Perils of Procreation
Authors: Leslie Mitch 
Published: August 2003

Repent, ye fornicators. Researchers have gleaned evidence that chastity pays--in beetles, that is. Mating inhibits an enzyme vital for defense against pathogens, according to a new study. The results might help scientists understand how reproduction shortens life.

Scientists have known for decades that bearing offspring diminishes survival, but they've rarely been able to show how. In some insects, for example, sex itself can kill. While mating, a male fruit fly pumps chemicals into the female that stimulate egg-laying but also trim her life-span. Other studies of insects suggest that juvenile hormone released during copulation depresses the immune system.

Probing that possibility, Jens Rolff and Michael Siva-Jothy investigated the effects of juvenile hormone on grain-nibbling mealworm beetles. The researchers drew samples of hemolymph, the insect version of blood, from beetles that had mated or abstained and measured two indicators of immune system strength: the abundance of a particular infection-battling cell and the activity of an enzyme called phenoloxidase, which helps kill and neutralize invaders. Although mating had no effect on numbers of the immune cell, it shrank phenoloxidase activity by more than 50% in males and nearly 50% in females.

To determine whether juvenile hormone provoked this decline, Rolff and Siva-Jothy performed a series of transplant experiments using the corpora allata, a pair of tiny glands that, like the vertebrate hypothalamus, squirts out hormones in response to nervous stimulation. Mating prods the glands to release juvenile hormone. "You basically have to do brain surgery on these beetles," says Siva-Jothy, a physiological ecologist at the University of Sheffield, U.K. The team first transplanted glands from mated or virgin beetles into unmated ones. Phenoloxidase activity was lower in animals whose transplants came from mated bugs. In further experiments, the researchers soaked some glands in a chemical that inhibits juvenile hormone before transplanting them. Unmated beetles that received the treated glands showed higher phenoloxidase activity than those given untreated glands, suggesting that copulation-induced juvenile hormone stifles the immune system.

The researchers are now testing whether plummeting enzyme activity increases vulnerability to disease--a likely prospect. Other studies have found that the intensity of the immune response depends on phenoloxidase concentration in fruit flies and damsel flies. Dwindling phenoloxidase could undermine the health of both sexes in these promiscuous beetles. Siva-Jothy says that females probably mate whenever they need sperm to fertilize their eggs--usually about every 2 days, according to other studies--and that this behavior could chronically weaken their immunity. Males are always ready for action, so the libidinous fellows could be scuttling around in an even more severe state of immune suppression. "Some of the particularly successful males could be in really bad shape," Siva-Jothy says.

Many of the earlier studies on tradeoffs between reproduction and survival were descriptive, says James Carey, a demographer at the University of California, Davis: "We're getting close to a mechanism, which is good." Now, he says, researchers need to figure out how juvenile hormone benefits reproduction to compensate for this decline in immunity. That might tell us why sickness is the price of love.

--Mitch Leslie

J. Rolff and M. T. Siva-Jothy, Copulation corrupts immunity: a mechanism for a cost of mating in insects.
Proc. Natl. Acad. Sci. U.S.A.
, 3 July 2002 [e-pub ahead of print].
[Abstract]
[Full Text]