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Biology and evolution of hemispheric and latitude-dependent effect of season of birth on behavior

Page history last edited by Alex Backer, Ph.D. 12 years, 5 months ago

For millennia, people have ascribed personality effects on the month of birth. Now people have been wrong before. But where there is smoke, there is often some fire, and a persistent credence for millennia may well have an explanation --whether one rooted on the roots of personality and the seasons or one on the neural basis of belief systems.



Perinatal exposure to winter-like seasonal light cycles induces persistent elevated depressive and anxiety-like behaviors in rodents14, and winter-born humans exhibit enhanced rates and severity of seasonal affective disorder, as well as elevated risk of bipolar disorder and schizophrenia6. In addition, individuals with seasonal affective disorder also exhibit enhanced circadian responses to seasonal light cycles15.


A summary of the experiment's findings

A recent paper published in Nature Neuroscience may shed some light. In the experiment, groups of mouse pups were raised from birth to weaning in artificial winter or summer light cycles. After they were weaned, they were maintained in either the same cycle or the opposite cycle for 28 days. Once they were mature, the mice were placed in constant darkness and their activity patterns were observed.

The winter-born mice showed a consistent slowing of their daily activity period, regardless of whether they had been maintained on a winter light cycle, or had been shifted to summer cycle after weaning. When the scientists examined the master biological clocks in the mouse brains, using a gene that makes the clock cells glow green when active, they found a similar pattern: slowing of the gene clocks in winter-born mice compared to those born on a summer light cycle.


In addition, their experiments found that the imprinting of clock gene activity near birth had dramatic effects on the reaction of the biological clock to changes in season later in life. The biological clocks and behavior of summer-born mice remain stable and aligned with the time of dusk while that of the winter-born mice varied widely when they were placed in a summer light cycle.

“The mice raised in the winter cycle show an exaggerated response to a change in season that is strikingly similar to that of human patients suffering from seasonal affective disorder,” McMahon commented.


My Conclusions

It seems to me from a preliminary reading that there are two interpretations of the results: 1. That the circadian clocks of mice exposed to 'winter' lighting perinatally are more sensitive to seasonal variations in daylight length, showing longer behavioral activity duration & associated cellular phenomena during long day seasons. 2. That the circadian clocks of mice exposed to 'winter' lighting perinatally and stark seasonal variation between birth and development (as would be experienced in higher latitudes) show longer behavioral activity duration & associated cellular phenomena on all seasons. Both could be distinguished by measurements during subsequent seasonal variations after your developmental and continuation photoperiods. 


So mice born in winter in circumstances typical of high latitudes (i.e., those that experience a marked effect of season on daylight length) show longer hours awake in summer than mice born in summer show in winter, or that mice under constant daylight length conditions (no seasons, as near the Equator).


This has several interesting implications:

1. There may be a scientific basis for some aspects of the zodiac and astrology.

2. What's relevant to personality effects is the season, not the position of the stars. Of course, when astrology developed, people did not travel between hemispheres, so those were one and the same. Today we know better. So if you're looking for compatibility based on the zodiac, treat a Sagittarius of the Northern hemisphere as you would a Gemini of the Southern one, and viceversa. 

3. Any effects of birth month are likely to increase with variations in day length with the seasons, and thus with latitude. If you're an astrologer, you may have more success with your predictions in Sweden than in the Congo.


From an evolutionary point of view, animals that stay awake for longer in summer months in high latitudes, where those months present a rather unique opportunity to feed in the sunlight without freezing, would indeed have a selective advantage over those that don't. Why this would be true for those born in winter but not those born in summer is less clear. But these experiments leave open the possibility that an animal born in summer, developed through a subsequent winter and tested in a subsequent summer would also show longer behavioral activity days during the summer. This was simply not tested in the experiment. I have written to Dr. McMahon, and am looking forward to reading about the results of that experiment.



Perinatal photoperiod imprints the circadian clock

Nature Neuroscience 14,25–27 (2011)


Keywords: science behind astrology - science behind the zodiac - biological & evolutionary basis of hemispheric latitude-dependent effect of season of birth on personality


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