Science Digest

Virus in Chimps Tied To AIDS-Like Illness

Chimpanzees infected with the simian immunodeficiency virus (SIV), the precursor of HIV, do develop an AIDS-like illness, according to a study published in last week's Nature. Scientists previously thought that SIV did not cause AIDS.

Beatrice Hahn of the University of Alabama at Birmingham and colleagues studied 94 chimpanzees, 17 SIV-positive and 77 not, in the Gombe National Park in Tanzania for nine years. They found a tenfold to sixteenfold higher death rate among the SIV-positive chimps. Also, the SIV-positive females were less likely to give birth and had a higher infant mortality rate than uninfected females. The scientists performed noninvasive testing, using samples of the monkeys' feces to detect SIV.

In a necropsy of a female who died three years after acquiring SIV, researchers found that she had depleted T-cells and a destroyed immune system, which are both hallmarks of end-stage HIV.

More than 40 strains of SIV infect primates in Africa; two of these have jumped from chimpanzees to humans. HIV crossed the species barrier less than 100 years ago.

Researchers have rarely observed SIV progressing to AIDS in chimpanzees in captivity. Noah, the first chimpanzee to be identified with SIV, is still alive and healthy nearly 20 years later. When SIV does progress to AIDS, it does so much more slowly than HIV -- one sooty mangabey in captivity took 18 years to develop AIDS after naturally contracting SIV.

"The finding that [SIV] is pathogenic in Gombe provides a unique opportunity to compare disease-causing mechanisms of two closely-related viruses in two closely-related species," Hahn wrote.

-- Rachel Saslow

Cloud Cover and Climate Change

Global warming leads to fewer clouds, fewer clouds lead to more sunlight, and more sunlight leads to global warming, according to a new study in Science. Amy Clement of the University of Miami and colleagues observed this vicious cycle in cloud and climate data from 1952 to 2007.

"What we're interested in is what that means for future climate change," Clement said.

Current climate change models predict that the Earth will warm by 1.5 to 4.5 degrees Celsius by the end of the 21st century. A primary reason for the wide range -- which has severe consequences at the upper end -- is scientists' uncertainty about cloud patterns.

To find out whether low-level clouds, which have a cooling effect on the climate, increase or decrease with warmer temperatures, the researchers studied cloud-cover data as recorded by human eyes and satellites over the Pacific Ocean near Baja California and paired it with weather data, including sea surface temperatures. They found natural climate shifts in 1976 and the late 1990s -- a warming and a cooling, respectively -- which are explained by a known pattern called Pacific decadal shifts. During the 1976 warming, the clouds dissipated and during the 1990s cooling, the clouds increased.

The scientists then matched the data with 18 climate change models, looking backward to see which models would have best tracked the actual cloud and climate data. The most accurate was Britain's Hadley Centre for Climate Prediction and Research's model, which predicts that the Earth will warm by about 4.5 degrees Celsius by 2100, Clement said.

-- Rachel Saslow