Many males of an Australian crayfish species consistently fake their way through macho confrontations, a new analysis of rivalries indicates, even though evolutionary theory says that such bluffing should be rare.
When two male slender crayfish (Cherax dispar) encounter each other, the one waving bigger claws typically sends the smaller-clawed creature fleeing, say Robbie Wilson of the University of Queensland in St. Lucia, Australia, and his colleagues. Yet the researchers’ measurements show that the biggest claws don’t necessarily deliver the strongest pinch.
That the oversized but feeble claws retain their menace represents “one of the first demonstrations of dishonest weaponry on a widespread scale,” says coauthor Michael J. Angilletta Jr. of Indiana State University in Terre Haute.
Slender crayfish, about palm size, stake out homes in streams, and males face off when they encounter each other. “They do this odd little dance,” Angilletta says. One, adopting a pose that reminds Angilletta of a knuckle-walking gorilla, plants his claws tip down in the sand, then lets the other male tap them. Then the poser and tapper switch roles. After several posing bouts, one male usually retreats. Only rarely does the encounter escalate into a wrestling match.
The researchers studied various confrontations among 32 males. Angilletta says that his coauthors—from Brazil and England as well as Australia—paired the males in a series of contests arranged to create “the crayfish World Cup.”
A detailed analysis showed claw size to be a much stronger predictor of dominance than strength and body condition, the researchers report in an upcoming American Naturalist.
To see what a large claw might mean to a challenger, the researchers presented the crayfish with a tweezerlike device that measured grip force. Their claw size correlated with force only “very weakly,” says Angilletta.
Coauthor Rob James of Coventry University in England dissected crayfish muscles to measure their force. He found that a section of male muscle tissue was only half as strong as a same-size sample of female muscle tissue. “What it suggests to us is that males are making crappy muscle,” says Angilletta. The males apparently put a lot of resources into size even if quality suffers.
On the rare occasions when males resorted to wrestling, claw strength did matter, the researchers found. So there ought to be an evolutionary advantage for males calling the bluff of fakers, which would over time reduce the frequency of cheating. “It’s a puzzle,” says Angilletta.
The result contrasts with the findings of a 2006 paper on threat behaviors among collared lizards in the southwestern United States. Male lizards spend time “just sitting there with their mouths open,” says Jerry Husak of Virginia Polytechnic Institute and State University in Blacksburg. The gape reveals mouth muscles, and Husak and his colleagues found that muscle width proved a reliable indicator of bite strength.
Husak notes, though, that evolutionary theory suggests that cheating may arise under certain circumstances. He calls the new crayfish study “a good step forward in our understanding of reliability and deception.”