A new study shows that men's testosterone levels drop when they become fathers.

Researchers at Northwestern University followed a group of 624 men in the Philippines for four and a half years, according to the university press release. The men ranged in age from 21 to 26.

Northwestern researchers noted this study was different because it observed males both before and after they had children.

The study was published in the journal Proceedings of the National Academy of Sciences (PNAS).

Humans are unusual among mammals in that our offspring are dependent upon older individuals for feeding and protection for more than a decade, study co-author and Northwestern anthropology professor Christopher W. Kuzawa said in the press release. Raising human offspring is such an effort that it is cooperative by necessity, and our study shows that human fathers are biologically wired to help with the job.

However, this does not mean that men who have lower testosterone levels to begin with are more likely to have children, the researchers noted.

On the contrary, the men who started with high testosterone were more likely to become fathers, but once they did, their testosterone went down substantially, co-author and anthropology doctoral candidate Lee Gettler said in the press release. Our findings suggest that this is especially true for fathers who become the most involved with child care.