If you put a captive-raised deer mouse that has never seen dirt into a sandbox, it'll quickly begin to dig itself a burrow. That burrow will feature a short tunnel sloping about eight centimeters down to a cozy nest, just like the burrow a wild deer mouse would build. The oldfield mouse, a closely related species, will do the same -- but its nest, whether in the wild or in captivity, will feature a longer entrance tunnel, plus a stealthy emergency escape passageway that ends just below the surface. And if you switch litters of mice at birth, deer mice will always build deer mouse style nests, even when raised by oldfield mouse mothers (and vice versa.)
Those clues have caused biologists over the years to suspect that genes play a role in the burrowing behavior of the two mouse species, which are thought to have diverged from each other around 100,000 years ago -- a blink of an eye, evolutionarily speaking. Now, writing in a recent issue of Nature, Hopi Hoekstra and colleagues report that they've tracked down some of those genetic triggers. By studying mice of both species, as well as hybrid mice and mice produced by mating hybrids with pure-breds, the team has identified four regions of DNA that contribute to the differences in tunneling behavior. "There are three regions that all affect tunnel length," says Hoekstra. When active, each genetic region adds about three centimeters to the length of a mouse's entrance tunnel, the team found. A fourth and independent region controls the presence or absence of an escape tunnel.
The genetic regions involved, says Hoekstra, may be connected to how the brain handles the neurochemical dopamineand the brain's motivation system. While other burrowing species likely don't share these same specific genetic codes, the biochemical pathways involved could well be the same in different diggers. The team is now trying a variety of approaches to attempt to identify the specific genes involved, and to figure out the exact mechanism by which they affect the mouse's tunnels. "I think this area—seeing how changes in a few genes can affect behavior—is the next big frontier in biology," says Hoekstra.
Take a look at the video below, to see just how to measure a mouse burrow in the lab.
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