Tough Tradeoff: Beetle brains show how sex shortens life

Neurosurgery in beetles uncovers yet another way that having sex can make life shorter.

Many insects and some other animals tend to die younger if they mate than if they don’t, says Michael Siva-Jothy of the University of Sheffield in England. After 200 or so organ transplants in mealworm beetles, he and Sheffield colleague Jens Rolff propose that a burst of so-called juvenile hormone triggered by mating revs up the insects’ reproductive system but with a dire cost: a weakened immune system.

The many insects that depend on juvenile hormone may have to deal with this lifespan-for-sex tradeoff, say the researchers in an upcoming Proceedings of the National Academy of Sciences.

In recent years, scientists have been tallying up the costs of sex. There’s the drain on energy and the risk of sexually transmitted diseases. Predators take advantage of animals distracted by mating activities. In one act of seeming ingratitude, when a male fruit fly dopes his ejaculate with compounds that encourage the female to lay eggs faster, he shortens her life.

Siva-Jothy says that earlier work with a variety of insects led him to suspect that mating sabotages the immune system via juvenile hormone, which drives vital functions such as metamorphosis and the production of eggs and sperm. It can also suppress the activity of phenoloxidase, a major player in immune defenses.

To test whether hormones mediate the life-shortening effects of sex, Siva-Jothy and Rolff experimented with Tenebrio molitor. Pet stores sell the larval form of this beetle species as mealworms.

Juvenile hormone comes from a pair of endocrine organs known as corpora allata. About 0.2 millimeters long, they lie at the stem of the mealworm beetle’s brain.

Siva-Jothy and Rolff permitted some of their beetles to mate, then transferred the insects’ corpora allata to virgin beetles. For comparison, the researchers also transferred some corpora allata from virgins to virgins. All transfers were between insects of the same sex.

“We’re doing brain surgery in beetles,” says Siva-Jothy. “You can’t have a cup of coffee and do this.”

When virgins received corpora allata from mated beetles, their phenoloxidase activity dropped–about a fifth for males and a half for females–compared with virgins receiving transplants from unmated donors.

The researchers did a second round of transplants, this time treating some of the mated beetles’ corpora allata with a compound that blocks the production of juvenile hormone. The immune systems of recipients that received these transplants showed more phenoloxidase activity than did those in beetles getting untreated organs.

The findings link sex with juvenile hormone and impairment of the immune system. The researchers propose that a weakened immune system shortens beetle life.

Another investigator of mating costs, Locke Rowe of the University of Toronto, praises the experimental design of the beetle study. “The transplants are the cleverest part,” he says. He considers the work the first to show a physiological mechanism for internal costs of mating for both sexes.