This is some wonderfully strange new research. Girls are more likely to be born at tropical latitudes than in temperate or subarctic climates. This study provides the first global look at human sex ratios and could shed light on how temperature and day length influence human reproduction. This article reminded me of temperature-dependent sex ratio of reptiles, and I'm not at all surprised that the length of day and temperature would induce a change in sex ratio.
Strangely, in reptiles there is polar variation in the effect temperature has on the outcome of the offspring among taxonomic groups. In crocodiles and alligators, for example, eggs incubated in temperatures below 30 °C produce female offspring and eggs incubated above 34 °C produces male offspring. In turtles, however, at cooler temperatures ranging between 22.5°C and 27 °C mostly male turtles are produced and at warmer temperatures, around 30°C, only female turtles arise. Many other animals adjust the sex of their offspring based on environmental cues such as temperature, but to confirm whether humans do to some extent is difficult to measure. There are multiple variables that have been confirmed to also affect the human sex ratio, such as stress imposed on the mother during pregnancy. How would one overcome this obstacle?
Reproductive endocrinologist Kristen Navara of the University of Georgia in Athens analyzed global sex ratios over 10 years, enough time to be sure that short-term social or economic crises weren't driving the results. Navara had previously found that Siberian hamsters raised with fewer hours of daylight produced more male offspring, and she wondered if the same might be true for humans. Navara gathered sex ratio data for every nation within a decade of uninterrupted statistics. She then analyzed the figures from 202 countries based on latitude, average temperature, day length, and socioeconomic status.
She found that humans living in areas with long, dark winters have more boys: 51.3% of babies born in temperate and subarctic regions are male, compared with 51.1% in the tropics. No one knows exactly when or how sex selection happens, but Navara says that some animals skew sex ratios as early as fertilization. "I suspect that all of this has something to do with melatonin," a hormone that regulates sleep-wake cycles and the production of female reproductive hormones, says Navara, noting that melatonin release varies in response to day length and season.
Navara, Kristen J. 2009. Humans at tropical latitudes produce more females. Biology letters.