Andrew Zink/San Francisco State University Male maritime earwig
Throughout the animal kingdom, symmetry is considered beautiful.
Female house finches like a male bird’s plumage to be the same shape and color on the right and left side of the body. White-tailed zygaenid moth females prefer males with symmetrical genital claspers, hind wings and antennae. And even in our own species, scientists have shown that men find women with evenly sized and leveled eyes more attractive than asymmetric ladies, and vice versa.
Evolutionary biologists say it makes sense: Symmetry signals good health and good genes.
But there are exceptions. And researchers at San Francisco State University have found a case in which asymmetry provides a benefit to an animal: earwigs.
“This is the first example where we have seen asymmetry provide a benefit in terms of weaponry,” said Andrew Zink, co-author of the study and a behavioral ecologist at SFSU.
The study appears in the journal Ethology.
Zink said the research was conducted primarily by his former graduate student Nicole Munoz, who is now at UCLA. She could not be reached for comment because she is working in the field with marmots.
Zink said he has been interested for years in female earwig behavior and its evolutionary implications. And while researching female nesting behavior, he and Munoz noticed that male earwigs often sport very asymmetrical forceps.
Their forceps are like two protruding, curved points at the end of their abdomen. Earwigs use them to grab one another in a fight, and some male earwigs have more asymmetrical forceps than others.
To figure out what was going on, Munoz and Zink captured a few off the beach in Tiburon and Richmond and brought them into the lab. The researchers wondered if the more asymmetrical forceps would provide the insects with better fighting ability.
To figure this out, they waged a few earwig battles. They glued a piece of food in the center of the fighting rink (a plastic box), dropped in two males and let them go at it.
What they found is that bigger males generally beat out smaller males, which wasn’t surprising, Zink said. And forcep asymmetry didn’t really seem to matter much in large male-on-large male battles – that was a contest of brute strength.
But what was interesting, Zink said, is that if you dropped two small males into the rink, the one with the more asymmetrical forceps generally turned out to be the better fighter. Not only did the asymmetry provide the earwig with better leverage, but it also seemed to endow better fighting ability.
To explore this further, Munoz and Zink tried manipulating the forceps, making them more asymmetrical or symmetrical.
What they found is that aggressiveness seemed to go along with asymmetry: A naturally asymmetrical male was more aggressive than a symmetrical male. So even if they gave a symmetrical male asymmetrical forceps, he still wasn’t as good a fighter as the naturally asymmetrical male.
Zink and Munoz are wondering, however, if the asymmetry comes at a cost. Are asymmetrical males not as attractive to females as their symmetrical counterparts?
It’s an area they are looking into now, though research from Australia suggests that maybe female earwigs are agnostic on the asymmetry front.