A Dog’s Dilemma

    Finding a babysitter while you go out to work is, for example, an inconvenience. For the African wild dog, one of the continents’s most endangered carnivores, it’s a matter of life and death. New research shows that once packs fall below a certain size, they 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 CambridgeUniversityhas found a reason why. The dog’s weakness lies in its social organization.

    Within each pack of up to 20 adults and pups, only he dominant male and female bread. 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 ¹. 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 modeling 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 he number of hunting excursions—which itself carries a cost to the pack.

    Field observations inZimbabwesupported 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 ², who is now atParis-SudUniversity. “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 atLondon’sInstituteofZoologyagrees. Maintaining the integrity of wild dog packs will be vital in preserving the species, he says.

词汇

carnivore /"ka: nivɔ:(r)/ n．食肉动物

pup /pʌp/ n．小狗

predator/"predətə(r)/n．捕食者

integrity /in"tegrəti/ n．完整

hyena /hai"inə/ n．鬣狗

prey /prei/ n．被捕食的动物

culminate /"kʌlmineit/ n．告终；达到顶点

Zimbabwe /zim"ba: bwei/ n．津巴布韦

注释：

1.A smaller hunting party is less able to tackle large prey and to defend the kill.：一个较小的捕猎群体不太容易捕捉大动物，也不太容易保卫猎获物。
2. A pack which drops below a critical size becomes caught in a vicious circle, says Courchamp：Courchamp说，“一个群体的规模降到警戒线以下时就会陷入恶性循环。”

The spotted hyena is on the verge of extinction.

A:Right B:Wrong C:

Not mentioned

Across the Deserts

　　The SaharaDesertis the largest desert in the world. It stretches across Africa fromSenegaltoEgypt. TheSaharaDesertis an unfriendly environment. During the day it"s very hot, and at night it’s sometimes very cold. It is also difficult to find water in theSahara.

　　In 2006, Kevin Lin, Ray Zahab, and Charlie Engle decided to do something very difficult. They made the decision to run across the Sahara Desert 4,300 miles (6,920km). It seemed impossible to do, but they wanted to try. The three men liked to test themselves, and this would be a very big test.

　　On the morning of November 2, Kevin, Ray, and Charlie started their trip across theSahara. Every morning they began running at 5:00. At11 a.m. they stopped and rested until 5 p.m. Then they ran again until 9:30 in the evening. Each day they ran about 40 miles (64 km). Every day it was the same thing. They got up and ran. They listened to music on their iPods, and they ran and ran.

　　Kevin, Ray, and Charlie needed to eat a lot of food during their trip. Most people need about 2,000 calories of food each day. Kevin, Ray, and Charlie needed between 6,000 and 9,000 calories every day. That"s a lot of food! They also needed to drink a lot of water.

　　The three men had some problems on their trip, and many times they wanted to quit and go home. It was often very hot (140°F/60°C) during the day, and the heat made them sick. Their legs and feet hurt. Sometimes it was very windy, and they couldn"t see. One time they got lost. But they didn"t quit. After 111 days, Kevin, Ray; and Charlie successfully finished their trip across theSaharaDesert. They hugged each other and put their hands in the water of theRed Sea. Then they ran to a hotel to take a long shower.

　　词汇：

　　Stretch / strɛtʃ / v. 延伸，伸展

　　Calorie / "kælərɪ / n. 卡(路里)，小卡，大卡

　　Quit / kwɪt / v. 停止，放

　　注释：

Each day the men ran for approximately eight hours.

A:Right B:Wrong C:mentioned

Across the Deserts

　　The SaharaDesertis the largest desert in the world. It stretches across Africa fromSenegaltoEgypt. TheSaharaDesertis an unfriendly environment. During the day it"s very hot, and at night it’s sometimes very cold. It is also difficult to find water in theSahara.

　　In 2006, Kevin Lin, Ray Zahab, and Charlie Engle decided to do something very difficult. They made the decision to run across the Sahara Desert 4,300 miles (6,920km). It seemed impossible to do, but they wanted to try. The three men liked to test themselves, and this would be a very big test.

　　On the morning of November 2, Kevin, Ray, and Charlie started their trip across theSahara. Every morning they began running at 5:00. At11 a.m. they stopped and rested until 5 p.m. Then they ran again until 9:30 in the evening. Each day they ran about 40 miles (64 km). Every day it was the same thing. They got up and ran. They listened to music on their iPods, and they ran and ran.

　　Kevin, Ray, and Charlie needed to eat a lot of food during their trip. Most people need about 2,000 calories of food each day. Kevin, Ray, and Charlie needed between 6,000 and 9,000 calories every day. That"s a lot of food! They also needed to drink a lot of water.

　　The three men had some problems on their trip, and many times they wanted to quit and go home. It was often very hot (140°F/60°C) during the day, and the heat made them sick. Their legs and feet hurt. Sometimes it was very windy, and they couldn"t see. One time they got lost. But they didn"t quit. After 111 days, Kevin, Ray; and Charlie successfully finished their trip across theSaharaDesert. They hugged each other and put their hands in the water of theRed Sea. Then they ran to a hotel to take a long shower.

