Evolution: Score 1 for earwig's odd claw
Published 10:19 pm, Wednesday, August 22, 2012
The maritime earwig, a bumptious bug known to sometimes devour its own offspring, has the kind of personality only an entomologist can like.
The cannibalistic creature has scary-looking pincers that can almost break human skin and, although the bug doesn't normally climb into orifices, it certainly feels at home in dark, moist ear-like places.
Intriguingly creepy as that may be, it is the insect's protruding forceps that have a team of San Francisco State University researchers giddily bouncing around on their laboratory chairs.
Graduate student Nicole Munoz and biology professor Andrew Zink have discovered that small male earwigs with severely curved right claws win more fights, indicating an evolutionary advantage in asymmetry, probably inspired by the sexual preferences of the females.
That, to the uninitiated, may not seem like much, but it is a revelation in the animal kingdom, according to the Munoz-Zink study published last week in the Journal Ethology. Most studies to date have shown that female wolf spiders, fish and other species prefer bodily symmetry in their mates, meaning essentially that they avoid suitors sporting growths, protrusions or distorted features.
Humans are no different, Zink said. Studies in which pictures of men are subtly altered digitally have consistently shown that women prefer the men with symmetrical faces.
The earwig study is roughly equivalent to finding that short men with club feet on the right side are suddenly getting all the ladies because of the uniquely forceful way in which they kick other men's behinds.
"This was the first instance of an asymmetric weapon having a benefit," Zink said.
The maritime earwig is one of the larger earwig species, growing to more than an inch long. The black or gray insects have pincers or forceps protruding from the tip of their abdomens. The species, believed to originally have come from Asia, was introduced around the world on commercial ships and now lives all over North America.
Maritime earwigs hang out on the shoreline and prey on invertebrates, including fleas, crickets, ants, small beetles, sow bugs and each other.
Contrary to popular belief, earwigs do not crawl into people's ear canals while they are sleeping and bore into their brains, but they do exhibit other revolting habits. The females, for example, live in colonies and spend their time invading other females' nests and devouring their eggs when they are not guarding their own babies. When they really get hungry, Zink said, they consume their own eggs.
Zink and Munoz noted that the female earwigs, which are larger, have symmetrical pincers, while the males have the one sharply curved forceps. The biologists figured that the curved pincer was somehow an advantage in the constant fighting among male earwigs for the right to mate, so they decided to test their theory.
The researchers glued a piece of cat food to the middle of a tiny arena and staged more than 90 fights between starving male earwigs. They found that the biggest earwig was the victor when two large bugs were paired, but that the earwig with the most pronounced curve in his pincer prevailed when both contestants were small.
Zink said more research has to be done to figure out why the curved pincer was an advantage only among smaller males. Still, he said, there is a suggestion among the findings that perhaps that ungainly appendage that we see as a physical disability may ultimately be an evolutionary leg up.
"Here is an example of how asymmetry has an actual function," he said. "It suggests that we should look at asymmetry not always as a negative."