Australasian Science: Australia's authority on science since 1938

Neighbours Keep Crabs Honest

By Stephen Luntz

Urban crabs are more honest than their country cousins, new evidence suggests.

Fiddler crabs put considerable resources into making large fighting claws. Since crabs will often avoid conflict with a larger-clawed opponent, the temptation exists to build large but poorly muscled claws to bluff opponents.

“When there are lots of crabs living in one area, there is lots of competition for resources like females and food. High competition means there is a greater chance of males having to fight each other to win resources compared to when there are not many crabs about. Those crabs might not have to fight at all,” says Candice Bywater, a PhD student in the Performance Lab at the University of Queensland.

“We wanted to have a look at lots of populations that vary in the level of contact they have with other crabs and see if there was an obvious variation in the level of honesty,” says lab head Dr Robbie Wilson. This was indeed what happened, with males from highly populated areas generally having claws whose strength matched their size.

“We found that when there are not many other male crabs in a population (low competition), males produce large but relatively weak claws as they don’t have to fight as often and ultimately can get away with it,” Bywater adds.

Wilson says that not enough data has been collected to determine whether a sharp cut-off exists at a particular crab density, or if the results lie along a continuum.

Wilson’s conclusion seems obvious: “Whether it’s a soccer player diving to fool a referee or a crab trying to intimidate a rival with weak claws, our lab has shown that individuals cheat more when their deception is likely to go undetected”.

However, he adds: “Behavioural biology has gone from one end of the spectrum to the other. At one time it was thought animals would never tell the truth, but we have found most examples, whether it be lizards intimidating their rivals with their size or male birds trying to show how sexy they are to females, are honest. If there is too much lying no one believes the signal and it becomes useless.”

Crustaceans have proven a good way to investigate this question because the strength of their claws can be easily tested, while Wilson notes it is hard for humans to determine whether other animals are lying if their own species cannot.

Wilson is keen to test crab self-awareness and see if those carrying large but weak claws know when to avoid fights or if they are also fooling themselves. He admits to not having solved the puzzle of why slender male crayfish sometimes cheat with oversized claws while the females of the species are honest, yet the reverse occurs in common yabbies (AS, July 2009, p.13).