Friday Weird Science: Female Water Striders Make them Work for it

Jun 19 2009 Published by under Friday Weird Science

Sci is stuck in the car dealership waiting for her baby to get her checkup. There are good things about this (free wireless!), but the coffee here is TERRIBLE. Bringing own coffee next time. But it also means there's plenty of time to bring you your daily dose of weird science!

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Now, in most insect species, the female doesn't have a lot of choice whether or not she gets mated with. The time comes, the males fight each other, and the poor female is stuck for it. But in other species, a male has to prove himself, and it's up to the female to accept him or not. Perhaps the best-known example is a dung beetle, who makes up a nice, tasty ball of crap for his special lady friend. If it's big enough, he's good to go. Otherwise, no thanks, try again, and make sure the dung is FRESH this time.

Well it turns out that there are several species of female insects that can't be "coerced" into mating. Rather, they have to be persuaded. The question is, what is the difference between those who are coerced and those who are persuaded. Scientists have hypothesized that the difference might be hidden genetalia. If the male can't find it, he can't force it. And it turns out, in water striders, this may make the difference.
ResearchBlogging.org Han and Jablonski. "Female genetalia concealment promotes intimate male courtship in a water strider" PLoS ONE, 2009.


About the first main thing I noticed about this paper was the pictures. At first glance, the gonadal opening of the female water strider looks a lot like a human vagina. Weird, indeed.
So basically, the people working on this paper took some detailed electron micrograph pictures of female water strider genitalia, and then they watched a whole bunch of water striders getting it on. Probably not remotely hot, but very fun to explain to the other people in your department.
And they found that the female water striders of the species Gerridae gracilicornis, a water strider which has some odd female morphology. Unlike other females of similar species, G. gracilicornis has a weird butt. Where other females have an ovipositor that is positioned downward (like it's ready to lay eggs), leaving the gonadal opening exposed, G gracilicornis has its ovipositer and vulva flipped UP, covering the gonadal area.
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In the picture above (top left, middle, and the right electron micrograph, the male parts on the bottom aren't important here) you can see the comparison of a "normal" water strider (A), with G. gracilicornis (B), and an electron micrograph of what the opening actually looks like (E). You can see that the ovipositor and vulva are over the gonadal opening, (the tip is toward the anus, which in this picture is at the bottom). This means that their genitalia is "hidden", and that a male can't just hop on and thrust, or he won't get anywhere.
In order for the male to get a piece of that, the female has to release her ovipostiro and vulvar opening, revealing the gonadal area.
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To Sci, this looks a lot like a drawbridge coming down. In this case, the anal opening is at the top. Once the ovipositor is fully extended, the male can hop on and propagate the species.
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But how does the male persuade the female to...drop ovipositor? He has to send courtship signals. In the case of the water strider, this results in mounting the female, and then tapping their legs on the water, creating little pulses of ripples that spread out over the surface. And these taps change in rhythm and frequency, depending on how well the male is doing.
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In the top panel above, the male approaches the female. He's tapping at various frequencies and strengths, perhaps trying out which pick-up line works. "Can I have your digits now, or do I get them off the phone in the morning?" In the second panel, was the male mounts and tries to get the ovipositor to opne, the taps are rhythmic and constant. Finally, as copulation begins, the male produces fast bouts of tapping, followed by long periods of silence, as perhaps he is distracted.
So how did they KNOW that the tappings were associated with gaining entry to the female's most private parts? They blocked them. The scientists allowed water striders to mate, only this time they put a tiny bar on the back of the female, which prevented the male from reaching the water with his toes when he was mounted. So he tapped away, but the female couldn't tell. When they tried this, mating took 3 times longer than matings performed without the bar on the back.
So why do the males need to be tapping out their devotion in Morse? The authors give several possible hypotheses. The first is that females may be able to get around more efficiently when there's a male already on their back, keeping other males away. So she wants to keep him on as long as possible to allow her to forage in peace. The authors admit, though, that this is rather silly. A second, more probably scenario is that females are delaying in order to assess the male's quality, as a healthier male will produce more taps and hold on longer than one that is unhealthy. A third possibility is that the female is attempting the throw the male off (as frequent copulations are not in her best interest), and the longer it takes him, the more likely she is to succeed.
The researchers actually think that none of these are the right answer. Instead, they postulate that the males are tapping to create disturbance and attract predators. This makes the female lower her ovipositor quickly, to spend as little time attracting predators as possible. I'm not so sure about this, but apparently they are preparing some convincing further publications to support their hypothesis.
So females, you also have hidden genitalia. Make the guy tap, so he can tap that. Eventually.
Han, C., & Jablonski, P. (2009). Female Genitalia Concealment Promotes Intimate Male Courtship in a Water Strider PLoS ONE, 4 (6) DOI: 10.1371/journal.pone.0005793

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