How Humans Created the Ultimate Superpests

Bolt your windows all you want—nature’s greatest invader will stop at nothing to enter your home. Even the NYPD is flummoxed in the face of Brooklyn's most devious masked bandit: the urban raccoon.

Whether they’re drunkenly waddling through warehouses or setting off frenzied rabies alerts in metropolitan areas, raccoons sure know how to hold the spotlight. Major cities are their own private amusement parks. In the past few years, raccoons have instated a veritable reign of terror in Brooklyn, diving in and out of dumpsters, nesting inside chimneys and walloping neighborhood cats. Inquiries about raccoon control to the city help line went up by almost 70 percent between 2014 and 2015.

And these invaders aren't going anywhere. The very things that make these species so successful allow them to intrude more intimately into our lives,” says Bob Wong, a behavioral ecologist at Monash University in Australia.

As a whole, human development has a negative impact on wildlife, and the culling of natural ecosystems remains the greatest threat to global biodiversity. But for certain species, the ability to adapt to and even exploit human resources makes them more likely to proliferate in our midst. More and more creatures have become seasoned city dwellers, feasting on our garbage and nesting in the nooks and crannies of our homes. What doesn’t kill them only makes them hardier—like the antibiotic-resistant microbes of animal pests.

In other words, we are creating our own worst enemies—by manufacturing the very conditions that encourage them to be better, faster, stronger and more adaptive.

We have a term for these critters: “nuisance wildlife,” a group that includes familiar vandals like crows, raccoons and coyotes. Entire animal control industries are devoted to removing these creatures from urban environments, with ranging from enmeshing gardens in deer-proof netting to the purging of pests with poisons, like the billion-dollar war currently being waged against possums in New Zealand.

But despite our efforts, wild animals continue to infiltrate urban centers: coyote removals increased over 15-fold in the Chicago metropolitan area in the 1990s, while urban Nevada suffered a 10-fold increase in complaints of black bear invasions around the turn of the 21st century.

Raccoons, with their five-fingered paws and scrappy brand of moxie, are in a league of their own. They continue to terrorize the inhabitants of major cities across the country as their numbers literally climb the ladders, and pass each other intel on the best sources of food and shelter, increasing the likelihood of nightly raids.

“Anyone who’s a homeowner knows the goal of every raccoon: ‘Whatever you’ve got, I want that,’” says Suzanne MacDonald, an animal behavior psychologist at York University in Canada.

The secret to these savvy species’ success? Street smarts, says Sarah Benson-Amram, a zoologist at the University of Wyoming. These creatures are adapting not in an evolutionary sense, but in a behavioral one. Natural selection simply does not operate on the same timescale as industrialization, so to keep pace with anthropogenic change, wild animals have resorted to altering their behavior rather than their genes. It’s a way to circumvent the need for genetic shifts: for example, rather than darkening its coat, an animal can simply learn to hide better.

And the flexible fare well. These behavioral adjustments “can buy crucial time for genetic changes to accrue,” says Wong. The animals most likely to invade urban environments are, unsurprisingly, the cleverest, and tend to exhibit plucky behavioral traits such as curiosity about new things, boldness and the ability to innovate in unfamiliar situations.

Oftentimes, they use these traits to take advantage of the resources humans hoard. Certain male birds decorate their nests with colorful human rubbish. Green herons native to North and Central America pilfer bread from unsuspecting pedestrians and use the morsels to bait fish close to the shore. In Bali, macaques steal trinkets from tourists and hoard them as bartering tools for food (in this case, the exchange rate appears to be one pair of eyeglasses for a slice of bread). Keas, inquisitive green parrots native to New Zealand, unabashedly pry open trash can lids, strip the insulation from power lines and duel unarmed windshield wipers on cars.

Unfortunately, when animals become overly reliant on human food and shelter, the results are bad for both parties. Scavenged human food, rich in sugar, fat and chemicals and low in nutrients, can make up over half the diets of city-faring foxes, raccoons, and birds, putting them at risk for vitamin deficiencies, metabolic syndrome and the inadvertent ingestion of plastic, rubber and metal.

And as animals come to associate the smell of humans with rich resources, their fear of people plummets, putting them in the crosshairs of the disgruntled defenders of urban jungles. Some counties in Ohio now permit recreational hunters to shoot down invasive deer within town limits. A controversial war has been waged for decades against city-colonizing coyotes across the country. In New Zealand, humans have slaughtered keas to the point of endangerment.

But the ways in which wildlife suffer at our hands can be far subtler. Not all urban critters are provoked into clambering up our fire escapes and rooting through refuse when humans come to call. Instead of using guile to reclaim their homes, some species are wising up to the idea of avoiding us altogether.

The truth is, most wild animals are far more terrified of us than we are of them. The mere scent or sound of humans can lower libido, dissuade predators from pursuing prey or interfere with critical communication. For instance, birds and tree frogs must adjust their twitters and chirps just to be heard above the din of rush hour traffic. Other wild animals normally active during the day have become night owls on the outskirts of cities. These animals are forced to endure far more than a later bedtime, sacrificing reproduction and survival as they shirk from humans and retreat into their dwindling habitats.

To them, humans, equipped with intelligence, technology and an ever-increasing population, are the ultra-predators at the top of every food chain. And we have not ruled with reserve: rates of extinction are 1000 times what they would be in the absence of human intervention. In these species, cognition is still at play—but it tells them to flee instead of fight. As they are relegated to more and more restricted environments, their vulnerability only increases.

And so, rather than grappling with the binary of “cull or conserve,” we may need to acknowledge that respecting the intelligence of these adaptive creatures is the gateway to new solutions. For instance, where simple barriers fail, more complex deterrents that engage multiple sensory modalities—such as a lock that also incorporates a frightening noise—may suffice.

The adroitness of “pests” could even be co-opted for our benefit. For instance, captive elephants can be trained to dissuade wild elephants from raiding crops through social communication. It may also be possible to leverage the treat-centric motivations of certain animals: Some researchers have successfully designed trash receptacles that reward crows with food when they dispose of litter. Lauren Stanton, a PhD student in Benson-Amram’s research group, is currently designing tools that may one day be used to train raccoons in a similar fashion.

“If [some of these animals] are seen less as pests and more as intelligent animals that can solve problems, maybe that will help reduce conflict [as well],” adds Benson-Amram. “Humans might be more tolerant of sharing a space with a species they know more about.”

If we want to finally reach a treatise, it’s time to reevaluate our own cognitive framework. Perhaps “we’re the real pests,” according to MacDonald. After all, they were here first: “we’re the ones that moved into their land, stayed and continued to grow.” By initially forcing these creatures out of their natural habitats, we have goaded them into assimilating into ours. Their invasions are out of necessity in the fight for survival, rather than malice.

Maybe it’s time we stop expecting the world to evolve around us. Intentional or not, we made our own demons, and now it’s our responsibility to at least meet in the middle.

“It takes a change in our own perspective to accommodate these animals,” says Justin Brashares, an ecologist at the University of California at Berkeley. “Animals are constantly adjusting, but if we care about this and want coexistence, then we need to adjust too.”

Editor's Note, July 10, 2018: This article initially misstated that Benson-Amram’s lab designed trash receptacles that reward crows with food. Other researchers actually performed the reserach.

Katherine J. Wu | | READ MORE

Katherine J. Wu is a Boston-based science journalist and Story Collider senior producer whose work has appeared in National Geographic, Undark magazine, Popular Science and more. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University, and was Smithsonian magazine's 2018 AAAS Mass Media Fellow.