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Bioscience
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Bioscience
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Oxford University Press

10.1093/biosci/biae030
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Everyone scavenges: From pole to equator, nature's recyclers perform a widespread and largely unheralded service
Dybas Cheryl Lyn International League of Conservation Writers

Email: cheryl.lyn.dybas@gmail.com
8 2024
01 5 2024
01 5 2024
74 8 500506
© The Author(s) 2024. Published by Oxford University Press on behalf of the American Institute of Biological Sciences.
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (https://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com
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pmc “When good is hungry it seeks food even in dark caves, and when it thirsts it drinks even of dead waters.”

—Kahlil Gibran, The Prophet

The Kluane region of the Yukon in Canada, a UNESCO World Heritage site and part of the largest internationally protected area on Earth, is a glacial valley ringed by mountains. In its vast boreal forest—a seemingly endless expanse of white spruce, trembling aspen, balsam poplar, gray willow, and dwarf birch trees—one of the most common food sources for predators is the snowshoe hare. The species likely makes up more than half the area's vertebrate biomass. But the hare feeds more than predators. In death, snowshoe hare carcasses sustain scavenger species—nature's cleanup crew that consumes dead animal matter.

The lineup of scavengers in Kluane was recently revealed in detail. It includes some surprises. Among them is the diminutive Canada jay. Canada jays quickly notice any change in their territory “and were often the first species in our study to show up at a carcass,” says Thompson Rivers University researcher Emily Studd. “These jays are the species in the boreal forest that know everything going on, like those super-observant homeowners who keep tabs on everyone in the neighborhood.”

The golden eagle is one of 24 or more species that feed on carrion in winter in Canada's northern boreal forest. Photograph: Bohuš Číčel CC BY-SA 3.0.

In fact, a study in Kluane by a team including Studd and Michael Peers of the University of Alberta found that Canada jays were one of the top carrion eaters, making off with bits of more than 60% of snowshoe hare carcasses deliberately placed for the research. The scientists eavesdropped on these opportunists using cameras run during the fall, winter, and spring seasons. The researchers obtained dozens of snowshoe hare carcasses from roadkill and trappers’ donations.

The findings revealed many of the usual suspects: species ecologists usually think of as scavengers, such as common ravens, magpies, and wolverines, for example. But it also uncovered scavengers ecologists might not expect, such as Canada jays, along with lynx, red squirrels, northern flying squirrels, Arctic ground squirrels, and chipmunks. Earlier, in 2018, Peers documented another unexpected source of hare scavenging—the hares themselves.

“The scavenger community is filled with surprises,” says Studd.

The results show that prey species have more complex impacts on food webs than was previously thought. Prey numbers affect not only predators but scavengers too. Scavenging is often overlooked in food web studies, the scientists conclude.

“Our findings add to a growing body of research indicating that an omission of scavenging might substantially underestimate the connectedness of vertebrate food webs,” write Peers, Studd, and coauthors in their 2020 paper in the Journal of Animal Ecology. Scavengers, write the ecologists, warrant a closer look and should be a priority for research.

Scavenging: New viewpoints

In the Yukon case, the data suggest that the abundance of keystone species such as snowshoe hares may affect entire communities of scavengers. After Canada jays, next in line for the highest percentage of snowshoe hares scavenged were common ravens, red squirrels, black-billed magpies, and Canada lynx. In all, the scientists documented 24 species—including golden eagles, short-tailed weasels, and black bears—feeding from the hare carcasses in Canada's boreal forest. It's one of the most diverse scavenger communities known.

During a particularly cold fieldwork period, Peers happened on “an ermine scavenging a lynx carcass in the forest,” he says. “Magpies also tried to scavenge the carcass, but the ermine chased them away. Once, there was quite a standoff, however, which ended with the ermine hurling itself at a magpie to drive it off.”

The food resources provided by carrion are a critical subsidy for many vertebrates, explains Peers, especially in colder months, when carcasses are less susceptible to decomposition. Indeed, the biologists found seasonal differences in scavenging. “The longer persistence of carcasses during winter explained why the season supported more scavenger species per carcass,” says Peers. “Persistence time was the strongest predictor of the number of species scavenging.”

