Do mating strategies determine genital allometry in African mole rats (Bathyergidae)?

Abstract

Allometry describes the relationship of components of an organism with change in overall body size and has become the focus of numerous studies on the evolution of genitalia. Typically, negative allometry is observed in insects and is explained by stabilizing selection whereas the very few studies on mammals have shown a positive allometric relationship of genitalia in the body size, thought to have arisen as a result of sexual selection. However, all mammal species studied to date are thought to use mainly post-copulatory mating strategies. Across mammals, however, both pre-and post-copulatory strategies occur (although the two are not mutually exclusive). We propose that where pre-copulatory strategies are mainly used, no reproductive benefits would result from evolving positively allometric genitalia. As such, mammal genitalia are not typically positively allometric but rather allometry will, to a certain degree, be determined by mating strategy. We tested this prediction using four species of African mole rats (Bathyergidae) exhibiting variation in their life histories and mating strategies. Although generally supported, in that positive allometry did not occur in species that we assumed use mainly pre-mating strategies, positive allometry did not occur in either of the promiscuous species thought to use post-copulatory strategies. We suggest, therefore, that while mating strategies may tentatively determine genital allometry, whether positively allometric genitalia occur also depends on a number of complex interacting factors. In addition, this study provides further evidence and empirical support for the co-evolution of male and female genitalia in mammals.