办事指南

Some like it hot

点击量:   时间:2019-03-07 14:03:14

By Alison Motluk IF THE summer brings unusually hot spells, look out for a boom in baby boys next spring. According to a scientist in Germany, boys are more likely to be conceived just after a heat wave, while conception after a cold spell favours girls. Biologists have long suspected that there are seasonal variations in the ratio of boys to girls born in any population. Alexander Lerchl of the University of Münster reported last year that more boys are born in Germany between April and June, and significantly fewer in October. Experiments with rats and bats had hinted that environmental temperatures could affect the sex ratio of offspring. Lerchl hoped to find out whether this is also true for people. He looked at average monthly temperatures in Germany between 1946 and 1995, using data sets from the National Climatic Data Center in Asheville, North Carolina. He also calculated which months were unseasonably cold or hot. Then, using German birth records from the same 50-year period, he looked to see if there were any correlations between air temperature and the sex ratios of babies born. Sex ratio seemed to correlate with temperature about one month before conception. Hot summers or unseasonably warm patches during this period yielded more boys, while unusually cold weather favoured girls. Temperature deviations of just a few degrees centigrade had an impact. The timing suggests temperature may affect processes within the testes, Lerchl says (Naturwissenschaften, vol 86, p 340). Temperature seems to play its part when the father-to-be’s sperm start maturing. Lerchl speculates that hot spells may damage sperm carrying an X chromosome more than sperm carrying a Y, so more boys are conceived. The finding doesn’t mean that people from hot climates have a disproportionate number of sons. While the skin temperature of the scrotum varies with the season, it doesn’t vary greatly between one place and another because people adapt their clothing to the climate. “And the scrotum has the highest density of sweat glands and the highest capacity to cool,” says Lerchl. But the sex bias, although small, could have large-scale consequences. Lerchl speculates that global warming might further increase the ratio of males to females, which already favours boys by a few per cent. But it’s also possible that the effect of temperature is indirect and that balmy weather just makes people have more sex. Frequent sex increases a woman’s chance of conceiving as soon as she ovulates. This produces more sons, possibly because sperm carrying a Y chromosome are faster though less robust than X-carriers, which stand a better chance if they have to wait for ovulation (“Why Presidents have more sons”, New Scientist, 3 December 1994,