Combined effects of rearing and testing temperatures on sperm traits

Temperature experienced during early development can affect a range of adult life history traits. Animals often show seemingly adaptive developmental plasticity – with animals reared at certain temperatures performing better as adults at those temperatures. The extent to which this type of adaptive response occurs in gonadal tissue that affect sperm traits is, however, poorly studied. We initially reared male mosquitofish (Gambusia holbrooki) at either 18oC or 30oC, and then measured their sperm reserves as adults. We also looked at the velocity of their sperm, at both the matched and mismatched temperature. Although males reared at 30oC were larger than those initially reared at 18oC, there was no detectable effect of rearing temperature on absolute sperm number. Sperm swam faster at 30oC than 18oC regardless of the male’s rearing temperature. Therefore, we found no evidence of adaptive developmental plasticity. Rearing temperature did, however, significantly influence the relationship between male body size and sperm velocity. Larger males had faster sperm when reared at the warmer temperature, and slower sperm when reared at the cooler temperature. This suggests that rearing temperature could alter the relationship between pre- and post-copulatory sexual selection as male size affects mating success. Finally, there was a positive correlation between velocity at the two test temperatures, suggesting that temperature experienced during sperm competition is unlikely to affect a male’s relative fertilisation success.

Methods
Methods on our data were collected, processed and analysed are described in the associated paper