Other project scientists began to map the basics of rat population dynamics, concepts that, Glass says, inform the way we manage endangered species today. Researchers noticed, for instance, that wiped-out blocks took time to repopulate, even though there were rats aplenty in all the surrounding blocks. Eventually, though, the rats almost always bounced back to their original numbers, the “carrying capacity” for that block.
Scientists even pinpointed rats’ absolute favorite foods; they relish macaroni and cheese and scrambled eggs and detest celery and raw beets. Their tastes are, in fact, eerily similar to ours.
Glass – who started off studying cotton rats in the Midwest – traps the animals with peanut butter baits and monitors the diseases they carry. (Hantavirus, once known as Korean hemorrhagic fever, and leptospirosis – which can cause liver and kidney failure – are of particular concern.) Lately he’s been interested in cat-rat interactions. Cats, he and his colleagues have noticed, are rather ineffectual rat assassins: they catch mainly medium-sized rodents, when they catch any at all. This predation pattern may actually have adverse effects on human health: some of the deceased mid-sized rats are already immune to harmful diseases, while the bumper crops of babies that replace them are all vulnerable to infection. Thus a higher proportion of the population ends up actively carrying the diseases at any given time.
Rats still infest Baltimore and most other cities. A few years ago a city garbage truck was marooned in the very alley we were touring, Glass says: rats had burrowed underneath until the surface caved in, sinking the truck to its axles. The rodents soon overran it, and its fetid load furnished quite a feast.
Even the poshest neighborhoods are afflicted: rats, Glass says, gravitate to fancy vegetable gardens, leaving gaping wounds in tomatoes. (Celery crops, one assumes, would be safer.) Recent surveys suggest that the rat populations of Baltimore neighborhoods haven’t changed much since the Hopkins studies began in the 1940s.
Yet we hadn’t glimpsed a single one on our stroll. Glass stopped suddenly in front of a junked-up yard and listened. “I didn’t see a rat, but I heard one,” he whispered. Rats – though adept at scurrying furtively – are actually quite vocal: they squeak, shriek and hiss. They also emit a series of high-pitched chirps inaudible to humans, which scientists believe may be the equivalent of laughter.