Roof rats are hardy and breed like machines.
They can jump 3 feet in the air, fit through a hole the size of a nickel, climb like monkeys, swim and roam for miles.
They don’t need much water and can eat almost anything: Garbage, vegetation, even dog feces.
They even appear to have inside help in their steady march across the Valley — people who are releasing them, both accidentally and intentionally, into new areas.
"Unless you live in a heavy desert landscape area, chances are you are going to have roof rats at some point," said Larry Jech, Maricopa County Vector Control supervisor. "The message is ‘They’re coming.’ Don’t count yourself lucky yet."
Exactly when they’ll arrive in any particular back yard is less certain.
The rats first broke out in the Valley two years ago when they appeared in the Arcadia neighborhood in east Phoenix and Scottsdale.
This winter, roof rats were caught in Mesa, Glendale, Tempe and Ahwatukee Foothills. Their Phoenix and Scottsdale presence has expanded considerably. Jech said the rats have probably spread to Chandler, though he can’t prove it yet.
"They’ve been here longer than people imagine," he said.
Inside the vector control office, a portable building in a south Phoenix county complex, Jech and his co-workers are still trying to figure out exactly how the rats are spreading. They track public complaints and drive out to pick up live and dead rats that people catch.
The East Valley rats could be a different strain from the descendants of rats in the Arcadia area, Jech said. The Arcadia rats had "mousy gray" fur, and the rats now being found in other locations are mostly brown, he said.
Jech said he doesn’t know whether this means the latest rat findings represent a whole new introduction from some other part of the country, or just that the Arcadia rats have begun to change their look.
"They may be reverting back to their normal color," he said.
The county may eventually conduct DNA or other identifying tests on the rats, but that’s not being done now, he said. Blood from live rats obtained by vector control is drawn and tested, but only to determine if they carry hantavirus, plague or tularemia.
The "good news" about the growing rat infestation is that no trace of those diseases has yet shown up in the Valley or elsewhere in the United States, Jech said.
An outbreak of disease in roof rats could be a problem, but would likely be restricted to a small area, said Gregory Glass, a professor in molecular microbiology at Johns Hopkins University in Baltimore.
Glass, a former wildlife biologist, said he began studying rat behavior because no one else was doing it. Arizona may be the latest home of roof rats, but the problem began growing dramatically 20 or 30 years ago in communities across the country, he said. That’s about when federal rodent control programs that began after World War II dried up, he said.
Though the rats are attracted to semi-tropical areas such as Southern California, Glass said he couldn’t think of a single urban area that hasn’t had problems with them.
"It certainly impacts the quality of life in neighborhoods," he said. "It can be one of those situations that exacerbates existing problems, particularly in neighborhoods that are marginal and could go either way."
Glass said he had no better explanation for the rats’ method of expansion in the Valley than local experts.
"If they really have spread that far, either the rat populations were really astronomical from where they started, or they are being continually reintroduced and brought in from some places, or they were present at low levels for longer than people noticed," he said.
Now that the rats have managed to cross the desert by hitching rides in motor vehicles or airplanes, they seem to be finding the Valley quite a paradise. They love citrus and garbage to eat, oleanders and attics to hide in, and water to drink. Humans provide all of that.
"They disperse very effectively to wherever there is food, water and shelter," said Kevin Wright, director of conservation for the Phoenix Zoo. "Even when they are 3 to 4 months old, they can disperse about a kilometer."
Like many other animals, roof rats occasionally give birth to offspring programmed genetically to move in a straight line for as long as possible, Wright said. Called roamers, these rats look for mates that are not relatives and might travel for miles, opening new territory for successive generations of rats, he said.
In every pair of rats lies a potential baby boom. Females only 3 to 4 months old can start having offspring. They can have up to four litters a year, each containing five to eight young. In urban areas long cleared of predators such as coyotes and rattlesnakes, "you can assume a fairly high survival rate," Wright said.
Officials have encouraged Valley residents to take measures such as picking up fallen citrus and clipping tree branches away from roofs to slow the rats’ expansion and keep their numbers low.
As some people fight the rats’ natural progression, however, others seem to be accelerating it. Experts theorize that landscaping or garbage trucks loaded with debris and parked overnight may have transported some rats inadvertently within the Valley.
Another theory is that rogue pest control operators are catching and releasing live rats out of greed or ignorance. A Scottsdale woman last month reported seeing an exterminator doing just that at a park, though neither city nor county officials have investigated the incident.
Barry Paceley, a Phoenix resident who helps run antirat programs, said some callers to his group’s help line have admitted to catching live rats in a humane trap and then letting them loose in the desert.
Unfortunately, Paceley said, for some people that desert ended up being the Phoenix Mountain Preserves surrounded by residential neighborhoods.
"They just don’t know what to do with them," he said. "They aren’t going to drive an hour out to dump them."