Forget topping out at 10,000 steps: In a typical day, a mountain lion can walk the length of 65 US football fields, stalking and ambushing its prey. But that lifestyle isn’t replicated in a zoo—and such captivity may be changing cat skeletons. Research out today shows captive big cats have less dense bones than their wild counterparts, likely because of reduced movement.
The study is based on the bones of animals that lived in zoos in the mid-1900s, so the results may not completely translate to modern zoos with larger habitats and better enrichment programs.
Still, “This is a great paper and really excellent work,” says Adam Hartstone-Rose, who studies animal morphology at North Carolina State University. How captivity affects animal anatomy is “a completely open question,” he says, “and studies like this are exactly what we need to answer it.”
Zookeepers have long noted differences between wild animals and those in captivity, but most research has focused on cat skulls. Captive felines typically have bigger noggins and weaker teeth, Hartstone-Rose and others have found, because—instead of chewing through the tough skin, muscle, and bone of wild prey—they eat a well-balanced, but mushy, diet.
The rest of an animal’s skeleton is also likely affected. As in humans, repeated physical activity boosts bone strength and mass.
In the new study, Habiba Chirchir, a biological anthropologist at Marshall University, and her colleagues chose four big cat species with vastly different ranges: mountain lions (Puma concolor), cheetahs (Acinonyx jubatus), jaguars (panthera onca), and leopards (panthera pardus). Jaguars, for example, occupy territories as small as 25 square kilometers, whereas mountain lions have home ranges that stretch to 250 square kilometers, or about four times the size of Manhattan.
The team gathered leg bones from the skeletons of 14 mountain lions, 15 cheetahs, 13 leopards, and 12 jaguars from the Smithsonian Institution’s National Museum of Natural History and the American Museum of Natural History in New York City. The bones were from wild and captive animals that lived in the mid–20th century, when zoo enrichment was rudimentary at best, Chirchir says. For each skeleton, the scientists looked at the upper bone of the front limb, known as the humerus, and the thigh bone, or femur—both important for running, climbing, and hunting.
The researchers cut out a small section of each specimen near the joint. Then they used high-resolution x-ray scanning to take 3D pictures of each bone’s inner structure. In each image, they counted the number of white pixels, denoting bone, and black pixels, denoting empty space, and calculated their ratio to determine the bone’s density.
In all four cats, the captive animals had less-dense bones than the wild ones, the team reports today in Royal Society Open Science. The felines’ front legs were most affected; for example, the femurs of captive mountain lions were about four-fifths as dense as those of wild cats, whereas their humeri were only three-fourths as dense.
Chirchir and her colleagues blame a lack of physical activity. But other factors could be involved. “One concern is inbreeding,” Hartstone-Rose says, noting that captive animals have higher levels of inbreeding and that genetic overlap could negatively impact an animal’s skeleton. But the researchers don’t know the family histories of each animal in their study, so there was no way to test that, Chirchir says.
The scientists also don’t know how all the animals in their study died, and whether they were healthy when alive. Reduced bone density can result in brittle bones; the researchers say that, although the bones in the study are not from modern zoos, animals released back to the wild through conservation programs might be at some disadvantage as a result of their captivity.
Those results are relevant to any scientist who uses natural history collections, which often contain captive bones mixed in with wild ones, in their research, says Stephanie Smith, who studies animal morphology at the Field Museum. When comparing captive and wild bones, researchers should ask, “Is that going to mess up my study?” she says. “It’s something we should be paying attention to.”