The hormone that defines the male of the species slumps dramatically when men become fathers, researchers have found.

Blood tests on 624 men in the Philippines revealed that levels of testosterone dropped substantially over a five-year period in those who had children.

Men who devoted at least three hours a day to childcare had even less testosterone, suggesting that looking after dependent children helped suppress the hormone. Testosterone is responsible for the male body shape, the distribution (and loss) of hair and a man's sex drive.

Previous studies have shown that fathers tend to have lower testosterone, but it was unclear whether men with reduced levels were more likely to have children, or whether parenthood pushed testosterone down.

"It's not the case that men with lower testosterone are simply more likely to become fathers," said Lee Gettler, an anthropologist at Northwestern University in Illinois. "The men who started with high testosterone were more likely to become fathers, but once they did, their testosterone went down substantially."

Christopher W Kuzawa, a co-author on the study in Proceedings of the National Academy of Sciences, said fathers seemed "biologically wired" to help raise children.

"To see dramatic changes in response to family life is intriguing," said Allan Pacey, an andrologist at Sheffield University. "The observations could make some evolutionary sense if we accept the idea that men with lower testosterone levels are more likely to be monogamous with their partner and care for children. However, it would be important to check that link between testosterone levels and behaviour before we could be certain."