Male fiddlers get lazy if there's no competition

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In the absence of competition, male fiddler crabs invoke the inner slacker in them and do not put as much effort in their courtship displays as when other males are present, according to research to be published in a forthcoming issue of the journal Biology Letters.

Studying the Porcelain fiddler crab (Uca annulipes) in Tanzania, Richard Milner, Michael Jennions and Patricia Backwell found that the male crabs adjust the rate at which they wave their mating claw to attract females, depending on the presence of other males.

Male fiddler crabs have a greatly enlarged (and often brightly coloured) mating claw that they wave synchronously in conspicuous courtship displays in order to attract females to mate with them.

In the Porcelain fiddler crab, females show a marked preference for males with larger claws that wave faster and lead (the other males in waving) more often, but there are costs to the males for trying to impress females: growing a larger claw and waving it faster expends more energy, as well as making the males more conspicuous to predators.

The authors conducted an experiment that manipulated the social environment of a focal male. Wandering females within half a metre of a naturally-occurring group of males were first removed, and a tethered female was placed 10cm in front of a randomly-designated focal male.

Other males were manipulated to be visible (high-competition treatment) or not (low-competition treatment) to the focal male. This was done in the low-competition treatment by scaring all the crabs into their burrows, placing bottle caps over all of the burrows within half a metre of that of the focal male before allowing the focal male to exit the burrow and display to the tethered female.

In the high-competition treatment, not all of the burrows within 50cm were covered by bottle caps (only 10 or more burrows were randomly selected to be covered, none of which were immediately adjacent to the burrow of the focal male). The number of waves by the focal male produced over a 30-second period was counted and the wave rate calculated. Fifty males in 50 different groups of males were subjected to this experimental manipulation.

The authors found that males whose male neighbours were not on the surface produced 30% fewer waves than those whose male neighbours were present. Furthermore, males with many male neighbours displayed more often than those with fewer male neighbours.

Although the model of sexual selection in fiddler crab courtship displays suggests that males should always court females at their maximum intensity, the results of this experiment suggest that waving is extremely costly and that males can reduce these costs by adjusting their wave rate according to the level of competition and/or the likelihood of attracting a female.

For more information, see the paper: Milner, RNC, MD Jennions and PRY Backwell (2011) Keeping up appearances: male fiddler crabs wave faster in a crowd. Biology Letters doi:10.1098/rsbl.2011.0926

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