Dog's forebears paid for their size

Canines: The bigger they were, the faster prehistoric dogs in North America became extinct, on the heels of their favored foods.

Medicine & Science

December 01, 2003|By Dennis O'Brien | Dennis O'Brien,SUN STAFF

In the prehistoric world, being the Big Dog had its disadvantages.

A UCLA researcher says the big canines that roamed North America millions of years ago - some the size of grizzlies - went extinct quicker than smaller dogs. She blames their diet.

Blaire Van Valkenburgh's analysis shows that most species grew larger over time, but growth hurt survival rates when climate changes altered their habitats.

"Smaller dogs had a more varied diet and could survive on a wider range of foods," said Van Valkenburgh, a paleobiologist who studies prehistoric bones and presented her findings to the Society for Vertebrate Paleontology this fall.

Experts say her work could help define the debate about the threat that human activities pose to carnivores.

"These supercarnivores - the lions, tigers and wolves - are systematically being harassed by humans, and if you multiply that by a vulnerability because of a limited diet, it adds an urgency to the overall debate," said Craig Packer, an ecology professor at the University of Minnesota who studies lions in Tanzania.

Van Valkenburgh's findings also are consistent with widely accepted theories about what causes animals to disappear. A limited diet makes it more difficult for a species to survive, and among carnivores, big animals eat differently from smaller ones, scientists say.

In 1999, London Zoo researchers found that carnivores weighing 45 pounds or more generally take only prey at least their own size. Smaller carnivores will take prey less than half their size and are more likely to snack on plants, fruits and bugs.

"When you specialize, you're going out on an evolutionary limb, and as long as the limb stays there you're OK. But if the limb breaks, you're in trouble," said Anna K. Behrensmeyer, a paleobiologist at the Smithsonian Institution in Washington.

Dog populations reached a peak in North America about 30 million years ago, when 25 species roamed what is now the southern United States and western regions from Texas to Montana. Their survivors include only four species: the wolf, coyote, gray fox and red fox.

Although there are 150 breeds of domestic dog, they're all the same species, canis lupis familiaris. Recent genetic tests show they share a common ancestry with the wolf. Anthropologists believe dogs have been domesticated for at least 12,000 years.

No one knows for sure what killed off most prehistoric dogs. But Van Valkenburgh says a cooling trend that began about 30 million years ago killed off many of the animals that were the basis of their diets.

Thousands of prehistoric dog bones have turned up in the United States over the years. Van Valkenburg studies the teeth and jaws, which provide clues about their size and diet.

"When you have to start taking prey that's larger than yourself, that requires stronger jaws and sharper teeth," she said.

In her study, Van Valkenburgh focused on dogs known as hesperocyonines and borophagines. The former ranged in size from small foxes to large coyotes; the latter could be as big as large wolves, she said.

Both species grew larger over the eons, developed meat-only diets and eventually specialized in prey as big as or bigger than themselves.

But the hesperocyonines began growing bigger earlier - and paid the price, disappearing 15 million years ago. The borophagines had a 33 million-year run before they died out 2 million years ago.

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