Short winter days may make females more aggressive

Ladies, take note! If you feel unreasonably aggressive during the winters, you may blame it on your hormones, suggests a new study that found a link between short winter days and increased aggression in females.
The study on hamsters, which advances basic knowledge on the connection between certain sex hormones and aggression, could go on to advance research on the treatment of inappropriate aggression in humans, researchers said.
"The results show for the first time that 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," said lead author Nikki Rendon, a PhD student at Indiana University in US.

Melatonin is a hormone that rises in the body during darkness and lowers during daylight. The hormone from the adrenal gland is dehydroepiandrosterone, or DHEA, a sex steroid shown to affect aggression levels in mammals and birds, and possibly humans.
The new study shows that melatonin acts directly on the adrenal glands in females to trigger the release of DHEA.
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 in Siberian hamsters, or Phodopus sungorus, a species with a similar adrenal system to humans.

About 130 hamsters were exposed to long days for a week, after which 45 were exposed to shorter days for 10 weeks. A random subset also received an injection of Adrenocorticotropic hormone (ACTH), which is known to trigger the release of DHEA.
The hamsters were then placed in situations where one hamster was perceived as an intruder in the other's territory, sparking aggressive actions and short physical fights.
The scientists then tracked certain actions, such as the time until an attack, the number of attacks and the length of the attacks, to assign an "aggression score."
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, including those that had received an injection of ACTH.
Collectively, the results show that melatonin is the primary regulator of aggression in females.
"It's growing increasingly clear that sex hormones play an important role in controlling aggression in both males and females - but females, human and non-human, are traditionally vastly understudied in the sciences," Rendon said.
"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," she said.
The study was published in the Proceedings of the Royal Academy B.