Crayfish females lure males with urine, but then play hard to get

By Ed Yong | March 30, 2010 8:25 am

Fighting_crayfishReleasing a steady stream of urine to attract a mate and then fighting off anyone who still dares to approach you doesn’t seem like a great idea for getting sex. But this bizarre strategy is all part of the mating ritual of the signal crayfish. A female’s urine, strange as it sounds, is a powerful aphrodisiac to a male.

Fiona Berry and Thomas Breithaupt studied these courtship chemicals by organising blind speed-dates between male and female crayfish, whose eyes had been covered with tape. They also injected a fluorescent dye into the animals’ bodies, which accumulated in their bladders. Every time they urinated, a plume of green dispersed through the water.

If the duo blocked the female’s nephropores (her urine-producing glands), the males never showed her any interest. If they met, they did so aggressively. But when the duo injected female urine into the water, things took a more lustful turn, and the males quickly seized the females in an amorous grip. Female urine is clearly a turn-on for males.

But the female doesn’t want just any male – she’s after the best, and she makes her suitors prove their mettle by besting her in a test of strength. As he draws near, she responds aggressively, even though it was her who attracted him in the first place. No quarter is given in these fights. The female only stops resisting if the male can flip her over so that he can deposit his sperm on her underside.

Signal_crayfishFemale crayfish shoulder all the burden of raising the next generation, spending six long months rearing their offspring alone. Males, however, only contribute their sperm. Because the females make such a big investment in the next generation, it’s in their interest to choose the best partners.

Being nocturnal, they can’t see how strong a male is and chemical cues aren’t always reliable indicators of quality. The simplest way of discerning the strongest males is to test their strength for herself. By playing hard to get, she makes sure that she gets fertilised by the best mates, who will at least help to produce the fittest possible young even if they never help to raise them.

Urine typically has an aggressive meaning for crayfish. Males release it when they battle each other, and so do females. During courtship, the difference is that males are drawn to female urine and they stop releasing their own. Doing so might be their way of appeasing the violent female, his way of raising a chemical white flag in the hopes of getting a chance to mate.

In some ways, this is a surprising set-up. In species like crayfish, where females do all the work in raising the next generation, males usually have to be the persuasive ones during courtship while females are the choosy sex. But female crayfish have taken on both roles – seductress and selector. By sending out mixed messages with her urine, she can draw a pool of eager mates that she can then test.

Reference: Berry, F., & Breithaupt, T. (2010). To signal or not to signal? Chemical communication by urine-borne signals mirrors sexual conflict in crayfish BMC Biology, 8 (1) DOI: 10.1186/1741-7007-8-25

Images: fighting crayfish by Berry & Breithaupt; individual by MdE

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MORE ABOUT: courtship, crayfish, sex, urine

Comments (9)

  1. Gingerbaker

    I’m confused. What does this have to say about “No means no!” ;D

  2. I’m just imagining that the urine is the equivalent to the sound of bell to start a boxing/wrestling match. “Who’s ready to rumble?”

  3. Nathan Myers

    Yes, if there’s no one to fight with, there’s also no one to mate with. It’s not so different for humans.

  4. Jeremy

    What an amazing study. Blind folding crayfish, and then telling them to ‘get it on’. The most funny pic I’ve seen on this site by a good long ways.

  5. I wonder if synthesising female crayfish urine in the lab and putting the chemical in traps could prove to be an effective way of controlling the invasive signal crayfish in UK waters.

  6. I have still to find time to read this paper thoroughly, but it will likely make me rethink the assumptions (and thus conclusions) of this study of mine (and another one that came after) in light of these findings. There is a reason why we (and others) used only male crayfish in our experiments, though we know that females also fight (with each other and with males). In more and more species it is found that aggressive behaviors, petterned or even stylized, may have evolved out of mating behavioral patterns. Which brings out the questions of the real goals of the aggressive behaviors which, in some species, was found not to be aggressive after all but rather “male-male bonding”. This is unlikely in crayfish (in which male-male fights may result in death of one of the individuals, not friendship), but what if such fight-to-death behaviors in male crayfish are just male-bonding behaviors gone awry? A mistake? Gotta think about this some more…

  7. Tom

    Ed, that’s the best/worst lede in the history of science journalism.

  8. If it’s simultaneously the best AND worst, doesn’t that automatically make it the best? ;-)

  9. Krey

    While the urine attracts the males, it doesn’t necessarily mean that the female is urinating in order to attract the male. It seems more likely that the attraction to urine is so that the males can differentiate between males and females and likely also contains chemical signals showing a females readiness to mate. Though not discussed here, I would guess that a pregnant female, or one already involved in raising young, would exude a urine that does not incite lustful feeling in the male.

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