When males roar and bellow all the time, it turns females on.

At least that seems to be the case among red deer, a European species closely related to the wapiti, or American elk.

Red deer stags are famous for roaring. At certain times of the year, a stag with a harem of hinds will roar twice a minute around the clock, sometimes for several weeks.

In a report in last week's Nature, Karen McComb, a Cambridge University zoologist who did experiments on a red deer farm in New Zealand, said she has found out why stags roar. It makes the hinds come into heat faster.

Similar effects have been known for birds, among which the male's song helps bring the female into heat, but McComb said her findings are the first to show the effect in mammals.

Her experiment involved playing a tape recording of a roaring stag to one group of penned hinds before introducing a stag. The following spring the serenaded group calved earlier than a control group that heard no roaring before a stag was put in with them.

Biomedical researchers say it is not surprising that sex hormones are affected by environmental stimuli. The sex organs are under the control of hormones produced by the pituitary, a gland situated within the brain and linked to it by nerve fibers.