Vermin: The story is told of a couple of American scholars of Yeats's poetry coming to Ireland in the early days, before the tourist board had discovered the pulling power of dead Irish writers, and arriving in Sligo at the shores of Lough Gill in search of the Lake Isle of Inisfree.
The pair had been advised to consult a particular local boatman who was said to have the most comprehensive knowledge of the lake. Inisfree was an elusive place, and even Yeats, searching for it years after the composition of the poem, was unable to find it, to his frustration and no little embarrassment. And indeed, when the Yeatsians located their boatman, he was at first unhelpful, saying he had never heard of an island called Inisfree. However, after an interval of rumination the light dawned, and he said brightly: "Sure, it must be Rat Island you're looking for!"
Robert Sullivan would not have been disillusioned, as no doubt the scholars were, to hear that Yeats's magical isle was known locally as the haunt of rodents - in fact, he would have been fascinated, and furthermore would have hired the boatman to take him there immediately. Sullivan, an author and journalist and a regular contributor to the New Yorker, freely confesses to feeling the usual human revulsion in the face of rats, which "can sometimes symbolize anxiety, for me, fear of the worst", yet he also sees them as clever, tenacious, brave and, in many surprising ways, just like us. He even presents the feat of the "large and scraggly rat" which, in what may be only an urban legend, swims through the sewer system and up through the toilet bowl into your house, as an "act of sublime magnificence" that should give us hope for the future of mankind. This, as might be guessed, is a man writing in desperate times, in the absent shadow of the Twin Towers.
Rats is the record of a year-long nocturnal rat-watch which Sullivan conducted, beginning in the summer of 2001, in Edens Alley, a filthy defile backed on to by a refuse-rich Chinese restaurant and an Irish pub, situated a block or two from Wall Street, from Broadway, and from what at the start of his vigil was still the World Trade Centre. He claims no expertise in the matter of rodents, and his book is no way scientific. It reads rather like an account by one of those 19th-century amateur naturalists of an expedition into a Conradian heart of darkness, except that in his patch of urban jungle Sullivan finds much that is illuminating. "All I did," he writes, "was take a spot next to the trash and wait and watch, rain or no rain, night after night, and always at night, the time when, generally speaking, humans go to sleep and rats come alive."
The spark for his fascination with rats was a painting of a group of them in a barn stealing a chicken's egg, done by the man he calls one of the patron saints of American naturalism, John James Audubon, author of the legendary Birds of North America. Born in the Dominican Republic, Audubon settled in New York on today's 157th Street, a neighbourhood that is now settled largely by immigrants from the Dominican Republic - "coincidence is the stuff of ratting!" Sullivan crows, with one of his many endearing exclamation marks. Audubon's trademark claim was that all his wildlife studies were "drawn from nature", and Sullivan, realising that "rats are ignored or destroyed but rarely studied, disparaged but never described", determined to follow his hero's example and, as he writes in excited italics, "draw the rat in nature".
He opens with some essential statistics on the brown rat, Rattus norvegicus - in fact, it is likely to have reached American shores from Denmark rather than Norway - the species which pushed out the notorious plague- carrying black rat, Rattus rattus, to become the King Rat of New York City. The brown rat is brown or grey, "with a belly that can be light gray, yellow, or even a pure-seeming white". It has poor eyesight, but a keen sense of smell - it is thought that the reason they bite young children on the face is because of the residues of food around their mouths which the rats can scent.
The brown rat's teeth are yellow, the front two incisors being especially long and sharp, like buckteeth. When the brown rat bites, its two front teeth spread apart. When it gnaws, a flap of skin plugs the space behind its incisors. Hence, when the rat gnaws on indigestible materials - concrete or steel, for example - the shavings don't go down the rat's throat and kill it.
The rat's incisors, which grow at a rate of five inches a year - that is a lot of gnawing - are as tough as steel, and can exert a pressure of 7,000 pounds per square inch. When it is not chewing on water pipes or electricity cables, eating, or digging holes, the rat is copulating. "Most likely," Sullivan cheerfully informs us, "if you are in New York while you are reading this sentence or even in any other major city in America [or, presumably, Ireland], then you are in proximity to two or more rats having sex." According to one report, "a dominant male rat may mate with up to 20 female rats in just six hours". Nor is Mr Rat particularly choosy: he has been known not to notice, or care, that the female with which he is consorting is dead, and in all-male colonies he will happily turn to, or turn-on, his fellows.
