Researchers from Indiana University have discovered a hormonal mechanism in hamsters that causes females to be more aggressive during short winter days. The study was conducted on Siberian hamsters, which have a similar adrenal system to humans.
The study, which was published in the journal Proceedings of the Royal Academy B, exposed more than 100 hamsters to long days of sunlight, mimicking summertime, for a week, and 45 hamsters to shorter days, mimicking winter, for 10 weeks.
The hamsters were then placed in situations where one was perceived to be intruding on one another’s territory. Scientists discovered that the “winter” female hamsters were more aggressive than the “summer” female hamsters.
The winter hamsters also had increased levels of melatonin (a hormone that rises in the body when it’s dark and lowers when it’s light) and DHEA (a sex steroid that’s known to impact aggression levels in mammals, birds, and possibly humans) in their bodies.
The findings show that melatonin acts directly on the adrenal glands, i.e. glands located on top of each kidney that produce hormones, and flips a “seasonal aggression switch” in females, lead study author Nikki Rendon, a PhD candidate at Indiana University, tells Yahoo Health.
Study co-author Gregory Demas, PhD, a biology professor and associate chair for research at Indiana University, tells Yahoo Health that the findings may have an implication for humans. However, he adds, “we don’t know for sure.”
Demas says this hormonal response may have evolved in our bodies to allow people to successfully compete for food during a season when it’s often limited (in the wild, at least).
But humans do secrete and respond to melatonin more during the winter vs. the summer. “It appears to play a role in mood disorders such as seasonal affective disorder (SAD),” Demas says. “The link between seasonality and aggression, however, has not been properly explored — but should be.”
Research has shown that more women than men are affected by SAD, which is marked by symptoms of depression during winter months. According to Mental Health America, three out of four SAD sufferers are women.
Both Demas and Rendon say they hope their work will lead to more research on aggression in females and what causes it.
“Most studies have focused on aggression in males and the role of testosterone as the primary cause,” Demas says. “Our findings suggest that females also show significant aggression and that hormonal systems other than testosterone may play a significant, yet under-appreciated role.”