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Grumpy men could soon start getting the kind of knowing glances that women of a certain age have long been used to.

  It is all in their hormones, according to a scientist. When levels of the male sex hormone testosterone begin to drop, irritation increases.

  Gerald Lincoln of the Medical Research Council's Human Reproductive Sciences Unit in Edinburgh first identified the syndrome in Soay sheep. He found that in the autumn, the ram's testosterone soars and they rut, but in the winter it plummets. High testosterone is supposed to mean more aggression, but it turned out that the rams were more likely to injure themselves when testosterone was low.

  Dr. Lincoln monitored eight rams, counting how often they struck out with their horns. As testosterone levels fell, the rams changed from well-balanced males who treated each other in a formal, ritualistic fashion, to nervous, withdrawn animals who struck out irrationally.

  Red deer, reindeer, mouflon and Indian elephants also show clear signs of what he has christened irritable male syndrome, when testosterone levels fall off at the end of their breeding seasons.

  Dr. Lincoln's colleague, Richard Anderson, has found that when men who cannot produce testosterone are taken off hormone replacement therapy they become irritable and depressed, but their mood improves when they resume treatment.

  Dr. Lincoln told New Scientist that stresses such as bereavement, divorce or life-threatening illnesses could send testosterone levels plummeting. He says there are few human studies on stress and testosterone, but numerous studies on animals, including primates, show that testosterone levels fall when stress sends corticosteroid levels up.

  David Abbott, a reproductive specialist from the Wisconsin Primate Research Center in Madison, said: "It's right on the money. Testosterone effects have been missed. When a bloke gets grumpy and irritable researchers try and explain it only in terms of cortisol levels and depression. They ignore the fact testosterone levels are probably also falling."

  David Handelsman, a male hormones expert a Sydney University, is not so sure. He says that the changes in testosterone levels in normal adult men are far smaller than the dramatic swings seen in Soay rams.

  The big exception is men who undergo castration for advanced prostate cancer, where levels fall by at least 90 per cent.

  Kenn-Hun Tai, of the Peter MacCallum Cancer Institute in Melbourne, said: "The wives notice it first. The men become more withdrawn but more emotional. They laugh and cry more easily."