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Eat More Meat, Go Extinct
24 October 2003 (All day)
ST. PAUL, MINNESOTA--Being top dog may not be so great after all. A new study of the fossil canids of North America shows that as dogs evolved to be bigger, they began to hunt larger prey--and tended to go extinct sooner. "It's a very clever blend of modern ecology and paleontology," says Christine Janis of Brown University in Providence, Rhode Island. The findings, presented here last week at the annual meeting of the Society for Vertebrate Paleontology, could have implications for conservation biology.
The study was inspired by a 1999 survey of living carnivores. It noted that carnivores bigger than 21.5 kilograms tend to eat differently. Although smaller carnivores can subsist on invertebrates and prey less than half their own size, larger predators hunt prey that's as big as they are, or bigger; this lifestyle, dominated by meat-eating, is known as hypercarnivory. The reason is likely that for a lion, mice don't have enough calories to be worth pursuing. Blaire Van Valkenburgh of the University of California, Los Angeles, wondered if this pattern held true for the past, and what long-term consequences might result from a picky diet.
Luckily for Van Valkenburgh, a detailed study of an extinct family of North American dogs had recently been published. As with many other ancient species, she found that in both subfamilies the dogs tended to evolve to larger sizes. Then she looked for signs of hypercarnivory, such as the depth of the jaw. That's known to correlate to larger prey size in modern dogs. It turned out that hypercarnivory was much more common among the larger dogs, whereas smaller ones showed signs of being omnivorous. This difference had a bearing on extinction. The hypercarnivore species went extinct after 5 million years or less. In contrast, species of dogs that took down smaller prey lasted for up to 12 million years.
"It's the clearest demonstration that hypercarnivorous dog species live a shorter time," says Lars Werdelin of the Swedish Museum of Natural History in Stockholm. One possible explanation, he says, is that competition may be stiffer for larger prey, which is less common than fruit, bugs, or mice. Given that so many hypercarnivores today are threatened by extinction, Werdelin says it's important to conduct research like this, which can help explain why they're vulnerable. "At the very least we need to know what aspects of extinction are natural and not caused by man," he says.