At the end of my PhD, I had a manuscript sitting on my desk in which we explored temporal variation in the opportunity for selection in red junglefowl (Gallus gallus). We had found that variance in reproductive success – a prerequisite for sexual selection – decreased sharply over a 10-day mating trial. This was true for variance in all components of reproductive success, namely, mating success (M), paternity share (P), and partner’s average fecundity (N). We had also developed simulations to check whether these temporal changes could be explained by an accumulation of random matings, rather than by competitive processes. It turns out that they could. Great! Results were interesting, the manuscript was almost ready for submission, but how general were these conclusions? After all, the red junglefowl – albeit the most majestic and fascinating animal in the planet – is just one species, and there are quite a few others out there with very different mating dynamics. It was at this stage that my supervisor gave me an option that proved crucial: we could either submit the paper as it was and hopefully get a decent publication out of it, or we could search the literature for longitudinal mating data across a range of species and explore the generality of the red junglefowl results. The choice was tricky: as an early career researcher, there was a big pressure on my shoulders to publish quickly and increase my odds of securing a good postdoctoral position. However, if we delayed the publication and included more species, the manuscript would become conceptually much stronger so it could be submitted to a better journal, and the scientific community would likely benefit. In the end I went for the latter option, which meant I had to work for another year on the manuscript while simultaneously looking for a job.
A year and a half later and you will have realised that the extra work paid off. It turned out that behavioural studies reporting longitudinal mating data are scarce, but we still managed to locate an additional 6 studies with suitable data. We found out that our results for red junglefowl were consistently true for other animal species. The opportunity for precopulatory sexual selection (IM) decreased sharply over the span of just a few days in vertebrates and invertebrates, and this erosion could be mostly explained by individuals mating at random. This is interesting because variance in mating success has often been used as a proxy of the actual strength of sexual selection, which could be misleading. Our results supported an earlier body of literature critical of the opportunity for selection metric due to its inability to distinguish variation in mating success arising from stochastic processes vs through sexually selected traits or strategies. To add insult to injury, we also showed that short sampling periods could lead to another source of bias in temporal estimates of opportunity for selection. Sampling durations substantially shorter than the length of a selective window (e.g. mating season or breeding event) would likely result in overestimations.
This is not to say variance-based metrics of selection should be abandoned, however. We confirmed previous suggestions that comparisons between empirical data and null models assuming random mating can begin to disentangle the causes of variation in mating success. In fact, we found evidence that temporal declines in IM are shallower than expected by chance in some species, likely due to intrasexual competition. Ultimately, the utility of a metric will depend on how it is used. In the case of IM, investigators must be careful not to treat it as a measure of the actual strength of sexual selection, which is better encapsulated by selection gradients (i.e. the regression of measures of reproductive success over given phenotypic measures among members of the same sex within a population). It is also important to clearly define null models to contrapose the empirical findings, and the use of simulations assuming random mating is a promising approach.
I hope the story behind this paper may serve as a motivator to other early career researchers, which is why I decided to share so many details of the background that led to this paper. Sometimes it is worth to take risks and delay our gratifications. However, what constitutes a right decision will surely depend on one’s circumstances. In my case, the decision to delay the publication was made easier by the fact I had already published two of my chapters before the end of my PhD. Would I have done differently, had I not published anything? Probably. So take my story with a pinch of salt, and always consult with senior authors around you, particularly your supervisor, who is usually the most keen on seeing your career take off. I hope my article will prove useful, and I wish all other early career researchers the best of luck in their endeavours!