Single mums are better at raising their kids than two parents 一 at least in the bird world. Mother zebra finches ¹ 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 factors 2 such as hunting and food supply. With two parents around, there"s always a conflict of interests 3，which can have a detrimental effect on the quality of the offspring.

    In evolutionary terms 4，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 buck 5 to each other. But Ian Hartley from the UniversityofLancasterand his team wondered how families solve this conflict, and how the conflict itself affects the offspring.

    To find oiit 6，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 atCambridgeUniversity, 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.

词汇：

evolutionary /,i: və"lu:ʃənəri/adj．进化的

detrimental /detrimentəl/adj．有害的

offspring /"ɔfspriŋ/n．（单复数同形）儿女，子孙，后代

empirically /em"pirikli/adv．以经验为根据地

ecological /i:kə"lɔdʒikəl/adj．生态的

reproductive /,ri:prədʌktiv/adj．生殖的

experimental /eks,peri"mentəl/adj．实验的，根据实验的

clutch /klʌtʃ/ n．一次产的或孵的卵

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：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，把为难的事推给别人。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.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.  

According to the passage, people believe that a female"s reproductive strategy is influenced by________.

A:an evolutionary driving force B:a conflict of interests C:ecological factors D:the quality of the offspring

The Family

    The structure of a family takes different forms around the world and even in the same society. The family"s form changes as it adapts to changing social and economic influences. Until recently, the most common form in North America was the nuclear family，consisting of a married couple with their minor children. The nuclear family is an independent unit. It must be prepared to fend for itself. ¹ Individual family members strongly depend on one another. There is little help from outside the family in emergencies. Elderly relatives of a nuclear family are cared for only if it is possible for the family to do so. In North America，the elderly often do not live with the family ; they live in retirement communities and nursing homes. ²  

    There are many parallels between the nuclear family in industrial societies，such as North America，and of families in societies such as that of the Inuits，who live in harsh environments. The nuclear family structure is well adapted to a life of mobility. In harsh conditions，mobility allows the family to hunt for food. ³ For North Americans，the hunt for jobs and improved social status also requires mobility.

    The nuclear family was not always the North American standard. In a more agrarian time，the small nuclear family was usually part of a larger extended family. This might have included grandparents，mother and father，brothers and sisters，uncles，aunts，and cousins. In North America today，there is a dramatic rise in the number of single-parent households. Twice as many households in the United States are headed by divorced，separated，or never-married individuals as are comprised of nuclear families. The structure of the family，not just in North America, but throughout the world，continues to change as it adapts to changing conditions.

词汇:

nuclear [ˈnju:kliə(r)] adj. 原子核的，中心的

harsh [hɑ:ʃ] n. 严酷的

emergency [ɪ"mɜ:dʒənsɪ] n. 紧急状况

agrarian [əˈgreəriən] adj. 土地的，耕地的

注释:

1. lt must be prepared to fend for itself. 它必须能够照料自己。

2. In North America，the elderly often do not live with the family; they live in retirement communities and nursing homes. 在北美洲，老人很少和家人一起生活;他们一般住在退休社

区和养老机构。

A nuclear family is defined as_____.

A:a married couple with their minor children B:a single father with，minor children C:parents，grandparents，and children D:parents，children，and aunts and uncles

My father is a farmer, but ______ is yours

A:who B:how C:which D:what

What makes father no longer be the most important in a family

A:Father does much less for his children today than he used to B:The number of married women in employment has increased now C:There are many choices of employment for mothers and children D:With their earning, mother and children do not need to depend on father for their life

What makes father no longer be the most important in a family ______.

A:Father does much less for his children today than he used to B:The number of married women in employment has increased now C:There are many choices of employment for mothers and Children D:With their earning, mother and children do not need to depend on father for their life

A:a single father with minor children B:parents, children, and aunts and uncles C:a married couple with their minor children D:parents, grandparents, and children

A:a single father with minor children B:parents, children, and aunts and uncles C:a married couple with their minor children D:parents, grandparents, and children