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How do rats and mice differ?

September 14, 2009

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What’s the difference between a mouse and a rat?

Rats and mice are distinct species, though they are both rodents and in that way are related. Typically, when people think of mice they think of house mice, and when they think of rats they mean Norway rats. In this case, there is a very clear size difference, with the rat being a good deal larger.

Some proportions differ. Rats have smaller ears in relation to the rest of their bodies. Rat heads tend to be blunter or more rounded than those of mice, which look more triangular.

Some care is needed, since mouse and rat are common names rather than scientific ones, and many rodents are called rats when they are only similar-looking and are not very closely related. For the record, house mice are Mus musculus, while Norwegian rats are Rattus norvegicus.

One unambiguous way to distinguish between these species is that mice, both male and female, have five pairs of nipples, while rats of both sexes have six. A less visible difference is that rats have 22 pairs of chromosomes, while mice have 20.

Rats and mice do not mate and produce offspring, and it’s doubtful one could make any sort of meaningful hybrid, even in a laboratory. Rats have been known to eat mice.

This question has had a recent spike in interest from the scientific community. After years of labs’ using rats, mice came to dominate in many fields as genetically engineered specimens became available, with various genes “knocked out’’ so that one could study what happened with bits of the mouse biochemistry taken out of action.

Many similarly engineered rats may be on the horizon. While rats take longer to grow and take more effort to feed and clean up after, they tend to be better models of more complex diseases that afflict humans.

Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.