A mouse's tale

IT'S A LONG AND TORTUOUS ROAD from the steppes of Asia to a rustic New England farmhouse and thence to superstardom in the world of modern science, but that is exactly the journey made over thousands of generations by the humble house mouse. This small creature, known to science as Mus musculus, adopted humans about 10,000 years ago, when the development of agriculture led to the invention of grain storage bins, and permanent settlements grew up around the fertile swath formed by the Tigris and Euphrates Rivers in what is now Iraq. Every crack and crevice in the new civilization provided the tiny rodents--able to squeeze through spaces less than half an inch wide--access not only to a cornucopia of food but to relative safety as well. Such a life must certainly have been a pleasant change from searching out seeds on open, unpredictable, and hazardous Asian grasslands.

From that time on, the house mouse has earned its common name, splitting its time between living with us and seeking adventure in the wild. M. musculus spread across the steppes of Asia from Turkey to China and hitch-hiked with humans to colonize much of the rest of the world. Today house mice are found from the equator to subpolar islands. They span the climatic range of modern civilization, too, having been discovered living, and even breeding, everywhere from heating ducts to refrigerated meat lockers.

Their zeal for cohabitation has not been generally reciprocated. Over the centuries, much human energy has been expended to keep mice out of homes and cupboards--to build a better mousetrap. Eventually, though, some humans began to think of house mice less as pests and more as pets. Our own species' compulsion to selectively breed any animal that can be kept as a pet led to a trade in "fancy" mice, those with odd coat colors and forms (including tailless, Manx mice). Bred for centuries in China, fancy mice caught on in a big way in Victorian England. Clubs promoted, and continue to promote (see "Fancy That!" page 69), officially recognized varieties. Fanciers hold regular competitions and shows in many countries, where ribbons, trophies, and prestige are awarded to the proud owners of the best-bred mice.

Breeding fancy mice became popular in the United States as well. Around the turn of the last century, Abbie Lathrop, a retired schoolteacher, turned her hobby into a business when she began to sell fancy mice from her farmhouse near Granby, Massachusetts. In 1902 Harvard geneticist William Castle purchased some of them to test new theories of mammalian inheritance. Thus was born the laboratory mouse. Today these descendants of house mice are indispensable to biomedical genetic research. It has become commonplace to turn individual mouse genes on and off or to soup up their normal activity in order to understand their function. As part of the research into the role of specific genes, researchers even insert human genes into the mouse genome. The mouse has indeed become an international superstar.

The two stages of our relationship with the house mouse have had very different biological consequences for the little creature. Stage one, the commensal stage, when mice lived furtively among us, probably did not change them very much. The warmer temperatures indoors and the steady supply of food did allow them to reproduce throughout the year, but as an opportunistic species, they had always been able to respond quickly to changes in their environment. Stage two, domestication, when we took control of their breeding to favor certain traits, was a different story. Selective breeding is the essence of evolutionary change. Usually nature provides the selection, but humans can as well. Animals evolve under domestication. They may grow larger or smaller than their wild ancestors. They may develop faster, become more fertile or docile, produce richer milk, a thicker coat, or more tender meat. This sort of evolution is obvious: these are the traits for which we breed.

The trait that breeders of fancy mice wanted first and foremost was docility. When handled, wild mice bite and attempt to escape. Anyone who has been the recipient of such a bite knows that the mice tend to hang on with the tenacity of bulldogs. And the escape jump of a wild house mouse is reminiscent of overheated popcorn. However, over the years, mice that bit ferociously usually came to a bad end at the hands of breeders, and escape artists soon round themselves back hiding among the cracks and crevices--leaving behind their less agile, gentler relations to form the next generation of pets.

Another trait selected for by the pet trade was an ability to thrive in comfortable confinement--a shoe-box-sized cage, for example. We humans have bred for gentleness, tolerance of confinement, and bizarre appearance in a number of other species, such as chickens, guinea pigs, and dogs. (How else does one explain the existence of the pug?) Observations of this sort of selective change under domestication were pivotal to Charles Darwin's work on his theory of evolution by natural selection.