We have published the final chapter of Pragya Singh’s PhD thesis. This one was quite a while in the making, and I am super happy that it has finally been published, after having been a preprint for several years.

This is a really cool comparative story. Pragya examined the mating behavior of 64 Macrostomum species to understand how their behavior and in particular the suck behavior evolves.

What’s So Special About These Flatworms?

These tiny hermaphroditic creatures offer fascinating insights into sexual evolution and conflict. The most intriguing aspect of their reproductive biology is what we call the “suck behavior.” After mating, some species of these flatworms do something remarkable - they place their pharynx over their own female genital opening and appear to suck out the recently received ejaculate. Yes, you read that correctly!

Sexual Conflict in Hermaphrodites

Why would an organism remove sperm it just received? This behavior represents a perfect example of sexual conflict, where what’s beneficial for one mating partner isn’t necessarily good for the other. Even though these animals are hermaphrodites, their interests in the “male” and “female” roles can differ dramatically. During reciprocal mating (where both partners mate simultaneously in both roles), an individual might receive sperm it doesn’t “want”. The suck behavior appears to be an evolutionary adaptation allowing the recipient to exercise post-mating choice over which sperm gets to fertilize their eggs. However, until now we did not have proof that the behavior actually removes ejaculate.

Now using video evidence, we could show the suck behavior in Macrostomum hamatum genuinely removes ejaculate! We also found that species that engage in reciprocal mating are much more likely to exhibit the suck behavior. In contrast, species that use hypodermic insemination (where one partner injects sperm through the body wall) don’t show this behavior. Consequently the reproductive anatomy of these flatworms also correlates with their mating behavior. This means we can make reasonable predictions about a species’ mating strategy just by examining its reproductive structures, which was an important assumption of my previous work. Therefore, this paper strengthens our general picture on sexual trait evolution in Macrostomum.

Why This Matters

This research demonstrates sexual antagonistic coevolution - essentially an evolutionary “arms race” between reproductive strategies. It shows how hermaphroditic organisms have evolved complex behavioral adaptations to maintain control over fertilization. This research journey, spanning several years and examining dozens of species, highlights the value of comparative approaches in evolutionary biology. By looking across related species with different behaviors, we can reconstruct evolutionary history and understand how complex traits emerge.