To understand the rats around you, look in the mirror. Every human society determines how rats live among them. Portland, Oregon, made choices early in its history that have resulted in a more benign relationship between people and rats than most cities enjoy.
The founders of Portland seem to have understood that the life of a city is shaped by how it deals with the garbage it creates. As far back as 1890, residents sorted their garbage (into burnable and non-burnable) and scolded each other for not doing it right.
The law most relevant to Portland's rat population, passed in the 19th century and still enforced, requires that trash be kept in sturdy rat-resistant containers with lids that fit tight. Compliance and enforcement have, predictably, drifted over the decades. But this custom has by now constrained over a hundred generations of rats.
Meanwhile, all that many other cities require for refuse is a plastic bag, which a rat can pierce with a stern look. Urban refuse is a cornucopia of carbs, fat, and protein. According to one study, rats' favorite food in garbage is scrambled eggs, followed by macaroni and cheese. Beets are their least favorite.
Portland rats, according to Christopher Roberts, the rat specialist at Oregon's Multnomah County Vector Control, make do on a leaner diet of birdseed, backyard compost, pet food left outside, and garden veggies—including beets.
Sometimes rats luck out and discover unprotected garbage at a negligent home or business. Roberts has the power to fine violators, but he would rather educate than punish. "I give them a sense of, 'You can do this,'" he says.
Whether or not rats have free access to garbage matters to a city because the more rats eat, the more babies they make. The genius of this species is its rapid and precise adjustment of population to food supply.
That makes hunger the quintessential rat contraceptive. So a place like Portland, which keeps them on a diet, should have fewer rats.
In 1949, scientists from the Rodent Ecology Project of Johns Hopkins University spent six months counting the rats of New York. They examined backyards and basements in representative city blocks, interviewed residents, and toured markets. Extrapolating from blocks to districts, they estimated the rat population of New York's five boroughs to be 250,000, one rat for every 36 New Yorkers.
The researchers were impressed. They had previously counted rats in Baltimore, where they found more than twice as many per capita: one rat for every 15 Baltimoreans, or 60,000. The scientists praised the pest control professionals of New York for "practic[ing] sanitation in addition to the temporary killing procedures. Thus, the pests are really reduced." To these experts, poison and snap traps only accomplished "temporary killing"! Ecology is all.
The 1949 paper also gives credit to New York's "history of 10 to 20 years of efficient health and sanitation services," meaning slum demolition, housing improvement, and trash collection.
New York has, unfortunately, become more favorable to rats since then. The city used to require actual garbage cans, but switched to plastic bags for convenience in the 1960s, according to Benjamin Miller, an expert on NYC trash history.
How many rats a city has isn't that important. What matters to city-dwellers is how crowded the local rats feel, because a crowded rat and a rat with room to stretch out might as well be two different animals. The denser a population of rats, the more desperately they defend what they have and try to capture more—just like people.
In the absence of crowding, rats are civilized and family-oriented. We know this because in 1947 a young scientist named John C. Calhoun set up a rat colony on a quarter-acre outside Towson, Maryland. He tracked the colony for over two years, charting the life history of every descendant of his ten original pioneers.
For the first year or so, Calhoun's rats lived bucolic lives. Females dug connected burrows and shared childcare. Males controlled and protected harems of several females. When a female rat went into heat, she mated with her man. Females produced healthy litters once a month in the warm seasons. When the pups grew up, they had room to establish new colonies, dig burrows, and raise the next generation. Not a bad life.
The good life didn't last, because the point of Calhoun’s research was to simulate conditions in the wild—the urban wild of bottomless garbage bags, that is. So he provided them with unlimited rat chow. Life in the colony went on, converting chow into more and more rats, with less and less room to put down burrows.
Most young males no longer established territory—they joined roaming bachelor packs. When a female in heat stepped out of her burrow, hordes of males ran her down and mated with her hundreds of times a night. Such stressed lovemaking stunted fertility. Pregnancies miscarried. Females who delivered live pups raised few of them, because their sickly babies died—and because mom might relieve her own distress by eating them.
Overcrowding made social status central to existence. High-ranking rats held the best territory. In these good neighborhoods, alpha males protected top-ranked females. Those females mated with their alpha male and bore healthy litters. Some offspring stayed near the high-class burrow, while the rest left for the slums.
