We are using within-species studies and comparative analyses to determine the effects of sperm competition on male investment into reproduction.
When females mate with more than one male, the sperm of those males must compete to fertilise a limited supply of eggs. The selection pressure this generates has far-reaching consequences for male behaviour, morphology and reproductive physiology.
Using fishes, frogs, house mice, insects, molluscs and other broadcast-spawning invertebrates as model systems, our work is showing how across animal taxa, male expenditure on the ejaculate increases with the strength of selection from sperm competition.
We are exploring how within-population variation in the level of sperm competition affects the strategic allocation of sperm to their ejaculates, depending on the reproductive value of their mates, and the prevailing risk of sperm competition. We seek to uncover the genetic basis of traits involved in sperm competition, and use a range of approaches from experimental evolution to quantitative genetic breeding designs to understand their evolutionary potential.
Sperm are the ultimate soldiers in the battle to fertilise eggs, and our research is exploring how sperm competition affects sperm form and function. This work attempts to link the morphology of sperm – the length of the tail or size of the head – to their swimming speeds, and their competitive fertilisation abilities.
Ejaculates consist of both sperm and non-sperm components but most research on sperm competition has focused on its effects on sperm numbers. Our work is showing how males adjust the competitive ability of sperm, in terms of their viability and swimming speed, in response to current levels of sperm competition. The most likely mechanism is via adjustments in seminal fluid composition. We are using proteomic techniques to identify proteins within the seminal fluids that influence the competitive ability of sperm.
Females are not inert vehicles in and around which males fight out their battles for fertilisations. They can exert strong influence over which males sperm ultimately fertilise their eggs, via cryptic female choice. Our work is showing that females can control fertilisation events by differentially accepting or rejecting sperm from different males, based on male attractiveness or relatedness.
Finally, male adaptations to sperm competition can be costly to females, inflicting genital damage on females, or killing their eggs because of fertilisations by more than one sperm. Such sexual conflict can generate female adaptations to reduce male harm.
Our research is exploring how females respond to sperm competition to ensure their own reproductive success.