ScienceWise - Spring 2010

The no.1 lady detection agency

How crabs eavesdrop on their rival’s courtship moves

Males of a species often perform blatant exhibitions of power or skill to impress a female: think antler-locked clashes, the dance of a bird of paradise or arm-wrestling at the local pub. Male fiddler crabs, with their oversized claw and elaborate waving ritual, are no exception. PhD student Richard Milner has recently discovered a more subtle strategy employed by these clever crustaceans: by ‘eavesdropping’ on their rivals’ courtship displays, these males gain a head start in the bid to charm the ladies.

The mudflats of the mangrove forests near Darwin are hectic, bustling places; hundreds of tiny fiddler crabs, less than two centimetres long, scuttle about, brawling over burrows, fighting over females and foraging for food. It is here within this exciting melee that Richard has witnessed the clever tactics applied by male fiddler crabs as they vie to out-compete their rivals for female attention.

When ready for a mate, female fiddler crabs abandon their burrows and set out across the flats looking for Mr Right. Male crabs have one oversized, brightly coloured claw which they wave in an eye-catching courtship display when they spot a female on the prowl. One of the key factors females consider when choosing a mate is wave rate: the faster the wave, the more attractive the male.

Due to the conspicuousness of this courtship display, wandering females often spot males before the males have spotted them; thus it would be advantageous for a male to begin waving before the female comes into his field of view. It might be expected, then, that males be evolutionarily attuned to any cue that potential mates are nearby.
With this in mind, Richard measured the wave rate of male fiddler crabs under different conditions to see whether they ‘eavesdropped’ on other males’ courtship displays to detect the presence of females. “In other words,” says Richard, “do males use other males as ‘female-detectors’?” 

Richard began by placing a small block of wood in front of an individual ‘focal’ male crab in a group of males. He then - using a very short length of string attached to a nail embedded in the sand - tethered a female crab behind the wooden barrier. In this set-up, the focal male could see all of his male neighbours (who were busy waving to the female) but could not see the female himself.

Richard observed that the focal male waved 12 times faster when the female was present than when she was absent. How was this male alerted to the presence of this potential mate? By peering at his peers. Richard also found that, remarkably, the eavesdropper modified his behaviour according to the reliability of the information he received: when the barrier was removed so he could see the female with his own eyes, he waved faster still. 

Given the importance of waving in the intense competition for female attention, there is an obvious advantage for males in detecting and reacting to the presence of a female as early as possible. “Females are very drab looking and rather well camouflaged,” says Richard. “Eavesdropping is so important because it allows the male to start courting the female even before he has seen her, which basically increases his chances of winning her over.”

Information acquired by eavesdropping is used in an extraordinary variety of situations. Some bird species avoid danger by recognising cohabiting species’ alarm calls in response to nearby predators, while female fighting fish gain inside knowledge on mate quality by spying on male-male confrontations, subsequently spending more time in the company of the victor.

Although the use of information acquired by eavesdropping on other males’ advertisement calls to attract mates has been observed in some cricket and frog species, Richard’s research is the first evidence of such an approach in a species with a visual signalling system and has recently been published in Biology Letters with his supervisors and co-authors Associate Prof Pat Backwell and Prof Michael Jennions.

This publication marks the completion of Richard’s 9th paper, an impressive accomplishment for any PhD student. It’s hard to imagine where the 23-year-old finds the time alongside his passion for cricket and baking his own sourdough bread. The enthusiastic researcher is off to Darwin again later this year, as well as Broome and Zanzibar, to look at other fascinating fiddler crab habits. “One of the projects that I am quite excited about will be looking at fight choice,” he says. “In fiddler crabs, males often form territorial coalitions with their neighbours. What I want to know is: do males, when choosing somebody to fight for their territory, take into account the size of that male’s neighbour?”

Life is not all slaving in the 40°C heat of the mudflats; Richard was recently married at his family’s property near Braidwood, which will also provide a lovely setting in which to write his PhD thesis. “I quite often go down to the farm to write,” he says. “The house is about 180 years old and it has four or five big open fireplaces that are brilliant to sit down in front of and write, especially in winter.” Richard plans to submit his thesis in July next year.

 

By Tegan Dolstra

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