Science & Technology

Here's Why Coyotes and Cougars Outlived Dire Wolves and Saber-Toothed Cats

Imagine taking out your trash at night and startling a pack of giant dire wolves, or going on a hike and running into a snarling saber-toothed cat and her kittens. Luckily, that's not our reality. It's scary enough to encounter a bobcat or worry about keeping your pets safe from coyotes. But considering that ancestors of those small predators lived at the same time as the massive ones, why aren't they all living among us today? Why did coyotes survive and dire wolves die out? A new study published in Current Biology says a lot of it comes down to what they ate. When it comes to a predator's diet, it doesn't pay to be a picky eater.

Dragged Through the Mud

For the study, paleontologist Larisa DeSantis and her team turned to a treasure trove of fossil remains: the La Brea Tar Pits. For 50,000 years, this was a place where natural asphalt bubbled up from beneath the Earth's surface and trapped unlucky animals for eternity — or at least until scientists had a chance to dig them up. More than 3.5 million fossils have been found there in the century or so we've spent excavating, and 90 percent of the mammals discovered have been carnivores.

For a paleontologist like DeSantis, that's huge. "Typically, carnivores are rare. Carnivores are rare in the landscape today, and carnivores are rare in fossil localities," she told Curiosity.

As you might expect, you generally get the same pyramid-shaped ratio of organisms in fossils as existed in the landscape: few carnivores, more herbivores, and tons of plants. But in La Brea, that pyramid is turned on its head. That's because it essentially acted as a carnivore trap. Herbivores would get caught in the gooey muck, giving carnivores what they thought was an easy meal — until they got stuck themselves.

Add up all the carnivores, herbivores, plants, and other critters that met their untimely demise at La Brea, and you get a truly breathtaking spread of fossil evidence. That evidence is especially important because it spans the mass mammal die-off known as the late Pleistocene extinction event. Scientists are still untangling the causes of this event, but they know that the climate was warming and humans were entering North America, and those elements likely caused major disruptions to the ecosystem. Studying how carnivores responded to a warming planet during that period might tell us more about how to help carnivores survive climate change and human impacts today.

You Are What You Eat

DeSantis and her team suspected that diet might be the reason why smaller carnivores survived this extinction event while the big ones didn't. To find out, they collected fossils of coyotes, cougar, dire wolves, and saber-toothed cats that lived at different points over time and looked for two things in their teeth: wear patterns and carbon isotopes.

By analyzing the microscopic patterns on the fossil teeth, scientists can figure out whether a carnivore mostly ate fresh meat or tended to scavenge on bones. Looking at carbon isotopes — different types of carbon atoms, basically — in the enamel gets even more in-depth: the type of carbon in the enamel tells scientists specific things about what that animal ate.

"Everything you eat is incorporated into your fingernails, into your hair, into your blood, into your liver and your kidneys, into your bone, and also in your teeth," DeSantis said. "The great thing about teeth, as opposed to say blood or organs or fingernails, is that it essentially locks in that chemical signature based on different carbon ratios."

That's because, unlike bone, which degrades as it mineralizes into a fossil, teeth are mostly mineral to begin with. That means that the type of carbon paleontologists find in enamel is the same type that was incorporated into that enamel when the animal was alive. The teeth of herbivores take in different carbon isotopes depending on the animal's choice of plants, which in turn contain different carbon isotopes depending on the way they photosynthesize. Forests are dominated by plants that photosynthesize in one way, while grasslands are dominated by plants that have adapted to use a different method. If a carnivore hunts those herbivores in open fields rather than dense forest, that particular flavor of carbon isotope will get into its teeth, too.

So what did they find? First of all, while past studies that analyzed bone collagen had concluded that sabertoothed cats and dire wolves competed for the same food, the tooth enamel data demonstrated that wasn't the case. Instead, the big cats kept to the forest and the wolves hunted in grasslands.

But these big predators were highly specialized. Saber-toothed cats had evolved to take down large prey, and those big teeth wouldn't have been as effective at pivoting to smaller prey as their main food source became scarce. The dire wolves, for their part, likely took down large herbivores like bison and horses — and these megafauna also went extinct. Gray wolves, however, were smaller than dire wolves and may have already been eating smaller prey. "A wolf is a wolf," DeSantis said. "Before the extinction and after the extinction, they basically did the same thing."

But smaller cats and dogs were less discerning in their diets. Extinct cougars had identical carbon isotope ratios in their teeth to those of modern pumas, which are known to be opportunistic hunters that will dine on anything from armadillo to elk.

But the researchers were surprised by the coyote fossils: They weren't the opportunistic hunters they are today. They were once much more specialized. "We know they were larger during the Pleistocene and became smaller more recently," DeSantis says. "So what this suggests is that once you have this large extinction event, these animals were likely able to survive because they were able to decrease in size and just become true opportunists."

So what does this mean for carnivores on today's warming planet? DeSantis says it means we have to think on an entire-ecosystem level. "One of the things that we find that's really important is that the extinction of a large number of prey was potentially why a lot of these carnivores went extinct. So I think it's really important that when we think about conservation, we need to think about the entire ecosystem. We need to worry about lions as much as we're worrying about the prey of the lion."

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Learn more about the Pleistocene extinction in "End of the Megafauna: The Fate of the World's Hugest, Fiercest, and Strangest Animals" by Ross D.E. MacPhee. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer August 22, 2019

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