　　词汇：

　　Stretch / strɛtʃ / v. 延伸，伸展

　　Calorie / "kælərɪ / n. 卡(路里)，小卡，大卡

　　Quit / kwɪt / v. 停止，放

　　注释：

In the middle of the day: the men usually stopped running.

A:Right B:Wrong C:mentioned

Across the Deserts

　　The SaharaDesertis the largest desert in the world. It stretches across Africa fromSenegaltoEgypt. TheSaharaDesertis an unfriendly environment. During the day it"s very hot, and at night it’s sometimes very cold. It is also difficult to find water in theSahara.

　　In 2006, Kevin Lin, Ray Zahab, and Charlie Engle decided to do something very difficult. They made the decision to run across the Sahara Desert 4,300 miles (6,920km). It seemed impossible to do, but they wanted to try. The three men liked to test themselves, and this would be a very big test.

　　On the morning of November 2, Kevin, Ray, and Charlie started their trip across theSahara. Every morning they began running at 5:00. At11 a.m. they stopped and rested until 5 p.m. Then they ran again until 9:30 in the evening. Each day they ran about 40 miles (64 km). Every day it was the same thing. They got up and ran. They listened to music on their iPods, and they ran and ran.

　　Kevin, Ray, and Charlie needed to eat a lot of food during their trip. Most people need about 2,000 calories of food each day. Kevin, Ray, and Charlie needed between 6,000 and 9,000 calories every day. That"s a lot of food! They also needed to drink a lot of water.

　　The three men had some problems on their trip, and many times they wanted to quit and go home. It was often very hot (140°F/60°C) during the day, and the heat made them sick. Their legs and feet hurt. Sometimes it was very windy, and they couldn"t see. One time they got lost. But they didn"t quit. After 111 days, Kevin, Ray; and Charlie successfully finished their trip across theSaharaDesert. They hugged each other and put their hands in the water of theRed Sea. Then they ran to a hotel to take a long shower.

　　词汇：

　　Stretch / strɛtʃ / v. 延伸，伸展

　　Calorie / "kælərɪ / n. 卡(路里)，小卡，大卡

　　Quit / kwɪt / v. 停止，放

　　注释：

They sometimes felt sick because it was so hot.

A:Right B:Wrong C:mentioned

Across the Deserts

　　The SaharaDesertis the largest desert in the world. It stretches across Africa fromSenegaltoEgypt. TheSaharaDesertis an unfriendly environment. During the day it"s very hot, and at night it’s sometimes very cold. It is also difficult to find water in theSahara.

　　In 2006, Kevin Lin, Ray Zahab, and Charlie Engle decided to do something very difficult. They made the decision to run across the Sahara Desert 4,300 miles (6,920km). It seemed impossible to do, but they wanted to try. The three men liked to test themselves, and this would be a very big test.

　　On the morning of November 2, Kevin, Ray, and Charlie started their trip across theSahara. Every morning they began running at 5:00. At11 a.m. they stopped and rested until 5 p.m. Then they ran again until 9:30 in the evening. Each day they ran about 40 miles (64 km). Every day it was the same thing. They got up and ran. They listened to music on their iPods, and they ran and ran.

　　Kevin, Ray, and Charlie needed to eat a lot of food during their trip. Most people need about 2,000 calories of food each day. Kevin, Ray, and Charlie needed between 6,000 and 9,000 calories every day. That"s a lot of food! They also needed to drink a lot of water.

　　The three men had some problems on their trip, and many times they wanted to quit and go home. It was often very hot (140°F/60°C) during the day, and the heat made them sick. Their legs and feet hurt. Sometimes it was very windy, and they couldn"t see. One time they got lost. But they didn"t quit. After 111 days, Kevin, Ray; and Charlie successfully finished their trip across theSaharaDesert. They hugged each other and put their hands in the water of theRed Sea. Then they ran to a hotel to take a long shower.

　　词汇：

　　Stretch / strɛtʃ / v. 延伸，伸展

　　Calorie / "kælərɪ / n. 卡(路里)，小卡，大卡

　　Quit / kwɪt / v. 停止，放

　　注释：

On their trip across the desert: the three men ran through five countries.

A:Right B:Wrong C:mentioned

物重P，用细绳BA，CA悬挂如图所示，θ=60°，若将BA绳剪断，则该瞬时CA绳的张力为（）。

B:0.5P C:P D:2P

物重P，用细绳BA，CA悬挂如图4-59所示，θ=60°，若将BA绳剪断，则该瞬时CA绳的张力为（）。

B:0.5P C:P D:2P

如果函数在x=0处连续，则p、q的值为（）

A:p=0，q=0 B:p=0，q=1 C:p=1，q=0 D:p=1，q=1

图示梁，欲使C点的挠度为零，则P与q的关系为（）。

A:P=qL/2 B:P=5qL/8 C:P=5qL/6 D:P=3qL/5

就绪队列中有5个进程P1，P2，P3，P4和P5，它们的优先数和需要的处理机时间如下表所示。 假设优先数小的优先级高，忽略进程调度和切换所花费的时间。采用“不可抢占式最高优先级”调度算法，进程执行的次序是（）。

A:P2P3P4P1P5 B:P2P5P1P4P3 C:P3P4P1P5P2 D:P3P2P5P1P4