Risk-spreading by mating multiply is plausible and requires empirical attention

Abstract

We appreciate Henshaw & Holman's [1] (henceforth HH) comment regarding our original article [2]. We understand the points made by HH, but we have reservations about their applicability to our data, as explained below. Before addressing the specific details in HH's commentary, however, it is important to note that the substance of their article deals with technical aspects of our statistical analysis, not the underlying theoretical framework or the empirical design employed in our study. Indeed, we are happy to see that HH acknowledge that our work offers a valid proof-of-principle approach for studying the role that bet-hedging plays in determining the benefits of multiple mating in isolation from other factors, namely sexually selected mechanisms. Indeed, that was the main motivation of our study, rather than to specifically document the benefits of bet-hedging and sexual selection in the subject species.To briefly recap, our study revisited the concept of bet-hedging in the context of explaining female multiple mating, identified common misunderstandings surrounding its interpretation and offered a novel experimental approach to test for its existence. Our study system, the sea urchin Heliocidaris erythrogramma armigera, offers remarkable levels of experimental control via in vitro fertilizations, thus enabling us to analyse temporal (geometric mean) fitness among females assigned simultaneously to a polyandrous and monandrous mating strategy. In this way, we controlled for the effects of female genotype and maternal effects on fitness outcomes, while separating 'pure' bet-hedging effects from sexually selected paternity-biasing mechanisms by manipulating the fertilization conditions (presence or absence of competition among the gametes of different males). The underlying question addressed in our paper was to determine whether the intergenerational fitness of females is higher when they follow a strategy that increases mate sampling (i.e. polyandry) compared with a non-bet hedger strategy (monandry). By simulating reproductive bouts across generations, we uncovered the potential for bet-hedging, in addition to paternity-biasing mechanisms (e.g. sexual selection), to provide increases in fitness for multiply mated females.HH's first comment suggests that technically, Gillespie's measure may provide a more appropriate fitness measures for our data. We appreciate the suggestion here, but note that Gillespie's measure hardly deviates from our use of geometric mean fitness, which can be understood as a valid proxy of intergenerational fitness (e.g. compare, for each trait, the data in the first and third rows in the first column of HH's table 1). Second, HH acknowledge that the analysis of alternating environments of the form ABA and BAB requires complex analysis. However, in the event, HH do not carry out such analysis. Instead, they apply bootstrapping to estimate confidence intervals (CIs) on mean effect sizes using our measure of evolutionary fitness (geometric mean) and Gillespie's measure. If the main objec...

Sexual selection after gamete release in broadcast spawning invertebrates

“…This latter finding suggests that under multiplemale spawning, fertilizations are biased in favour of sperm from the most compatible male (i.e. sexual selection rather than a 'bet-hedging' strategy, where fertilizations would be more equally shared by competing ejaculates [49,50]). Indeed, as predicted, the average fertilization rates of the male that did best under monandry were not significantly different from those achieved when ejaculates from both males competed for fertilization ( [47], but see [48]).…”