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Davis, California

Saturday, December 9, 2023

Column: Insect sexy-time

Fall is here, so, naturally, the earwigs of Davis are cruising for sex. Like horny college students, earwigs hook-up, break-up and even move in together.

An earwig love story:

They meet under a rotten log. His shiny brown abdomen catches her compound eye. She’s been with other males, but he seems special.

He caresses her with his pinchers (called cerci). Mmmm. Tactile stimulation. He whips out his earwig penis, which can be as long as his entire body. For several minutes, they make sweet earwig love. It’s too soon for baby earwigs, so she stores his sperm in an organ called a spermatheca.

One day, over a breakfast of bark and dead aphids, she asks him to move in. Let’s dig a burrow together, she says. They’re a pair now – earwig soul mates.

“Fall is the time of love for earwigs,” said Amanda Hodson, graduate student in entomology at UC Davis.

Earwig mating is complicated. Mate selection is ladies’ choice, and male desirability depends on factors like cerci length and pressure from parasites.

There are two types of male earwigs. The alpha males are more aggressive, and they’ve got bigger, spikier cerci. Then there are males with smaller, more slender cerci – the nice guys. Not surprisingly, the alpha males get more action.

So why didn’t all earwigs evolve to be alpha males?

Hodson and recent UC Davis graduate Lily Wu spent last year researching the reactions of earwigs to parasitic worms called nematodes. While Wu was infecting earwigs with nematodes, she noticed something: alpha males were easier to kill. The wimpy nice guys often survived – they had a stronger immune system.

“It looks like the bigger male earwigs are more likely to die from a nematode infection,” Hodson said.

No one is sure why there are two types of males, but Hodson said the split could be a way to increase species survival.

“It probably depends on parasite pressure,” Hodson said.

When there are fewer parasites around, alpha males flourish. They pass on their genes while the nice guys look on. But when parasites move in, the alpha males die, and the females turn to the trusty nice guys.

Having two types of males, each with its pros and cons, helps the species adapt to changing environments.

If the earwig dating scene sounds familiar, that’s no coincidence. Biologists today think primate populations also evolved to have different types of males.

In a study from the National Institutes of Health’s Laboratory of Comparative Ethology, scientists that found most male rhesus monkeys have an “assertive” personality, but about 20 percent are shy nice guys. The confident males get laid more often, but the nice guys provoke fewer fatal conflicts with other monkeys. Using different strategies, both types of males get to pass on their DNA.

The researchers found that shy monkeys (and humans) have a gene that can make them more resistant to depression and anxiety. The gene regulates the levels of a chemical called serotonin. More serotonin means they are calmer – and chicks dig an emotionally stable monkey.

Earwigs don’t have serotonin, but their familiar behavior shows the roots of human evolution.

Like humans, earwigs also show maternal instincts. While many insect species just lay eggs and leave, female earwigs take care of their young.

Once the earwig couple digs a burrow together, the male gets pushed out (he’ll eat the eggs). He spends the winter with a group of other males while the female attempts single motherhood.

The female turns her eggs regularly and makes sure they stay at a constant temperature. She also licks the eggs. Scientists aren’t sure why she does this, but it seems to stop fungal infections – either her saliva is fungicidal or she’s actually licking off fungal spores.

Pesky scientists have tried scattering a female earwig’s eggs to see what she’ll do. Like a good mother, she’ll track down the eggs and gather them back into a tidy clutch. If two groups of eggs accidentally get mixed up, one earwig mom will actually steal the eggs from the other mom. It’s survival of the cut-throat.

“They’re programmed to take care of eggs,” Hodson said.

By spring, baby earwigs enter the world. It’s a new generation of alpha males, nice guys and picky females.

In early fall, a female meets a male under a moldy tree branch. She bats her antennae and he flexes his cerci. True love.

MADELINE McCURRY-SCHMIDT had to Google “earwig penises” while writing this article. Awkward. E-mail her at memschmidt@ucdavis.edu.


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