Venison hawk and meat bird, the Canada jay has been called. Here, this small but enterprising scavenger pecks away at a deer carcass in the Adirondack Mountains of New York in the United States. Photograph: Larry Master.

Here a scavenger, there a scavenger, everywhere a scavenger

Canada's boreal forest is just one ecosystem where familiar and lesser-known scavengers play important roles. Researchers now suggest that, at one time or another, everyone scavenges.

Worldwide, facultative scavengers, or those that subsist on other food in addition to carrion, include jackals, hyenas, lions, foxes, wolves, bears, lynx, eagles, ravens, and crows. Obligate scavengers rely entirely or almost entirely on carrion for food. Among terrestrial vertebrates, only vultures are considered obligate scavengers. In the ocean, benthic species such as hagfish rely on carrion for a large part of their diets and may be obligate scavengers too. Scavenging is active when the predator responsible for the kill is chased away and most of the meat on the carcass is eaten by the interloper. It is passive when bones and meat are discovered and collected, explains ecologist James Beasley of the University of Georgia.

Until recently, Beasley says, the importance of scavenging-derived nutrients to many vertebrate species was largely unknown. He is part of a growing cadre of researchers underlining the role vertebrate scavenging plays in the food web dynamics of microbes such as bacteria and fungi and of invertebrates such as burying beetles, relationships whose importance has been greatly underestimated.

In some ecosystems, vertebrates may assimilate as much as 90% of the available carrion, according to Beasley, Zach Olson, and Travis DeVault in the chapter “Ecological Role of Vertebrate Scavengers” in Carrion Ecology, Evolution, and Their Applications. That such a substantial proportion of carrion provides food for vertebrates challenges the traditional paradigm that microbes and invertebrates dominate nutrient recycling. It “suggests that intensive interkingdom competition exists for access to carrion nutrients,” they write.

Indeed, add Beasley and colleagues in their Carrion Ecology chapter, “because carcasses represent a rich source of nutrients to any organism that can use the resource, there is a race for organisms to find and consume carrion before other organisms can monopolize it. That race can lead to sometimes intense competition among microbes, invertebrates, and vertebrate scavengers for the resources sequestered in a carcass.”

An unexpected array of vertebrates forages on carrion, including the hippopotamus, white-tailed deer, and pileated woodpecker. Many species that scientists have traditionally thought of as exclusively predators also consume carrion. For example, great horned, snowy, and short-eared owls often make meals of carcasses, says Denver Holt, director of the Owl Research Institute, in Montana. In western Montana, near Charlo, Holt observed “short-eared owls scavenging a road-killed deer during a winter with above-average snow,” making the owls’ usual vole prey harder to find, he says. Holt has also watched snowy owls feasting on caribou meat in Utqiagvik, Alaska, after a local hunter left behind caribou remains. “One snowy owl was a male that brought caribou pieces to a female,” says Holt.

Humans also have a history of scavenging. In the distant past, “efficient distance running in early humans may have evolved to allow our ancestors to capitalize on… carcasses to supply their diets with protein,” speculate Beasley and colleagues in Carrion Ecology. Hominins on the Iberian Peninsula 1.2 million years ago may have successfully outcompeted giant hyenas for carcasses, suggest Ana Mateos and Jesus Rodriguez of the National Research Center on Human Evolution in Burgos, Spain. The researchers developed a computer model that simulates competition for carrion between giant hyenas and hominins. In ancient times, saber-toothed cats likely left behind half-eaten carcasses, generating resources for scavengers, including hominins, found Mateos and Rodriguez. They published their results in a 2023 paper in Scientific Reports.

Unlike facultative scavengers, obligate scavengers must locate carrion meals to survive. Most but not all obligate scavengers are vultures. “Obligate scavenging birds exhibit spectacular adaptations to a lifestyle dependent on carrion,” write Beasley, Olson, and DeVault in Carrion Ecology. These adaptations include fewer feathers on the head to protect against fouling, acidic guts to speed digestion and presumably protect against pathogens, excellent visual and olfactory perception, and efficient travel via soaring.

