Evolution of the Major Claw in Fiddler Crabs

One of the most fascinating questions in evolution is why some species have evolved extremely large secondary sexual structures, such as the antlers of the extinct Irish elk or the horns of beetles. In all fiddler crab species the males have one enormous major claw, nearly half of the body weight of the animal, which is used in combat with other males and in complex waving displays that attract females to the male's burrow. Why so large a claw? Does the claw detract from efficient movement of the animal? Does the claw function in damaging opponents, or is it merely a display device, with little function in combat? The male's other claw is small and is used for feeding only. Females have two feeding claws, which gives them an advantage in gathering food.

We have been studying the function of the male's giant claw, by morphometrics, by studies of closing force in the laboratory, and by videotaping of crab activities in the field. Like the imagined behavior of Tyrannosaurus in the film Jurassic Park, fiddler crabs react swiftly to movement. But if a student keeps still (even while being bitten by mosquitoes!), the crabs will behave normally. This allows us to videotape their activities in the field. We can measure running velocity, waving patterns, and allocation of time to display and feeding.

We have found that the claw is a fully functioning fighting organ, but it does not seem designed to allow males to badly damage other males. Instead, the major claw is designed mechanically to grapple with opponents. The male's major claw has reduced mechanical advantage, hence reduced closing force. But reduced mechanical advantage increases gripping speed. You can visualize this by imagining a seesaw. If the board of the seesaw is not evenly placed, it will take more force to push the short side than the long side. But a given angular movement of the short side will produce a much larger angular movement of the long side, and at a greater speed. In this way, the major claw of the fiddler crab has a movable finger that is relatively long and moved by muscles inserting and acting on a much shorter arm on the other side of the fulcrum. This moves the movable finger rapidly, but with little closing force. Like a college wrestler, the crab seeks to grapple the opponent rapidly, so it can drag it away.

Our future work will concentrate on the relationship of claw function to social system, which varies greatly among species of fiddler crabs. In species that live at low density is the claw used less for damage than in species living under crowded circumstances? Does communal waving by males result in less combat? What is the evolutionary origin of waving and hypertrophied structures such as the major claw of fiddler crabs?