U-M study shows American pronghorns evolved for speed long before the American cheetah arrived on the scene
Study abstract: Family-level ecometrics reveal functional trait stability in Miocene artiodactyls despite environmental change in the Mojave region (DOI: 10.1093/jmammal/gyaf089)
The fastest land animal in North America is the American pronghorn, and previously, researchers thought it evolved its speed because of pressure from the now-extinct American cheetah.
But recently, that theory has come under fire. Now, a University of Michigan study examining fossilized ankle bones of ancient relatives of the American pronghorn has shown that the pronghorn was evolving to be faster more than 5 million years before the American cheetah appeared on the continent. The study, supported by the Michigan Society of Fellows and the U-M Rackham Graduate School, is published in the Journal of Mammalogy.
"There was a long-standing idea that pronghorn are so much faster than every predator in North America because of a predator-prey arms race between the pronghorn and the American cheetah," said U-M paleontologist Anne Kort and co-author of the study. "What our work was able to add to this story was that not only was the American cheetah not as cheetah-like as previously thought, but that pronghorn have this build for running that existed well before the American cheetah came about."
The researchers say this sheds light on how current artiodactyl relatives-artiriodactyls include camels, cows, deer and antelopes-and other mammals may adapt as humans push farther into wild landscapes and as the climate warms. First author Fabian Hardy, assistant professor at Slippery Rock University, says the findings have implications for how we develop future wildlife and livestock management practices, as well as conservation practices.
"This tells us something about what animals are going to succeed going forward, and where we are going to find them. Is the modern pronghorn going to continue throughout the region that it's found in now?" said Hardy, who completed the work as a graduate student and postdoctoral researcher at U-M. "They're still traveling long distances. They were set up for that 12 million years ago. So they'll probably do all right, even with urbanization and fragmentation, because they can move around efficiently."
The study focused on fossils found in the Mojave Desert's Dove Spring Formation, which was deposited from 8 million to 12.5 million years ago. During this time, the region underwent great environmental change. In a valley called the El Paso Basin, the landscape transformed over time, shifting from once unbroken forest into a patchy mosaic of woodlands separated by more arid grasslands.
Examining the ankle bones of early pronghorn relatives, the researchers expected to see the bones shorten in comparison to the point of rotation of the ankle. This would suggest that they adapted to traveling efficiently across grasslands and between patches of forest. Instead, the researchers found that their ankle bones, specifically a blocky bone in the middle of the joint called the astragalus, remained unchanged. This suggests the pronghorns were able to move between patches of forest effectively rather than adapt to open grassland.
Working in a fossil assemblage from a geologic epoch called the Miocene, Hardy collected a range of bones from a span of about 4 million years. He met with Kort, who studies the interaction between environmental change and mammals' locomotor adaptations, to develop a project centered around them.
"From about 30 million years ago to present, we see mammals becoming increasingly cursorial, which means they're adapted to running," Kort said. "I was curious to see if you could detect that in this kind of smaller scale pattern of drying and opening environments."
The researchers compared the astragalus using a study that had been done on modern artiodactyls such as antelope and cows. That study showed that these animals, which lived in an open habitat, had a shorter astragalus, adapted for efficient running.
"We expected to see longer astragali in the beginning, and then it would transition to more running-adapted astragali in the end. But we did not find that," Kort said. "Instead, what we found was a community that just stays the same through the whole section."
The pronghorn relatives, which were the size of a slender, long-legged beagle, were large enough that they could move in and out of the El Paso Basin, Kort said, seeking patches of forest elsewhere that were still suitable habitat.
"Our guess is that they could just move around to compensate for this aridification and landscape change," Kort said.
Although the astragalus had not changed over the time period Kort and Hardy were examining, they did show evidence of already adapting to running-5 million years before the cheetah appeared. In fact, the earlier pronghorns have "effectively the same ankle ratio as the modern pronghorn. So they're really built very similarly, and really built for speed well before this American cheetah shows up," Kort said.
Eventually, toward the end of the Miocene, the small pronghorns died off-perhaps as a result of a "tipping point" in the ecosystem that led to irrevocable changes in the ecosystem.
"It's easy to expect evolution and this gradual change over time, but I think this idea that you might not even see a problem until it's too late is a good reminder of how these things work," Kort said. "Our findings also may provide contextual information for biologists who are in wildlife management and doing direct conservation. They may not directly use it, but it's almost like having a historical context to understand a political problem."
