The House Mouse

(This column was first published in the February 24, 2003 issue of The Buffalo News.)

There is a word in the biological vocabulary that is often used to describe the house mouse's relationship with us. It is commensal.

Derived from the Latin mensa for table, in Middle English commensal meant sitting at the same table or sharing a meal and the contemporary technical meaning of the word reflects that use: a close association in which one species is benefited while the other is unaffected.

Indeed house mice do share our meals. They are represented by the City Mouse of the La Fontaine fable who dines richly on our leftovers, deeply impressing his country cousin -- until the cat shows up.

That may be but there is slippage in their commensalism. House mice will indeed eat what we eat but their diet goes well beyond ours to anything organic as well. They prefer grains and seeds but will also eat insects, soap, paper and the hardened glue of bookbindings. Some are even cannibals: one was observed eating a family member caught in a mousetrap.

Worse for us, they strip electrical insulation, creating in the process a serious fire hazard. (Although the insulation is chewed, little of it is digested, most carried away for nest construction.)

And there is a final slippage in house mice being our commensals. Especially in warm weather they often desert our homes and barns to take residence in wheatfields, cornfields or woodlands.

Whatever defines our relationship, as soon as civilization began, house mice associated with humans in Asia and later Europe, their rodent ancestors a wild species inhabiting Turkestan. They then came to North America with the earliest explorers.

They should not be confused with our native mice, voles and shrews, some of which also invade our homes. The house mouse is grayish brown, somewhat lighter or buffy on the belly. Its fur is short and occasionally appears glossy. It has beady eyes, a long, pointed snout, large naked ears and a naked, scaly tail. You can often identify it before you see it by that unpleasant odor.

They are, you must admit, remarkable little rodents. The National Pest Control Association has called them "the Houdinis of the animal world" for one can dive through a hole scarcely more than a quarter inch in diameter. They can climb stairs as well as we can as they can leap a foot straight up. They can dash around at up to 8 mph. They can even sing a canary-like trilling.

And, wow, are they prolific. A female can mate at two months of age and nurture up to 14 litters a year, each litter producing 5-11 young. Making conservative assumptions, I calculated that a single female and her progeny could add over 4600 mice to the population in a single year. No wonder geneticist Lee Silver has described them as, next to us, the "most successful mammalian species living on earth today."

Of course, that population estimate assumes no predation by hawks, owls, snakes, foxes, coyotes -- and us. When predator populations are reduced, however, house mouse populations can soar. In California, for example, over 80,000 per acre were found in one field and two tons were killed in a single granary. (Each mouse weighs only a fraction of an ounce.)

Bad news, but we shouldn't forget that albino house mice have served as pets and laboratory experimental animals since 1900 when retired schoolteacher Abbie Lathrop began to breed them on her Massachusetts farm for sale through pet shops. Her neighbor, Harvard scientist William Castle, found them perfect for research, and soon Lathrop was meeting orders from what became an 11,000 mouse colony.-- Gerry Rising