Effect of vomeronasal organ removal on male socio-sexual responses to female in a prosimian primate (Microcebus murinus).

In most mammals, olfactory cues play an important role in regulating socio-sexual behaviors, but the relative contributions of the main olfactory system and the vomeronasal system remain unclear. The lesser mouse lemur, a nocturnal prosimian, possesses well-developed chemosensory structures, including a functional vomeronasal organ (VNO). In this primitive primate, social communication and competition between males for priority access to receptive females includes reliance on urinary chemical cues. To assess the role of the VNO in mediating males' behavioural responses to females, sexually-experienced intact males (C, n = 8) or males deprived of their VNO (VNX, n = 8) were put in pairs and their socio-sexual behaviors in response to a preoestrous female were monitored. Independent of social context (with or without female), VNX males exhibited less sniffing behaviors than C males, but their marking behaviors, although reduced, were not significantly different. Removal of the VNO dramatically reduced the frequency of both sexual behaviors (anogenital investigations, mounts) and intermale aggressive behaviors. However, VNO removal did not impair successful mating and had no effect on plasma testosterone levels. Lastly, VNX males always exhibited a significantly lower general activity level than C males. The decrease in sexual behaviors and the lack of aggressive intermale competition in responses to a preoestrous female possibly proceed from functional disturbances of central nervous areas connected to the vomeronasal system rather than from a chemosensory deficit per se.