Fiddler Crabs Reveal Honesty Is Not Always The Best Policy

The signals animals send each other about their fighting prowess - and the honesty of these signals - is a long-standing problem in evolutionary biology. Despite their size - they are just two centimetres across - fiddler crabs are ideal for studying dishonesty in signalling. This is because males have one claw that is massively enlarged (which they use to attract females or fight rival males) and if they lose this claw during fights they can grow a replacement. In most species the new claw is identical to the lost one, but some species “cheat” by growing a new claw that looks like the original but is cheaper to produce because it is lighter and toothless.

According to lead author of the study, Dr Simon Lailvaux of the University of New South Wales: “What’s really interesting about these 'cheap' claws is that other males can’t tell them apart from the regular claws. Males size each other up before fights, and displaying the big claw is a very important part of this process.”

Dr Lailvaux and colleagues from the Australian National University measured the size of the major claw in male fiddler crabs, and two elements of fighting ability - claw strength and ability to resist being pulled from a tunnel. They found that while the size of an original claw accurately reflects its strength and the crab's ability to avoid being pulled out of its burrow, this relationship does not hold true for a regenerated claw.

“This means that while males can gain an idea of the performance abilities of males with original claws from the size of those major claws, regenerated claws don’t reveal any information on performance capacities. Males with regenerated claws can 'bluff' their fighting ability, like bluffing in a poker game. They’re not good fighters, but the deceptive appearance of their claw allows them to convince other males that it’s not worth picking a fight with them. The only time it doesn’t work is when regenerated males hold territories, which means they can’t go around choosing their opponents - they have to fight everyone who challenges them, and eventually someone will come along and expose their bluff.” Lailvaux explains.

The study is important because it helps shed light on an issue - dishonesty - that is by definition hard to study. “One of the reasons we don’t know a huge amount about dishonesty is because it’s tough to pick up on it. Dishonest signals are designed to be difficult to detect, so to have a system like fiddler crabs where we’re able to do experiments and test hypotheses about dishonesty is pretty cool,” he says.

The results also have important implications for individual reproductive success and survival, as understanding the mechanisms and consequences of dishonesty is essential to uncovering the full story of how these and other animals live, die and reproduce.

According to Lailvaux: “By studying exactly how animals fight, and what physiological and performance capacities enable males to win fights, we’re getting closer to identifying which traits are likely to be generally important for male combat. Honest signalling is important for several reasons, primarily because it’s important that fights don’t always escalate into bloody violence. Fighting can be costly in terms of time and energy, and it’s in an individual’s best interest to avoid risking being injured in a fight, so one of the reasons why we think honest signalling has evolved is because animals need to have a diplomatic option for settling disputes, as opposed to duking it out with every male that comes along. If there’s a way for individuals to assess beforehand which males they are likely to lose to in a fight and which ones they are able to beat, then that allows them to plan accordingly.”

Fiddler crabs live in mangrove swamps and mudflats. There are around 100 species worldwide. Despite their propensity for dishonesty, the name fiddler crab comes from the fact that while waving their big claw to attract females they look like they are playing the violin.