Animal carcasses are regularly left at this garbage dump near Udaipur, India. Huge concentrations of Egyptian vultures, nature's cleanup crews, circle above the carrion. Photograph: Sarusscape, CC BY-SA 4.0.

Obligate scavengers also dwell in the sea. Hagfish—eel-shaped, jawless fish that produce copious amounts of slime to defend against predators—can go a year or more between meals. Their sensitive chemoreceptors allow them to detect distant carrion in deep-sea ecosystems. That carrion is often the flesh and bones of sunken whales.

When whale bodies first fall to the ocean floor, they're reduced to near skeletons by hagfish. The process can take as little as 4 months and is the first stage in a succession of creatures that feast on a sunken whale. Once first responder scavengers quit the scene, second-stage creatures such as polychaete worms and crabs move in for 4 to 30 months. They largely dine on particles that waft from carcass to seafloor.

After this stage, little is left but bones. The final cleanup corps includes bacteria, worms, clams, and mussels. These third-stage creatures make use of sulfide-rich compounds to help decompose whale bones. “Such communities can thrive for years and years,” says David Wiley, a cetacean biologist at the Stellwagen Bank National Marine Sanctuary, in Massachusetts. “In death,” Wiley says, “whales provide life for hundreds of other marine animals.”

Deep-sea hagfish are likely obligate scavengers; they feed only on carrion, as far as scientists know. After a meal, hagfish can survive a year or more until the next one. Photograph: Peter Southwood, CC BY-SA 4.0,

Scavengers: Welcome neighbors

Who wouldn't want neighbors who take out your trash, clean your road better than any street sweeper, and protect you from disease, all at no cost?

Enter the scavengers. Egyptian vultures thrive in Socotra, Yemen, “where they are valued for removing livestock and human waste that would otherwise cause water contamination and are expensive to remove,” write Christopher O'Bryan of the University of Queensland and colleagues in a 2018 paper in Nature Ecology and Evolution.

Elsewhere, vultures outcompete stray dogs in finding and consuming carrion, sometimes to the vultures’ detriment. Studies by wildlife veterinarian Martin Gilbert, of Cornell University, and others linked a decline in vulture populations in India—a 92% loss from 1990 to 2000—to the widespread use of diclofenac, a veterinary drug used in cattle. The vultures fed on cattle carcasses containing the drug. The findings were reported in September 2004, in the Journal of Applied Ecology.

A sharp increase in the number of stray dogs followed the deaths of vultures that had eaten diclofenac-laced carrion. Without vultures consuming carrion such as dead farm animals, stray dog populations will continue to rise, increasing dog bites and exposure to rabies in humans, writes O'Bryan in Nature Ecology and Evolution. Unwelcome facultative scavengers such as gulls, rats, and invasive foxes may enter the picture; all are potential disease hosts.

In Spain, vultures have saved farmers and others an estimated US$50 million, writes O'Bryan. Like good neighbors, the vultures are “supplanting [commercial] transport of livestock carcasses to processing facilities.” The vultures are the processors. Similarly, in the Tigray region of northern Ethiopia, high numbers of spotted hyenas coexist with humans. The hyenas consume cattle and donkey carcasses in urban areas, reducing disease risk for people.

Then there are golden jackals, which carry off more than 3700 tons of domestic animal waste in Serbia each year, including road-killed animals and those contained in waste dumps, says Miha Krofel, of the University of Ljubljana, in Slovenia. “Across Europe's urban landscapes, these jackals remove more than 13,000 tons of organic waste each year,” he says. “That amounts to more than half a million euros in saved waste removal that would otherwise cause groundwater contamination and other health risks.” Krofel and colleagues published the results in Biological Conservation in July 2016.

