Crayfish fight all the time. They battle over food, shelter and females, but is more going on than meets the eye? Thomas Breithaupt knew that some wrestling crustaceans send each other warning signals in their urine. Could crayfish do the same? Having found a way of visualising the transparent urine, Breithaupt watched the crayfish fighting and realised that both crayfish urinated, but the victor signalled his physical superiority by urinating most (p. 1221).

Crayfish are nocturnal, and often live in cloudy water, so visual communication can be unreliable. Which is why Breithaupt suspected they were signalling to each other with their urine. But seeing an invisible signal is tricky. Breithaupt wasn't sure how to visualise the clear liquid until Eger suggested trying a diagnostic method that she had used when she was a nurse; injecting a coloured dye into the animal's circulatory system to colour its urine.

Once they'd successfully tinted the crayfish's urine, getting the animals to fight was simple. Just put two adult males together in a tank and they start gesturing at each other, ready to a battle. Breithaupt and Eger blindfolded the fighting crayfish, so they wouldn't be distracted by the clouds of visible urine, and videoed each crayfish clash.

After watching over 60 fights, Breithaupt realised that the winner always urinated more than the loser. When he analysed the loser's response, he saw the loser became more defensive every time its opponent urinated, although this wasn't obvious from watching the animals fighting. He also realised that no matter whether the crayfish had fought before, the loser always lost more quickly during the second duel, even if it was fighting a new adversary.

This tied in well with other experiments where Breithaupt had found that fights between crayfish that couldn't urinate went on and on! No matter how much the crayfish gestured and wrestled, they wouldn't give up fighting until they picked up the urine signals. Even though a crayfish's gesturing looks impressive, the visual message is nowhere near as intimidating as the chemical message from the urine. Breithaupt realised that the urine clearly signalled which of the fighters was going to win.

Breithaupt thinks that gesturing can make a timid crayfish appear more aggressive than it feels, but its urine cannot lie. The animal can't disguise the metabolic products that signal its weaker physical state. Once a weak opponent picks up a whiff of the superiority-signal, it knows that it's up against a real champion and decides to quit before the going gets really tough.

Having proved that crayfish urine carries the smell of success, Breithaupt wants to isolate the pheromone that puts the weaker crayfish in their place. He believes that crayfish pheromones could prove to be a good starting point for unravelling the complex neural circuitry that links the crayfish's olfactory system to the well-established behaviours that they provoke.