The ‘super-rat’ a new concrete threat?

A 1982 Canadian film, Deadly Eyes, presents an invasion of Toronto by concrete-gnawing rats plumped up to the size of dachshunds by steroids.

Their proportions were no coincidence — some of the stars were dachshunds dressed in rat suits. However, recent headlines might lead even a reasonable person to suspect that the world is falling prey to a breed of super-rats.

Last year, Swedish newspaper Aftonbladet reported on a so-called "Viking Rat" that gnawed though the concrete wall of a home belonging to the Bengtsson-Korsas family. Measuring 40 centimetres long from nose to rump, the rat reportedly terrorized the family cat before succumbing to an industrial-strength rat trap.

Rat expert Steven Belmain, professor of ecology at the University of Greenwich in England, frequently finds himself dispelling myths about rats. While individual specimens may tip the scales, most rats are standard issue.

"Stories of rats of alarming size have been with us for a long time," says Belmain.

"This is partly because we often don’t get a good look at rats, and when we do see them, they look big.  When it’s cold, rats have thicker fur coats, but also many urban rats are particularly well-fed and in environments with relatively little predation can reach large mature sizes.  Many of these reports are about the larger Norway rat, however, escaped pet rats sometimes get the blame — normally African pouched rats."

The Norway rat, also called the brown rat, is the most common found in Canada and typically grows to 25 centimetres long, tail excluded. The slightly smaller black rat is restricted to the coast of British Columbia. Thanks to vigorous pest control measures, rats don’t live in Alberta — at least not for long.

The notion of rats "eating" their way through concrete isn’t exactly accurate, Belmain says. They "gnaw" their way through construction materials, mouths closed.

"Rats need to gnaw as their teeth grow continuously and gnawing keeps them sharp," says Belmain. "Rats can certainly gnaw through concrete and metal, most normally soft metals such as tin, aluminium, copper and lead, but I have seen gnaw marks on steel, various hard plastics such as waste pipes and terracotta pipes — as well as concrete walls."

Alice Sinia, an entomologist and technical advisor with pest control specialist Orkin Canada, notes, however, that all concrete is not created equal, nor is it equally well-maintained. Substandard cinderblock may not be an impediment to a motivated rat.

"Rats will usually only gnaw at material softer than their tooth enamel," she says. "They’d prefer to gnaw through softer material instead of concrete."

She notes that an integrated pest management (IPM) program begins with exclusion — ensuring that rats can’t easily get inside a structure. That’s the responsibility of architects, builders and eventually owners.

Concrete construction should be free of gaps. Even a hole 13 mm in diameter is large enough to allow a rodent to squeeze through. Gaps around pipes, vents and utilities are prime entry points.

Foundations should also be designed with no loose soil around the perimeter, where rats might dig.

Belmain notes that some construction products, such as plastic, incorporate unpleasant tasting materials to discourage rats.

"These do not always work as rodents do not taste everything they gnaw through," he says. "They close off their mouths as they chew through things they have no intention of eating."

Eliminating food sources is the second major strategy of IPM. If there’s no food inside the building, there’s no incentive to gnaw at concrete.

"Rats generally seek shelter only in places where they first find food," says Sinia.

"If they are already living inside, we prefer to use mechanical traps and avoid rodenticide to manage the rat population."

Finally, it’s unlikely that rats are becoming larger, says Belmain. While they’ve quickly developed resistance to anticoagulant rodenticides over the past 50 years, growing larger isn’t exactly an evolutionary advantage.

"The abundance of food in urban areas may drive large sized rodents, but other factors will drive towards smaller rodents," he says.

"The premise of rats getting larger does not account for other needs such as being able to get into small spaces or escape predators. We simply don’t know what factors will be more important in a changing environment."

Sinia says she agrees, noting that there’s no evidence of the development of Canadian super-rats.

"However, nature is very interesting," she says. "In nature, never say ‘never’."