Increasing temperature is known to affect sex in some reptiles and fish. However, researchers from the University of Oxford analyzed more than five million births in 33 countries in sub-Saharan Africa and India and found that as the average air temperature increases, the ratio of girls born to boys increases. The Oxford researchers found that in both regions, temperatures above 20°C were consistently associated with a lower number of male births. The gender ratio in humans is not exactly 1:1. Globally, approximately 101 to 102 boys are born for every 100 girls.
“Extreme heat is not only a major threat to public health. We show that temperature fundamentally shapes human reproduction by influencing who is born and who is not. Our findings suggest that temperature has measurable consequences for fetal survival and family planning behavior, with implications for population composition and sex balance. Understanding these processes is key to predicting how the environment affects societies in a warming climate.” said the lead author of the study Dr. Abdel Ghany from the University of Oxford.
However, the mechanisms differ by region. In sub-Saharan Africa, a decline in the number of male births has been linked to prenatal mortality due to maternal heat stress. In India, the effects appeared later in pregnancy. “Higher second-trimester temperatures are associated with fewer male births, particularly among older mothers, higher-order births, and among women without sons in northern states,” the authors add.
Scientists from the University of Manchester have also drawn attention to the connection between climate and reproduction. They analyzed the sperm of more than 15,500 men in Denmark and Florida. “We were struck by how similar the seasonal pattern was in two very different climates. Even in Florida, where temperatures remain high, sperm motility peaked in summer and declined in winter, suggesting that ambient temperature alone is unlikely to explain these changes.” said Professor Allan Pacey.
