely fused outer labiae and a clitoris ( 'pseudo-penis') that closely mirrors the male penis in size and erecta bility. The evolutionary origin and adaptive value of these structures have eluded scientists for centuries. The example of virilized genita lia of female spotted hyaenas demonstrates that questions about the ev olutionary origin of a trait benefit from adopting non-adaptive hypoth eses: the virilized genitalia are most likely a non-adaptive by-produc t of selection pressures related to the evolution of siblicide. Field studies suggest that virilized genitalia have fitness costs in the con text of parturition and convey benefits in communication (greeting cer emonies) and mating. The virilized genitalia of spotted hyaenas highli ght the need to pay attention to three aspects in the study of adaptiv e processes. First, it is important to study current use and fitness c onsequences in the field. Second, an important empirical question is w hether an observed trait is evolutionarily simple or complex in the se nse that the trait may be composed of a number of sub-traits subjected to independent selection events. In this context comparative studies may be useful. Third, the evolutionary history of a trait may be stron gly influenced by interactions with other traits.