An increased proportion of male African buffalo1(水牛,野牛) are born during the rainy season. Researchers writing in the open access journal BMC Evolutionary2 Biology collected data from over 200 calves3 and 3000 foetuses(胎儿) , finding that rain likely exerts(运用,发挥) this effect by interaction with so-called sex ratio (SR性别比率) genes4, which cause differences in number, quality or function of X- and Y-bearing sperm5. Pim van Hooft, from Wageningen University, The Netherlands, worked with a team of researchers to study animals in the Kruger National Park, the scene of the famous 'Battle at Kruger' wildlife video. He said, "Here we show temporal(现世的,暂时的) correlations6 between information carried on the male Y chromosome7 and foetal sex ratios in the buffalo population, suggesting the presence of SR genes. Sex ratios were male-biased during wet periods and female-biased(有偏见的) during dry periods, both seasonally8 and annually9".
The researchers studied data collected between 1978 and 1998 to investigate the associations between rainfall, birth rates/ratios and genetic10 information. Ejaculate volume, sperm motility(运动性) and proportion of normal-shaped sperm decrease significantly during the dry season. This decline in quality is likely due to decreasing availability and quality of food resources. According to van Hooft, "These observations may point towards a general mechanism11 in mammals whereby semen-quality related sex-ratio variation is driven by SR genes".