No scientist that has studied rat colonies has allowed only limited amounts of food—the Portland condition. We do have evidence, albeit indirect, that Portland rats are different from rats in cities that allow free access to garbage.
Victims of rat bites are usually children, usually sleeping children, and the wounds are usually to the face and hands.
Emergency rooms in the five boroughs of New York City see at least 400 rat bites a year. Nationwide, ERs treat about 3,000 people a year for rat bites, on average one person per 100,000.
Calculating from that statistic, Portland-area emergency rooms should expect to see 15 patients a year with rat bites. But according to DeBess, the state public-health veterinarian, Oregon registered only 17 rodent bites over the two years from 2010 to 2012, not one of them in or near Portland. When I called a half-dozen local clinics for poor and homeless residents, no clinic employees could remember any rat-bitten patients, ever.
Maybe Oregonians taste funny. (It's probably all that kale.) Or, maybe Portland rats behave less aggressively because they're not densely overpopulated.
Rats excel at infecting others. They catch and pass on dozens of horrifying diseases, including salmonella, hepatitis, tularemia, plague, and a handful of parasites.
Sickness spreads faster and further through crowds. The lower the population density—that is, the farther away the next rat—the harder it is for a pathogen to jump from one rat to the next.
One rat-borne bacterial disease—leptospirosis—is increasing in the Pacific Northwest, according to Emilio DeBess, Oregon's public health veterinarian. Lepto-infected rats deposit the bacterium in puddles, creeks, and wet dirt when they urinate. Humans and dogs catch the disease by playing in or drinking infected water. Lepto attacks the liver and kidneys, most often causing a flu-like illness, but it can be fatal.
In Detroit, 90 percent of rats carry lepto bacteria. Sixty-five percent of Baltimore alley rats are infected, 60 percent of Atlanta rats, and 53 percent of Copenhagen sewer rats.
DeBess studied Portland rats, and found that just 14 percent carried lepto.
Early 20th-century rats brought bubonic plague to almost every major West Coast city. The bacterium first came to North America in 1899, by ship from Hong Kong to San Francisco. An epidemic in the city proper began in 1900, but officials kept it secret to protect business. The outbreak fizzled out in 1904 after 122 died.
The 1906 earthquake that shook San Francisco to ruins provided a healthy environment for the plague bacillus and the fleas and rats that carted it around. A second epidemic began in 1907, this time not a secret. Every port city in the West quaked at the prospect of the Black Death heading their way, and acted to bar the pestilence with quarantines and rat-killing campaigns.
In Portland, Esther Pohl, the city health officer, launched a citywide rapid response. The City Council passed emergency laws to fumigate all ships coming into port, install screens on every building containing food, and pay bounties for rats, which were burned immediately, together with their deadly fleas.
Few other cities moved urgently enough or rigorously enough to keep this little bacterium out. Seattle, for example, moved slowly while the infection took hold. Three people died there, and the plague bacterium escaped into the countryside.
Over a century later, Portland and its environs have never seen a case of plague in humans or any other animals.
Another major influence on Portland's rats, after garbage control, is a tradition of practical collaboration, brought over with the covered wagons on the Oregon Trail and reinforced on the frontier.
Many of the dangers of life in a frontier settlement, such as fire and dysentery, arose from the faults of a few but threatened everyone. Working together to avert those dangers clearly benefited all. From its first years, Portland staged regular cleanup campaigns to keep waterways sewage-free and to clear debris from streets. Settlers of the town, rich and poor, knew from experience their lives depended on the well-being of all their neighbors.
Historian David Alan Johnson, in his book Founding the Far West, finds Oregon's founding ideas—civic duty and the common good—in the situations people left behind when they started out on the Oregon Trail. Most of the emigrants were farmers, ruined by financial volatility in the 1830s and '40s. They wanted nothing to do with rampant self-interest or the panics it produced.
Rat control, then and now, depends on the well-being of other people. Rats reflect human social class, but they do not respect it. The rich family is vulnerable to rats nourished by garbage in the poor family's yard.
Trash collection and enforcement have by no means been consistent or equitable across all neighborhoods for all of Portland's history. But rats there have been kept hungry enough, over the long haul, to suppress population growth.
Citizens anywhere can do what Portland has done—and they can start by helping their neighbors put up rat-proof screens and clean up their yards.
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