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最新研究表明，那些花较长时间怀孕的人比那些快速怀孕的人更容易生男孩。
荷兰人研究显示，在用了一年多才怀孕的500个女性里生男孩的几率为58％。但是在4800个怀孕少于一年的女性里这个比例下降到51％。
研究统计表明，一对夫妇尝试自然生产每增加一年，生男孩的几率将提高4％。这个比例是在考虑吸烟，饮酒和月经周期差异后得出的。
荷兰Maastricht大学的流行病学家Luc Smits领导了这项研究，他相信这项结果支持携带Y染色体的精子游的速度要快于携带X染色体的，且更容易穿过粘性子宫颈粘液的想法。
他告诉New Scientist先前的研究显示子宫颈粘液的黏性在不同女性之间存在变化并且那些更粘的粘液会降低受孕。
配偶质量
新西兰奥克兰大学的ValerieGrant曾经研究过母亲的角色在人类生产性别比例中的作用他说, “这是研究中的精华”
例如，一种理论表明被剥夺较多低等动物会产生更多的雌性。这是因为生产和养育一个雄性在能量上考虑耗费更多。生育一个雌性后代在时间的压力下是一个更好的选择，甚至在多配偶动物中雄性比雌性更倾向于产生更多的后代。
“这些发现与理论并不相符。但是他们同不容易生产的女性不一定更不健康的理论相矛盾”Smits说。“对此我们没有进化上的解释。”
为延续雄性而努力
但是Grant并不欣赏Smits关于试图生孩子越长时间越容易生男孩是因为Y－精子更容易穿过子宫颈粘液的理论。
她转向研究组中另一种解释，子宫颈质量常常与激素问题有关，激素问题同时还会增加自发性流产的几率。Grant自己的研究表明孩子性别的影响取决于母亲而不是父亲，这同许多进化生物学家的想法一致。
她强调从先前的概念我们知道雄性有更高的死亡率。因此在有压力的时段保证一定数量的雄性，母亲更倾向于提高男孩的比例。Grant说：“任何可以加强母亲这中行为的机制都将毫无疑问的增加适应性。”
她告诉NewScientist“这项新的研究表明在生男性可能有更大的阻碍，这包括更严苛的母性的要求甚至是仅仅启动它。”
Women who take longer to get pregnant are more likely to bear sons than those who conceive quickly, a new study indicates.
The chance of a woman having a baby boy was 58% in a group of 500 women who took more than a year to conceive, the Dutch study showed. But among the 4800 women who conceived in less than a year this fell to 51%.
For every additional year a couple tries to conceive a baby naturally, there is an almost 4% higher probability of having a baby boy, the researchers calculate. The increased chance remains after allowing for confounding factors such as smoking, alcohol and variability in the menstrual cycle.
Epidemiologist Luc Smits of Maastricht University, the Netherlands, who led the study, believes the results support the idea that sperm bearing the male Y chromosome swim faster than sperm with the female X chromosome, and can travel through viscous cervical mucus more easily.
He told New Scientist that previous studies have shown that the viscosity of cervical mucus varies between women and that those with more viscous mucus get pregnant less easily.
Mate quality
“This is an elegant piece of research,” says Valerie Grant at the University of Auckland, New Zealand, who has researched the mother’s role in determining the sex ratio of human offspring.
Many animals are able to influence the sex of their offspring according to the quality of their mate and environmental conditions. Evolutionary biologists believe this maximises the number of their surviving descendants.
For example, one theory suggests that low-status animals, which may suffer more deprivation, have more female offspring. This is because bearing and rearing a male costs more in energy terms. Having female offspring in times of stress could be a better bet, even though in polygamous animals males are likely to produce more offspring than females.
“These findings are not in agreement with the theory. But they also do not contradict this theory as less fertile women are not necessarily less healthy,” says Smits. “We do not have an evolutionary explanation for this.”
Struggling to be male
However, Grant does not favour Smits's explanation that the skew towards having a boy after trying for a baby for a long time is due to the Y-sperm being better at penetrating viscous cervical fluid.
She veers towards an alternative explanation offered by the team – that poor mucus quality is often related to hormonal problems, which in turn increase the chance of spontaneous abortion. Grant’s own research has suggested that any influence on the sex of a child is exerted by mothers and not fathers, as many evolutionary biologists think.
Males are known to have greater mortality rates from conception onwards, she notes. So to ensure a given number of male offspring in stressful times, the mother may skew the ratio towards boys. “Any mechanism that allowed the mother to do so would surely increase fitness," Grant says.
“This new study suggests that there may be even more hurdles to overcome in the struggle to be male, including slightly tougher maternal requirements even to get started,” she told New Scientist.
Journal reference: BMJ (vol 331, p 1437)
帖子在2005-12-20 8:25:43被编辑过了 | 最初发表时间：2005-12-17 |  | | 未填 |
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