Over 50 million years a succession of large carnivores evolved in North America, diversified, and then died out.
Researchers studying canids - the family that includes dogs and wolves - report in Thursday's issue of the journal Science that as these predators grew larger they tended to become more specialized, making them more vulnerable to extinction.
"What happens is, for a variety of reasons, predators tend to get large," explained Blaire Van Valkenburgh of the University of California at Los Angeles. Being large is advantageous, being the dominant animal at feeding time, she explained.
But, she went on, as meat-eating animals become larger they are forced to prey on animals larger than themselves - catching little animals like mice and rabbits requires more energy than it's worth.
Once they cross that line and start to prey on larger animals, the predators develop larger jaws and teeth and generally become more specialized. In addition, they need bigger ranges, Van Valkenburgh explained in a telephone interview.
Needing larger ranges and eating only limited prey in the long run makes them vulnerable when conditions change, resulting in periodic disappearance of these large carnivores.
Even today, she noted, large predators like tigers and wolves are often found on endangered species lists.
The series of species studied by Van Valkenburgh evolved before the arrival of people, so the extinctions cannot be blamed on humans, she noted.
Such is not the case for animals that disappeared more recently, according to a separate paper in the same journal.
In that report, paleobiologist Anthony D. Barnosky and colleagues studied large animal extinctions that occurred between 50,000 and 10,000 years ago, an era called the late Pleistocene. These extinctions included animals such as mammoths and mastodons, the saber-toothed cat, ground sloths and native American horses and camels.
While many researchers have blamed humans for those extinctions, the team led by Barnosky, of the University of California, Berkeley, said humans and climate change share the blame.
"Humans and climate change were the one-two punch that drove extinction between 50,000 and 10,000 years ago, and the same thing is happening in a major way today," Barnosky said in a statement.
Climate change is occurring more rapidly today than in the late Pleistocene and growing human populations are taking over former large-animal habitats, Barnosky said.
By The Associated Press