Abstract
BackgroundStudies of animal mating systems increasingly emphasize female multiple mating and cryptic sexual selection, particularly sperm competition. Males under intense sperm competition may manipulate sperm quantity and quality through masturbation, which could waste sperm and decrease fertility. I examined the factors influencing masturbation by male Cape ground squirrels (Xerus inauris) in light of a number of functional hypotheses.MethodologyObservational data on a marked population of squirrels were collected in east-central Namibia using scan and all-occurrences sampling.FindingsMasturbation was far more frequent on days of female oestrus and mostly occurred after copulation. Masturbation rates were higher in dominant males, which copulate more, than in subordinates and increased with number of mates a female accepts.ConclusionsThese results suggest that masturbation in this species was not a response to sperm competition nor a sexual outlet by subordinates that did not copulate. Instead masturbation could function as a form of genital grooming. Female Cape ground squirrels mate with up to 10 males in a 3-hr oestrus, and by masturbating after copulation males could reduce the chance of infection. Sexually transmitted infections (STIs) can profoundly affect fertility, and their consequences for mating strategies need to be examined more fully.
Highlights
Male reproductive success can be strongly affected by the quantity or quality of sperm produced, especially in promiscuous mating systems with high levels of sperm competition
Masturbation rates were higher in dominant males, which copulate more, than in subordinates and increased with number of mates a female accepts. These results suggest that masturbation in this species was not a response to sperm competition nor a sexual outlet by subordinates that did not copulate
Instead masturbation could function as a form of genital grooming
Summary
Male reproductive success can be strongly affected by the quantity or quality of sperm produced, especially in promiscuous mating systems with high levels of sperm competition. Males with high risk of sperm competition are selected to increase sperm viability, ejaculate volume, or sperm concentration, suggesting that high sperm numbers are important under intense sperm competition [3,5,6,7]. Sperm counts decrease with successive ejaculations in many species [5,12,13,14], suggesting that the rate of sperm production or maturation may be limited [13,15,16,17]. Males under intense sperm competition may manipulate sperm quantity and quality through masturbation, which could waste sperm and decrease fertility. I examined the factors influencing masturbation by male Cape ground squirrels (Xerus inauris) in light of a number of functional hypotheses
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