This is quite possibly the most awesome biology photo ever taken. It is two blind-folded crayfish battling each other in clouds of fluorescent green urine. It’s a good thing it’s just a picture, because if it were a video, in 3D, with the Star Trek fight music playing in the background, every science nerd in the world would have to lie down and die in ecstasy. Don’t click on the little arrow below it! I won’t be responsible for the consequences! (Fortunately, I can’t put it into 3D motion, so I won’t be slaughtering my readership here.)

The premise of this work is a small dilemma in sexual selection theory. The theory predicts that females are generally the choosy, limiting sex, so they should invest less in courtship — they can simply lie back, look alluring, and let the males fight it out over them before picking a winner. Males are expected to invest the most in courtship, because after all, winning gets them the big payoff with little expense, while females get the big expense of egg laying/pregnancy. That boys are traditionally expected to ask girls out on dates, not vice versa, makes a lot of sense in the context of this theory.

A possible contradiction to the theory, though, is the production of female pheromones to invite courtship. Many arthropods in particular — consider moths that produce olfactory signals that males have evolved enormous antennae to detect — use female-initiated signals to initiate courtship behavior in males, as if every day were Sadie Hawkins Day.

One way to resolve the contradiction, though, is to discover that the signal evolved for some other purpose than triggering courtship. Perhaps ancestral female moths were flying about smelling generically mothy, and male moths are simply homing in on something the females can’t help but produce.

Enter the crayfish, Pacifastacus leniusculus. Females in this species initiate courtship with a grand aromatic puff of urine, a kind of crustacean invitation to dance that I am quite glad our species does not emulate. In the photo above, it’s been visualized by injecting the crayfish with a dye, fluorescein, that can be seen against a black background with the proper illumination. The crayfish have been blindfolded so that their behaviors aren’t triggered by visual cues.

And the answer they discovered is that the copious urination is an aggression, not courting, signal. Males spew it out when they’re fighting with other males, females do it at the start of courtship, and males actually reduce the amount of urine produced during courtship. So they’re not saying, “I love you, come get me,” they’re saying “Grrr, fight, fight, fight” when they spray the tank with urine.

What’s the advantage to the ladies here? It’s the incitement. The crayfish live in high population densities, and stirring up a little trouble and getting the males to fight provides an opportunity to select a winner. It may also produce a local population of desirable contenders: a whiff of urine may encourage wimpy males to run away and avoid potential trouble, while the more aggressive males may home in on it.

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Berry FC, Breithaupt T (2010) To signal or not to signal? Chemical communication by urine-borne signals mirrors sexual conflict in crayfish. BMC Biology 8:25.