Rats are nature’s mobsters and serial killers with phenomenal disease-carrying pandemic abilities; they are feared and for good reasons.

Rats, like people, have conquered all habitable continents. They feast on human garbage and survive in human effluvia.

Rattus norvegicus — the Norway rat or brown rat — is a rodent. This rat is large, 16 inches from the tip of its nose to the end of its tail, and weighs about a pound. Brown rats vary from gray to brown with a belly that ranges from light gray to yellow or white. Pet or laboratory rats are Rattus norvegicus and are born in captivity. Wild rats are fearless.

Black rats, or Rattus rattus, once inhabited all North American cities until the brown rat pushed them out. In some southern West Coast cities like Los Angeles, the two species still coexist. Black rats live in attics and palm trees, and are gray to almost black.

Rats are nocturnal. They forage at night and depend upon a keen sense of smell and touch because they have poor nighttime eyesight. In addition, they have an acute sense of taste that is able to detect poison down to one part per million.

Brown rats have strong feet outfitted with four clawlike nails. They are fast runners, agile climbers and outstanding swimmers able to navigate rivers, bays and sewer streams, and they surface in toilet bowls.

They have yellow teeth with two ever-growing, long, sharp incisors. They grow at a rate of 5 1/2 inches a year and their teeth are as hard as steel. When a brown rat bites, its two front teeth spread apart. When it gnaws, on the other hand, a flap of skin plugs the space behind its incisors.

Their bite is alligator-like and they exert an astonishing biting pressure of up to 7,000 pounds per square inch.

Rats are attracted to wires — utility, computer, vehicle — and gas and water pipes. It’s believed that the wires resemble vines and stalks of plants; in a way cables are the vines of the city.

Twenty-six percent of all electric cable breaks, 18 percent of all phone-cable outages and 25 percent of all fires of unknown origins are rat-caused.

When not gnawing or feeding on garbage, brown rats are legendary excavators.

They dig to enter buildings and burrow underneath concrete sidewalk slabs. Rats are able to collapse their skeleton and squeeze into a hole as small as 3/4 of an inch wide — about the width of their skull.

Tunnels lead into nests. At the back of the nest is a long tunnel that opens to another hole on the street — an emergency escape. Rats nest in basements, sewers, manholes, abandoned pipes of any kind, floorboards and in trash-strewn corners of subway stations or alleys.

A male and female will copulate up to 20 times in a day. A male will have sex with as many females as possible. One dominant male will mate with 20 females in six hours.

Females gestate for 21 days, and litter sizes range from eight to 10 pups. Females with access to a constant source of garbage will have up to 12 litters of 20 rats each year. One rat nest can become a rat colony of 50 within six months. And one pair of rats can potentially produce 15,000 offspring in a year!

Rats destroy one third of the world’s food each year.

When a flea, likely Xenopsylla cheopis, infects a rat with a bacteria carrying the bubonic plague or black death, the rat eventually dies, the flea senses a lower rat body temperature and jumps to a new live rat.

Rat fleas prefer to feed on rats. But when rats and humans live in close proximity, the fleas will feed on humans. The Roman Emperor Justinian witnessed 25 to 50 percent of his people perish in the first recognized black death pandemic in the 540s.

Black death struck Europe in 1338, and by 1365 only about one third of Europe’s population remained alive.

The plague arrived in Honolulu via Asia early in January 1900. It was terribly mishandled when it arrived in San Francisco on March 6, 1900, although immediately and correctly diagnosed by Dr. Joseph J. Kinyoun. Frighteningly, it still exists in the wild in North America today.

Alberta, Canada, is the only vast territory that is rat-free due to the world’s most formidable rat control program.

Dr. Reese Halter is a conservation biologist at Cal Lutheran University, public speaker and founder of the international conservation institute Global Forest Science. Follow him @ twitter.com/DrReeseHalter.

Thumbnail photo: markfftang/Flickr

The loathsome rat
They live in walls, eat our garbage and spread disease. But did you know they also can squeeze into a hole 3/4 of an inch wide?