Male water striders summon predators to blackmail females into having sex

By Ed Yong | August 10, 2010 11:00 am

Waterstrider_backswimmer

If you want to see some sex, violence and blackmail, don’t bother with soap operas – try looking at the surface of your local lake or stream. There, you’ll find small insects called water striders (or pond skaters), skimming across the water on outstretched legs. These legs can pick up the vibrations of prey, predators and mates, but they can also produce vibrations by tapping the water surface. And males use this ability to blackmail their way into sex. It’s a drama of sexual tension played out across the surface tension.

Water strider sex begins unceremoniously: the male mounts the female without any courtship rituals or foreplay. She may resist but if she does, he starts to actively strum the water surface with his legs. Each vibration risks attracting the attention of a hungry predator, like a fish or backswimmer (above). And because the female is underneath, she will bear the brunt of any assault. By creating dangerous vibes, the male intimidates the female into submitting to his advances. Faint heart, it is said, never did win fair lady.

A male water strider doesn’t have to go through the hardships of pregnancy and he plays no role in raising the next generation. It’s a theme that echoes throughout the animal kingdom and it means that the best strategy for him is to mate with as many females as possible. After all, he has plenty of sperm to go around. A female, however, has a limited supply of eggs and mating opportunities. When she has sex, it has to count, so it suits her to be choosy.  And she has the right equipment for the job.

Last year, Chang Han and Piotr Jablonski from Seoul National University found that female red-backed water striders (Gerris gracilicornis) can block their vaginas with hard genital shields. This defence is important because once the male manages to insert his penis, he can inflate it to make him harder to throw off. The female’s only hope is to prevent him from getting through in the first place.

Hyper-violent males can sometimes wear the female down but some opt for a subtler approach – they tap intricate rhythms on the water with their legs. When Han and Jablonski discovered these rituals last year, they suggested that the males might be trying to demonstrate their quality, by tapping out the most consistent rhythms. Now, they have another explanation – the tapping is a form of blackmail, a way of coercing sex from the female with the threat of death.

The duo studied the preferences of the backswimmer – a predatory bug that floats upside-down at the water’s surface and listens out for the vibrations of potential prey. When given a choice between a silent male water strider and a mating pair with a tapping male, the backswimmer always headed towards the vibrating duo. And since these predators attack from below, the female was always the one who was injured while the male strode off to tap another day.

The backswimmer menace is so potent that after a few minutes of tapping from the male, the female relents by opening her genital shield. If she had been previously attacked by predators, she gave in almost instantly. And only when she relented did the male stop his threatening taps.

But all of this changed when Han and Jablonski prevented the males from sending out their deadly summons by gluing a bar on the females’ backs. This stopped the males’ legs from touching the water and it made the females much less forthcoming. On average, they only gave in after 20 minutes and some held out for a full hour.

Of course, it’s possible that the males tap the water for an entirely different reason, and the threat of predators is just a happy side-effect. But Han and Jablonski found that the males, particularly large ones, actually tap faster when backswimmers are around – they’re clearly trying to attract these predators.

This predator-summoning strategy probably evolved from more innocuous behaviours. Water striders already tap their legs to communicate with each other during fights. And the males of another species, Aquarius najas, also tap their legs during sex but without causing harsh ripples and without affecting female resistance. It’s possible that these signals originated as a way of demonstrating male quality, as Han and Jablonski initially suggested, before they were co-opted for more sinister purposes.

The battle of the sexes between male and female water striders has led to a whole suite of adaptations and counter-adaptations. Some males have evolved special grasping structures to give them a better hold of females, while females have responded by evolving spines and other defences to weaken their grip. Females evolved their impregnable genital shields, which males have countered with a behaviour that makes females more likely to lower their defences.

To be honest, the female water strider has an easy time of it. In other insects, where females have evolved an upper hand in the war of the sexes, males have developed even more extreme counter-strategies. Look no further than the common bedbug – the male bypasses the female’s genitals altogether and stabs his sharp penis straight into the female’s back, a technique known appropriately as traumatic insemination.

Reference: Nature Communications http://dx.doi.org/10.1038/ncomms1051

Image by Chang S. Han

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Comments (20)

  1. Ah, sexual selection, the craziest guy in town

  2. ChH

    This is extortion, not blackmail.

  3. Bob Carlson

    The backswimmer menace is so potent that after a few minutes of tapping from the male, the female relents by opening her genital shield. If she had been previously attacked by predators, she gave in almost instantly. And only when she relented did the male stop his threatening taps.

    The concept of female water strider being more inclined to relent to a relentless male of the species depending upon prior experience with predators seems like something one would find in the Onion. Moreover, I was unable to link to the Nature Communications page.

