Spatiotemporal Variations in Human Birth Weight Are Associated with Multiple Thermal Indices

Human populations are scattered worldwide and live under widely different climates. Like other mammals, humans respond to climatic influences through various processes involving behavior, physiology, and various forms of adaptation. Human populations can be explored in investigating patterns of adaptation because many of their biological attributes have been monitored for over a century. Here, we evaluated the association between several thermal indices and human birth weight (BW) and offered some initial observations on the temporal integration of thermal cues associated with pregnancy outcomes. We compiled three datasets: (1) a dataset with global coverage of recent BWs; (2) an extended time series for seven European countries; and (3) a time series for four countries in equatorial Africa. Each dataset was analyzed for associations between BW and mean annual temperature, as well as seasonal and daily amplitudes. Mean annual temperatures, as well as seasonal and daily amplitudes, delivered consistent and comparable impacts in our analyses. The thermal indices can explain approx. 80% of the global variation in BW and 25–50% of the BW variation in time series covering the last 70 to 120 years. Mean BW in larger aggregates of humans (i.e., millions) is associated with several thermal indices, likely associated with systematic differences in proximate factors (e.g., maternal height, weight, food intake) between populations. This study underlines the diverse impact of the thermal environment on human reproduction, but it also underscores that this impact is less pronounced for differences in mean BW with respect to different communities, and it is possibly undetectable and/or irrelevant with respect to differences between individuals.