After a man becomes a father his testosterone level drops, a new study finds.

High testosterone levels have been linked to prostate cancer and high cholesterol levels.

If fathers have lower testosterone levels, this might protect them against certain chronic diseases as they age, said Dr. Christopher Kuzawa, a study co-author and an associate professor of anthropology at Northwestern University.

The study suggests that men’s body evolved hormonal systems that helped them stay loyal to their families once children were born. It also suggests that men’s behavior can affect hormonal signals their bodies send, not just that hormones influence behavior.

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, said Kuzawa, the lead investigator of the work carried out in the Philippines.

The study was led by Northwestern University researchers. The study was funded by the National Science Foundation and the Wenner-Gren Foundation.

Scientists measured the testosterone levels of 624 Philippine men who were 21 years old.

Study has shown that men who devoted the most time to child care had the lowest testosterone levels. The study was published in the Proceedings of the National Academy of Sciences.

Testosterone declined in all the subjects, which is normal as men age. But the largest decline occurred in married or partnered men with children as compared with men who remained single.

The study found that men who had a newborn had even lower testosterone levels than those with slightly older children.

Testosterone is a hormone associated with perceived hallmarks of masculinity such as libido, aggression and musculature.

In men, testosterone plays a key role in the development of male reproductive tissues such as the testis and prostate as well as promoting secondary sexual characteristics such as increased muscle, bone mass and the growth of body hair. On average, an adult human male body produces about ten times more testosterone than an adult human female body.

Testosterone is necessary for normal sperm development. It activates genes in Sertoli cells, which promote differentiation of spermatogonia.

Since testosterone affects the entire body the brain is also affected by this sexual differentiation.