Male Fiddler crabs eavesdrop to find the ladies

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Scientists in Australia have discovered a new strategy that male fiddler crabs employ to attract females: eavesdropping.

In research to be published in an upcoming issue of the journal Biology Letters, Richard Milner, Michael Jennions and Patricia Backwell of the Australian National University have demonstrated that male fiddler crabs (Uca mjoebergi) eavesdrop on the courtship display of nearby males in order to locate mate-searching females. 

Fiddler crabs attract females by waving their single greatly enlarged claw in a highly conspicuous display, with receptive females approaching the displaying males based on wave rate, wave leadership and claw size. 

The authors hypothesise that other males could use the conspicuous waving displays of the male fiddler crab to alert them to the presence of a mate-searching female (i.e. with the displaying males serving as a ‘female-detector’) and carried out an experiment to test this hypothesis.

Utilising the fiddler crabs at mudflats in East Point Reserve, Darwin, Australia, the authors first located a naturally occurring group of at least four courting males and removed any wandering females within 50 cm of the group. 

Randomly selecting a focal male, they then measured its wave rate (by counting the number of waves over a 30-second period). 

The authors then anchored a piece of plywood vertically into the mud in front of the male, preventing it from seeing the area directly in front of the barrier. 

They then counted the wave rate of the focal males again. In the third treatment, they tethered a mate-searching female crab behind the plywood visual barrier in such a way that the female was not visible to the focal male, but could be seen by other nearby males. Again, the wave rate of the focal male was counted. 

Lastly, they removed the plywood barrier and counted the wave rate of the focal male that was now able to see the tethered female clearly.

After testing 50 groups of males in this manner, the authors found that the focal male eavesdropped on other males and began waving even though it could not see the female. 

Furthermore, the males apparently adjusted their wave rate based on the information received: 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.

The authors surmise that "…detecting receptive females as early as possible is likely to be of great importance in determining male success at mate attraction. Through eavesdropping, males are effectively increasing the distance at which they can detect receptive females, thereby increasing their conspicuousness and consequently elevating their likelihood of being sampled by a mate-searching female."

For more information, see the paper: Milner, RNC, MD Jennions and PRY Backwell (2010) Eavesdropping in crabs: an agency for lady detection. Biology Letters doi:10.1098/rsbl.2010.0384.