Ohio State University researchers have studied how a naturally occurring chemical in the bodies of mice affects aggressive behaviour in accordance with the length of the daylight they face.
The researchers found that a class of hormones called 'estrogens' increase aggression in the animal during the short days of winter, while the same chemical decreases aggression among male mice, when daylight increases in the summer.
Published online this week in the Proceedings of the National Academy of Sciences, the study is the first studies to show the possible impacts of simple environmental factors, such as the length of daylight, on how genes influence behaviour.
"We found that estrogen has totally opposite effects on behaviour in these mice depending only on how much light they got each day. It is quite a surprising finding," said Brian Trainor, co-author of the study and postdoctoral fellow in psychology and neuroscience at Ohio State University.
The researchers castrated male mice to stop production of testosterone, and then fitted in them implants that controlled their testosterone levels.
Upon treatment with a drug that halts the production of estrogen, it was found that normally aggressive mice became less aggressive when faced limited daylight as in winter. On the other hand, normally docile mice became more aggressive when faced long-day conditions as in summer.
Moving one step further in their research, the researchers identified the mechanisms that may underlie how the length of daylight affects the way estrogen works to either increase or decrease aggression.
They treated one group of mice with an estrogen-like drug that attaches primarily to estrogen receptor alpha, and another group of mice with a different estrogen-like drug that attaches primarily to estrogen receptor beta. The two receptors are like docking stations that send signals from the estrogen molecules into the cells.
It was found that both the alpha-receptor and the beta-receptor played a key role in increasing aggression in short days and decreasing aggression in long days.
In another study, the researchers then injected mice living in winter-like short-day and summer-like long-day conditions with estradiol, a type of estrogen. The injection increased aggression in mice in winter-like short days almost immediately, while it had no noticeable immediate behavioural effect on the mice living in longer day lengths.
Through the string of studies, the researchers came to the conclusion that estrogen increases aggression in short-day mice by working through non-genomic pathways in the brain, but the hormone decreases aggression in long-day mice through genomic pathways.
The researchers say that their findings may have many implications for humans.