Testosterone may be a guardian of your genetic heritage.

A new study has shown that while men with high testosterone levels are more likely to have mating success and father children, after the child is born their testosterone levels drop.

And if they take care of the child, their levels may drop even further.

These drops are temporary and anthropologists suggest are a way of the fathers remaining focused on the viability of their progeny and not on wandering off, at least until the baby is well established.

The study tested a group of 21-year-old single men in the Philippines before and after they became fathers. About 600 of them were followed for almost five years; some became fathers and some didn’t.

Published in the Proceedings of the National Academies of Science, the results showed those who became fathers had a 40 to 50 per cent drop in their testosterone levels within one month of their child being born.

And those who spent more than three hours a day caring for their child had the lowest levels.

But as their child grew, their testosterone increased.

This is the first large study to track testosterone levels and their role in the biology of fatherhood. It suggests a trade-off, with high testosterone driving men to mate and lower testosterone helping them keep the family going.

A previous study showed young men in committed relationships have lower levels than their uncommitted peers. Another showed testosterone rises after men are divorced.