Sperm competition and ejaculate evolution

Under sperm competition, production of large numbers of sperm is often advantageous, either due to numerical superiority or by displacing a larger volume of rival sperm. However, sperm and ejaculate production are inevitably associated with costs, which at times can directly limit male mating rates. As a consequence of sperm limitation, males are expected to strategically allocate their ejaculates to maximise fertilisations. Here I review the evidence for sperm production costs and male ejaculate tailoring showing that males are sensitive to cues relating to varying levels of sperm competition and female fecundity and modulate their ejaculates accordingly. Variation in ejaculation production costs are also shown to influence male ejaculates in relation to outbreeding, cost of inbreeding, and reproductive parasites. There is evidence that females suffer reduced fertility as a consequence of male strategic ejaculation, potentially resulting in sexual conflict over number of sperm delivered. The cost of ejaculate production and the necessity of producing large quantities of sperm may have promoted the evolution of 'manipulative' ejaculates, (e.g. mating plugs, non-fertile sperm, cooperative sperm, and seminal peptides). Many of these ejaculate components are designed to reduce or remove the risk of sperm competition altogether and hence the need to transfer large numbers or elaborate sperm to the female. Some of these adaptations are costly to females since they interfere with their reproductive rate or physiology.