Studies have previously shown that fathers have lower testosterone levels. However, it has been unclear whether men with low testosterone were more likely to become fathers or whether fatherhood might itself lead to lower testosterone levels.
Dr Christopher Kuzawa and colleagues from the Memorial University of Newfoundland examined data from a group of 624 men from the Philippines.
They showed that over a 4.5-year period those who became fathers were more likely to have large declines in waking testosterone levels.
Dr Kuzawa and his team said their findings suggest that testosterone mediates trade-offs between mating and parenting in adults.
High testosterone levels predict subsequent mating success, but then decline rapidly once fatherhood begins.
The findings may also partly explain health disparities between partnered fathers and single men, the researchers said.