Temperature-related birth sex ratio bias in historical Sami: warm years bring more sons

• Published in: Biology Letters
• Main Authors: Helle, Samuli, Helama, Samuli, Jokela, Jukka
• Format: Article in Journal/Newspaper
• Language: English
• Published: Royal Society 2008
• Subjects: climatic reconstruction; parental age; time-series analysis; Northern Finland; sami
• Online Access: https://doi.org/10.1098/rsbl.2007.0482

The birth sex ratio of vertebrates with chromosomal sex determination has been shown to respond to environmental variability, such as temperature. However, in humans the few previous studies on environmental temperature and birth sex ratios have produced mixed results. We examined whether reconstructed annualmean temperatures were associated with annual offspring sex ratio at birth in the eighteenth to nineteenth century Sami from northern Finland. We found that warmyears correlated with a malebiased sex ratio, whereas a warm previous year skewed sex ratio towards females. The net effect of one degree Celsius increase in mean temperature during these 2 years corresponded to approximately 1% more sons born annually. Although the physiological and ecological mechanisms mediating these effects and their evolutionary consequences on parental fitness remain unknown, our results show that environmental temperature may affect human birth sex ratio.