Scavenger Animals Are in Trouble, and That Could Spell Bad News for Human Health
More than one-third of species that eat some amount of carrion are threatened or declining, a new analysis finds, and that could lead to a rise in zoonotic diseases
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Scavengers are in trouble—and their decline could be harmful to human health.
With many of these creatures that feast on dead animals struggling to survive, scientists say their downfall could lead to a rise in infectious diseases among humans, according to a paper published Monday in Proceedings of the National Academy of Sciences.
For the study, researchers looked at 1,376 vertebrate species known to eat some amount of carrion, ranging from tiger sharks and spotted hyenas to cane toads and common shrews. When they investigated each animal’s status on the International Union for Conservation of Nature (IUCN) Red List, they found that 36 percent are threatened or decreasing in number.
When the team dug even deeper into the data, a more nuanced picture emerged. Large species and those that rely on carrion for survival tend to be imperiled, while smaller species and those that scavenge occasionally are thriving.
Obligate scavengers, or those that only consume carrion, are especially vulnerable. Half of the 17 obligate scavenger species included in the study are considered “vulnerable” or “critically endangered” by the IUCN.
That’s a bad dynamic, because in the wake of their disappearance, some smaller, occasional scavengers—known as mesoscavengers—are proliferating. These creatures, such as rodents and feral dogs, have a tendency to transmit diseases to humans. For example, places where mice and rats are the most abundant scavengers are more vulnerable to outbreaks of illnesses like the bacterial disease leptospirosis and the diarrhea-causing cryptosporidiosis.
In addition, mesoscavengers cannot adequately fill the roles left vacant as their larger counterparts disappear. Smaller scavengers often need to follow larger ones to find rotting carcasses on the landscape. And once they arrive, they typically must wait for the bigger creatures to rip open the decaying remains before they can dig in.
“As we went through the literature, it was a reoccurring pattern that mesoscavengers cannot functionally replace the carrion consumption,” lead author Chinmay Sonawane, a biologist at Stanford University, tells Science News’ Bethany Brookshire.
Why are some species proliferating while others are struggling? Some are just better at adapting to humans, the researchers write. Apex scavengers are disproportionately affected by activities like intensive livestock production, land use changes and the wildlife trade.
Hunters, for example, are more likely to target large animals for consumption and trade, according to the study. Beyond that, apex scavengers are sometimes killed accidentally—hyenas and lions can get caught in wire snares meant for herbivores in Africa. And when hunters kill too many prey animals, there’s often not enough left for scavengers to eat.
Apex scavengers may also accidentally consume toxic substances, such as poison intended for predators threatening livestock, or veterinary drugs found in livestock carcasses.
Though scavengers tend to get a bad rap, they are vital for human well-being. In India, for example, vulture populations dropped by 95 percent from the mid-1990s to early 2000s, after ranchers started treating their livestock with the painkiller diclofenac. Without the birds around to help clean up carcasses—and curb the spread of diseases in the process—an extra 500,000 people died between 2000 and 2005, researchers estimated in a 2024 study. The birds’ absence also allowed feral dogs to proliferate, which led to an uptick in dog bites and rabies cases.
Past research has also found that scavengers help keep livestock healthy, such as by preventing the spread of diseases like anthrax, bovine tuberculosis and brucellosis. Improving livestock health has benefits down the line for humans.
“I think as we become more aware of these connections, we’re going to be more inclined to protect these species, because, in turn, we are ultimately protecting our health,” Sonawane says in a statement.
Scavengers also help communities save money on waste management. As nature’s clean-up crew, these creatures quickly and efficiently dispose of unsightly, smelly and disease-causing carcasses—without charging a dime.
“I have personally observed a group of eight to ten Andean condors (Vultur gryphus) removing an entire wild boar carcass in less than five hours,” says Pablo Plaza, a biologist and veterinarian at Argentina’s National University of Comahue who was not involved with the new research, to Science News.
In Spain, for instance, vultures have saved farmers up to $67 million by eating dead livestock they would have otherwise had to collect and transport to processing plants, according to the paper. And across the Americas, turkey vultures save nearly $1 billion in removal costs by eating approximately 1.5 million tons of waste each year.
Looking ahead, the scientists hope their study will lead to more policies that benefit apex scavengers and help their numbers rebound. They also hope their work changes a few hearts and minds, clearing up some of the stigma surrounding these misunderstood and much-maligned animals.
“There is this prejudice that these scavengers—vultures and hyenas and the like—are nasty animals and that we should get rid of them,” says study senior author Rodolfo Dirzo, a biologist and environmental scientist at Stanford University, in the statement. “But they are important not only for the functioning of ecosystems, but also for human well-being.”
