Solar activity at birth predicted infant survival and women's fertility in historical Norway
Gine Roll Skjærvø, Frode Fossøy, Eivin Røskaft • The Royal Society Proceedings B (London)
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.
Environmental factors during the early development of an organism can have downstream effects on the phenotypic quality and reproductive performance of that organism . Several long-term studies on a wide variety of species , including humans [3–5], have revealed that the environment an organism is exposed to early in life may influence adult life-history traits, such as survival, fertility and lifetime reproductive success (LRS). Individuals may differ in their sensitivity to stressors during early development, which can be influenced by gender and life stage. First, it is generally accepted that males are more vulnerable to environmental stressors than females , and second, such effects may vary at different life stages, with greater vulnerability appearing during early development .
Exposure to high levels of ultraviolet radiation (UVR) is one such type of environmental stressor that can affect later survival and reproductive performance [7,8]. Levels of UVR vary with solar activity , latitude/altitude  and photoperiod . The detrimental effects of high UVR exposure during development are unclear but may act via multiple molecular (degradation of folate and DNA damage) and cellular (membrane damage) mechanisms [10,12–14] in the developing organism. Such effects may lead to detrimental consequences later in life . However, organisms can exhibit differential defences against UVR damage, including behaviour (avoidance), accumulation of photo-protective compounds (pigmentation, e.g. melanin and carotenoids) as well as cellular defence mechanisms (DNA-repair and antioxidants) [16,17], and specific genotypes . Moreover, the presence of such UVR defence mechanisms is often costly and indicates that UVR represents a potential environmental factor in life-history evolution . Indeed, a number of aquatic studies indicate that ambient UVR exposure during early development affects life-history traits later in life [7,8]. However, to our knowledge, such research with respect to terrestrial vertebrates is lacking (but see ). A few studies have focused on how periods of high solar activity (as a proxy for UVR) during gestation may adversely affect humans through fetal loss [19,20] and through the onset of diseases in adult life [21–23]. Furthermore, some studies have investigated how solar activity during embryonic development predicts lifespan. Whereas one study found that individuals born during years with high solar activity experienced a reduced lifespan , another study found there to be no effect . While certain studies suggest stronger long-term effects in men than in women with respect to both lifespan and disease owing to early life exposure to high UVR [9,24], no study has yet considered the effect on survival across the complete lifespan or captured changes in effects across age and gender. This approach makes it possible to identify the specific phases where selection occurs as such information would help determine the causal mechanism responsible for long-term consequences of high UVR during development. However, thus far, no human study has explored the fitness consequences of this association in terms of both survival and reproductive performance.
Herein, we use an individual-based dataset from two human populations in Norway that spans 160 years. The data include information regarding natural fertility and mortality and enable us to test the hypothesis that high solar activity during early development affects human life-history traits (survival, fertility and LRS) and that these associations are modified by gender and age. Specifically, we examine whether the solar activity at birth is (i) related to the probability of survival from birth to the age of 20 years; (ii) related to fertility; and (iii) related to reproductive success (LRS). We analyse each gender separately as UVR has been found to affect men and women differently [9,24]. In contrast to data used in previous studies, the longitudinal individual-based data enable us to control for factors known to affect survival and fitness, such as socioeconomic status, age at first reproduction and ecology .…
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[Hat Tip: Nachos.]