A male fiddler crab displaying courting behaviour.
Credit: Tanya Detto
Male fiddler crabs competing for mates.
Credit: Tanya Detto
A male fiddler crab courting a female.
Credit: Tanya Detto
SYDNEY: Understanding the impressive strategies that male fiddler crabs use to attract females may shed new light on how animals communicate.
Male fiddler crabs attempt to attract a mate by waving their exceptionally large mating claw in the air, which can be a costly exercise in terms of predation visibility and physical exertion, since the claw weighs about one-third of their total body weight.
New research published in Biology Letters today shows that male fiddler crabs adjust the rate at which they wave their claw depending on the level of competition at the time. "Male fiddler crabs eavesdrop on other male's courtship displays to determine how hard they should try - that is, how fast they should wave," said evolutionary biologist and lead author Richard Milner from the Australian National University in Canberra.
Because studies in the past have focussed on animals, such as frogs, that court acoustically, this study based on visually courting animals is a first. "I think the results illustrate an impressive level of social information used by fiddler crabs, which opens up future areas of research within animal communication," said Milner.
The effect of competition
Fiddler crabs can be any of the 100 species of semi-terrestrial marine crabs in the genus Uca, and can vary in colour from tan to blue. They are close relatives of the ghost crabs and grow no bigger than 5.1 cm across. They are found in mangroves, salt marshes and on sandy or muddy beaches everywhere from West Africa and the Western Atlantic to the Indo- and Eastern Pacific.
Previous studies have shown that female fiddler crabs Uca annulipes prefer males with faster waving rates and more waving leadership when they are in a crowd. To see whether competition affects a courting male, Milner and his team manipulated the social environment of a focal male fiddler crab (Uca annulipes) and tested high and low competition situations.
They found that male fiddler crabs have 30% lower wave rates when isolated away from competition. "We measured [the] male wave rate under either low or high competition pressure. Under high competition male wave rate was significantly higher than under low competition," said Milner. The team observed that the crabs that were most economical about their wave while keeping it attractive were also successful in scoring a mate. "Sexual selection is a fascinating area of research and fiddler crabs are an incredibly easy study system to work on," said Milner.
Fake it to make it
"By asking the question, 'Do males put a lot of effort into signalling to attract females if they can get away with less?' the authors are able to show that busy-looking male fiddler crabs are in reality just like most of us, in that they will do the bare minimum to get by unless forced by circumstances to do more," said Simon Lailvaux, an evolutionary biologist at at the University of New Orleans in the U.S. who was not involved in the study.
"This result makes a lot of sense and will appeal to the inner slacker in us all - why put a lot of effort (and energy) into standing out from the crowd if the entire crowd consists of you and a handful of other guys?"
Fiddler crabs are one of the easiest yet most fascinating animals to study the sexual selection pressures of, which could inspire research on more complex animal signalling behaviours. "When a result like this comes along, it really highlights how much we don't know about signalling behaviour and how much we still have to learn," said Lailvaux. "Certainly it raises a lot of questions about what factors animals are paying attention to when they are displaying, and about what influences the intensity of those displays."
