Single mums are better at raising their kids than two parentsat least in the bird world.Mother zebra finches1 have to work harder and raise fewer chicks on their own，but they also produce more attractive sons who are more likely to get a mate.
The finding shows that family conflict is as important an evolutionary driving force as ecological factors2 such as hunting and food supply.With two parents around,there's always a conflict of interests3，which can have a detrimental effect on the quality of the offspring.
In evolutionary terms4，the best strategy for any parent in the animal world is to find someone else to care for their offspring，so they can concentrate on breeding again.So it's normal for parents to try to pass the buck5 to each other.But Ian Hartley from the University of Lancaster and his team wondered how families solve this conflict,and how the conflict itself affects the offspring.
To find oiit6，they measured hoW much effort zebra finch parents put into raising their babies.They compared single females with pairs,by monitoring the amount of food each parent collected,and removing or adding chicks so that each pair of birds was raising four chicks，and each single mum had two — supposedly the same amount of work.
But single mums,they found,put in about 25 per cent more effort than females rearing with their mate.To avoid being exploited,mothers with a partner hold back from working too hard if the father is being lazy,and it's the chicks that pay the price.“The offspring suffer some of the cost of this conflict,"says Hartley.
The cost does not show in any obvious decrease in size or weight,but in how attractive they are to the opposite sex.When the chicks were mature,the researchers tested the"fitness" of the male offspring by offering females their choice of partner.Those males reared by single mums were chosen more often than those from two-parent families.
Sexual conflict has long been thought to affect the quality of care,given to offspring,says zoologist Rebecca Kilner at Cambridge University,who works on conflict of parents in birds."But the experimental evidence is not great.The breakthrough here is showing it empirically.”
More surprising,says Kilner,is Hartley's statement that conflict may be a strong influence on the evolution of behaviour,clutch size and even appearance.“People have not really made that link," says Hartley.A female's reproductive strategy is usually thought to be affected by hunting and food supply.Kilner says conflict of parents should now be taken into account as well.
注释：
1.zebra finch:斑胸草雀，一种澳大利亚小鸟，它有黑白色条纹，而且是受欢迎的笼养鸟。
2.family conflict is as important an evolutionary driving force as ecological factors:家庭冲突是与 生态因素同样重要的进化推动力。
3.a conflict of interests：(双亲之间的)利益冲突。
4.in evolutionary terms：从进化的角度讲。in.?.terms,in terms of…：从方面来讲;就而言。例如：It has been a terrible year in terms of business.就生意而论，这是很糟糕的一年。
5.pass the buck (to)：推卸责任，把为难的事推给另!|人0 例如：Bus companies are just passing the buck by saying their drivers are responsible for delays.公交公司说司机应该对误点的事负责，这只是在推卸责任。
6.to find out：这是一个承上的短语,从逻辑上讲，它的宾语应该是:how families solve this conflict,and how the conflict itself affects the offspring。
译文：
单亲幼儿最出色
单身母亲在抚养孩子方面比双亲要好——至少在鸟的世界是这样的。母斑胸草雀必须要非常 努力地亲自抚养较少的小鸟，但是她们抚养的雄性后代更有吸引力,更容易得到配偶。
这个发现显示家庭冲突是与捕食和食物提供这样的生态因素同样重要的进化推动力。身边有 双亲就总会有利益冲突，这会对后代的质量产生有害的影响。
从进化的角度讲，在动物界，任何一个父亲或母亲的策略是让他人来照顾后代，这样，他们 .就可以集中精力再次繁殖后代了。所以父母之间很自然就出现了相互推卸责任的现象。但是兰卡 斯特大开云kaiyun(中国)的Ian Hartley及他的工作组想知道家庭是如何解决这个冲突的，以及冲突本身是如何影响 后代的。
为了找到答案，他们衡量斑胸草雀抚养它们的子女花了多少力气。他们监控每个母亲或父亲采 集的食物量,移动或增加小鸟，因此每对草雀养4只小鸟，每只单身母雀养两只——假定这是相同的工作量。通过这些方法他们对单身母雀和成对草雀作了比较。
但是他们发现单身母雀比与配偶共同抚养后代的母雀多付出25%。如果公雀太懒的话，为了避免筋疲力尽，有配偶的母雀并不十分努力工作。这样做的后果就是后代付出代价。Hartley说: “后代要为这种冲突付出一些代价。”
这种代价不是表现在身材和体重的明显减少，而是表现在它们对异性的吸引力上。当小鸟长大 后，研究者通过提供给雌性它们所能选择的伴侣的方法检验雄性后代的健康状况。那些由单身母亲抚养大的雄性比由父母抚养大的雄性更多地被选中。
研究鸟类父母冲突的剑桥大开云kaiyun(中国)动物开云kaiyun(中国)家Rebecca Kilner说，很久以来人们就认为两性之间的冲
突会影响对后代的照顾。“但是试验证据并不充分。这个突破主要是以经验来显示这种影响的。”
Kilner说，更令人惊讶的是Hartley说这种冲突可能会严重影响行为的进化、窝卵数目，甚至外 表。Hartley说:“人们并没真正确定这种联系。”人们通常认为雌性的繁殖策略受捕食和食物提供的 影响。Kilner说父母之间的冲突现在也应该被考虑在内了。
