Aging and Cancer / Zoo Animals Missing Out / Largest Rodent / Rats Cloned

I'm Bob Doughty with Sarah Long, and This is SCIENCE IN THE NEWS, from VOA Special English.

This week -- researchers try to learn why the risk of cancer rises with age ... a new theory about what some animals in zoos are missing ... a report about the largest rodent that ever lived ... and, cloned rats are here at last!

Studies of yeast may help scientists better understand why the risk of cancer increases as people get older. Yeast cells do not get cancer. But they could be a useful tool in the study of genetic changes that happen as human cells age.

Yeast is a one-celled fungi. It is found in soil and on plants. It is also found on the skin and in the organs of warm-blooded animals. Some kinds of yeast cause infections. But others are used to make bread and wine.

The magazine Science recently published findings by researchers at the Fred Hutchinson Cancer Research Center in Seattle, Washington. They found strong similarities in the changes that take place as genes age in both humans and yeast.

Daniel Gottschling (GOTCH-ling) led the study. Graduate student Michael McMurray assisted. The researchers observed several kinds of simple baker's yeast.

Most yeast cells survive for about thirty or thirty-five cell divisions. The whole process takes about five days.

Cells divide to produce new ones to keep an organism alive. Scientists talk in terms of mother cells and daughter cells. Each generation is represented by a mother cell's production of a new daughter cell.

The researchers found that genetic mistakes started to happen as the daughter cells reached the equivalent of middle age. Doctor Gottschling says these changes could damage the ability to control cell growth. Cancer is uncontrolled cell growth.

The researchers found that the damage always happened around the twenty-fifth generation of cell division. The researchers hope to find some way to turn off whatever "switch" causes the genetic changes.

Getting older is considered the biggest risk factor for cancer. Middle age is when humans start to develop genetic changes that can lead to the disease. But researchers note that it will be much more difficult to find out how the process works in people.

The American Cancer Society says almost eighty-percent of cancers are discovered after the age of fifty-five. It says men face a fifty-percent chance of developing cancer after they reach late middle age. Women have a thirty-five percent chance.

Recent findings show that animals that normally live in wide open areas have the most problems in zoos. Nature magazine published a report by two researchers at Oxford University in England. Georgia Mason and Ros Clubb studied more than one-thousand reports by other scientists. These dealt with observations made at five-hundred zoos worldwide in the past forty years.

The researchers say animals such as polar bears, lions, tigers, and cheetahs are more likely to show signs of boredom in zoos. The signs can include walking back and forth, over and over. In addition to pacing, these animals also have higher death rates among their babies compared to other zoo animals.

Polar bears move within areas as large as eighty-thousand square kilometers. Zoos, however, usually limit them to an area one-millionth of that size.

In the past, other scientists have suggested that the problems are because meat-eating animals in zoos are not able to hunt. The new report says the problem is because their territory is restricted.

The researchers found the situation better for animals that normally need less territory. Grizzly bears, Arctic fox and American mink were some of the animals that had lower death rates among their babies. Their pacing rates were also lower than among the animals that normally move in wide open areas.

A number of zoos in North America and elsewhere are taking steps to provide more room and more activities for their animals. But Georgia Mason at Oxford University says most zoos need larger, more complex living areas. She says another solution is for zoos to replace animals, when they die, with other kinds that need less space to move around.

Scientists have been studying what they say is the most complete skeleton of the largest rodent that ever lived. They estimate that the animal weighed about seven-hundred kilograms. That is about ten times the size of the South American capybara, the largest rodent alive today.

The discovery is expected to help scientists better understand what the northern part of South America was like millions of years ago.

Scientists are calling the ancient animal Phoberomys pattersoni. They link it to modern guinea pigs, an animal small enough to hold in one hand. They believe that the huge rodent lived about eight-million years ago. Scientists first described it more than twenty years ago. At that time, however, they had only small pieces of bone or teeth to study.

Now, Science magazine has published the results of a new study of ancient fossil remains. These include the nearly complete skeleton discovered in a desert area of western Venezuela three years ago.

Scientists recovered the skeleton near the town of Urumaco, about four-hundred kilometers west of Caracas. They say fossils of large fish, crocodiles and other creatures also were found in the area.

Marcelo Sanchez-Villagra is a researcher at the University of Tubingen in Germany. He says tests of soil suggest that the area near Urumaco was rich in water and plant life. He says the rodent probably ate grasses and could swim.

Phoberomys belongs to a group of rodents called caviomorphs. Scientists have evidence that these developed in South America about forty-million years ago. At that time, South America was not connected to any other continent.

A land bridge formed between North and South America only about three-million years ago. One theory is that the rodent might have been killed off by animals that came from North America.

Science magazine also published a commentary by McNeill Alexander of the University of Leeds in Britain. He notes that small rodents like mice have short legs -- but they also have big hands. These are good for digging holes to escape attackers.

Mr. Alexander writes that large mammals can dig holes too. But they have long legs and generally escape by running. He wonders if large rodents like Phoberomys were just too slow for their own good.

We just talked about an ancient rodent the size of a cow or buffalo. Now let us talk about a better known rodent. Lots of people think of rats as ugly and dirty animals that spread disease and eat food meant for humans.

Now scientists have found a way to help rats reproduce. Not that rats need any help. But medical scientists have been trying for some time to create more of the rats they use in laboratories. These provide a way to study drugs and disorders before doing tests on people.

Researchers at the National Institute of Agricultural Research in France succeeded in cloning rats. They published their work in Science.

Scientists had already cloned sheep, pigs, cows, cats, mice and horses. But no one had produced a genetic copy of a rat before. Their reproductive eggs age too quickly for scientists to make changes to the nucleus.

The French scientists, however, were able to halt the activation process in the eggs. They removed the genetic material from the nucleus and replaced it with DNA from an adult rat cell. The result was more than one-hundred live embryos. These were placed into two female rats. Three male rats were born. One of these later died.

The researchers repeated the experiment and produced two healthy females. Then they mated the cloned males and females and produced healthy babies.

The next step is to genetically design rats for studies of human diseases. The French team plans to put a human gene into a group of cloned rats. Then they plan to use the animals to study possible treatments against diseases linked to that gene.

SCIENCE IN THE NEWS was written by George Grow, Nancy Steinbach, Chi-Un Lee and Cynthia Kirk, who was also our producer. This is Bob Doughty. And this is Sarah Long. Join us again next week for more news about science in Special English on the Voice of America.