  4. if we can’t look to insects for ethical sexual behavior, then where are we to turn?

  5. Sociobiology FTW… c’mon Ed, let’s have some eve psych next! ;-)

  6. charle

    A teen age woman confessed to me that a man had told her if she didn’t have sex with him, that he would tell her boyfriend that he had had sex with her. She had sex with him.

  7. stomp_stompclap

    @Michael Meadon:

    I believe you’re using that word incorrectly.

  8. prokaryotistan

    Only NERs can satiate my fetish for insect sex. this sounds soo date-rape to me

  9. Vox Clamantis

    It kind of throws into disarray the whole human notion of love and procreation as naturally among the most noble of drives. But I suppose darker themes can be found anywhere. If you take it far enough, the personal threats of aging and mortality provide the starkest call among human females to reproduce. How selfish is that!

  10. JJ

    just to take one paragraph’s stupidities:

    You don’t know that pregnancy is a “hardship” for a strider.

    I doubt that any striders “raise” the next generation.

    Striders brains are not capable of “strategy”. They do what they do because natural selection made those behaviors successful.

    Bugs are not in the “animal kingdom”.

    It doesn’t have to “count” for a female. There are no mother-in-law striders puting pressure on them.

    She is not capable of being “choosy”. Her actions are involuntary and instinctive.

    You seem to be confusing bugs with humans, trying to prove preconceived opinions about human behavior.

    To an intelligent person you sound like an idiot.

  11. Mark

    Ed,

    Thanks–very interesting stuff.

  12. Steve

    It’s not clear why natural selection would result in females who are more difficult to impregnate.

  13. Julie D

    JJ, which kingdom were you planning to include insects in? Fungi? Protista?

    Insects are definitely capable of being picky in their mates and courtship is very common. Males of many insect species perform song or dancing to attempt to attract a mate. Cricket chirping is a mating call, and males compete to produce the best call. The hangingfly males bring a gift of a meal to a female and she can choose to reject the gift or accept and mate.

    The males of the vast majority of animal (yes, insects are animals. wow) species compete for the ability to mate with females.

    The female has to consume extra food while producing eggs, which indeed puts a hardship on her. The only intelligent thing you said is that water striders do not raise their young. Good work.

  14. AlexB

    @JJ,

    the only one who sounds like an idiot is you.
    who would take the personification of an insect absolutely literally?
    words like “hardship” and “choosy” are used to help the average person better understand the article.

    it’s so insanely obvious that water striders do not literally endure “hardships” that it goes beyond saying. clearly all behavior is influenced by instinct and genetics.
    if you were as intelligent as you’d like to think, you would realize that.

    “hardships” is not to be taken in a human sense, although you’ve again failed to see that.
    sexual reproduction in any species takes time and energy, even if it’s just in producing gametes. plus a female’s eggs are more “expensive” than sperm (and i don’t mean in a monetary sense). This is the “hardship” Ed was referring to.

  15. Derpus Herpus

    @JJ, this reminds me of the article that stated that people of below average intelligence tend to overestimate their abilities.

  16. Demonsthenes

    @Derpus Herpus – I believe I read an article similar to that and I was about to mention that to “JJ”, but you already did my job, so thank you. :)

  17. Hmm it looks like your site ate my first comment (it was extremely long) so I guess I’ll just sum it up what I submitted and say, I’m thoroughly enjoying your blog. I as well am an aspiring blog writer but I’m still new to the whole thing. Do you have any tips for newbie blog writers? I’d certainly appreciate it.

  18. JJ you did NOT seriously say “bugs aren’t in the animal kingdom” and then refer to yourself as intelligent.

    Insects are animals in every single way. They share all the same senses, many of the same organs and tissues as you, they have two sexes, they eat, they poop, they breathe oxygen…does a grasshopper look like a vegetable to you? Not only are arthropods animals, but they make up the vast MAJORITY of all life on Earth and have been around longer than us vertebrates. Therefore, an insect is not only an animal, but the most normal and typical example of what an animal is. They are the true default definition of animal.

    I knew all this when I was in kindergarten. Everyone I know of every age group knows insects are animals. It’s a mistake I would even admonish a very young child for, it’s so freaking obvious. I would address the stupidity of your other points, but I can’t possibly get over that one…that’s a whopper of a stupidity.

  19. Alright, you got me, I’ll address your other stupidities too, JJ:

    All of your complaints are meaningless nitpicking and semantics. This article is written with journalistic flair that does not at all stray too far from hard science. You’re mincing words just to be a jerk, AND making yourself look like a moron in the process, what with the “bugs aren’t animals” thing. Nope, can’t stop harping on that.

  20. ONE MORE for the road:

    lol “bugs aren’t animals” lol I guess they’re a kind of mushroom or something lol

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