Fiddler Crabs Use Dishonesty to Deter Rivals

Dishonesty in ecology, as a policy for deterring potential rivals, has not been thought of as a common strategy across the animal kingdom until recently.

Dishonesty has been a long-standing conundrum in evolutionary ecology. Previously, well-respected researchers such as Harper & Maynard Smith (2003) and Zahavi (1975) have conceded that cheating is unlikely to evolve as an effective strategy because of the costs of producing dishonest signals. However, new research published in the British Ecological Society’s Functional Ecology journal sheds light on how animals can feign their fighting prowess.

Research focusing on fiddler crabs Uca mjoebergi – so-called because when waving their oversized claw to a female they appear to be playing a fiddle – suggests dishonesty could be much more widespread than previously thought. The research is all the more exciting because, by definition dishonesty is notoriously hard to detect.

There are around 100 species of fiddler crabs world wide, and they tend to live in mangrove swamps and mudflats.

Fiddler crabs provide a good model species to resolve the question of whether armaments, such as claws, are capable of being dishonest. This is because they possess an overtly enlarged claw, not only used in battle to defend territories but to assess fighting ability prior to an encounter, in order to prevent a costly fight. The claw is also used by females to identify high quality partners, so there is a two-fold advantage to possessing an oversized claw in terms of signalling.

The lead author of the research Simon Lailvaux from the Australian National University said: “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.”

If male fiddler crabs lose a claw in battle, they are able to regenerate a new claw. The potential for cheating lies in their ability to produce a new claw that is similar in size and impressiveness to the previous claw, but lacking in equivalent strength and effectiveness when fighting.

The researchers pitted males caught from the wild against each other under controlled laboratory conditions, to determine the effectiveness of original vs. regenerated claws in signalling (deterring a rival male from fighting) and fighting (defeating a rival male). Losers of encounters between rival males left the territory, making it easy to identify the victor.

The researchers found that, although size was generally correlated with strength and fighting ability, weaker regenerated claws did not perform as well as original claws in fights.

Lailvaux said: “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. “

This research also exposes the cost associated with bearing a dishonest signal. Generally, males tend to challenge other males of similar claw size. When males are forced to defend intruders possessing a strong original claw from burrows , the bluff is exposed and they tend to lose. There is also possibly an evolutionary pressure to keep cheating to a minimum, as has been documented in yeast (Greig and Travisano 2004). Since dishonest males are in the overwhelming minority (~7% of the study population), there is clearly sufficient scope for them to get away with it.

Source: Simon P Lailvaux, Leeann T Reaney and Patricia R Y Backwell (2008).
Dishonesty signalling of fighting ability and multiple performance
traits in the fiddler crab Uca mjoebergi. Functional Ecology, doi:
10.1111/j.1365-2435.2008.01501.x, is published online on 12 November 2008.


Greig D., andTravisano, M., 2007, The Prisoner’s Dilemma and polymorphism in yeast SUC genes, Proceedings of the Royal Society B, Vol 271, pp 25-26

Harper, D. & Maynard Smith, J. (2003) Animal Signals. Oxford University
Press, Oxford.

Zahavi, A. (1975) Mate selection: a selection for a handicap. Journal of
Theoretical Biology, 53, 205–214.