14 Days of Genitals, Day 12: Arms Race Between the Sexes

Survival of the fittest doesn't always mean survival of the nicest. Sometimes assholes are the best at reproducing, and water striders are really, really good at reproducing. 

Not the biggest asshole in the animal kingdom (wait 'til day 14), but close.

You see dear readers, in evolutionary terms sperm is cheap. Like so often in the animal world the male water strider's only role in the next generation is that little drop of baby juice. Eggs, on the other hand, are more expensive in terms of energy, physical investment, and all that shit that mammas do after the baby is born.  Even for invertebrates (which typically do not care for their young) the females bear the greater burden of reproductive energy, which means males and females have different reproductive strategies. Males can afford to inseminate every female they find, while the female water strider is more choosy about who she lets into her chitinous cookie jar. 

Male water striders can be pretty pushy, but females do have a defense: they have a genital shield, which sounds like the best name for a new condom brand ever. 

Can you trademark a phrase on a public blog post? Cuz DIBS. 

Unless the female lowers her shield, the male can't get in. However, here is where the story gets a lot more "fuck you, male water striders." See, when water striders are doing it, sometimes the male will tap on the surface of the water. Everyone thought this was some sort of cute insect courtship ritual. Maybe the equivalent to a dog's uncontrollable leg shake when you scratch its belly. 

In reality, it was sexual blackmail.

Hans & Jablonski (2010) found that males that were "signaling" i.e. tapping on the surface of the water, were actively attracting the local predator, the backswimmer. Because the females were always on the bottom of this tryst, if a backswimmer attacked they were the one who got injured.

But if a female lowered her genital shield, the male stopped tapping (and therefore, stopped summoning her death). Han and Jablonski found that females usually gave into this tactic within a few minutes but if a female had mated with a "tapper" before, she retracted her genital shield almost immediately. 

"Lower your shield or I'll use you as a shield" - Evolution

But they're still better than otters.


Missed the previous days of genitals?! Have no fear, check out the links right here: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11!

Works Cited

Han, C. S., & Jablonski, P. G. (2010). Male water striders attract predators to intimidate females into copulation. Nature communications, 1, 52.