"Fatherhood and the demands of having a newborn baby require many emotional, psychological and physical adjustments. Our study indicates that a man's biology can change substantially to help meet those demands."
The researchers think that lower testosterone levels might also protect against certain chronic diseases, which might help explain why married men and fathers often enjoy better health than single men of the same age.
Testosterone is the hormone which makes men go out and find a mate, often competing to do so.Professor Ashley Grossman, spokesman for the Society for Endocrinology, said: "this shows the hormonal and behavioural trade-off between mating and parenting, one requiring a high and the other a low testosterone level."
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said: "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."
Children North East publishes a series of guides for Dads for sale from our Father's Plus website, look out for the latest one about Dads and Midwives.