The gargantuan mastodons, mammoths, and giant ground sloths that once roamed North America seem much too big for most prehistoric predators to contend with. But a new study suggests that the lions and saber-toothed cats that once roamed North America did indeed attack these beasts and may have significantly reduced their populations. If so, these carnivores may have had a much more dramatic impact on ancient ecosystems than previously believed.

“This is a landmark paper because it really puts together an unusually broad range of lines of evidence on this issue,” says John Damuth, paleobiologist at the University of California (UC), Santa Barbara, who was not involved in the study.

The scientific thinking that large herbivores are immune to predators goes back to Charles Darwin. In his 1859 Origin of Species, the famed naturalist wrote that “… with the elephants and rhinoceroses, none are destroyed by beasts of prey. Even the tiger in India most rarely dares to attack a young elephant protected by its dam.” But Blaire Van Valkenburgh wasn’t so sure. “I just said, no, I don’t believe that,” says the UC Los Angeles paleoecologist and lead author of the new study. Predators might think twice before attacking an adult mammoth, she theorized, but a baby could have been a different story.

To find out what ancient predators were capable of, Van Valkenburgh and her colleagues first had to figure out just how big the large herbivores and carnivores were that lived during the Pleistocene epoch, 12,000 to 2.5 million years ago. The researchers used thousands of measurements of fossil bones and compared them to the bones of modern elephants to infer the range of body masses of both adults and juveniles of the different species of extinct megaherbivores, including mammoths and mastodons. They used a similar method to infer the size ranges of extinct carnivore species, comparing saber-toothed cats to modern tigers, for example.

The analyses indicated that the juvenile megaherbivores weighed from 200 to 2000 kg. Adults weighed several tons; estimates for some adult males exceed 10,000 kg. The prehistoric carnivores ranged from 150 kg to more than 400 kg.

So could ancient carnivores have actually taken down these herbivores? To answer this question, the researchers compiled information from thousands of records of prey killed by big modern carnivores such as lions and tigers hunting solo and in groups—a task that took several years. Conservation ecologist and co-author Matt Hayward of Bangor University in the U.K. used these data to develop a mathematical model to predict typical and maximum prey size for any given carnivore.

Finally, the researchers needed to determine how protected young mega-herbivores were. “There’s a sweet spot in there, from the predator’s standpoint, where the juveniles can wander away from mom frequently enough and far enough to be accessible, and where they are of a size small enough that they can actually be tackled,” explains co-author Louise Roth, a biologist at Duke University in Durham, North Carolina.

For this, the researchers relied on earlier studies of the chemical signature in the enamel layers that are added annually at the base of herbivore tusks as they grow out from the skull. The chemistry of the tusks changes when the animal’s diet switches from mother’s milk to vegetation. The researchers assumed that baby herbivores stayed close to mom while nursing, but wandered farther away once they began to forage for themselves. The team found that there was indeed a window when juvenile mammoths and other herbivores were weaned but were still small enough to be hunted by lions and other predators. “A solitary saber-toothed cat could have taken young mammoths aged 2 to 4—sometimes up to age 9—and a pride would have been able to take adult females,” says Roth.

All told, lions, wolves, and saber-toothed cats could have killed about 17% of young mastodons and other mega-herbivores, the team reports online today in the Proceedings of the National Academy of Science [url tk]. Although the researchers do not have direct evidence of predation on the bones of the extinct herbivores, for studies of prehistoric ecosystems, “getting smoking gun evidence is always difficult,” notes Kathleen Lyons, paleobiologist at the Smithsonian Museum of Natural History who was not involved in the study. But the evidence that Van Valkenburgh and her coauthors brought together to make the case is convincing, she says. “It’s a really nice, solid, piece of work.”

These predators didn’t just have dramatic impacts on prey populations—they could have had cascading effects on the entire ecosystem, Van Valkenburgh says. “Large carnivores provide carcasses that feed a whole array of smaller species like eagles, coyotes, and foxes,” she says. And keeping the megaherbivore populations in check meant that there was more vegetation for small mammals and birds. The predators might even have indirect effects on river ecosystems, she says, because the banks of the rivers were not being denuded by megaherbivores and eroding rapidly. “So aquatic species like beavers and otters did well and fish were abundant.”

The work may have important implications for today’s conservation efforts, Damuth says. “It’s important to know that the current ecological communities differ in critical ways from those in which those herbivores evolved,” he says. Modern megaherbivores like elephants and rhinoceroses are threatened, and “if we want to manage them, we have to understand how they worked naturally, as opposed to how they might be working under current conditions.”