Females fall for robot fiddlers

Female fiddler crabs have a thing for males who fiddle fast – or possibly just fancy a fling with a crustacean robot, research has found.

Male fiddler crabs (Uca mjoebergi) are well known for their very active and repetitive courtship displays. Inevitably, however, crabs who are unsuccessful in scoring the undivided attention of a female within a certain period of time find themselves flagging. Their energy reserves diminish, even if their desire does not, and the formerly frantic display slows to a more languid pace.

And this, say researchers led by biologist Sophie Mowles of the Anglia Ruskin University of Cambridge, UK, is a complete and utter waste of time. Unattached female fiddler crabs can’t stand a slowcoach.

To make this finding, Mowles and her colleagues made some robotic crabs and took them to a well-known fiddler crab hang-out on a beach in Darwin, in the north of Australia. The robots were made of accurately painted hydrostone, each equipped with a remote-controlled signalling arm.

The robots were arrayed on a little stage on a mudflat. The researchers then went off in search of unattached female crabs, scooped them up and took them to display.

The crabs meanwhile were set to operate according to a variety of protocols: some increased their signalling speed, while others slowed down, and a third group remained constant.

The scientists found that the female crabs “demonstrated a strong preference for escalating robots”.

In a paper published in the journal Biology Letters, Mowles and her colleagues suggest that the preference for escalating signals is perhaps more complex than a simple liking for raw vigour.

“In species such as fiddler crabs, where the male bears a formidable weapon, signalling rate could also indicate motivation to court, where males expending energy in a costly display are less likely to react with dangerous levels of aggression towards approaching females,” they write.

The team’s formal conclusion carries perhaps a whiff of observations made of species other than crabs.

“As in fiddler crabs, females of many species may be sensitive to changes in display rate and benefit from attending to prolonged dynamic repeated courtship displays, which provide more reliable information with which to accurately gauge male quality,” they note.