Twenty thousand years ago, a giant ground sloth the size of a large bear lumbered into a cave in Nevada and, well, went to the bathroom.

Its dung -- rich with the remains of the lilies and other plants it had been munching -- dropped to the floor of the cave. There, the softball-size lump joined thousands of others left by generations of sloths that had visited the cave.

In the millenniums since then, the dry air in the cave and relatively constant temperatures have preserved the deposits.

Now, for the first time, scientists braving an unrelenting barrage of bad jokes have used DNA sequencing to unlock the secrets of the dung from Gypsum Cave, near Las Vegas. They've identified the Ice Age animals that lived in the area, detailed the vanished ecosystems that supported them, and provided science with a valuable new tool.

"People called my Ph.D thesis my 'Ph.D feces,' " says Hendrik N. Polnar, 29, an American biologist working at the Max-Planck-Institute for Evolutionary Biology in Munich. There have been other, and worse, gags. Polnar shrugs it all off.

"Dung happens to be a great source of molecular information," he says. Not only can DNA analysis identify the animal that produced the droppings [from the DNA in intestinal cells sloughed off in the dung], it can also identify what it ate and the parasites that lived in its body, and suggest the climate it must have lived in.

"Scatology," as it is called, is no joke. It allows biologists to study the health and habits of animals without ever seeing or disturbing them in their natural habitat. It can produce results from droppings that are fresh, or millions of years old.

Paleontologists last month reported the discovery of a Tyrannosaur dropping in Saskatchewan. It had been fossilized, but microscopic studies revealed fragments of the chewed-up bones of the T. rex's last meal.

Polnar is analyzing DNA in dung samples sent to him from South America by a team sponsored by the British Broadcasting Corp. They are searching for evidence of a living giant ground sloth. Although paleontologists insist that the creatures are extinct, local people say some have survived in the Amazon rain forest.

With advances reported by Polnar's team today in the journal Science, scatology can now answer questions about dung that is neither fresh nor fossilized, just really old and dried-up.

Gypsum Cave and others like it in the Southwest have long attracted animals. Scientists believe the caves offered shelter from heat and cold. They may even have served as birthing rooms for some species.

The caves have yielded the bones of extinct sloths, llamas, camels and horses. But mostly researchers have found remarkably preserved lumps of ancient dung, called coprolites. For some reason, the vast majority were left by the plant-eating sloths.

"These were sloth toilets for 40,000 years," Polnar says. Rampart Cave in Arizona had 8,500 coprolites on the surface of the cave floor and more below. It has a low ceiling only because thousands of years of sloth dung gradually raised the level of the floor.

Modern sloths are cat-sized. Their Ice Age cousins, the extinct Gypsum Cave sloths, called Shasta ground sloths (Nothrotheriops shastensis), were "probably the size a pony or a big bear," Polnar says. The giant sloths of South America were as big as elephants.

The entire family of big sloths seemed to be doing fine. But about 10,000 years ago, just as the last glaciers began to retreat, the big sloths began a rapid decline toward extinction throughout the Americas.

Some paleontologists believe it is no coincidence that the big sloths -- like many other giant mammals of the Ice Age -- vanished from the Americas just as the first human big-game hunters arrived from Siberia.

Others, however, believe that the warming climate doomed these creatures. Polnar suspects it may have been a combination of hunting and climate change.

Scientists had long believed that the dung deposits in Gypsum Cave came from giant sloths, but they could not be sure. DNA sequencing techniques worked for modern dung, but failed whenever they were tried on dung that has sat around for thousands of years.

Polnar, assisted by Michael Hofreiter and Svante Paabo of the Max-Planck-Institute; Paul S. Martin, of the University of Arizona, and others, suspected the DNA might have been locked up by a chemical process called Maillard reaction. Their chemical analyses of the dung suggested they were right.

As the dung dried, sugar molecules in the plants consumed by the sloths began to fold up in a complex, extended reaction. It condensed the dung and sealed in a variety of proteins and DNA.

"It was a sandwich with a lead exterior, extremely durable," Polnar says. "What we needed was a key to unlock these sandwiches and let the DNA out."

By adding a chemical agent known from other applications to loose the chemical bonds formed by the Maillard reaction, Polnar and his colleagues found that they were able to free fragments of DNA.

Then, using conventional techniques (called polymerase chain reaction, or PCR) to isolate and copy the tiny bits of DNA, they were able to reproduce the strands in larger volumes. From that, they were able to make comparisons with the known genetic-code sequences for modern animals and plants.

They found close matches with modern two- and three-toed sloths, and even better matches with DNA fragments recovered from the bones and soft tissues of an extinct giant sloth from South America called Mylodon darwinii.

DNA sequencing of plant material in the dung produced matches with seven modern species, including capers, mustards, lilies, wild grapes, grasses and two desert plants -- yucca and agave.

Similar species are found today in Nevada, but at much higher altitudes than Gypsum Cave. The findings suggest that the Gypsum Cave area had an arid climate 20,000 years ago, but one cooler and wetter than the harsh desert that surrounds it today.

The great continental ice sheets were far away, but there were large lakes in Nevada, and an ecosystem that supported desert tortoises, camels, wild horses, kit foxes, llamas, coyotes and large predators called dire wolves.

"Now you're lucky to find snakes and some jack rabbits," Polnar says.

The wild grapes in the sloth's diet belonged to a family found today near springs and streams. The paleo-geology of the area suggests nearest springs were 6 to 12 miles from the cave. The sloths must have walked at least that far to dine on the grapes and drink.

Polnar says he has already used the new DNA techniques to link other ancient dung deposits with extinct species of goat and buffalo. He has begun work on some mammoth and mastodon droppings, and is eyeing reports of more material available in Africa and Europe, including feces in Gibraltar linked to Neanderthal man.

"There is going to be an explosion of interest in dung," he says.

Shasta ground sloth images can be found on the Web at www.lam.mus.ca.us/page/exhibits/fossils/ ' mammals-contents.html.

Pub Date: 7/17/98