Male moth's sperm package includes 'life assurance policy,' biologists find

The orb-weaving spider inspects a rattlebox moth containing defensive chemicals. Moments later the moth was set free by the spider. T. Eisner/Cornell

By Roger Segelken

An enduring nuptial gift is included in every sperm package from a male rattlebox moth (Utetheisa ornatrix) to his freshly mated female: a potent, plant-derived chemical that protects her for life against predatory spiders, biologists at Cornell have discovered.

The first (but almost certainly not the only) example of a sexually transmitted chemical defense to benefit a female animal is reported in the May 11 Proceedings of the National Academy of Sciences by Cornell biologists Andrés González, Carmen Rossini, Maria Eisner and Thomas Eisner. The protective chemical, pyrrolizidine alkaloids (PAs) that the adult male obtains by eating rattlebox plants (Crotalaria mucronata) while in the larval stage, reaches every part of the female's body within minutes after mating, the biologists say. It also protects her eggs.

"Only a human bridegroom would buy life insurance for his bride. This classy moth gives a gift she can really use -- a life assurance policy, if you will -- that keeps paying off every time her life is in danger," said Thomas Eisner, the Schurman Professor of Chemical Ecology. During courtship, Eisner added, the female rattlebox moth has a way of sensing which suitor offers the best chemical defense, and she chooses accordingly.

Native to central Florida, U. ornatrix earned another common name, the ornate moth, for its distinctive coloration, an intricate pattern of flamingo pink, black and white that adorns the inch-long adults. "They are one of the few moths to fly during daylight," Rossini said. "They literally flaunt their brilliant colors, as if to say: 'You should know from my pattern that I am distasteful.'"

Spiders that don't know the color code are quick to find out that the moth is inedible. One taste of the chemical-laden moth is enough to make a spider cut loose its erstwhile prey from the web, and the moth escapes unharmed. The Cornell biologists repeatedly tested moths on wolf spiders and orb-weaving spiders.

Every moth with PAs survived its close encounter with the spiders, while moths without the defensive chemical in their bodies were killed or eaten. That is because they were raised on a laboratory diet of pinto beans, which do not have PAs, or because the females mated with males that could not offer the nuptial gift of PAs.

Until the Cornell study, only U. ornatrix eggs were known to benefit from a male's nuptial gift. The grisly experiment with spiders was performed to prove that the female profits, as well.

"It didn't seem right that the female could not benefit from the male's alkaloidal gift," González said, detailing the contents of the spermatophore, as the insect sperm package is known. Weighing more than 10 percent of the male moth's body mass, the spermatophore contains enough sperm to fertilize hundreds of U. ornatrix eggs, nutrients to help the female produce numerous healthy eggs and PAs, now known to protect the female as well as her eggs, González said. "It makes sense that the male should contribute to the fe-male's defense. She is, after all, the recipient of the sperm."

"Of course the defensive alkaloids also protect the male," Rossini said, "and they have a secondary benefit for him during courtship. A chemical derivative of PA is released into the air from two brushes on the male's abdomen during the courtship dance, as a way of telling the female how much alkaloid he can offer." To initiate the courtship dance and attract males from a distance, the female releases her own sex pheromone, which is not related to PAs, Rossini noted.

"With this finding, we have added one more item to that very short list, 'What are males good for?'" Thomas Eisner quipped. "They evidently donate more than sperm."

He credits his wife, Maria Eisner, a research associate in the Section of Neurobiology and Behavior at Cornell, with carrying out the outdoor moth-spider tests. Also a wife-and-husband team, Rossini and González are graduate students in that department. The study appears in the National Academy of Sciences (NAS) journal as "Sexually Transmitted Chemical Defense in a Moth (Utetheisa ornatrix)." It was supported in part by a National Institutes of Health grant and by Johnson & Johnson Fellowships in Chemical Ecology to Rossini and González.