Washington, Sept 30 : After sex, female fruit flies ditch the usual siesta in favour of extra foraging and searching for places to lay her eggs, a new study has found.

The study showed that it is all down to a chemical or ''sex peptide'' produced by male fruit flies.

The sperm of male fruit flies are coated with the chemical, which inhibits the female''s usual afternoon siesta and compels her into an intense period of foraging activity.

Both male and female fruit flies (Drosophila melanogaster) – commonly seen hovering around rotting fruit and vegetables - are active at dawn and dusk, and have a deep sleep at night.

They also exhibit a marked ''resting state'' during the afternoon, which Isaac likens to a siesta that conserves the fly''s energy and reduces damaging exposure to the sun during hot afternoons.

"However, we noted that after mating, females still slept deeply at night, but ditched the usual siesta in favour of extra foraging and searching for places to lay her eggs,” said Professor Elwyn Isaac from the University of Leeds'' Faculty of Biological Sciences.

"This behaviour lasts for around eight days – and our research findings suggest that this change is not by choice. Females who mated with males that produced sperm without the sex peptide continued to take their siesta. So we''re certain that this change of behaviour is chemically induced by the male.

"Sleep is an ancient and essential mechanism in living creatures from worms to humans, so to inhibit this for such a long period and replace it with extra activity that exposes the female to environmental hazards and danger from predators must require a powerful mechanism," he added.

The sex peptide is produced in the males'' accessory glands and attaches itself to the surface of the sperm''s tail.

Previous studies have shown that the sex peptide encourages females to increase egg production - a mated female will lay up to 100 eggs a day compared with 1-2 eggs laid by a virgin female. It also inhibits her from mating with other males for around a week to ten days.

"It would appear that preventing sleep and inducing extra domestic-type duties to prepare for the birth of offspring in females is a further tactic used by the male to ensure successful paternity after mating," Isaac said. (ANI)