Does diet influence ejaculate expenditure under experimentally altered risk of sperm competition in guppies?

Abstract

According to the sperm competition risk (SCR) model, males should increase their investment in ejaculates when there is a high chance that their sperm will compete for fertilization with those from another male. However, as ejaculates are energetically costly to produce, the extent to which males can adjust ejaculate investment may depend on the availability of metabolic resources. We tested these ideas using the guppy, Poecilia reticulata, a livebearing fish known to exhibit strong condition dependence in ejaculate traits and among the highest recorded polyandry rates in any vertebrate. Specifically, we manipulated SCR by visually exposing mature males to females either in the presence (high SCR) or absence (low SCR) of a rival male over 1 month, which comprises a full spermatogenic cycle. To test for condition dependence in male response to SCR, both groups were further subdivided into two diet quantity treatments (high and low), enabling us to test fixed and interactive effects of SCR and diet treatment on ejaculate expenditure (sperm production, sperm length and sperm velocity). Contrary to our predictions, we found no evidence that males increase ejaculate expenditure under high SCR, and indeed report the opposite pattern (higher expenditure under low SCR) for sperm velocity. Furthermore, we found no evidence that dietary restriction (when administered to mature adult males over 1 month) affected any ejaculate traits, leading us to suspect that ejaculates may only be susceptible to dietary restriction effects during a critical ontogenetic stage of development. We discuss these unexpected findings in the context of guppy biology, which may render the species less likely to exhibit long-term adjustments in ejaculates when perceiving short-term changes in SCR. Moreover, the high polyandry rates in this species may mean that the visual presence of a rival is a cue of altered sperm competition intensity, rather than risk.