Social cues of sperm competition influence accessory reproductive gland size in a promiscuous mammal

Abstract

Theory predicts that males should increase overall investment in ejaculate expenditure with increasing levels of sperm competition. Since ejaculate production is costly, we may expect males to tailor their reproductive investment according to anticipated levels of sperm competition. Here, we investigate plasticity in ejaculate investment in response to cues of population average levels of sperm competition in a promiscuous mammal, the bank vole (Myodes glareolus). We manipulated the social experience of experimental subjects during sexual development via differential exposure to the odour of rival males, to simulate conditions associated with relatively high or low average levels of sperm competition. Males exposed to a high level of competition developed larger major accessory reproductive glands (seminal vesicles) than those that experienced a low level of competition, suggesting that an increased investment in the production of copulatory plugs and/or mating rate may be beneficial at relatively high sperm competition levels. However, investment in sperm production, testis size and sperm motility were not altered according to social experience. Our findings emphasize the importance of non-sperm components of the ejaculate in mammalian postcopulatory sexual selection, and add to the growing evidence linking plasticity in reproductive traits to social cues of sperm competition.

Interactions between the developmental and adult social environments mediate group dynamics and offspring traits in Drosophila melanogaster

“…In groups consisted of mixed male phenotypes, large males (from low larval density) might have perceived weaker intra-sexual competition (due to the presence of small males), and therefore may decrease their courtship investment to allocate resources in ejaculate and offspring health. Furthermore, the larval manipulation could have signalled the level of intra-sexual competition that individuals were likely to face in adulthood 19, 20 which in turn could have altered male and female offspring resource allocation 59, 60 and life-history strategies 33 . Scrutinising these hypotheses lies beyond the scope of this paper, but remains an important topic of research for future studies.…”

The seminal symphony: how to compose an ejaculate

“…Male moths, Heliothis virescens , alter their ejaculate composition in response to female mating status, transferring larger spermatophores but equal sperm numbers, to previously-mated females, indicating greater allocation to non-sperm components [15]. Further indirect evidence comes from male voles, Myodes glareolus , which develop larger accessory glands (but not testes) in the presence of other males [16], and male fruit flies, Drosophila melanogaster , which alter the expression ratios of Sfp genes in response to the presence of rival males in the pre-mating environment [17]. In the black goby, Gobius niger , males adopt either a parental or sneaking behaviour and consequently face variable or consistently high sperm competition (respectively).…”

Ejaculate economics: an experimental test in a moth

“…However, the majority of females copulate multiply [3]. Males of many insect [2] and non-insect species [4] including humans [5] have evolved strategies to adjust ejaculate investment in response to sperm competition because spermatogenesis is costly. In the natural environment, sex ratio is temporally and spatially dynamic [6], which may provide information about the risk and intensity of sperm competition at a given time and space [7].…”