I was in college when I saw my first truly chonky raccoon. It was perched on the rim of a trash can, a half-eaten tuna-salad sandwich clutched between its forepaws, its whiskers pinwheeling as it chewed. From across the quad, the raccoon fixed me with a beady-eyed stare of reproach, as if daring me to steal its already-filched fish. But I was much more interested in the creature, which looked twice as big as any raccoon I’d seen before. It was also a wild animal that had chosen a very unwild meal. And I couldn’t help but wonder if there was a link between the two.
As cities have grown and green spaces have shrunk, many wild animals, especially those in the Western world, have adopted diets that look an awful lot like ours. Squirrels snarf hard taco shells, and abscond with Nutella jars; subway rats chow down on pizza, while seagulls have ripped fries and even a KFC wrap straight out of human mouths. For at least some creatures, the menu changes seem to come with consequences. Raccoons that spend their days feasting on trash have higher blood sugar, heavier bodies, and crummier teeth than their wilder counterparts; bears that forage on human food hibernate less and show signs that their cells may age atypically fast. Vulture chicks nourished with scraps from landfills have lower levels of antioxidants in their blood. And when researchers repeatedly toss McDonald’s cheeseburgers to crows, the birds’ babies leave the nest chubbier, and with higher cholesterol.
It seems a worrying set of findings, especially considering the rising rates of obesity, diabetes, and heart disease among humans in many parts of the world—all conditions that have been linked to the consumption of ultra-processed foods. Maybe, in inheriting our mediocre diets, wild animals are picking up a bunch of our health problems too. But laying down such a verdict is trickier than it might sound. Humans barely have a grasp on how and why our own diets affect us, much less on how they’re shaping the bodies of creatures that share only some of our biology and live extraordinarily different lives than we do. “There’s so much basic physiological stuff we don’t know about wild animals,” says Maureen Murray, a wildlife-disease ecologist at the Lincoln Park Zoo. Some wild animals are certainly in worse shape from scavenging our scraps. Others, though, might be doing all right with urban diets—even adapting to them in ways that researchers are only beginning to understand.
Since life first appeared on Earth, it’s operated on a principle of scarcity: “Anytime you can get food you should get food,” says Shane Campbell-Staton, an evolutionary biologist at Princeton. Then humans came along, and “flipped that fundamental biological rule on its head.” Animals out in nature spend their days foraging, hunting, and competing with one another for whatever resources they can find. Meanwhile, “we can meet our day’s caloric requirements in a 10-minute trip to McDonald’s,” says Joanna Lambert, an evolutionary ecologist at the University of Colorado Boulder.
That life of energetic luxury spells trouble for us. Sugary, fatty, processed foods are easy to overeat—to the point that they can spark or exacerbate certain types of chronic disease. And some of the animals that live closest to us may be following in our footsteps. Pets such as dogs have been getting paunchier in recent decades, an issue that can put canines at higher risk of heart problems and high blood pressure. Many zoo animals, confined to enclosures and fed limited diets full of domesticated produce, tend to be sicker, sadder, and less eager to reproduce than those that roam free.
The outcomes get a lot grayer, though, when it comes to animals that are still technically wild, coming and going through our territory more or less as they please. Scientists have found evidence that human diets—which tend to be a little too carb-rich—can cause direct harm to animals: Young geese that nosh heavily on bread crumbs can develop angel wing, a type of malnutrition that can impair their ability to fly. And coyotes that scrounge on compost might be more susceptible to tapeworms and more likely to spread mange.
But in most cases, the math of modern animal menus just isn’t clear. Bianca Wist, an ecologist at the University of Hamburg, has found that her city’s squirrels, which frequently nibble on cupcakes, fries, waffles, and cookies, are scrawnier than the ones in the local forest, despite consistently eating more. White ibises—spindly, nomadic wetland birds that have evolved to chow down on crayfish—seem to do okay in parks full of people who toss or hand-feed them bread, bagels, pretzels, Froot Loops, and Flamin’ Hot Cheetos, Murray told me. The birds’ bodies get “kind of chunky, more round,” she said, and yet they weigh less than wilder ibises. And although their gut microbes are less diverse, the park birds seem quite content, spending their plentiful free time preening and loafing around in the sun.
The convenience of city eating has probably rescued some animals from terrible fates: individuals too old, sickly, or injured to keep up with wild prey or endure weather and predation in the wild, says Desirée Narango, a conservation scientist at the Vermont Center for Ecostudies. Many of Ethiopia’s dumpster-diving hyenas might have starved if not for their taste for trash; coyotes that lope into cities—where they’ve been spotted gnawing on Doritos, muffins, candy canes, and watermelons—“are definitely getting subsidized,” which allows populations to boom, Lambert told me.
