Fatherhood indeed makes you less of a man, it seems. Research finds the more you evolve as a better father, the more you lose your testosterone.

But the results also show that men are biologically wired to raise their children, unlike any other male mammals even though that dampens their hormonal level.

Northwestern University researchers studied a large group of men in their 20s and followed them for four and a half years. They  found a dramatic fall of their level of testosterone once they got partners and became fathers.

Testosterone is associated with man's libido, aggression and helps men find their mates and produce offspring. But lower levels help men become calm, attentive and evolve as a better father, the researchers found.

Earlier studies showed that fathers have lower level of testosterone than childless men. To discover whether parenthood is directly involved with decline in hormones, scientists studied the testosterone level of 624 Philippine men who were 21 years old. They again studied them after four and a half years when most of the single men found their partners and became fathers. Research on them showed a drop of almost 34 percent in the testosterone level compared with single, childless men who have a natural decline of 12 to 14 percent. Fathers with newly born babies and men who spend more than three hours a day in rearing children, showed the most decline.

The study was published in the Proceedings of the National Academy of Sciences.

There's something about being an active father that's contributing to these dramatic declines, said Lee Gettler, study leader and biological anthropologist at Northwestern University.

On the hand, the dip in the testosterone level has benefits as high levels can increase risks for prostate and testicular cancer, stroke and heart diseases, said researchers.