by– Carly St. James
A study done at Indiana University draws a correlation between the hours of sunlight received per day and the level of aggression displayed in test mice.
During the study, several test groups were established with all factors controlled for except for amount of sunlight exposure per day, the test variable. The study finds that female rats who were exposed to the least amount of sunlight displayed the most aggression. The aggression was channeled toward a perceived territorial threat and was not observed in male rats.
The findings indicate that rather than being controlled strictly by sex hormones (estrogen, testosterone and the like), aggression is strongly influenced by Melatonin. During the winter time, the level of sex hormones decreases sharply. Melatonin levels, however, increase as response to the change in sunlight. Despite this drop of sex hormones, aggression increased notably in the female rats, indicating that Melatonin levels affect aggression just as much, if not more than, estrogen and testosterone. Melatonin does this by acting on the adrenal glands, causing a release of dehydroepiandrosterone, DHEA. DHEA has consistently been linked to aggression in both mammals and birds.
Humans, too, experience seasonal changes in behavior. We have coined the term Seasonal Affective Disorder (SAD) for when these behavioral changes interfere with normal functioning. Interestingly enough, 3 out of every 4 SAD-sufferers are female*. This statistic could further imply the connection between Melatonin and female non-sex hormones (DHEA) and the behaviors that coincide.
The full study can be viewed in the Proceedings of the Royal Academy B.
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