VOA慢速英语 2007 0612b（在线收听）

VOICE ONE:

This is SCIENCE IN THE NEWS, in VOA Special English.  I'm Barbara Klein.

VOICE TWO:

And I'm Steve Ember.  On our program this week, we will tell about an animal that can reproduce without mating.  We also will tell about a Swedish man who developed a naming system for all living things.  And, we answer a listener's question about weight loss. 

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VOICE ONE:

 
File photo of a hammerhead shark, the kind involved in the first known case of asexual reproduction in sharks.
The birth of a shark in the United States has been confirmed as the first case of a female shark reproducing without a male shark.  Scientists from Florida, Nebraska and Northern Ireland studied genetic material taken from a baby hammerhead shark.  The baby shark was born six years ago at a zoo in Omaha, Nebraska.  At the time, its mother had been living without male sharks for three years.

Another fish was said to have killed the baby shark shortly after it was born.  Recently, tests showed the baby had no genetic material from a male shark.  The findings were reported in the publication Biology Letters.

VOICE TWO:

At the time of the birth, researchers believed that the mother had possibly used reproductive fluid received from a male shark years earlier.  Female sharks are able to store such fluid from male sharks.  However, no shark has been known to do this for several years.

Instead, the scientists found the mother’s own genetic material combined when her egg was produced.  This form of reproduction involving only one animal is called parthenogenesis.  It is also known as asexual reproduction. 

VOICE ONE:

Asexual reproduction has been known to happen in some animals, including snakes and lizards.  But it has never been confirmed in mammals.  The shark's birth was the first time asexual reproduction had been observed in a shark.  It has yet to be observed in animals that live in the wild. 

Scientists say the discovery may also explain growing numbers of sharks born in captivity without males present.  However, genetic material from these sharks must also be tested to confirm asexual reproduction.

VOICE TWO:

Scientists now wonder if animals living in the wild also reproduce in this way.  Many shark populations are decreasing because people are killing too many of the animals.  The asexual form of reproduction could help population numbers.  Yet scientists say it could also be harmful for the species itself. 

Biologists say that sharks born asexually with only their mother’s genetic material will have less chance of surviving than other sharks.  Genetic material from a male and female helps living creatures to better deal with disease and other threats.  If sharks in the wild are reproducing asexually, their young will be genetically weaker than those produced sexually. 

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VOICE ONE:

 
Carl Linnaeus
Last month, scientists around the world celebrated the birthday of an important man in the history of science.  His name was Carl Linnaeus.  He was born three hundred years ago in Sweden. 

Carl Linnaeus is remembered for developing a system of scientific names for all the living things on Earth.  Experts say his system continues to influence the way people think about the natural world.

VOICE TWO:

Linnaeus was a medical doctor.  He also was very interested in plants.  In seventeen thirty-five, he produced a book that listed all the known plants in the world by their sexual parts.  The book was called "Systema Naturae," or "System of Nature." 

Linnaeus later published two more books.  They proposed a system of dividing and ordering plants by groups.  These publications listed and ordered all the known plants and animals in the world.  That was more than seven thousand kinds of plants and more than four thousand kinds of animals.

Linnaeus continued to make changes in his system and publish books describing them.  The tenth version of "Systema Naturae" was published more than twenty years after the first one.  The naming system he described in that book is the one that has been used ever since by scientists.

VOICE ONE:

The system that Linnaeus used to organize all living things started with the largest group, called a kingdom.  He divided all living things into one of three kingdoms: plant, animal, or mineral.  Members of each kingdom were then placed into increasingly smaller groups.  Linnaeus' system was the first to place human beings in the same group as animals that walk on two legs.

The main part of the system that survives today combines two groups that have biological meaning.  It is the two-word description of an organism based on its physical appearance: the genus name and the descriptor.  A good example is the expression Linnaeus used to describe human beings and that we continue to use -- homo sapiens. 

VOICE TWO:

Scientists have been making changes to his naming system for more than two hundred years.  Today, some experts want to re-organize the system.  They say this is needed because of the scientific progress that has been made since Linnaeus' time.  For example, biology experts want changes because knowledge of genetic material has created much new information. 

Some people are calling for a system that would group organisms with a common history.  But others feel there is no real agreement as to how to place new discoveries in groups.  They say Linneus' system has been used for so long that it would be very difficult to change.

Scientists say the main reason the system survives is because it is simple.  They say its use makes it possible for persons who speak different languages to understand each other, and agree on what they are talking about.  The system also makes it easy for scientists today to identify all the ten million known species of plants and animals.  That is many more living things than were known during Carl Linnaeus' lifetime.          

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VOICE ONE:                              

Finally, a listener has written from China for advice about how to lose weight.  Michael in Shanghai says he is twenty-six years old and has been severely overweight for most of his life.

Doctors say this condition, also known as obesity, is complex.  A doctor may advise medical interventions in addition to changes in behavior.  But experts say the most successful weight-loss plans include a well-balanced diet and exercise.

VOICE TWO:

People who want to avoid weight gain have to balance the number of calories they eat with the number of calories they use.  To lose weight, you can reduce the number of calories you take in, or increase the number you use, or both.

Experts at America's National Institutes of Health say a person wishing to lose weight should have an hour of moderate to intensive physical activity most days of the week.  This could include fast walking, sports or strength training.

You should also follow a nutritious eating plan and take in fewer calories than your body uses each day.

VOICE ONE:

A recent study looked at four of the most popular dieting plans in the United States.  Researchers at Stanford University in California studied more than three hundred overweight women.  Most of the women were in their thirties and forties.

Each woman went on one of the four plans: Atkins, The Zone, Ornish or LEARN.  The women attended diet classes and received written information about the food plans.

At the end of a year, the women on the Atkins diet had lost the most -- more than four and one-half kilograms on average.  They also did better on tests including cholesterol levels and blood pressure.  The Journal of the American Medical Association published the findings. 

VOICE TWO:

Christopher Gardner led the study.  He says the Atkins diet may be more successful because of its simple message to lower intake of sugars.  Also, he says the advice to increase protein in the diet leads to more satisfying meals.

He says there was not enough money to also study men, but that men would probably have similar results.

VOICE ONE:

Another report suggests that only a small minority of people have long-term success with dieting.  Researchers at the University of California, Los Angeles, medical school found that most dieters regained their lost weight within five years.  And often they gained back even more.  But those who kept the weight off generally were the ones who exercised.

The report was based on thirty-one studies.  It was published in the journal American Psychologist. 

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VOICE TWO:

This SCIENCE IN THE NEWS was written by Brianna Blake, Nancy Steinbach and Caty Weaver.  Our producer was Mario Ritter.  I'm Steve Ember.

VOICE ONE:

And I'm Barbara Klein.  Join us again at this time next week for more news about science in Special English on the Voice of America.