Paula White, a wildlife biologist, was examining lion skulls to estimate the animals’ ages when she noticed something strange about their teeth. Instead of just showing the normal, gradual wear that happens over time, a sharp, V-shaped notch was cut into the back edge of some of the big cats’ canines.

The bizarre markings, it turned out, had been caused by snares — circular wire traps that tighten like a noose around the neck or paw of an animal. Snares are typically a death trap, but the lions whose skulls Dr. White was examining had apparently managed to escape by pulling on the wire with their teeth.

Putting this together “was a real a-ha moment,” said Dr. White, a researcher affiliated with the University of California, Los Angeles. “It was kind of horrifying but fascinating at the same time.”

Dr. White and her colleague, Blaire Van Valkenburgh, a vertebrate paleobiologist also at UCLA, realized they had stumbled across a valuable data set — one that would allow them to calculate the frequency of nonlethal, human-caused injuries to large carnivores.

Their findings, published Thursday in the journal Frontiers in Conservation Science, were worse than expected. More than a third of the lions and a fifth of the leopards they examined had evidence of being snared at some point in their lives. More than a quarter of the lions also had lead shotgun pellets embedded in their skulls, indicating a prior run-in with poachers or other people.

“If we want these big cats to thrive, we need better data on the nature and magnitude of the threats they face, especially from humans,” Dr. Van Valkenburgh said. “We can get those data by documenting the history of insults recorded in their bones and teeth.”

African lions are in precipitous decline. Between 1993 and 2014, population numbers fell by 43 percent, to as few as 23,000 wild individuals remaining today. Less comprehensive evidence indicates that leopard numbers are declining as well.

Habitat loss is the primary threat to lions and leopards, but humans also pose a significant danger to the big cats through poaching. Although illegal international trade of lions and other felines is on the rise, for now, big cats killed in retaliation for livestock predation is the bigger problem. Others are killed by snares set for animals like gazelles and antelopes that are part of the bushmeat trade. The snares harm the survival of the big cats by taking out the prey they depend on for food, as well as incidentally catching and killing the predators.

Determining the full extent of the impact of snaring and conflict with people is challenging, though. Animals are often killed in very remote locations, and cases go unreported. “Most often, animals just disappear and you don’t know what happened to them,” Dr. White said. If wildlife officials do manage to collect any data, they usually pertain to deaths rather than injuries.

The researchers are not certain about how the injuries they have documented may be affecting lions and leopards, but they suspect it must be significant.

“You’re looking at an animal that depends on its ability to hunt and bring down large prey, which is not an easy way to make a living,” Dr. White added. “Any kind of physical injury is going to make its life more difficult.”

The new research — likely the first to systematically document such injuries — came about by chance. Dr. White had originally been working with the Zambian government on a project that photographed the skulls and hides of adult male lions and leopards that had been legally hunted as trophies in Zambia’s Luangwa Valley and Greater Kafue Ecosystem.

The archived photos were the starting point for the new study. While examining the skulls, Dr. White and Dr. Van Valkenburgh found that a startling 37 percent of 112 lions and 22 percent of 45 leopards had evidence of being snared at some point in their lives. And 27 percent of the lions had been hit in the face with shotgun pellets.

The results are almost certainly an underestimate. For the snaring calculations, the researchers only considered animals that had both tooth wear and corresponding scars on their hides — not those that had only tooth wear. For the shotgun estimates, they were only able to examine images of the animals’ skulls, not their entire skeletons. As Dr. White said, “Our reported numbers are conservatively low, even though they are high.”

While the findings cannot be directly extrapolated to other lion and leopard habitats, Dr. Van Valkenburgh pointed out that the approach “is easily repeatable in countries without deep pockets or access to sophisticated technology.”

Joel Berger, a biologist at Colorado State University and the Wildlife Conservation Society, who was not involved in the study, agreed that the novel method has “truly broad value.”

“This is a remarkable paper that uses an imaginative diagnostic approach,” he said.

Carnivore deaths caused by humans are a significant problem around the world, yet very few studies give insight into this issue at the local level, Dr. Berger continued. That kind of fine scale information is needed, though, if wildlife managers wish to do more than make blind guesses about how to best help carnivores survive.

According to Amy Dickman, a conservation biologist at the University of Oxford, Dr. White and Dr. Van Valkenburgh’s “valuable and alarming” findings “suggest that snaring and conflict may be even more intense threats than previously estimated.”

This highlights the need to prioritize finding solutions for these problems, Dr. Dickman added. As an example, she said people would be less inclined to snare animals if they had access to sufficient food, and they would be less likely to retaliate against predators if they were given education and support to better protect themselves and their livestock. Engaging people directly in conservation and ensuring that they receive tangible benefits from living alongside wildlife are also parts of the solution, Dr. Dickman said.

Conservation is complex, though, and even in the best of circumstances, wildlife can still face pressure from humans. The landscapes in Zambia where the study took place, for example, are considered to be bastions of conservation, with strong anti-poaching and community engagement programs. The fact that snaring and human-wildlife conflict is so high, even in these places, was a sobering revelation, Dr. White said.

“It’s hard to say ‘increase efforts,’ because clearly, there is a lot of effort already on the ground,” she said. “But if it’s not working, you’ve got to rethink it.”