In polyandrous species, a male’s reproductive success depends on his ability to fertilize females, which, in turn, depends on his mating ability and his ability to produce competitive ejaculates. In many species, sperm traits differ between old and young males in ways that are likely to decrease the sperm competitiveness and fertility of older males. This age–ejaculate quality relationship is attributed to male ageing (i.e., senescence). In a natural setting, male age and mating history are usually confounded: older males have usually mated and replenished their sperm supplies more often, so they have made a greater lifetime reproductive effort. In principle, the costs of reproduction, independent of any causal effect of male age, could generate an age‐related decline in ejaculate quality. To date, only a handful of studies have determined how male age, reproductive effort or their interaction affect ejaculate quality. Here, we experimentally manipulated the long‐term mating history of 209 adult male mosquitofish (Gambusia holbrooki) over 14 weeks (N = 1,118 sperm samples). Males either had direct access to females and could mate freely, or had only visual and olfactory access to females. We documented the effect of mating history, adult male age (3, 9 and 14 weeks post‐maturation) and their interaction on sperm velocity, sperm reserves and the rate of sperm replenishment. For sperm velocity, we additionally examined the effects of sperm age, because when older males mate less (or more) often than younger males there will be a correlation between mean sperm age and male age. Sexually active males produced fewer sperm and replenished their sperm at a lower rate, and their sperm had lower velocity than males prevented from mating. Though older males produced more sperm, the rate of replenishment and velocity of their sperm was lower than the sperm of younger males. We also tested for a difference in the velocity of recently replenished (<24 hr) and older sperm (i.e., post‐meiotic sperm senescence). There was no evidence that male age or mating history affects the extent of sperm senescence, but older sperm swam faster than recently produced sperm. Crucially, complex interactions are evident between male age and male mating history with respect to sperm number and the proportion of sperm that are replenished. These results suggest that male age and mating history will interact to determine the reproductive success of a male under sperm competition. They reveal a complex relationship between a male’s age and his ejaculate quality. We suggest that both mating history and sperm age should be controlled for when measuring the intrinsic rate of senescence for male reproductive traits if the goal is to isolate effects that are solely attributable to a male’s chronological age. A plain language summary is available for this article.

Identifying targets of selection is key to understanding the evolution of sexually selected behavioral and morphological traits. Many animals have coercive mating, yet little is known about whether and how mate choice operates when these are the dominant mating tactic. Here, we use multivariate selection analysis to examine the direction and shape of selection on male insemination success in the mosquitofish (Gambusia holbrooki). We found direct selection on only one of five measured traits, but correlational selection involving all five traits. Larger males with longer gonopodia and with intermediate sperm counts were more likely to inseminate females than smaller males with shorter gonopodia and extreme sperm counts. Our results highlight the need to investigate sexual selection using a multivariate framework even in species that lack complex sexual signals. Further, female choice appears to be important in driving the evolution of male sexual traits in this species where sexual coercion is the dominant mating tactic.

Many animals undergo complete metamorphosis, where larval forms change abruptly in adulthood. Color change during ontogeny is common, but there is little understanding of evolutionary patterns in these changes. Here, we use data on larval and adult color for 246 butterfly species (61% of all species in Australia) to test whether the evolution of color is coupled between life stages. We show that adults are more variable in color across species than caterpillars and that male adult color has lower phylogenetic signal.These results suggest that sexual selection is driving color diversity in male adult butterflies at a broad scale. Moreover, color similarities between species at the larval stage do not predict color similarities at the adult stage, indicating that color evolution is decoupled between young and adult forms. Most species transition from cryptic coloration as caterpillars to conspicuous coloration as adults, but even species with conspicuous caterpillars change to different conspicuous colors as adults. The use of high-contrast coloration is correlated with body size in caterpillars but not adults. Taken together, our results suggest a change in the relative importance of different selective pressures at different life stages, resulting in the evolutionary decoupling of coloration through ontogeny.