The health risks of animal carcasses include brucellosis, a bacterial disease that causes female bison, elk, and cattle to abort their calves. Brucellosis is extremely contagious and is the bane of cattle producers. In the US Yellowstone region, scavengers such as coyotes, foxes, and eagles help wildlife and ranchers by carting away aborted remains and keeping brucellosis at bay. Wild animals and livestock can contract the disease through contact with the remains, which may stay infectious for weeks or months, found Kimberly Szcodronski and Paul Cross, of the US Geological Survey Northern Rocky Mountain Science Center, in Montana.

Szcodronski and Cross conducted a study using remote cameras to show that scavengers remove these remains in just days. Their results were published in the October 2021 issue of Ecosphere. The researchers found that the average time to disappearance of fetal materials is about 3 days in grassland habitats, 5 days in sagebrush habitats, and 5 days in forest habitats. The fastest removal, in grasslands, is likely related to better visibility across the landscape, helping scavengers find remains more quickly. At sites where scavengers were killed by human hunters, it took more than 12 days for infected tissues to disappear. Recreational hunting and targeted coyote and fox removal may be inadvertently harming cattle producers by increasing livestock exposure to contagious diseases, write Szcodronski and Cross. By reducing the risk of disease on rangelands, they add, scavengers provide an important and, until now, overlooked ecosystem service.

Puma kills: Smorgasbords for scavengers

One of the best sources for scavengers is carrion from puma kills, such as white-tailed deer in the Northern Hemisphere and guanacos, llama-like camelids, in the Southern Hemisphere. “Puma prey, carrion in particular, supports food webs of hundreds of species,” says biologist Mark Elbroch, of the global wild cat conservation organization Panthera.

Publishing their study in the January 2022 issue of Mammal Review, Elbroch and colleagues estimate that pumas contribute more than 1.5 million kilograms of meat each day to 281 species in scavenger communities across North and South America. “For carrion-dependent species such as Andean condors, puma kills are likely essential,” Elbroch says.

“Pumas provide a disproportionate amount of carrion to ecological communities,” the biologists state in their paper. Elbroch adds that “pumas are ‘ecological brokers.’ They're the most widespread terrestrial mammals in the Americas, and they play an essential role in linking an incredible number of species from many trophic levels.”

One of many sometime—or facultative—scavengers, a snowy owl peers at a carcass. With its brown flecks or bars, this owl is a female; males are all white. Photograph: Jongsun Lee, CC BY 3.0.

Possum feast in the Australian Alps

Half a world away from North and South America, unexpected scavengers feast on kangaroo carrion. In the Australian Alps in winter, brushtail possums, usually thought of as herbivores or insectivores, are consummate scavengers, report James Vandersteen and Thomas Newsome, of the University of Sydney, in a 2023 paper published in Wildlife Research.

“When we think of scavengers, large carnivores are what comes to mind—not brushtail possums,” they write. “It came as a surprise when these Australian marsupials turned out to be one of the most common scavengers in our study.”

No vertebrate Australian animals survive solely by scavenging. Carcasses are only “sometimes food,” the biologists observed. They wanted to better understand the seasonal use of carrion by vertebrate scavengers in Kosciuszko National Park, in the Australian Alps. Winter in these mountains is a time of heavy snow, but by summer, the landscape has changed. Bushfires often rage.

The researchers placed 15 eastern grey kangaroo carcasses in the alpine environment in autumn, winter, spring, and summer, for a total of 60 kangaroo carcasses, all from culls. The kangaroos were then monitored with remote cameras.

Brushtail possums mostly eat plants and insects—or so scientists thought. Much to the biologists’ surprise, brushtail possums are prime scavengers in winter. In 745,599 camera trap images, scavengers included spotted-tail quolls, feral cats, dingoes, pied currawongs, wedge-tailed eagles, ravens, red foxes, feral pigs—and brushtail possums.

Initially, the scientists thought carrion would be easier to find in summer. They were wrong. In that season, it took vertebrate scavengers longer to find carcasses. In winter, carcass visits peaked.

The explanation: In summer, a carcass is scoured by insects within minutes. These tiny scavengers can hasten decomposition so much that vertebrate scavengers have little time to find the leftovers before they're gone. Other food sources, such as leaves, flowers, and fruit, are also abundant in summer, perhaps affecting vertebrates’ need to scavenge. In winter, when these meals are scarce, brushtail possums alone accounted for 81% of all scavenging in the study.

