Abstract
Sexual selection, differences in reproductive success between individuals, continues beyond acquiring a mating partner and affects ejaculate size and composition (sperm competition). Sperm and seminal fluid have very different roles in sperm competition but both components encompass production costs for the male. Theoretical models predict that males should spend ejaculate components prudently and differently for sperm and seminal fluid but empirical evidence for independent variation of sperm number and seminal fluid volume is scarce. It is also largely unknown how sperm and seminal fluid variation affect future mating rate. In bedbugs we developed a protocol to examine the role of seminal fluids in ejaculate allocation and its effect on future male mating rate. Using age-related changes in sperm and seminal fluid volume we estimated the lowest capacity at which mating activity started. We then showed that sexually active males allocate 12% of their sperm and 19% of their seminal fluid volume per mating and predicted that males would be depleted of seminal fluid but not of sperm. We tested (and confirmed) this prediction empirically. Finally, the slightly faster replenishment of seminal fluid compared to sperm did not outweigh the faster decrease during mating. Our results suggest that male mating rate can be constrained by the availability of seminal fluids. Our protocol might be applicable to a range of other organisms. We discuss the idea that economic considerations in sexual conflict research might benefit from distinguishing between costs and benefits that are ejaculate dose-dependent and those that are frequency-dependent on the mating rate per se.
Highlights
Sexual conflict is a potent driver of biological diversity [1,2,3] and can arise and be maintained when male and female fitness maxima for a given trait, or process, are not identical [1,2]
volume is portioned (Vm): Decline of ejaculate volume over mating rate Sperm vesicle size and sperm density decreased in a different way over successive matings (Figure 3a): Sperm density showed the largest decrease at the first mating, while sperm volume showed the largest decrease from the third mating
The seminal fluid reserves of bedbug males declined faster than sperm reserves and we empirically confirmed that males stopped mating when they were depleted of seminal fluid but not sperm
Summary
Sexual conflict is a potent driver of biological diversity [1,2,3] and can arise and be maintained when male and female fitness maxima for a given trait, or process, are not identical [1,2]. Substantial ejaculate production costs [4,5,6,7] can lead to differential ejaculate allocation across successive matings because of ejaculate economics [4] or because of constraints on production [8,9,10,11]. There are several ways in which males allocate ejaculates. A male may not adjust ejaculate volume and partitions the sperm and seminal fluid reserves into equal portions. Without ejaculate replenishment, this strategy results in a linear decrease in ejaculate reserves over successive matings [12], leading to ejaculate depletion, the inability to mate and a reduction in future mating rate. In the presence of competition for females, males may tailor their ejaculate in strategic ways [13]
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