Conclusions
The results supported the previous finding that removing one cue of rival presence does not prevent male D. melanogaster from detecting rivals and responding to them by extending mating duration (Bretman et al. 2011b). This suggests that the cues signalling rival presence are at least partially redundant. The redundancy of cue components may confer benefits to the receiving male by preventing information from being compromised if one component is inaccurate or lost, thereby facilitating adaptive reproductive plasticity. It cannot be concluded whether alternative combinations of cue components signalling rival presence are equivalent in terms of the fitness benefits achieved by responding to them, as no increase in reproductive success among males exposed to a rival were detected. Males exposed to all rival cues or the restricted set of cues did not increase their paternity, either in the absence or presence of sperm competition, despite extending mating duration. The receptivity of females to remating was also not affected by male exposure to rival cues. The absence of any apparent fitness benefits of longer mating is inconsistent with previous studies (Bretman et al. 2010; Bretman et al. 2011b), but highlights that caution should be taken when indirectly extrapolating fitness benefits from behavioural responses alone. It is possible that the lack of increased offspring production following longer mating was caused by damage sustained from aggressive interactions with rivals impairing the male’s ability to increase ejaculate investment. Or, the fitness benefits of longer mating may have been obscured by homogenising effects other features of the social environment, such as male and female mating status. Alternatively, longer mating following rival exposure could have conferred ‘hidden’ fitness benefits not measured in this study, for example sperm displacement or delaying of female remating up to 24 h. However, it is also possible that behaviour can become decoupled from increases in the transfer of sperm and SFPs, and that this may be mediated the degree of male-male competition (Hopkins et al. 2019).