November 9th, 2015
Male fiddler crabs face two major challenges to their dominance; attracting females, and fending off other males competing for those females. The crabs rely on an extra-large claw to achieve both these goals. However, most scientists studying these crabs have assumed that as the male crab evolved, it was forced to make a tradeoff between an attractive, long claw, and a shorter one that’s more effective in combat.
However, function and beauty may reside in the same fiddler crab claw after all, according to a study in Evolution by Stefan Dennenmoser and John Christy, of the Smithsonian Tropical Research Institute. Studying two types of fiddler crabs (Uca terpsichores and Uca beebei) in Panama, the researchers found that males with longer, larger crab claws used the middle of the claws to grab their opponents, either flipping the opponents over or hurling them from their burrows. In fact, the larger claws have in turn developed larger bumps in the middle part of the claw, to better grab an opponent. The crabs use their claws in a very different way than scientists assumed; instead of stabbing with the pointed end of the claw, they grab the opponent with the claw’s strongest part.
Meanwhile, the longer claws continued to make the male possessor more attractive to females.
The scientists noted that if the crabs were to use their longer claws like humans would stab at food with chopsticks, they would have a mechanical disadvantage to crabs with shorter claws. But the crab’s fighting behavior has instead adapted to fully exploit the strength of the larger claw’s middle section. In fact, Dennenmoser and Christy noticed that, in 22 fights with Uca terpsichores and 24 fights with Uca beebei, none of the crabs used the outer tip of the claw. All of them, instead, grabbed the other crab with the middle claw and wrestled him away. And as the claw gets bigger (either from one generation to the next or as the male crab develops), the bumps so key to fighting function are situated closer and closer to the hinge of the crab, further strengthening the weapon.
Not every evolutionary compensation works as well as this, the authors noted. Many sexual characteristics can reduce performance, such as male barn swallows, which have longer tails that are more attractive, but reduce flight performance in the males. But in the case of the fiddler crab, it’s the overall utility of the claw for its many uses that appeared to have mattered most. For the crab, it’s a “both beautiful and powerful weapon,” the authors wrote.
Also the Otton frog has a body part that is used for mating and combat: a thumb. Read more about it here.
Source: Smithsonian Tropical Research Institute
Photo: Wikimedia Commons
Dennenmoser, S., & Christy, J. (2012). THE DESIGN OF A BEAUTIFUL WEAPON: COMPENSATION FOR OPPOSING SEXUAL SELECTION ON A TRAIT WITH TWO FUNCTIONS Evolution DOI: 10.1111/evo.12018
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