New York, Nov 18 (IANS): Now you know why your spouse or girlfriend suddenly gets hostile towards you as winter settles in. According to fascinating research, a hormonal mechanism may trigger aggression in females -- and not males -- during short winter days.

The hormone called melatonin acts directly on the adrenal glands in females to trigger a “seasonal aggression switch” from hormones in the gonads to hormones in the adrenal glands -- a major contrast to how this mechanism works in males,” explained Nikki Rendon, PhD student of biology from Indiana University.

The work on hamsters (rodents), which advances basic knowledge about the connection between certain sex hormones and aggression, could go on to advance research on the treatment of inappropriate aggression in humans.

"This study reveals a ripe area for research," Rendon added.

Melatonin is a hormone that rises in the body during darkness and lowers during daylight.

The hormone released from the adrenal gland is called DHEA - a sex steroid shown to affect aggression levels in mammals and birds and possibly humans.

Professional sports competitions have banned the use of DHEA in athletes.

Rendon and colleagues found that melatonin acts directly on the adrenal glands in females to trigger the release of DHEA, without the need for the pituitary hormone.

DHEA can be converted to androgens and estrogens, which affect aggression in both males and females.

In females, DHEA appears to compensate for low levels of estradiol -- a form of estrogen -- that occurs during the winter.

The research was conducted on Siberian hamsters, a species with an adrenal system similar to humans.

About 130 hamsters were exposed to long days for a week, after which 45 were exposed to shorter days for 10 weeks.

The female hamsters exposed to shorter days had increased levels of both melatonin and DHEA -- and higher aggression scores -- along with physical changes in their adrenal glands.

Females exposed to longer days did not experience these changes, the authors noted.

Collectively, the results show that melatonin is the primary regulator of aggression in females.

"By conducting this research on females, we are increasing our understanding of hormones and social behaviour in a field currently dominated by discussions on testosterone regulating aggression in males,” Rendon added.

The study appeared in the journal Proceedings of the Royal Academy B.