(Fleur Champion de Crespigny)

Researchers at the University of Exeter have found that female bruchid beetles (Callosobruchus maculatus, above) mate when they are thirsty.

Evolutionary biologist Martin Edvardsson kept some female bruchids with, and others without, access to water. All the females were given the opportunity to mate with a new male every day.

In the journal Animal Behaviour, Edvardsson reports that those females without access to water mated more frequently - up to 40% more - than those with access to water. This leads him to conclude that female bruchid beetles mate in order to obtain water from the male's ejaculate when they are dehydrated.

During mating, the female absorbs water through the walls of the reproductive tract. But mating also has its costs. For the female, it is a tricky and painful business: the male's genitalia (below) are covered with spines which unfurl during copulation, puncturing the female's reproductive tract.
 

When copulating, females use their hindlegs to kick their mates. This reduces the duration of copulation, and, therefore, the amount of damage inflicted by the male's spiky geitalia.

A cost/benefit analysis is therefore essential to the mating behaviour of the female. The number of mating events must be strictly limited because of the resulting harm. But at the same time, the female's needs for both sperm and water must be met.

Male bruchids are therefore amongst those insect species that have evolved very large ejaculates. In some species, the ejaculate can account for up to 10% of the insect's body weight. The larger a male's ejaculate is, the more water the ejaculate will contain.

During mating, the female receives the water in the ejaculate as a "nuptial gift". The larger the amount of water contained within it, the longer it will be before the female mates with another male, and the less likely is the possibilty of sperm competition.