The truth is, frogs bluff and crabs cheat

Two research teams suspect they’ve caught wild animals bluffing their own kind. The scientists know of only one other published example outside of primate antics.

When an intruder croaks, small male green frogs (Rana clamitans) drop their responding croaks extra low, like genuine big guys, say Mark A. Bee of the University of Missouri in Columbia and his colleagues. For an especially deep-voiced intruder, the little guy makes his deepest response, they report in the March-April Behavioral Ecology.

Similar moxie shows up in a study of an Afro-Asian species of fiddler crab known to take a short cut to regrowing a lost claw. Male Uca annulipes produce a substitute as long as the original but lighter and flimsier, explains Patricia R.Y. Backwell of the Smithsonian Tropical Research Institute in Balboa, Panama.

Yet this frail claw works just as well in threatening rival males and waving to the ladies, Backwell and her colleagues report in the April 7 Proceedings of the Royal Society of London B.

“It’s the first example of a naturally occurring cheat in attracting females,” she says. Both examples sound plausible, comments deceit theoretician Rufus Johnstone of the University of Cambridge in England. “I think, in a sense, deceit is likely to be quite widespread,” he predicts.

At first, scientists considered communications within a species largely a matter of animals cooperating to transfer information, he notes. About 20 years ago, an interest in evolution’s sway on individuals transformed this approach. “Once you take an individualized view, suddenly you start to see opportunities for cheating,” Johnstone says. Yet cheating can’t get too common or victims wise up.

Documenting deceit proved tricky, however. As Backwell puts it, “Cheats are designed to go undetected.”

In 1983, researchers found that a prawnlike Caribbean stomatopod that has just molted still threatens intruders by waving a claw. That claw is too soft and weak for a fight, yet the threat often works.

Bee and his colleagues checked for similar machismo among green frogs. Males “plunk, plunk” like a loose banjo string while splashing toward each other. If neither backs down, males can wrestle for an hour, treading water the whole time. The bigger male usually wins.

Until recently, scientists expected body size always to determine pitch. Now, they’ve found pitch shifting in several species, including the green frog.

Bee and his colleagues spent much of two summers standing in ponds in the dark playing frog “plunks” from a speaker floating on foam. The frogs that lowered their response calls the most were neither the feistiest nor in the best shape. Instead, the pattern suggested that they were little guys that were bluffing.

Among fiddler crabs, Backwell looked at flirting as well as fighting. At the sight of a female, males hanging around burrow entrances speed their claw waving to a fast, roughly synchronous display. They wave “more like the Queen Mother until a female shows up, and then they get frantic,” Backwell says.

Females prefer the longest claw and the wave that leads the pack. Males with light claws can wave them fast. “The females just love them,” Backwell notes. In experiments, these males lost fights, but in the wild, they still intimidated rivals with smaller but stronger claws. In one colony, about 20 percent of males cheated—a percentage predicted by theories. Another site had 44 percent cheaters, Backwell notes with consternation.

“There’s no magic frequency,” Johnstone says. He expects the percentage of cheats to vary with the cost of being cheated and the danger of calling a bluff. Backwell refuses to comment on how cheating theory might stretch from crabs to humans.

Johnstone, however, points out that most of biologists’ theoretical models of deceit “came from economic theories in humans.”