[Photo] Dehydrated female bruchid beetles can accept sexual invitations simply to get hold of the water in the male's seminal fluid. Credit: Fleur Champion de Crespigny

Female beetles mate to quench their thirst, literally.

In some insect species, including certain beetles, moths and crickets, the males produce exceptionally large ejaculates that can account for up to 10 percent of their body weight.

A new study, published in the August issue of the journal Animal Behavior, reveals that dehydrated bruchid beetles accept sexual invitations simply to snag the water in a mate's seminal fluid.

It's the insect-world version of gold digging.

Martin Edvardsson of the University of Exeter and his colleagues gave female bruchids (Callosobruchus maculatus) either unlimited access to water or limited water, with all females having free access to males for mating.

They found that thirsty females mated 40 percent more frequently than those with free access to water.

Like sponges, females can absorb the water from seminal fluid through their reproductive tracts. The more water they take in, the less frequently they need to mate, an activity that's physically damaging to the females. (The males have spines on their genitalia that puncture females' reproductive tract during mating.)

That decrease in mating frequency also can be advantageous for a male, since the longer a female goes without mating with another male, the greater will be his chances of successful fertilization.

By delivering a deluge of water with the sperm that thereby also slakes a female's thirst, a male can help ensure his sperm has more time to fertilize the eggs without competing sperm from future mates.

Like other insects, the female bruchid beetles can store viable sperm for long periods of time before fertilizing their eggs. Past research has shown that a larger proportion of eggs will be fertilized by the last male to mate when there are long intervals between matings.

"The large ejaculates may have evolved because males can make it less beneficial for females to remate by providing them with a large amount of water," Edvardsson said.

While traditional nuptial gifts are thought to entice a female to mate or invest in the resulting offspring, the beetles' watery offerings act to prevent females from mating with other males.