A protein named after the Hershey Kiss chocolate which stimulates a surge in testosterone is what causes male brains to develop differently from females.

A surge in testosterone in newborns hours after birth is triggered by a Hershey's Kiss protein that only affects male brains, a study by the University of Otago has found.

The study published in the Journal of Neuroscience observed a small group of neurons linked to fertility are only active in male newborn nice, and not females.

Gonadotropin-releasing hormone (GnRH) neurons are cells that control the release of reproductive hormones from the brain's pituitary gland.

Kisspeptin is a small protein which stimulates GnRH neurons, a process that researchers found only occurred in males.

The molecule was originally named after the Hershey Kiss chocolate by US cancer researchers based in Hershey, Pennsylvania, who at the time were unaware of its role in fertility.

The Otago study observed that when kisspeptin binds to specific nerves it kick-starts a brief but powerful testosterone surge in male babies within five hours of birth, causing their brains to develop differently from females.

This could have implications on the neurological disorders from which men and women suffer.

The study's leader Allan Herbison said the findings highlight the role kisspeptin can play in controlling fertility.

"Not only does kisspeptin signalling act as a master switch for puberty and ovulation, we now show how in the first hours of drawing breath it also triggers our brains to develop differently according to our sex," he said.

The study was carried out by Professor Herbison with Jenny Clarkson from the Centre for Neuroendocrinology in collaboration with researchers in Canada and Germany.

Copyright AAP 2014.