Beetle to Beetle: Will Mate for Water

Sept. 28, 2007 — An unusual benefit of mating may be taking place right now in your kitchen pantry.

A common food pest has evolved a unique way to survive in dry conditions — say, inside a bag of dried beans — says biologist Martin Edvardsson, who discovered that female bruchid beetles, also known as bean weevils, mate more when thirsty. Their male partners, it seems, give up a lot of water in exchange for the opportunity.

Mating as a means of survival (for at least one of the partners) could be widespread among insects, since males store up to 10 percent of their bodyweight as ejaculate, which has a high water content.

It's doubtful the same incentive exists for people and other mammals, although human semen does contain essential vitamins and minerals, as well as sugars.

"Female mammals are too big and need too much food and water for this to be feasible," said Edvardsson, who is currently working at the University of Exeter but conducted the beetle study at Sweden's Uppsala University..

He explained that men "would have to increase the size of their ejaculates by about 2,500 times to 2 gallons to be able to match the beetles.".

Bruchid beetles, also known as bean weevils, are tiny black, brown or mottled insects that bore into seeds or beans as young larvae. They become more visible when they mature and leave their legume hideaways.

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Since water and nutrients can be scarce after life in the legume, females may mate when thirsty, to obtain life-saving nutrients and water from the male's substantial offering. The females require more such resources to produce and lay eggs.

For the study, which has been accepted for publication in the journal Animal Behavior, Edvardsson maintained the promiscuous bugs on black-eyed beans in Petri dishes. Different groups of virgin females were kept with or without access to water and allowed to mate with virgin males.

Overall, the parched females mated more than the females with access to water.

"Basically, by giving his mate a big drink of seminal fluid, a male can buy his sperm more time to fertilize her eggs before they have to compete with the sperm from the next male she mates with," explained Edvardsson..

The drink also could make male beetles more tolerable to females, for whom the act seems less than pleasant. Male bruchid genitalia bear spines that unfurl during copulation and puncture the female's reproductive tract. Females must vigorously kick the males off during the last moments of mating to prevent physical damage.

Scott Sakaluk, a professor in the Department of Biological Sciences at Illinois State University, studied a similar phenomenon in crickets.

"Hydration benefits could indeed be a more important benefit of mating than previously thought," he said. "Moreover, the ability to secure water from a male's ejaculate could explain why females of some species mate more often than they need to fertilize their eggs."

But the real "acid test," Sakaluk said, will be if Edvardsson's findings are confirmed in another study with more rigid conditions.

Insect Mating

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