ANDERSON, Matthew/J; Zoological Society of San Diego: Comparative reproductive morphology in mammals: a Darwinian perspective

Sperm competition occurs when the gametes of two, or more, males compete for opportunities to fertilize a given set of ova. This phenomenon is most frequently observed in mammals possessing a multiple partner mating system where females mate with more than one male during a single peri-ovulatory period. Previous studies have demonstrated that certain morphological characteristics are affected by selection pressures which relate to sperm competition (for example, testes size). This research reveals that morphological areas appertaining to sperm morphology and sperm delivery are also affected by sperm competition intensity. Comparisons of sperm morphology and vas deferens length (a highly muscular duct which rapidly transports sperm from the cauda epididymis and the ampulla during sexual activity) were undertaken between mammalian species possessing single partner mating systems and those with multiple partner mating systems. Subsequently, histological sections of the vas deferens were also compared to investigate possible evolutionary effects upon its musculature. Sperm midpiece volume was found to be significantly larger in mammalian species with multiple partner mating systems when compared to their single partner counterparts. The anatomy of the vas deferens was also found to be affected by sexual selection pressures with shorter, more muscular duct being observed in species with multiple partner mating systems. Further, strong correlations were also found when relative testes size was used as a surrogate for nominated mating system. Evolutionary effects observed are thought to relate to increases in sperm motility and more efficient sperm delivery. Such morphological markers are extremely useful in testing the efficacy of captive breeding programs and questions relating to animal housing.