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Ancestor: Mus musculus interstellaris (Ship Mouse)

Descendants: Spring Mouse
Hairball Mouse

Size: 10 -  14 cm not including tail, 21 -  29 cm with tail.

Dietary Needs: Has a more seed and grain-based diet than it's ancestors due to a scarcity of soft fruit and fewer insects up in the mountains, although elderberry and mulberry is present to a lesser extent than the lowlands. They may dig just on the leaf or humus layer to unearth the dwarfed earthworms or mealworms in the thin mountain soil. They also include river snails into their diet, and young trout alevins if they can catch one, or unprotected crayfish larvae gone astray - basically any tiny prey they can find in the water. When not seed-and-grain eaters they are opportunists.

Life Cycle: Mice are placental mammals. The females have a very short estrous cycle of only about two weeks, and the presence of such a cycle depends on the presence of males. Males have a chirping ultrasonic call they use when searching for or tracking a potential mate. They secure a territory and defend it while leaving urine trails all over the area, and he will allow the female he mates with to live and forage there. Females are usually the ones going around in search of a suitable mate and territory, although she will sometimes mate with travelling males and have to birth wherever she can find in whatever nest she can cobble together, at risk of attacks from the claimant of that territory. Their life cycle only takes around two-and-a-half months to complete, sometimes less, and females can produce multiple litters a year each averaging seven pups. Pups are helpess, blind and naked. Their mothers build a warm insulated nest before giving birth to them and feed them on milk for around four weeks.

Other: Climbing ability has improved for multiple surface types. Their feet morph to the shape of flatter surfaces when certain foot muscles relax, but when those same muscles contract it forms grooves between the callouses and the traction of the foot improves. They still have claws to help with their grip and can use them to scatch when fighting other mice for territory, or grip slippery prey like worms or snails.

They are also good swimmers, with dense greasy water-resistant fur to prevent becoming chilled by the mountain river water and short webbing between their toes. They need to be able to cross rivers or risk becoming geographically isolated by them. The push to find territory is a motivator to cross rivers.

They mainly use their tail as a counterbalance when climbing and tight-walking, or steer when swimming. They can coil their tails around an anchor somewhat but the anchor has to be not too thin as well as not too wide, the tail is not as flexible as some other climbing mouse tails.