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Cyber crabs shed light on sexual selection

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Physical prowess matters to female fiddler crabs when it comes to choosing a mate but the quality of a suitor's home is more important, according to research enlisting robotic crustaceans.

Biologist Richard Milner has investigated the basis of mate choice in fiddler crabs in work with implications for evolutionary psychology.

Milner, a postgraduate student in the Australian National University's School of Botany and Zoology, wanted to find out what physical traits were attractive to females and whether mate choice was influenced by the environment.

The colourful male fiddler crab poses outside his burrow and waves his single enlarged claw in a courtship ritual. Females check out up to 100 males in the colony before selecting a prospective mate.

In experiments in Darwin, Milner presented each of several female crabs with the choice of two robotic males that were identical in claw and signal attributes but which were in different environmental settings. "We make moulds of the claws and stick them onto a metal arm, which produces a wave identical to that of real crabs," he says. "This allows us to control numerous variables."

The females were attracted to males with big claws, and those which waved them more vigorously and in a particular way . Males use the claws in territorial battles around their burrows, so females have evolved mate choice behaviour that will get their precious genes into the gene pool.

Milner altered the environment of one of the robotic males by simulating mangrove roots, and changing the lighting levels and colour of the background to see if mate choice was affected. However, the females were not influenced by the setting.

"This was surprising," says Milner, whose work is being supervised by evolutionary ecologists Dr Pat Backwell and Associate Professor Michael Jennions. "In other species that use courtship displays, including some birds, the males put a lot of effort into making their displays more conspicuous."

However, the standard of the male's burrow, where mating and incubation occurs, was the main deciding factor, and females knocked back otherwise eligible males if this haven from predators and the incoming tide was not up to scratch. Most of the small tropical crabs mate underground, with the male leaving the burrow after the eggs have been extruded. The female stays on until the eggs hatch two weeks later.

Milner, who grew up on a farm near Canberra, did his BSc at ANU, choosing the institution because of its strength in behavioural ecology and the opportunity for fieldwork, even at the undergraduate level. "The transition from the country to Canberra was easy," he says.

In 2008, he was about to start a PhD on other aspects of fiddler crab behaviour, work that was to take him on fieldwork in Mozambique

 

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