Male house mice do not adjust sperm allocation in response to odours from related or unrelated rivals

Sperm competition theory predicts that males should adjust the number of sperm they ejaculate adaptively, according to sociosexual cues of sperm competition at the time of mating. Specifically, it is predicted that ( 1) males will respond to an increased risk of sperm competition from rivals by increasing sperm allocation, and ( 2) the increase in allocation will be lower when rivals are related. In species that use odour for communication, scent-based cues provide information on the presence and identity of conspecifics, and could thus serve as a basis for adjusting sperm allocation. We tested these predictions in the house mouse, Mus musculus domesticus, a species for which scent is critical to many aspects of social and reproductive communication. Sperm allocation was measured for subject males mating immediately following exposure to odour stimuli deriving from ( 1) themselves ( control treatment), ( 2) a brother or ( 3) an unrelated male. The behavioural responses of subjects to conspecific odours indicated that males detected the presence of these stimuli in their environment, but contrary to theoretical predictions we found no evidence that they increase the number of sperm ejaculated in response to either of the odour cues indicating an elevated risk of sperm competition. Similarly, we found no significant differences between treatment groups in other traits linked to ejaculate investment, including copulatory plug size and copulatory behaviour. These findings contrast with previously studied rodent species, suggesting that responses to cues of sperm competition risk may display considerable interspecific variability. (C) 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.