Experimental evidence that investment into ejaculation and mating effort impose different life history trade-offs

Abstract

When males compete, sexual selection favors reproductive traits that increase their mating or fertilization success (pre- and post-copulatory sexual selection). It is assumed that males face a trade-off between these two types of sexual traits because they both draw on investment from the same pool of resources. Consequently, investment into mate acquisition or ejaculation should create similar trade-offs with other key life history traits. Tests of these assumptions are exceedingly rare. Males only ejaculate after they mate, and investment into ejaculation is therefore highly correlated with mating effort. Consequently, little is known about how each component of reproductive investment affects a males future performance. Here, we ran an experiment using a novel technique to distinguish the life history costs of mating effort and ejaculation for mosquitofish (Gambusia holbrooki). We compared manipulated males (mate without ejaculation), control males (mate and ejaculate) and naive males (neither mate nor ejaculate) continuously housed with a female and two rival males. We assessed their growth, somatic maintenance, mating and fighting behavior, and sperm traits after 8 and 16 weeks. Past mating effort significantly lowered a males future mating effort and growth, but not his sperm production; while past sperm release significantly lowered a males future ejaculate quantity, but not his mating effort. These findings challenge the assumption that male reproductive investment draws from a common pool of resources such that, all else being equal, greater mating effort reduces ejaculate investment, and vice versa. Instead, we provide clear evidence that investment in traits under pre- and post-copulatory sexual selection have different trait-specific effects on subsequent male reproductive performance that occur both early and late in life.