Promiscuity is the new black. For many years, biologists believed in the Victorian fairy-tale that females were monogamous whereas males were not. The truth is females are promiscuous, and evidence shows this to be the norm, not the exception.

However, in science – as in society – our perception of females has been slow to change. Even after knowing the widespread occurrence of female promiscuity, very little was known on how it shapes evolution. Theory predicted that promiscuity should decrease male competition before mating and increase competition after mating, but no direct test of this hypothesis had been performed.

I started my DPhil (the Oxfordian name for a PhD) looking at how larval nutrition affected flies’ reproduction and sexual selection¹. Upon having my first real experience with data analysis, we discovered that larval nutrition could strongly influence the strength of sexual selection in groups. That’s when we wondered if we could use any of our genetic mutant fly lines to manipulate fly sexual behaviour in more extreme (fun!) ways.

That was when we hosted the best undergraduate student Oxford had to offer – Emelia Smith. For her project, we used mutant lines of flies, Drosophila melanogaster, in which females were more promiscuous than normal. We then designed a study to directly test whether theory was right in predicting that female promiscuity should decrease male competition before mating and increase competition after mating. To our knowledge, this was the first time a direct genetic manipulation of female promiscuity was used to investigate the effects on sexual selection acting on males. But even though the mutant makes the overall idea of the experiment more feasible and exciting, there is more to it than what meet the eyes.

The experiment actually involved paint marking flies – hundreds of them – with some very thin forceps (brushes are just too big!) with different colours under the microscope. Our ‘Picasso-Entomologist’ moment. Then following groups of painted flies for hours in a windowless room, and counting – one by one – some thousands of those flies’ offspring based on their eye colour in a phenotypic paternity test for flies. All for the sake of science!

But it was worth it.

We confirmed the theory: female promiscuity weakens selection on males before mating while increasing the importance of male-male competition after mating. But that wasn’t enough, we wanted more than that. We wanted to know the behavioural mechanisms underpinning the phenomenon. Luckily, Grant C McDonald worked just next door on methods to tease apart some of the different behavioural mechanisms that could contribute to the patterns we observed. Together, we used the data to test more sophisticated models of sexual selection that take into account how males and females interacted in the groups. We showed that males do not – at least in our design – attempt to mate with more or less promiscuous females. Instead, to compensate the stronger competition after mating, males mated repetitively with the same promiscuous female more often, which increased male’s success in fertilising females’ eggs.

We were excited, and few months later here I was standing in front of a large crowd to present my results at the ESEB conference in 2015. As a second year DPhil student, that was my first ‘real’ presentation. Big names were there. Things went relatively well…until the end. When the floor opened for questions, some senior colleagues criticised some aspects of the experiment. Mostly that out genetic mutation could have affected sperm competition and sperm usage in ways that undermined the findings. Fair enough – we did not have any data on that at the time. But as I walked out of the building, a senior colleague came to me and said:

‘I don’t believe in anything you said up there’. I tried to defend my argument, but after some discussion, he finished off with ‘you are a liar’. Ouch. As a note, please never treat a young DPhil student like this – it is definitely not encouraging.

Anyway, we were concerned about the criticisms regarding the effects of the mutation on sperm competition. While we were trying to find ways to address this conundrum, Damian Smith and my academic grandparent Prof Tracey Chapman published their manuscript with data that could be used to address our concerns². Perfect timing! We then collaborated with Damian and Tracey and re-used their data to show that changes in sperm competition were unlikely to explain our original results. It is female promiscuity that is the key.

 Our findings provide an important advance towards a better understanding of how female promiscuity – and female behaviour more generally – influences evolution. We now need to go beyond flies, into other species and other experimental designs to investigate whether theory and experiment continue to agree. Please have a look at our manuscript published in Nature Communications³.

References

[1] Morimoto, Juliano, Tommaso Pizzari, and Stuart Wigby. "Developmental environment effects on sexual selection in male and female Drosophila melanogaster." PloS One 11.5 (2016): e0154468.

[2] Smith, Damian T., et al. "Sexual conflict over remating interval is modulated by the sex peptide pathway." Proc. R. Soc. B284.1850 (2017): 20162394.

[3] Morimoto, Juliano, McDonald, Grant C., et. al., "Sex peptide receptor-regulated polyandry modulates the balance of pre- and post-copulatory sexual selection in Drosophila." Nature Communications (2019) doi: 10.1038/s41467-018-08113-w