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).