Image copyright Linda Freshwaters Arndt/SCIENCE PHOTO LIBRARY Image caption The eastern chipmunk (Tamias striatus) drinking water in bird bath

Its stripy back makes it one of the most recognisable of rodents - but until now it has been unclear exactly how the chipmunk earned its stripes.

Now, scientists have found the evolutionary gene change responsible for the distinctive markings of both the chipmunk and an African mouse.

The gene normally makes the bellies of many rodents light in colour.

The stripes may have helped the animals hide from daytime predators with keen eye sight, such as birds, they say.

Prof Hopi Hoekstra, of Harvard University, US, who led the research, said: "What these two rodents have in common is that they are both diurnal [active during daylight], when one could imagine stripes could be more valuable than if they were nocturnal.

"It is notable that of the rodents that are striped, most are diurnal - again consistent with them being important for evading visual predators (for example, raptors and mammalian carnivores)."

Mystery patterns

The African striped mouse has a pattern of two dark-light-dark stripes on its back.

The dark and light stripes can be seen as soon as the mouse pups are born.

Research shows a gene change causes skin cells called melanocytes to stop making dark pigment in a particular area of the mouse's back, causing a white stripe.

A similar thing happens in the chipmunk.

Image copyright J. F. Broekhuis Image caption The African striped mouse Rhabdomys pumilio (Kagalagadi, Botswana)

Image copyright Elaine Kruer Image caption Stripes are thought to give animals a fitness advantage

In some other animals, white patches are caused by a lack of melanocytes in a particular area of the body - such as the blaze on the nose of a horse or a white bib on a cat.

"Overall, we know very little about how pigment patterns form, especially in mammals," said Prof Hoekstra, an evolutionary biologist at Harvard University.

Evolution must have "tinkered" with the gene that normally makes the bellies of many rodents light in colour, she said.

"In other words, a gene working to make bellies light in colour was co-opted and reused in both chipmunks and mice to also make light-coloured stripes," Prof Hoekstra said.

Chipmunks are part of the squirrel family, and split off from mice and rats about 70 million years ago.

Scientists think they probably evolved their stripes independently.

The research is published in the journal Nature.

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