欢迎光临月亮飞船的博客

 Wondering about the gender of your future offspring? Check your GPS. Girls are more likely to be born at tropical latitudes than in temperate or subarctic climes, according to new research. The study provides the first global look at human sex ratios and could shed light on how temperature and day length influence human reproduction.

 想知道你未来的后代是什么性别吗？查看一下地理位置就行了。一项新研究表明：比起温带和亚北极地区，在热带地区生女孩的可能性更大。该研究首次在全球范围内观察了性别比率，也许能够解释温度和白天的长度是如何影响人类生殖情况的。

 Many animals adjust the sex of their offspring based on environmental cues such as temperature, but whether humans do the same is difficult to measure. On average, about 105 boys are born per every 100 girls, a ratio that varies by continent. Differences are difficult to nail down; however, due to insufficient data and stressful conditions like war that potentially affect rates of fertilization and miscarriage. Until now, the only studies done on the connection between human sex ratios and latitude--one in North America and one in Europe--were geographically limited and reached opposite conclusions.

 许多动物可以依靠环境因素（如：温度）来调整后代的性别，但是很难确定人类是否也可以这样来调整后代的性别。出生的男孩和女孩的平均比率大约为105︰100，这个比率在各个大洲有所差异，然而由于资料不足，由于象战争之类的紧张环境对受孕率和流产率的潜在影响，这些差异不容易确定。到目前为止，在人类性别和纬度高低之间的关系方面仅有两项研究，一项在北美，另一项在欧洲，两项研究受到了地域的限制，结果得出了相反的结论。

 To overcome these problems, 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. Using the CIA's The World Factbook and other government publications, she gathered sex ratio data for every nation with a decade of uninterrupted statistics. She then analyzed the figures from 202 countries based on latitude, average temperature, day length, and socioeconomic status.

 为了弥补上述缺陷，佐治亚州立大学的生殖泌尿学家克里斯滕·纳瓦拉在10年的时间里分析了全球的性别比率，这个时间长度足以确保短期的社会危机或经济危机不会影响到研究结果。起初纳瓦拉发现西伯利亚在阳光较少的环境下长大的仓鼠多繁殖雄性后代，所以她想知道人类是否也会存在同样的情况。纳瓦拉利用中央情报局的《世界概况》（The World Factbook）一书以及其他国家政府的出版物，搜集了连续10年期间有关各国性别比率的统计资料，然后根据纬度、平均温度、白天时间的长短和社会经济状况分析了202个国家的统计数据。

 The analysis, reported today in Biology Letters, found a clear pattern. Just like hamsters, 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.

 该分析结果报道在今天出版的《生物学快报》（Biology Letters）上，通过分析发现了一个清晰的性别比率模式。跟仓鼠一样，人们居住在冬季持续时间长、夜间持续时间长的地区，则生养的男孩更多——在温带地区和亚北极地区出生的婴儿中有51.3%是男婴；相比之下，在热带地区出生的男婴只有51.1%。没有人确切地了解性别的选择是何时发生以及是如何发生的。但是纳瓦拉称，一些动物早在受孕的时候就调整了性别比率。纳瓦拉注意到一种调节睡眠和清醒周期的激素——褪黑素，同时也能够控制雌性生殖激素的产生，而且这种激素的释放随着白天的长短和季节的变换而发生变化，因而她指出：“我认为所有这一切都跟褪黑素有关。”

 Reproductive biologist Thane Wibbels of the University of Alabama, Birmingham, says the results suggest the ability to match offspring gender to environment may be adaptive for humans, though no one knows why boys might have an advantage outside the tropics. "She's uncovering something that now we have to address physiologically as well as evolutionarily," he says. One key to understanding why sex ratios differ, suggested reproductive biologist Mary Mendon?a of Auburn University in Alabama, might be to investigate why some countries like Venezuela buck the tropical girl trend.

 阿拉巴马大学的生殖生物学家塞恩·威伯尔斯说，尽管没有人了解为什么男孩会在热带以外的地区拥有优势，但该分析结果表明：对人类来说，把后代的性别与环境匹配起来的能力或许是一种适应性本领。“她的分析揭示了一个问题，我们现在不得不从生理学以及进化论的角度去解决该问题，”威伯尔斯说。阿拉巴马州奥本大学的生殖生物学家玛丽·孟冬萨指出：要想明白为什么性别比率会有所不同，也许一个关键的办法是需要调查一下为什么某些象委内瑞拉那样的国家在极力扭转热带多生女孩的趋势。