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

Abstract

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 annual mean temperatures were associated with annual offspring sex ratio at birth in the eighteenth to nineteenth century Sami from northern Finland. We found that warm years 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.

Feeling the Heat? Substantial Variation in Temperatures Does Not Affect the Proportion of Males Born in Australia

“…Indeed, variation in environmental factors such as climate, ambient temperature and rainfall appear to predict fluctuations in the human SSR, with more males being born within more favourable ecological conditions (Catalano et al 2008;Helle et al 2009). This pattern is in accord with evolutionary theory that fewer males will be born during stressful periods, as a weaker male would not survive to reproduce where a female might (Trivers and Willard 1973), and suggests a role for ambient temperature in sex allocation of human births (Catalano et al 2008;Helle et al 2008Helle et al , 2009). …”

“…In humans oscillations in ambient temperature are also associated with greater mortality in adults (Basu and Samet 2002;Lerchl 1998;Young and Mäkinen 2010) and predict the survivability of new-borns, with both increases (Catalano et al 2008) and decreases (Young and Mäkinen 2010) negatively associated with survival during the first year of childhood (Catalano et al 2008(Catalano et al , 2012Young and Mäkinen 2010). These effects appear to be more pronounced for males than females and thus it is argued that climatic shifts cause gestational stress that affects sex allocation in humans (Catalano 2011;Catalano et al 2008;Helle et al 2008Helle et al , 2009Navara 2009). Indeed, variation in environmental factors such as climate, ambient temperature and rainfall appear to predict fluctuations in the human SSR, with more males being born within more favourable ecological conditions (Catalano et al 2008;Helle et al 2009).…”

“…These findings differ from some previous studies in which time-series analyses were used to explore the relationship between the secondary sex ratio (SSR) and fluctuations in annual mean ambient temperature. For example, in Finland and elsewhere in Scandinavia more males are born in warmer years (Helle et al 2008(Helle et al , 2009Catalano et al 2008). However, unlike in Scandinavia, fluctuations in annual mean ambient temperatures in New Zealand are unrelated to rates of male live births or stillbirths (Dixson et al 2011).…”

“…Globally, the proportion of males born is greater in subarctic and temperate climates than in tropical climates (Navara 2009). There appears to be a positive relationship between ambient temperature and male births in subarctic climates (Catalano et al 2008;Helle et al 2008Helle et al , 2009) and a neutral effect within temperate climates (Dixson et al 2011(Dixson et al , 2013. It may be that the conditions within tropical climates have the reverse effect on sex allocation, so that increases in ambient temperature induce maternal stress during gestation and result in a lowering of the proportion of male births (McLachlan and Storey 2003;Navara 2009).…”

“…Time-series analyses conducted within subarctic climates of Finland and across Scandinavia using long-term historical datasets demonstrated that more males are born in years with relatively higher annual mean temperatures (Catalano et al 2008;Helle et al 2008Helle et al , 2009). …”