Home & Garden Columns

It’s the Year of the Rat again‚—but which rat? For most of us “rat” signifies Rattus norwegicus, the Norway, brown, sewer, or wharf rat, progenitor of all those rats in all those labs, whose original homeland was northern China. But a case could be made for a less-well-known relative with roots in Asia: Rattus rattus, the black, roof, house, or ship rat.  

I once interviewed Garland Buckner, an East Bay exterminator (doing business as Rat Patrol), for an article in another venue. He said 80 percent of his clients had roof rats: “They’re more of a problem in newer neighborhoods. Norway rats are more in older neighborhoods, up in the hills or in inner city areas.” 

That makes Berkeley sound like Norway rat territory, but I suspect we have black rats as well. Once, traveling down Haste Street in mid-afternoon, I saw a rodent shape scurrying along a telephone line overhead. Squirrel, was the first reaction, then: no, the tail’s wrong. Black rats are more adept climbers than brown rats, more prone to occur in trees and upper stories of buildings. Conversely, they avoid sewers and other damp places. 

Behavior is not an infallible clue, though. “A sewer rat will climb trees and a roof rat will go underneath houses,” Buckner told me. “A rat is a rat.” Be that as it may, there are ways to distinguish black rats from brown. Many black rats are in fact black-furred, with or without lighter bellies, although white, gray, and agouti variants exist. The clinching detail seems to be the shape of the upper first molar, if you’re that close.  

When the two species meet, brown rats dominate black rats and have been known to kill them. Although they’re not really from Norway, brown rats are also more cold-tolerant than their relatives. 

Socially, black rats live in hierarchical groups with a semi-dominant male. Home range is typically about 120 square yards. Females are said to be more aggressive than males. The species is omnivorous, but prefers vegetable matter if available.  

Despite competition from the brown rat, the black rat has done well for itself. It started out in South Asia. Archeological evidence indicates that it had reached the Middle East by 3000 years ago, and the western Mediterranean in another 600 years. Ships have been the black rat’s second home, taking it to remote oceanic islands (where it has raised havoc with nesting birds), even the Antarctic. 

Australian geneticists recently used mitochondrial DNA—the stuff we all inherit from our mothers—to trace the black rat’s travels. Ken Aplin of the Commonwealth Scientific and Industrial Research Organization says six different lineages have been identified, originating in India, East Asia, the Himalayas, Thailand, the Mekong Delta, and Indonesia. According to Aplin, it’s the Indian stock that reached the Americas, as well as Africa and Australia, during the Age of Sail. East Asian black rats island-hopped from Taiwan to Japan and the Philippines, then moved out into the South Pacific. The other four lineages haven’t traveled as far. 

The black rat, as a vector of plague and other diseases, has a lot to answer for. But it’s a third species, the Polynesian rat (R. exulans), that’s being blamed for major environmental destruction on some Pacific islands. Among many other things, Polynesian rats eat palm fruit. Excavations on Easter Island found thousands of rat bones and rat-gnawed seeds of the Jubaea palm. Archeologist Terry L. Hunt says fruit-eating rats, not improvident islanders, were responsible for deforesting the island. Ironically, the Polynesian rats of Easter Island were driven to extinction by late-arriving ship rats. 

As for our local rats, black or brown, it’s understandable that no one wants them underfoot, or in the attic, or in the fan palm in the front yard. But—and yes, it’s soapbox time—there are better ways of controlling them than heavy-duty rodenticides. Buckner, the exterminator, doesn’t use poison: “When they eat poison they go out somewhere else to die.” Last summer two juvenile Cooper’s hawks found dead in West Berkeley tested positive for brodifacoum, the lethal ingredient in D-Con, Talon, and Havoc rodent baits. The raptors had likely eaten poisoned rodents.  

Brodifacoum, a second-generation anticoagulant, may take several days to kill a rat. This widens the window of vulnerability for wild rodent-eating predators, as well as household pets. The American Bird Conservancy has compiled hundreds of records of birds of prey—including red-tailed hawks, great horned owls, and golden eagles—poisoned by brodifacoum. Many of these birds are dedicated rodent-eaters, our best natural allies. 

The consensus among environmental agencies, bird advocates, and many pest-control professionals is that rodent problems are best handled preventively, through exclusion and sanitation. Trapping may be necessary if rats breach the barriers. And no, they don’t get a break because it’s their year. 

Joe Eaton’s “Wild Neighbors” column appears every other Tuesday in the Berkeley Daily Planet, alternating with Ron Sullivan’s “Green Neighbors” column on East Bay trees.