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Animals are by nature seasonal creatures: Deer mate during the fall, so their fawns are born just in time for the spring season when food becomes more abundant. Polar bears look for mates during late spring and early summer and give birth to cubs between December and January.
But what about humans? Are we the only animals that don’t have a true mating season? After all, humans have sex throughout the year, without worrying whether their babies will have enough food to survive the winter.
It appears we do—kind of.
This heat map created by Visme, based on the latest UN data on live births, reveals a surprising link not only between peak birth months and seasons, but also between peak birth months and latitude (which is the distance north or south of the equator, measured in degrees):
Do you notice a consistent pattern across high-latitude countries in the Northern hemisphere? The months with the greatest average number of births per day are July, August and September.
As you move down the list of countries, ordered from highest to lowest latitude, you can clearly observe that the peak birth months shift farther and farther to the right, occurring later in the year.
Once you’ve entered the middle latitudes, or the tropical zone, September and October become the highest-ranking months by average number of live births per day, followed by November and December, with some spilling over into the next year.
At the bottom of the list are the countries in the Southern hemisphere, which register the highest average number of births per day occurring between March and May.
When this data is translated into dates of conception, using a 40-week gestation period, we also see that the peak time for baby making is October in high-latitude countries such as Russia, Norway, Finland and Denmark, as seen in the graphic below made with Visme.
According to this same data, December is another peak conception month in a large percentage of countries, including the United States, United Kingdom, Australia, Canada, Mexico and Japan.
While it is easy to attribute this to the long-held assumption that colder temperatures lead us to find warmth in physical intimacy, possible scientific explanations are a bit more complex.
According to a study published in The Journal of Reproductive Rhythms, the ideal time of the year to conceive is when the sun is out for 12 hours and the temperature is between 50 and 70 ° F. For reasons that are not totally understood, these are the perfect conditions for conception, either because they stimulate sperm production or ovulation--or a combination of both.
The findings of other studies have also been consistent with the insights gleaned from this heat map. For example, a comprehensive analysis of human birth seasonality published in the Proceedings of the Royal Society B concluded that peak birth months occur later in the year the farther south you travel, as clearly seen in the graph below.
Boer Deng, in her article for Slate, also did the math and concluded that countries in northern latitudes were conceiving babies more often during the fall, while those farther south were doing so during the winter, which is also somewhat consistent with our findings.
Other studies such as this one on the seasonal variations in sexual activity concluded that there is in fact an “increase in sexual activity and unsafe sex occurring at or around the Christmas period.” Even an analysis of Google searches, as a proxy measure of sexual activity, indicates that queries related to sex and mating behaviors peaked during the winter and early summer.
All this seems to indicate that there is a confluence of factors, both environmental and biological, at work here and that, in the end, we may not be able to pinpoint one exact reason why these patterns exist.
What we can know for sure is that even though it appears humans may have a quasi-mating season, it is not really a true one as women are receptive to sex year-round and ovulate every 28 days, not annually. Unlike other animals, humans have concealed ovulation because they don't show any outward sign of biological fertility, which is still a mystery to scientists.
To create the heat map above, we filtered the latest UN data on live births by month to obtain figures between 2000 and 2015 (which is the time period with the greatest amount of data for all countries listed).
In order to account for differences in the number of days in each calendar month, we then calculated the average number of live births per day in each month and ranked every month of the year relative to each other.
For example, for the United States, the average number of live births per day for each month was calculated. Each month was then ranked relative to each other in terms of the average number of live births per day, from highest to lowest. Since the month of September registered the greatest average number of births per day for this time period, it was assigned a ranking of 1; August exhibited the second highest total, and so received a ranking of 2; and so on.
The heat map was then created using a color scale corresponding to the rank value of each month, from 1 to 12. This way, the color-coded visualization would allow for easy comparisons across countries and hemispheres. Otherwise, the vast disparities between live birth numbers from one country to another would generate a color scale too broad in scope that would not allow for quick visual comparisons by viewers.
What are your thoughts on these conception and birth trends? Do you think other external factors besides climate and season are involved? Let me know in the comments section below!