Waving robotic crab arm attracts females
10 Sep BBC
Female fiddler crabs prefer males who can out wave their neighbours,
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.
Ms 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. |
