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A Dog’s Dilemma 狗的兩難境地

　　Finding a babysitter while you go out to work is, for example, an inconvenience. For the African wild dog, one of the continent’s most endangered carnivores, it’s a matter of life and death. New research shows that once packs fall below a certain size, there are not enough animals to both hunt food and stay at home protecting the young.

　　The African wild dog has declined drastically over the past century. Habitual loss, persecution and unexplained outbreaks of disease have all been blamed. Only 3,000 to 5,000 animals remain, and the species is expected to go extinct within decades if the trend continues.

　　Other large carnivores such as the spotted hyena face similar pressures, yet are not declining. Now Franck Courchamp of Cambridge University has found a reason why. The dog’s weakness lies in its social organisation.

　　Within each pack of up to 20 adults and pups, only the dominant male and female breed. The remaining animals help raise the pups, cooperating to hunt prey and defend the kill from other carnivores.

　　Because pups can’t keep up on a hunt, large packs leave an adult behind to protect them from predators, which include lions and hyenas. But leaving a babysitter also carries costs. A smaller hunting party is less able to tackle large prey and to defend the kill. There is also one less stomach in which to carry food back to the den, and one more mouth to feed when they get there.

　　Courchamp investigated this awkward trade-off by modelling how the costs of a babysitter change with decreasing pack size. This showed that packs of more than five adults should be able to feed all the pups and still spare a babysitter. But with smaller packs, either the hunting or the babysitting suffers, or the animals have to compensate by increasing the number of hunting excursions － which itself carries a cost to the pack.

　　Field observations in Zimbabwe supported the model. Packs of five animals or fewer left pups unguarded more frequently than larger packs did. There was also evidence that when they did leave a babysitter, they were forced to hunt more often.

　　A pack which drops below a critical size becomes caught in a vicious circle, says Courchamp, who is now at Paris –Sud University. “Poor reproduction and low survival further reduces pack size, culminating in failure of the whole pack” And deaths caused by human activity, says Courchamp, may be what reduces pack numbers to below the sustainable threshold. Mammal ecologist Chris Carbone at London’s Institute of Zoology agrees. Maintaining the integrity of wild dog packs will be vital in preserving the species, he says.

　　譯文：

　　例如，當你出去工作時找一個臨時照顧孩子的保姆是一種不便。而對于大陸上最有滅絕危險的食肉動物之一非洲野狗來說，這是一件生死攸關的事情。新的研究顯示一旦一個動物群低于某種規(guī)模，他們就沒有足夠的動物既捕獵又留在家里保護幼兒。

　　在過去的一個世紀里，非洲野狗的數(shù)量急劇下降。原因是慣常的喪失、迫害和莫名其妙的疾病的爆發(fā)。目前只剩下3 000到5 000只野狗，如果這個勢頭繼續(xù)下去的話，在數(shù)十年里，這個物種就會滅絕。

　　其他的大型食肉動物，如帶斑點的袋狼也面臨著類似的壓力，但數(shù)量并沒有下降�，F(xiàn)在，劍橋大學的富蘭克·顧尚已經(jīng)發(fā)現(xiàn)了原因。狗的弱點在于它的社會組織。在一群可以多達20只大小狗中，只有占支配地位的母狗和公狗可以產(chǎn)仔。其他的狗幫助養(yǎng)小狗，共同捕獵和防御其他食肉動物的捕殺。

　　因為小狗在獵食時跟不上來，大的狗群會留下一只成年的狗來保護它們而不受包括獅子、袋狼在內(nèi)的捕食動物的傷害。但是留下一只看護狗也是要付出代價的。一個較小的捕獵群體不太容易捕捉大動物，也不太容易保衛(wèi)獵獲物。而且少了一個帶食回窩的胃，回到窩還多了一張吃東西的嘴。

　　顧尚通過模擬隨著狗群的減小看護狗的代價的變化研究這種兩難境地。這顯示多于五只成年狗的狗群應該能夠喂養(yǎng)所有的小狗，并仍然能留一只狗做看護狗。但是規(guī)模稍小的狗群中，捕獵或看護小狗都會遭受損失，要么這些狗就得增加出獵的次數(shù)作為彌補——這本身就需要狗群付出代價。

　　在津巴布韋的野外觀察支持這一模式。五只或五只以下的狗群比規(guī)模大些的狗群更經(jīng)常地把小狗沒有看護地獨自留下。還有證據(jù)表明當它們留下一只看護狗時，它們不得不更經(jīng)常地捕獵。

　　現(xiàn)在在巴黎大學的顧尚說，低于某特定規(guī)模的狗群會陷入一種惡性循環(huán)。“少量的繁殖和低存活率進一步減小了狗群的規(guī)模，最終導致整個狗群的消亡。”顧尚說人類行為導致的死亡可能是使狗群成員數(shù)量低于可維持的最低點的原因。倫敦動物學研究所的哺乳動物生態(tài)學家克里斯·卡波恩也同意這種說法。他說保持非洲野狗群的完整對保護該物種是重要的。

　　練習：

　　★1).The African wild dog has been endangered.-R

　　★2).The spotted hyena is on the verge of extinction.-W

　　★3).The remaining lions will die out within decades.-N

　　★4).The dominant females is always left behind to protect the young.-W

　　★5).There is a tension between babysitting and hunting.-R

　　★6).The size of a pack must be big enough for it to survive.-R

　　★7).Steps will be taken to protect the African wild dog.-N

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