Australia conducts extensive culling of “overabundant” species such as kangaroos, leaving their remains behind, making understanding the fate of carcasses important, according to the researchers. In the Australian Alps, they found, brushtail possums are the main members of the removal team.

A spotted hyena scavenges a wildebeest carcass in Amboseli National Park, Kenya. Photograph: Kandukuru Nagarjun CC BY 2.0.

It's not easy being a scavenger

It's not all free meals for scavengers. According to Beasley and colleagues writing in Carrion Ecology, “any shift in the availability or distribution of carrion could have a profound impact on the composition of scavenging communities and distribution of carrion resources throughout food webs.” Habitat loss, the disappearance of apex predators that leave behind kills, and climate change are a few of the many threats to scavengers.

“Slight changes in temperature could have profound impacts on energy flow in ecosystems as carrion decomposition rates roughly double for every 10-degree-Celsius increase in ambient temperature,” write Beasley and coauthors. “Consequently, the availability of carrion to vertebrate scavengers could be reduced by as much as 20% to 40% over the next century based on projections from current climate change models.”

Polar bears may be among the most affected species. Dead whales washing ashore are valuable sources of fat and protein for polar bears, but they're likely not enough to sustain the bears in ice-free Arctic summers, which may occur routinely by 2040, write Kristin Laidre, of the University of Washington, and other scientists. Their findings were published in a 2018 issue of Frontiers in Ecology and the Environment.

In September 2017, dozens of polar bears found their way to the shores of Wrangel Island, Russia. There, they fed on the remains of a bowhead whale washed up along the coast. More than 180 bears made a meal of the whale carrion. Scavenging on large whales may buffer polar bears from sea ice loss in some areas. But carcasses of large whales, Laidre believes, won't replace polar bears’ usual prey—seals—as the Arctic has less and less ice. The environmental changes are becoming too large, according to Laidre, and the carcasses too few.

A cougar, or puma, feasts on a dead deer; cougars contribute meals to more than 280 species in scavenger communities in North and South America. Photograph: US National Park Service, via Wikimedia Commons.

Brushtail possums are surprising winter scavengers in the Australian Alps; there they survive the cold season by feeding on kangaroo carrion. Photograph: Andrew Mercer, CC BY-SA 4.0.

New insights leave unanswered questions

Such studies are an example of new directions in the field of scavenging ecology. “One of the greatest recent advances has been an understanding of the importance of scavenging and of carrion-associated nutrients to a vast array of vertebrates,” says Beasley. “We now recognize that scavenging isn't limited to vultures or the occasional scavenging behavior by predators but that most vertebrates are facultative scavengers who frequently engage in this behavior.”

Scientists know that carrion resources are critical to the survival of obligate scavengers, but researchers have begun investigating how carrion affects the survival, reproduction, and health of species that opportunistically feed on carcasses. “How does climate change influence scavenging dynamics and complex interactions among microbes, invertebrates, and vertebrates?” Beasley asks. “We also need to better understand the role scavengers play in both reducing and facilitating the spread of diseases.” These questions are important, he says, in every ecosystem around the globe.

Wherever they call home, scavengers’ lives surely aren't easy ones. Picture searching for a meal, wandering from place to place on a frigid day or night, then finding meat frozen to bones embedded in inches—if not feet—of snow and ice. From the shadows, other eyes watch, also waiting to snatch morsels. Or on a too-warm day, imagine that your food has spoiled before you can eat it. No wonder, as Kahlil Gibran wrote in The Prophet, a scavenger “seeks food even in dark caves and… drinks even of dead waters.”

Polar bears scavenge the carcass of a bowhead whale along the coast of Wrangel Island, Russia. Photograph: Chris Collins/Heritage Expeditions.

Author Biography

Ecologist and award-winning science writer Cheryl Lyn Dybas (cheryl.lyn.dybas@gmail.com), a fellow of the International League of Conservation Writers, often covers conservation biology for BioScience.
