Abstract
Social circuitry of the mammalian brain can influence male reproductive physiology. This often manifests as plasticity in sperm production or allocation, particularly in response to male-male competition. However, socially mediated testicular plasticity has not been investigated with respect to mating and parental strategy. Testis mass and sperm production of sexually naïve and female-exposed adult male individuals of three rodent species were compared: the socially monogamous and paternal prairie vole (Microtus ochrogaster), the promiscuous meadow vole (Microtus pennsylvanicus), and the promiscuous house mouse (Mus musculus). Monogamously paired prairie vole males exhibited significantly larger testes and greater sperm production than naïve prairie vole males. Comparatively, there were no significant differences between naïve and monogamously paired male meadow voles or mice. To investigate the role of olfactory cues for regulating this phenomenon in prairie voles, a group of naïve males exposed to soiled bedding from novel females was used. These males were more similar to paired males than to naïve males not exposed to novel female odors, demonstrating a strong role of the social olfactory system. Further, the predictions of sperm competition theory (species with greater female promiscuity have larger testes than closely related species with less female promiscuity) are not observed between prairie voles and meadow voles. This demonstrates the complexity of intraspecies social dynamics and reproductive pressures which socially monogamous paternal males face and the evolutionary adaptations that may develop in response.