With so much sex occurring, reproduction rates are predictably high. The gestation period is 21 days, the average litter is eight to 10 young, and the female, never daunted, can become pregnant again immediately after giving birth. One pair of rats, therefore, can have 15,000 descendants in a year. This is a large number of rats, Sullivan concedes, but then goes on mildly to point out that Hans Zinsser, in his classic study of the history of human diseases, Rats, Lice, and History, "suggests that the fertility rate of the human can rival the fertility rate of the rat". Man, be not proud.
Our notions of the prevalence of rats are curiously confused. Many people in New York firmly believe that there is a rat for every one of the eight million inhabitants of the city - in fact, the number of rats in New York is more likely to be about 250,000, a surprising figure, given their busy sex lives - yet few of them realise in what close proximity they live to rats, or how shakily the human environment rests upon the subterranean tunnel-works of rat colonies. Sullivan reminds us repeatedly that the rats we see above ground are merely those which have been driven by their stronger brothers to come up from the underworld and forage for food. He cites one exterminator who points out that there are three sewerage systems underneath New York, the one built in the 1900s, below that the one built in the 1700s, and below that again the one that dates from time immemorial, in the last of which there must live a breed of rats who have never seen a human being.
Although it is difficult to do so, we should keep in mind that rats mean us no harm. This is not to say that they do not harm us, but the harm they do is inadvertent. Their energies are devoted to survival - getting food, having sex, rearing their young - and despite the odd and rare exception, they will avoid us when they can. They are not the malevolent, murderous force that haunts the popular imagination. So far as can be ascertained, man, and some of the higher apes, are the only animals which will kill for no reason other than fun. All the same, in the past century, according to Sullivan, rats have been responsible for the deaths of more than 10 million people, and rats may devour as much as a third of the world's food supplies.
Then, of course, there is plague, although even the spread of that dread disease is not the doing of rats themselves, but of the rat flea - "about the size of this letter o and . . . shaped like a miniature elephant" - which, as with rats and sinking ships, will leap from a dead rat on to a live human being, "as a kind of second choice", and inject a few million plague bacilli into his or her bloodstream. And do not imagine that all this is history: "there are more rodents currently infected with the plague in North America than there were in Europe at the time of the Black Death, though the modern rodents infected (prairie dogs, for example) tend to live in areas less populated by humans . . ."
Rats is haunted by the events of September 11th, 2001. Sullivan, who is either a primitive in the style of the Douanier Rousseau or a sly old fox like Mark Twain - it is hard to decide which, such is the oddness of his prose style - makes many seemingly accidental connections between the city's resilience after that atrocity and the irrepressibility of the rats under his observation. In his chapter on the aftermath of 9/11, aptly titled 'Winter', he writes of returning to Edens Alley to find that the health department had cleared the trash, sealed the rat holes and put down poison - had, in a word, "sanitized" the area. The rat-watcher is inevitably cast down: "I didn't have much hope for the rats. I also had mixed feelings about having any hope for rats." Life will assert itself, however, and soon young rats begin to appear again in the alley, bringing promise of "rat regeneration, of rat life reborn in this still-winter alley, in the wounded but healing city". Sullivan is aware of the possibility of bathos in such musings, yet he is unapologetic: "I know that if you look deep down into the darkness, even in a rat hole, there is some life down there, some fecund spark, like it or not."
In his notes, Sullivan describes one of his source-books as being "gloriously serene in its nonhysterical description of rats", and the same might be said of his own account of his year in the alley with nightscope and Thermos flask, studying the desperate and relentless struggles of Rattus norvegicus to live and thrive and procreate. "I think of rats as our mirror species," he writes, "reversed but similar, thriving or suffering in the very cities where we do the same. If the presence of a grizzly bear is the indicator of the wildness of an area, the range of unsettled habitat, then a rat is an indicator of the presence of man."
John Banville's new novel, The Sea, will be published this year by Picador
Rats: A Year with New York's Most Unwanted Inhabitants, By Robert Sullivan, Granta, 243pp. £12