At low tide, a female fiddler crab (Uca perplexa) cruises the beach in search of a suitable mate. When she spots a reasonably attractive male, she approaches for a closer look. Encouraged, the male treats her to a bit of the old I’m too sexy for my shell. That is, he engages in a display. He flaunts his desirability as a mate by waving his greatly over-sized major claw – raising it high and lowering it several times. If the female is impressed, she will enter his burrow, inspect it, and perhaps mate with him. If not, she passes him by. Female crabs tend to approach larger than average males, and there is evidence that females respond specifically to the size of the major claw. Whether the approached male is deemed worthy of a “visit” – phase 2: the home inspection – is apparently determined by his claw waving ability. The higher the male raises his claw the better his chances seem to be.

Both claw size and display behavior are thought to be subject to sexual selection – that is, subject to adaptation driven by preferences of females. A critical factor in sexual selection is the reliability of the selected trait as a predictor of genetic quality and/or physical condition. A preference in mate selection should be favored by evolution if it tends to increase the likelihood of securing good genes for one’s offspring. In a report published online this week, by the journal Evolution, biologists Minoru Murai of the University of the Ryukyus (in Japan), and Patricia Backwell and Michael Jennions of The Australian National University, experimentally evaluated the reliability of the fiddler crab display.

The researchers filmed the crabs in their natural environment, in order to quantify the heights attained in the waving displays. Then they captured males that had been approached by females, measured them, and used them as experimental subjects. Regression analysis revealed that neither claw size nor wave height affected female choice; what mattered was wave height relative to size.

Now, repeatedly hoisting a claw that can account for half an individual’s total body mass is no mean feat. It requires a non-trivial energetic investment. Evolutionary theory holds that a sexually selected signal is reliable (as defined above) if it involves a cost to the signaler and if the cost is more easily borne by a higher quality individual. This is referred to as the handicap principle – according to which a superior individual is able to waste resources and still outperform the competition.

To test the hypothesis that the waving display is more costly to some males than to others – and might therefore be a reliable signal of quality to a prospective mate – the researchers attached metal weights to the claws of half their male fiddler crabs. The weights were proportional to the sizes of the individual crabs. The other half - the control group - they outfitted with lightweight pieces of plastic. They returned the males to their natural habitat and again filmed their interactions with females. For each male, they compared wave height before and after manipulation.

The control group exhibited no change in wave height. In the experimental group, though, they observed an interesting trend. The males that displayed below average wave height prior to the treatment suffered the greatest decreases in wave height when artificially burdened. The males with above average initial wave height were barely affected by the additional weight. These results indicate that the males with the boldest moves did, in fact, have more strength to spare than their more restrained rivals. The results are consistent with the prediction that wave height is a reliable signal of quality to potential mates in the fiddler crab – and therefore potentially beneficial as a target of sexual selection.

REFERENCE: M. Murai, P.R.Y. Backwell, M.D. Jennions, “The Cost of reliable signaling: experimental evidence for predictable variation among male in a cost-benefit trade off between sexually selected traits”, 30 April 2009, Evolution, (Accepted Article, doi: 10.1111/j.1558-5646.2009.00726.x)