One kind of female butterfly mates only once in its life -- and then closes its wings to avoid "harassment" when pursued by persistent and unwanted males, a Japanese researcher said.

Observations of the Small Copper Butterfly, a colourful orange and black butterfly, showed that some females closed their wings when males flew by, but only when they were males of their own species, said Jun-Ya Ide, an associate professor at Kurume Institute of Technology in Fukuoka, western Japan.

"When we looked into why, the males were courting the females. They do this very stubbornly, so the females sometimes try to get away -- or prefer to stay unobserved," he added.

Ide and his colleagues brought a model of a male butterfly near the females to test their reactions and found that young females that had yet to mate kept their wings open, but those that had already mated closed them.

Not only can the persistent attentions of unwanted suitors harm the delicate females, fleeing from them -- with the male often in pursuit -- uses up strength and can even shorten their lives, he added.

"Closing their wings makes them less visible, helping them avoid communication with their own species. We figured they were trying to avoid sexual harassment by males," Ide said.

(Read by Nelly Min. Nelly Min is a journalist at the China Daily Website.)