Waving robotic crab arm attracts females

A vigorous wave of the claw can be the key to mating success for male fiddler crabs, report researchers at the 13th Congress of the European Society for Evolutionary Biology.

Male crabs advertise their quality as a potential mate to passing females by waving their large yellow claws.

Using robotic arms, researchers evaluated how the size and speed of the waving claw affected mating success.

The results may help explain why males protect their smaller neighbours.

To the fiddler crab Uca mjoebergi, the Australian mudflats in the north of the country are a heaving dance floor, where a male must rely on his moves to attract a mate.

Males stand outside their burrows and use their enlarged claw to attract females by moving it in circles.

If a female likes the look of a male, she will come closer and disappear down his burrow in the sand, possibly staying to mate.

Wave of waving

When a female wanders through a neighbourhood, “you see part of the mudflat light up” with waving yellow claws, said ecologist Sophie Callander from the Australian National University in Canberra.

Dr Callander and her colleagues used a fully adjustable robotic arm – called Robocrab – to determine what female crabs are looking for in a mate.

Dr Callander set up three robotic arms around a female crab, and sat beneath the unforgiving Australian sun for many hours recording the females’ reactions to different combinations of wave speeds and claw size.

Females approaching from 20cm preferred males with a higher wave rate and larger claws. Intriguingly, this preference increased in strength when the male was flanked by more slowly waving, smaller-clawed crabs.

Fiddler crabs also use these claws in displays of dominance and fighting prowess.

Previous work has shown that larger males sometimes go to the aid of smaller males when an intruder is trying to steal a smaller male’s burrow.

This behaviour is unlikely to be an altruistic form of neighbourhood watch, and Dr Callander thinks that her experiment could offer an explanation.

“If larger males can retain smaller neighbours they might… increase their mating success,” she told BBC News.

For fiddler crabs at least, it pays to keep close to the small and weak.

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