In the world of fruit flies, being sexy can be a curse, especially for the ladies.

A new research has shown that the most attractive female fruit flies are constantly harassed by mates, affecting their fertility.

The harassment could lead to smaller families and affect fruit fly evolution, say researchers from the University of California, Santa Barbara.

Study''s lead author Dr Tristan Long, from the University of Toronto in Canada, says in many species, including fruit flies, males find large-bodied females ''''attractive'''' because they have greater capacity to produce offspring.

According Long and colleagues, too much mating is harmful to the females because seminal fluid from the male has toxic side effects. Too much courtship can also hinder the female''s ability to forage effectively.

"When they court the females, the males sing to them; they do this by vibrating their wings. They dance and sing at the same time. This might sound romantic, and it would be if it only happened once. But males are doing it all the time," said William Rice, biology professor at UCSB.

"This courtship is unrelenting -- like mosquitoes on a warm summer night -- as the male fruit flies try to persuade females to mate. The males are so persistent that they get them to mate almost every day," he added.

In many species, females are frequently subject to intense courtship "harassment" from males attempting to obtain additional matings, according to the researchers.

These coercive activities can result in attractive females becoming less fit to reproduce -- a factor that has a major effect on the entire population.

Long said: "We found that when harmful courtship behaviours were directed predominantly toward larger females of greater fecundity potential -- and away from smaller females, of lesser fecundity potential -- this resulted in an overall reduction in the variation of lifetime reproductive success of females in the population."

The experiments clearly showed that the evolutionary adaptation of fruit flies is hindered by this mating situation.

"This change in the distribution of fitness represents a previously unappreciated aspect of sexual selection -- one with important implications for the ability of beneficial genetic variation to spread through the gene pool, and ultimately for a species'''' capacity to adaptively evolve," Long said.

The study has been published in the December 8 issue of Public Library of Science Biology.