Say that you want to track down, once and for all, why all those massive Pleistocene mammals disappeared after the end of the last Ice Age.

What really happened to the mastodons, mammoths, giant bison and other megafauna that had thrived in ice-free areas such as Beringia up until about 10,000 years ago?

Where did the predators go -- the short-faced bears and the sabre-toothed tigers as well as the human hunters, the paleo-Indians with their beautiful fluted stone points?

Well, when you really have a mystery, bring in the professionals -- Skylark Holmes and Dr. Janet Watson in this case. These Ice Age sleuths have neatly dispatched the existing theories of climate change and human overkill, and surmised that this case really hinges on arboricidal megaherbivores.

In lay terms, that would translate into "tree-killing big plant-eaters."

And if this mystery story still remains murky, it might be necessary to bring in a more academic angle. "The Case of the Arboricidal Megaherbivores" is an invention of Elin Whitney-Smith, a geobiologist/anthropologist at George Washington University in Washington, DC.

She has used the mystery approach as an entertaining way to explain the background to the Pleistocene extinction story, the problems with the existing theories, and her alternative hypothesis, which she calls Second Order Overkill.

For her more skeptical colleagues, she has also developed a computer model that can simulate the scenarios that she proposes.

Whitney-Smith presented her hypothesis at the annual meeting of the Canadian Quaternary Association, which was held in Whitehorse in 2001. Quaternary refers to the present geologic period and includes the Pleistocene Epoch, the time of the last Ice Age.

Whitney-Smith -- and her two detective colleagues -- have problems with both of the existing hypotheses used to explain why the Pleistocene megafauna disappeared. The climate change hypothesis assumes that mammals could not adjust to the warmer temperatures that ended the last Ice Age, and they disappeared along with the glaciers.

"The reason I'm disregarding climate change is that we know that these animals got through other changes in climate," she says.

Mammoths, sloths, mastodons and other animals that went extinct had survived similarly warm periods during previous interglacials. Also New World horses that went extinct at the end of the Ice Age thrive in the same climate today.

Her model does address the overkill hypothesis, but with a significant twist. In the traditional version humans sweep into the New World and hunt the herbivores to extinction, and then the carnivores die out because they have nothing left to eat.

Whitney-Smith says this hypothesis flies in the face of everything we know about traditional predator-prey models, in which predators decline in synchrony with their prey. She thinks that human predators took a cold hard look at the competition, and decided that the only good carnivore was a dead carnivore.

She thinks that Homo sapiens kicked off the wave of extinctions by killing the predators that had kept the numbers of herbivores in check. The herbivore populations then boomed, changing the landscape around them by overgrazing and knocking down trees to get at the remaining leaves at the top.

The landscape at this time was park-like with mixed trees and grass. Whitney-Smith points to African studies that prove the huge impact that large herbivores like elephants can have on a mixed woodland environment.

"If elephants can turn mixed woodland into grassland, I propose that mammoths can do the same thing. Classically we have assumed that the environment shapes animal evolution, but I am suggesting that we have it backwards."

As the vegetation changed, the animals started to disappear. Animals such as mastodons that fed on the slow-growing trees would have been the first to go, followed by grass- eaters such as mammoths and sloths.

The ruminant herbivores fared the best as they could efficiently extract the maximum amount of nutrition out of the minimum amount of poor quality forage. Small animals that could get by on a minimum amount of food would also have survived.

Generalist carnivores and scavengers such as wolves and smaller cats would have survived, while the specialists who depended on the megafauna would have followed their prey into extinction.

And what happened to the paleo-Indians that kicked off this whole chain of events? They entered a sort of dark ages, scattering into small isolated groups, no longer with the resources and the leisure time to make their beautiful fluted points.

Whitney-Smith's model simulates the interrelationships between different types of vegetation, different types of herbivores, carnivores and humans. She says it shows that people could easily have set off the extinctions by killing their animal competitors.

"You do not have to kill a lot of carnivores to set off extinctions," she says. "I'm talking about a rate where a quarter pound of carnivores are killed for every one pound of humans. It would have taken 10 to 12 people to kill a big cat."

Her research is primarily modeled on the Prairie environment, where she thinks bison played a big role in maintaining grasslands once the trees were gone. She thinks that arboricidal megaherbivores could have played the same role in the North, breaking up any permafrost and helping to form well-drained soils that would have maintained productive grasslands.

She expects that holes will be found in her hypothesis, and invites the scrutiny from other scientists and researchers. After all, Holmes and Watson did not always work alone either.

The full detective story can be found at www.quaternary.net. The model can be downloaded from this site, and it is also available on CD. Elin Whitney-Smith can be contacted at elin@quaternary.net.