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.