Even animals that seem unhealthily chubby may not be unilaterally worse off. In Ontario, female urban chipmunks move less and weigh more than rural ones do—but have fewer stress hormones in their blood. And several studies have found that garbage-raiding primates in Kenya have higher body weight, cholesterol, and blood-sugar levels than those that have to forage in the wild—but may also reach reproductive maturity earlier, and get pregnant again more quickly after giving birth. Animals that get heavy and stay heavy may benefit from their caloric cushion when food becomes scarce; they might have more energy to invest into surviving harsh conditions, eluding predators, and producing and raising their young. “It’s a question of whether the costs outweigh the benefits,” says Albrecht Schulte-Hostedde, a behavioral and evolutionary ecologist at Laurentian University.
And although it’s easy for humans to make snap judgments, there are no universal standards on what makes an animal “too heavy.” It’s one thing to notice that a creature has put on some weight, or even that the levels of lipids or sugars have gone up in its blood. Labeling them “obese” or “diabetic,” though, turns those observations into value judgments, says David Allison, an obesity researcher at Indiana University at Bloomington. “What does it mean to be obese? So much fat that it’s a problem?” That’s a threshold that researchers haven’t satisfactorily defined even for people. The further animals get from us on the tree of life, the less we can assume about what’s going on in their tissues, or what’s to blame. For a dietary generalist such as a crow, “having a little more cholesterol in their diet might not be a big deal,” Narango told me, especially if the birds are still getting by well enough to couple up and hatch eggs.
Animals may not even be prone to metabolic disease in the same way that people are. In humans, weight- and diet-related issues can take decades to manifest, long past the life span of many of the animals rooting around in our garbage bins, says Julie Young, a behavioral ecologist at Utah State University. Maybe those timelines are compressed in wild creatures, and health problems could arise for an animal that’s quite short-lived. But the speed of illness could be slower to unfold in nature too—especially in creatures that are only supplementing their natural diets with our snacks, and still spending plenty of time running, flying, and swimming outdoors. Certain animals have also evolved resilience to major changes in physique way beyond what humans can handle: Bears, squirrels, and marmots, for instance, will essentially eat themselves into a high-glucose coma in advance of their long stints in torpor or hibernation, then happily and healthily return to a seasonal baseline.
Even if researchers were to define metrics of poor health across species, they’d still have a tough time blaming metabolic woes on diet alone. Urban living can come with plenty of other perils, says Christopher Schell, a behavioral ecologist at UC Berkeley. Cities are islands of heat, chock-full of pollutants and toxins, and lacking in plants. Lights stay on well into the evening; the din of traffic can drown out the sounds of life. Species that would never cross paths in the wild butt heads in alleyways, allowing infectious disease to spread. And then there’s the stress of frequently encountering us. All of that could be changing how animals’ bodies work. And there’s always the chance that sickness is affecting what animals eat, Murray pointed out. “It’s circular, how diet affects health, and how health affects diet.”
That murkiness might only be growing. Urban environments are some of the most extreme and fastest-changing habitats the world has ever produced, and perhaps in response, evolution in cities is playing out at accelerated rates. Storks in Spain are interrupting their migration patterns to congregate around garbage dumps. House sparrows are producing more starch-digesting enzymes, as dogs did when they split off from wolves; New York mice are churning out comparable digestive machinery that helps them better break down fatty, sugary food. Animals are adapting to city living—and blurring the definition of what truly counts as a “natural” diet in the age of the Anthropocene, says Tali Caspi, a behavioral ecologist at UC Davis. And if muffin-munching coyotes and pizza-pilfering rats end up being the ones producing the next generations, the standards for wild diets may continue to shift. Still, these animal adjustments aren’t necessarily a comforting trend—just one that’s clearly playing out on our streets. “Just because they are adapting to us doesn’t mean it’s beneficial to them,” Schell told me.
For now, as adorable as roly-poly squirrels might be, there are still plenty of reasons to keep human food out of animal paws and claws, experts told me. Our garbage contains many harmful substances, including toxins, metal, glass, and plastics that can sicken creatures or entangle their limbs. The stakes of separation aren’t just about them. Colleen Cassady St. Clair, a behavioral ecologist at the University of Alberta, and her team are starting to find evidence that urban coyotes that feast on too much human food might experience changes to their microbiome that make them more aggressive and likelier to tussle with people in problematic ways. Raccoons, which stick close to people to eat our trash, transmit a nasty roundworm in their feces that can cause serious harm to humans and dogs. And those non-migrating storks in Spain might now be contaminating nearby reservoirs with dump-heap germs, making the water less safe to drink. Garbage may not always directly damage the wild animals that eat it—but the worst of our trash does often find its way back to us.