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Crabs caught spying on rivals' love claws
Male fiddler crabs spy on their competitors to work out when a potential female mate is around, Australian researchers have found.

Their findings are reported today in Biology Letters.

"Males will use other males as female detectors," says behavioural ecologist Richard Milner of the Australian National University in Canberra.

"They'll eavesdrop on other males' courtship displays to detect the presence of a female."

Milner carried out the research for his PhD under the supervision of Associate Professor Patricia Backwell and Professor Michael Jennions.
Long arm of love

Male fiddler crabs have a large specialised claw that they use to fight and wave around to attract mates.

"When a female approaches a group of males they'll all start waving in synchrony and they'll all start trying to attract her," says Milner.

But knowing when females are around in the first place can be tricky because females are well camouflaged.

"The males are ridiculously conspicuous but the females look very bland," says Milner.

Milner wanted to see if males would use waving by other males as a sign there was a female around and start waving before they could actually see the female.

This would enable them to detect the presence of a female earlier than they otherwise would.
Experiment

Milner set up a field experiment involving a group of four or five male crabs.

Milner then measured the number times per minute a particular "focal male" waved his love claw, under three different conditions.

In the first case, there were no females around, enabling him to establish the "base rate" of waving.

In the other two cases he tethered a female crab using a piece of cotton string amongst the group of males.

But in one of these cases the focal male was hidden behind a barrier where he could see the other males but not the female.

Any increase above the base rate of waving would only be because he could see the other males waving, says Milner.

When there was a female around, the focal male waved his claw 20 times a minute.

This compared to a base rate of once a minute when there were no females around.

Interestingly, though, when the focal male could not see the female (but the other males could), he waved his claw 12 times a minute.

"Eavesdropping males wave 12 times faster than non-courting males, but only 1.7 times slower than males in full visual contact with the female," says Milner.

Milner repeated the experiment with different 50 focal males.

"That's how I worked out that they do use other males as female detectors."
Being first isn't everything

Milner says it is generally known that females prefer the males that wave the fastest, have the largest claw and wave first.

Waiting until others wave may not mean missing out on the ladies, especially if a male has a bigger claw or waves faster than them.

But females care about things other than the love claw.

Fiddler crabs mate in burrows, so females also rely on the male having a good quality burrow for incubating her eggs.

So what a male lacks in the claw-waving department, he may be able to make up in his home-building skills.

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