More sun means fewer children and grandchildren
Solar activity affects human fertility across generations according to historical records covering the years 1750 to 1900 and recorded in a church in Norway.
Gine Roll Skjaervoe of the Norwegian University of Science and Technology's (NTNU) Department of Biology, has studied church records from the period 1750-1900 looking at life variables. Her areas of interest were: How old women were when they had their first child, and their last? How many years passed between the birth of each child? How many of these children survived? How many of these children married and had children of their own?
All told, she studied information from more than 9,000 people listed in the church records.
When the church records were compared with environmental factors — including solar activity — Skjaervoe and her colleagues found that children born in the years with lots of solar activity died sooner than children born in the years with less solar activity.
On average, the lifespan of children born during high solar activity was 5.2 years shorter than children born during low solar activity. They also had a higher probability of dying in their first two years.
Children who were born in years with lots of sunshine and who survived were also more likely to have fewer children, who in turn also gave birth to fewer children. These findings show that increased UV radiation during years of high solar activity have an effect extending across generations.
Skjaervoe used the number of sunspots as an indication of the amount of UV radiation in a given year. The number of sunspots reaches a maximum every 11 years on average, which resulted in more UV radiation on Earth during years with high sunspot and solar activity.
UV radiation can have positive effects on human vitamin D levels, but it can also result in a reduction of vitamin B9 (folate). It is known that low folate levels during pregnancy are linked to higher child mortality.
The NTNU study showed that families from the lowest socio-economic groups were most affected by UV radiation. This is probably related to the time period Skjaervoe studied, which was a time of clear class distinctions in Norway, especially in rural areas. Women who worked in the fields were more exposed to the sun than other women. In many cases they also had a poorer diet.
Both climate change and variability in the ozone layer are expected to increase the amount of UV radiation reaching the Earth in the future. At the same time, there have been many societal changes since the 1900s. Nevertheless the NTNU researchers felt it prudent to caution women who want to have children.
"There are probably many factors that come into play, but having measured long-term effects over generations — the conclusion of our study is that you should not sunbathe if you are pregnant and want to have a lot of grandchildren."
Gine Roll Skjaervoe, Norwegian University of Science and Technology, Trondheim, Norway.
Scientists are particularly concerned about people with light skin who move to warmer climates with lots of sun.
The study was published in the 7 January issue of the Proceedings of the Royal Society B and is entitled "Solar activity at birth predicted infant survival and women's fertility in historical Norway."
Ultraviolet radiation (UVR) can suppress essential molecular and cellular mechanisms during early development in living organisms and variations in solar activity during early development may thus influence their health and reproduction. Although the ultimate consequences of UVR on aquatic organisms in early life are well known, similar studies on terrestrial vertebrates, including humans, have remained limited. Using data on temporal variation in sunspot numbers and individual-based demographic data (N = 8662 births) from Norway between 1676 and 1878, while controlling for maternal effects, socioeconomic status, cohort and ecology, we show that solar activity (total solar irradiance) at birth decreased the probability of survival to adulthood for both men and women. On average, the lifespans of individuals born in a solar maximum period were 5.2 years shorter than those born in a solar minimum period. In addition, fertility and lifetime reproductive success (LRS) were reduced among low-status women born in years with high solar activity. The proximate explanation for the relationship between solar activity and infant mortality may be an effect of folate degradation during pregnancy caused by UVR. Our results suggest that solar activity at birth may have consequences for human lifetime performance both within and between generations.
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