Fiddler Crab Claws: Good For Bling But Bad For Bang
July 17, 2013

Female Fiddler Crabs Like Their Men With Big Claws, But They’re Not So Great For Fighting

April Flowers for - Your Universe Online

New research from Gonzaga University and the Smithsonian Tropical Research Institute reveals male fiddler crabs walk an evolutionary fine line between bling and bang.

The study, published in Evolutionary Biology, finds that long light claws are better for attracting females (bling), but not for fighting (bang).

Fiddler crabs are sexually dimorphic, with the males having one large and one small front claw while the females have two small front claws. The small claw of the male is used for feeding and the large one for attracting females, threatening other males and as a weapon when fighting. To attract females, like a flag, the claw needs to be as large and light as possible to make it easier to move. As a weapon, however, it needs to be heavy, with a thick cuticle, short dactyl and polex pincers, and large muscles to close them.

The team, led by Dr. Brook Swanson, assistant professor of Biology at Gonzaga University, analyzed the morphology and mechanical properties of 21 species of fiddler crabs from the Pacific, Gulf and Atlantic coasts of the US. Different species have evolved a wide range of tactics. For example, U. argillicola has a small powerful claw but does not wave to attract females, while U. heteropleura and U. saltitanta have weak claws but intense waving behaviors.

Swanson explained, "These crabs dramatically show the evolutionary trade-off driven by competing traits. The conflicting requirements of the claw for mate attraction and fighting may be the force driving and maintaining the diversity between species."