Many people also know him as "G.S." in Peter Matthiessen's memoir, The Snow Leopard, about a winter trek the author took with Schaller through Nepal in 1973.
But for more than 20 years political instability in Tajikistan and Afghanistan prevented Schaller from surveying the wildlife there.
In 2003 he was able to join several local biologists on a Marco Polo sheep survey in Tajikistan. They found that the sheep population there might have shrunk to as few as 10,000 animals.
Schaller says indiscriminate hunting by poor livestock herders, as well as border officials and the military, is to blame.
"Local people have nothingthey have to sell livestock to get food," he said.
"To get meat, they go out and hunt Marco Polo sheep."
The severe toll that hunting has taken on the Marco Polo sheep again became clear when Schaller finally got to visit Afghanistan's Wakhan Corridor in late 2004.
In pursuit of the massive animals, which can weigh more than 300 pounds (135 kilograms), Schaller and his colleagues often observed only the flash of white rumps as the sheep raced out of sight.
The sheep, which were first reported by Marco Polo in 1273, prefer open terrain. When faced with danger, they don't run to hide under cliffs but instead take off across the rolling hills.
The weather also proved problematic for Schaller's expedition. After exploring 13 valleys in the Pamirs' lower regions, Schaller led his group across a 16,700-foot (5,100-meter) pass in knee-deep snow to continue the survey in the range's higher reaches.
But the local people aiding the expedition did not want to go because of the deep snow, and the team had to turn around.
In total, Schaller counted 624 Marco Polo sheep in 3,300 square miles (8,550 square kilometers) of the Pamirs.
He says that perhaps a thousand animals live on the Afghan side of the mountains.
That is less than half of the sheep estimated to have been there when the area was last surveyed in 1973 by Canadian biologist Ronald Petocz.
"We saw fewer sheep in most areas, and in some places they had vanished," Schaller said.
"There were various valleys where they used to be where the local people said they don't go anymore."
But Schaller says the sheep are not yet endangered, and he believes the population can be sustained through proper management.
"The rangeland is in reasonable condition, especially in Tajikistan and Afghanistan," he said.
"The only thing that is hampering things is a lack of protection."
There are encouraging signs, Schaller reports.
He and his Chinese colleague Aili Kang began a Marco Polo sheep survey on the Chinese side of the Pamirs last fall. They counted 2,175 animals before they had to cut the census short because of heavy snow.
Kang will finish the survey this spring. But there's no doubt that the sheep population in China has seen a healthy increase in the last two decades, Schaller says, due to China's antipoaching efforts.
Male Marco Polo sheep have the longest horns of any sheepthe world record is 190 centimeters (6.2 feet)making them highly seductive to hunters.
"They hold an almost mythical status for trophy hunters," Schaller said.
"Trophy hunters in China or Tajikistan will pay [U.S.] $25,000 or more to shoot one."
Schaller says he "personally would not care to shoot something just to have an ornament on the wall," but he believes trophy hunting can be a viable tool to help protect the sheep from extinction.
"Trophy hunting could provide considerable income for these countries," he said.
"If the money were spent on conservation and helping local communities, it would be really valuable."
But the hunting economy in the region remains small. Tajikistan currently sells from 40 to 60 hunting licenses a year. In Afghanistan, President Hamid Karzai has banned all forms of hunting for five years.
Schaller has been advocating the establishment of an international peace park in the region, which would incorporate four countries.
The park would help protect not only the Marco Polo sheep but the many other animals that wander across international bordersSiberian ibex, snow leopard, wolves, and bears.
Schaller says officials in Afghanistan, China, Tajikistan, and Pakistan have all endorsed the principle of such a project.
"I'd like to see each country deciding how it wants to use the land," he said.
"Some land can be a strict reserve, some can have hunting concessions.
"The most important thing is that there is proper rangeland management so that these animals are protected while traditional lives there continue, and everyone benefits."
"Making this park a unit and getting the countries to communicate problems and exchange information could be a real benefit," Schaller said.
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