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  3. The Koala’s Life Cycle The breeding season for koalas runs roughly from September to March. This is a time of increased acti

The Koala’s Life Cycle The breeding season for koalas runs roughly from September to March. This is a time of increased acti

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问题                       The Koala’s Life Cycle
    The breeding season for koalas runs roughly from September to March. This is a time of increased activity, and sound levels increase as males bellow more frequently. This is also when the young from the previous year are dispersing from their mothers. Where koalas live near suburban settlements and major roads, this period heralds the busiest time for koala carets, as koalas on the move cross paths with cars and dogs, and accompanying stress levels mean a higher incidence of sickness.
    Females generally start breeding at about three or four years of age and usually produce only one offspring  each year. However, not all females in a wild population will breed each year. Some produce offspring  only every two or three years, depending on factors such as the age of the female and the quality of the habitat. In the average female’s life span of about twelve years, this means that one female may produce  only 5 or 6 offspring over her lifetime.
    Once a female has conceived it is a short 35 days before the birth of the new baby, called a "joey". The tiny baby which is roughly 2 centimetres long and weighs less than 1 gram, looks rather like a pink jellybean ms it is totally hairless, blind and has no ears.
    The joey makes its way from the birth canal to the pouch completely unaided, relying on its already well-developed senses of smell and touch, strong forelimbs and claws and an innate sense of direction. Once inside the safety of the pouch, it attaches itself to one of the two teats, which swells to fill its mouth. This prevents the joey from being dislodged from its source of food The mother contracts her strong sphincter muscle at the pouch opening to prevent the baby from tailing out.
    The young koala drinks only mother’s milk for the first six to seven months and remains in the pouch for that time, slowly growing and developing eyes, ears, fur etc. At about 22 weeks, its eyes open and it begins to peep out of the pouch. From about 22 to 30 weeks, it begins to feed upon a substance called "pap" which the mother produces in addition to milk. Pap is a specialised form of faeces, or droppings, which forms an important part of the young koala’s diet, ’allowing it to make the transition from milk to eucalyptus  leaves, rather like a human baby is fed "mushy" food when it starts to eat solids. Pap is .soft and runny and thought to come from the eaecum. It allows the mother to pass on micro-organisms present in her own digestive system which are essential to the digestion of eucalyptus leaves, and is a rich source of protein.
    The joey Leans out of the pouch opening on the centre of the mother’s abdomen to feed on the pap, stretching it open towards the source of the pap, and therefore "downwards" or "backwards". This is why koalas are sometimes said to have a "backward-opening" pouch, although this is not strictly true.
    The baby feeds regularly on the pap and as it grows it emerges totally from the pouch and lies on its mother’s belly to feed. Eventually it begins to feed upon fresh leaves as it rides on its mother’s back. The young koala continues to take milk from its mother until it is about a year old, but as it can no longer fit in the pouch, the mother’s teat elongates to protrude from the pouch opening. Young koalas remain with their mothers until the appearance outside the pouch of the next season’s joey. Then it is time for the previous year’s joey to disperse and find its own home range. If a female does not reproduce each year, the joey stays with her longer and has a greater chance of survival when it does leave its mother.
    Females generally live longer than males as the males are more often injured during fights, they tend to travel longer distances with the resulting increase in risks such as cars and dogs, and they more often occupy poorer habitats. Putting a life span on the average koala can be misleading because some survive only for a period of weeks or months, while others survive to old age. Koalas living in an undisturbed habitat would have a greater life expectancy than those living in suburbia. Some estimates for the average life-span of a wild adult male koala are ten years, but the average survival rate for a dispersing sub-adult hale living near a highway or a housing estate is closer to two or three years.
Well-developed senses of vision and direction and strong forelimbs enable the joey to make its way from the birth canal to the pouch completely unaided.
选项 A、Y
B、N
C、NG
答案B
解析
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