Abstract
Sperm competition is often considered the primary selective force underlying the rapid and diversifying evolution of ejaculate traits. Yet, several recent studies have drawn attention to other forms of selection with the potential of exceeding the effects of sperm competition. Since ejaculates are complex, multivariate traits, it seems plausible that different ejaculate components vary in their responses to different selective pressures. Such information, however, is generally lacking as individual ejaculate traits tend to be studied in isolation. Here, we studied the macroevolutionary patterns of ejaculate volume, sperm number, sperm length and the proportion of viable normal sperm in response to varying levels of sperm competition, body size and the duration of female sperm storage in pheasants and allies (Phasianidae). Ejaculate volume, sperm number and sperm viability were all relatively higher in polygamous than in monogamous mating systems. However, whereas ejaculate volume additionally covaried with body size, sperm number instead increased with the female sperm-storage duration, in conjunction with a decrease in sperm length. Overall, our results revealed important details on how different forms of selection can jointly shape ejaculates as complex, composite traits.
Highlights
Sperm competition is often considered the primary selective force underlying the rapid and diversifying evolution of ejaculate traits
Whilst the response of sperm morphometry to these selection pressures has been studied previously[39], we focused on the ejaculate volume, total sperm number and the proportion of viable, morphologically normal sperm ( ‘proportion of viable normal sperm’) in ejaculates, as well as their relationships with sperm morphometry, to better understand the evolution of ejaculates as composite traits
In a phylogenetic generalized linear model (PGLM)[46] across all 32 species in our dataset, with phylogenetic relationships derived from Stein et al.’s47 time-calibrated, multi-gene phylogeny of Galliformes, the duration of female sperm storage was not affected by female body mass (r = 0.14 (95% confidence interval: -0.21 to 0.45), t30 = 0.78, P = 0.44) nor by the social mating system (monogamy/rare polygamy vs. obligate polygamy: r = 0.16 (-0.19 to 0.46), t30 = 0.90, P = 0.37; phylogenetic scaling factor λ = 0.97
Summary
Sperm competition is often considered the primary selective force underlying the rapid and diversifying evolution of ejaculate traits. Whereas increased levels of sperm competition are predicted to enhance male investment in ejaculates overall[25], approaching the capacity of sperm production under physical and energetic constraints[26,27] should, based on theory[28,29,30], generate trade-offs between sperm size and number Which of these two ejaculate traits is favoured over the other by selection is thought to depend at least in part on the density of sperm prior to fertilization[30] It is possible that, after accounting for sperm competition, males of species with extended oviposition periods transfer relatively more sperm to ensure that even the last egg is reached and penetrated by sufficient sperm This may be important given that avian ova typically require polyspermic penetration for successful embryo development even though their pronucleus fuses with only a single sperm[40]. Even if sperm morphometry was affected only by the female sperm-storage duration as reported[39], both sperm quantity and quality could, or instead, respond to sperm competition and other predictors
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