In downtown Boston, at one of the city’s finest restaurants, a roomful of well-heeled locals is tucking into plates of sesame-crusted salmon and filet mignon au poivre. Out back, meanwhile, a dozen or so diners are having a feast of their own: stinky cabbage matter to start, followed by fatty trimmings in a tepid grease sauce, all washed down with a nice drop of Château Urine de Drunkard. You can practically hear their little salivary glands kicking into overdrive as they skitter in and out of an open dumpster. Rudely interrupted by a couple of guys from the city’s Inspectional Services Department, the critters squeal and scatter. But they’ll be back. Rats always come back.

AT THIS TIME OF year, Boston’s rats normally aren’t out and about enjoying the weather. Normally, they are chattering away in their lairs — hungry, chilly, and miserable. Rats cannot stand the cold. It depletes their food supply. It freezes their drinking water. It blunts their otherwise ravenous sexual appetite. It leads to internecine combat, infanticide, and cannibalism. In short, cold weather kills rats off.

But it’s not only the weather that gets them. Winter is also the time when cities step up their rodent-control efforts. You hit the rats when they’re down, goes the argument. Get them in their nests. Bait traps with Slim Jims and peanut butter and break their little backs. Feed them blue-green blocks of poison and wait for their bellies to burst. Block up their burrows with cement and wire mesh. Leave the sewers strewn with rotting rodent corpses. Degrade their environment, deplete their numbers, interrupt their breeding patterns. What rodent-control people don’t want to be doing at this time of year is chasing rats around. But this is not, by any means, a normal year.

"In theory," says burly pest technician Stephen Trundle, as he sets a trap at 5 a.m. in the shadow of the Zakim Bunker Hill Bridge, "I shouldn’t even be out here now." No, he shouldn’t — and neither should the rats. But this year’s unseasonably warm weather has led to a rat jamboree in the city. This winter, the rats are having a ball — pigging out in the trash cans, cavorting in the alleyways, and copulating in full view of whomever happens to be around. "I’ve seen them mating in the street in January," says Frank Fothergill, the Central Artery Project’s resident rodent-control expert. He shakes his head ruefully. "Playtime for the ratties."

Two rats bonking away in the middle of the street might be a somewhat unsavory spectacle, but these trysts have implications far more grave than their gross-out potential. An average New England winter would generally lead to a 30 percent depletion in the city’s rat population. By contrast, a mild winter like the one we’re having now is likely to kill only about five percent. What this means is that there will be more rats having pups, who will also have pups, who in turn will have pups of their own. This could lead to an inundation of rats come spring — a prospect that has pest-control people feeling a little jittery.

"Not only are more rats surviving, but they are breeding more," says Bruce Colvin, a Boston-based ecologist and one of the world’s leading authorities on rodent control. Furthermore, Colvin adds, a healthy crop of springtime rats will necessitate a more "reactive" approach to rodent control, rather than the "proactive" approach that today’s pest technicians favor. "If you spend your resources trying to chase individual rats," he says, "you miss the opportunity to manage the bigger issue, which is the environment.

"Prevention," Colvin stresses. "Prevention. Prevention."

The war on rats in Boston has two fronts. There is the Central Artery’s rodent-control division — whose jurisdiction covers a four-block-wide, seven-mile-long area along the construction site — and there’s the city-run Inspectional Services Department (ISD), which covers the rest of the city (with some overlap). The Central Artery’s division, headed by trained ecologists and funded in part with federal money, was set up in 1991 to combat a rising sense of rat panic in the city. Thanks to overzealous media and a few ill-informed politicians, Boston residents were led to believe that the Big Dig would cause millions of rats to come pouring up from underground, teeth gnashing, eyes blazing. The reality was a lot less dramatic.

"There were all sorts of wild stories being espoused," says Colvin, who helped draft the Artery’s anti-rat strategy back in the early 1990s. "It was thought that people would need to get snow shovels out to shovel up the rats. I had a colleague in Bangladesh — one of the poorest regions on earth — who called me and said, ‘What’s going on in Boston? All those rats!’ The first thing we had to do was gain control of all that myth and misinformation."

Part of the mythology bandied around at the time involved how the city would go about dealing with its rat problem. "People had watched too many Schwarzenegger movies," Colvin says, "too many Raiders of the Lost Ark movies." When the Big Dig’s anti-rat pack set to work on Boston’s rodents, there were no flamethrowers, no battalions of rat catchers armed with pointy sticks, only a group of dedicated, somewhat wonkish scientists implementing a new kind of rodent-control program that people in the field now call Integrated Pest Management, or IPM.

Traditionally, rodent control has proceeded on a see-one, kill-one basis. A complaint comes in. A technician races to the scene and lays a trap. Snap. End of story. The Artery people introduced a much more systematic approach to the problem. IPM's main principle is to create an environment that is essentially hostile to rat life. To do this, you need to get a wide range of city departments involved in the effort. Rats love low-lying shrubbery, so you have to coordinate with the parks department to minimize such rodent-friendly flora. If parking restrictions aren’t strictly enforced, then trash removal is hindered, so you have to get the traffic people in on the act, and so on. In effect, IPM represents a Coalition Against Rats.