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Ancestor: Montane Climbing Mouse

Size: Smalles: 14 cm (20 cm plus tail), largest 23 cm long (30 cm including tail)

Dietary Needs: Carnivore of small animals, mainly small terrestrial and aquatic invertebrates (including snails) but also immature fish and on rare occasions smaller mouse species.

Life Cycle: Males are aggressively unsociable to other males. Since holding a good section of river-side territory can be a matter of life and death, fights can often get serious and bloody. And since none of their fathers, grandfathers and so-on passed their genes on by being meek, they are also fiercely aggressive to each other over the chance to mate with a female.

Since they are sparsely populated the male has a very loud call that can be heard across valleys which he uses to advertise his presence and location to females. While it's often the males who seek out the females, the female has a strong sex drive while in oestrus and the male can sometimes let the female find him using this song, as well as scent markings of urine in the riverside grasses.

Males and females keep separate territories, but the male allows females he has mated to overlap his territory.

Pregnant females make a nest or burrow, depending on the environmental resources available. Burrows are dug out from steep river banks comprised of fine but compact sediment, and lined with bedding such as dry grass, dry moss, fur and feathers. In environments with soil that is too gravelly, too rooty, too boggy or otherwise unsuitable, a nest made of grass and twigs will suffice. Such nests are usually hidden in grass or in a shrub, but might also take the form of a pile of small dead scavenged branches. More elaborate nest types are "inspired", that is, they are never taught to nest build but still usually try to replicate the nest type they grew up in (though some still default to burrowing) using some instinctive nest-building actions as a basis.

Like all mammals the young are fed on milk in their infancy. They take around 2 months to mature. Juveniles have a difficult first year as they are driven from territory to territory, but can sometimes get lucky  and establish a burrow without disturbance. While they do have an oestrus cycle as mentioned before, anoestrus can occur under stress from a lack of home security.

Other: A mouse adapted to seize opportunity in high, clear, cold mountain rivers. They have thick, waterproof fur, well oiled on the surface and kept dry and fluffy underneath. Webbed feet allow them to swim quickly with alternating kicks of the hind legs, and the front leg kicks are used to change direction. Their whiskers are quite sensitive to vibrations, though it's effectiveness can be dampened by very fast-flowing, crashing water. As semi-aquatic animals they usually understand where it's safe to swim and where it isn't. However this doesn't stop an individual from having accidents and ending up in powerful rapids or down a waterfall. Because the surface of their fur is hydrophobic it traps air and this gives the mouse some extra buoyancy, giving it a chance to survive such accidental mishaps. Under such powerful forces the air usually doesn't stay in the mouse's fur for long, but sometimes it's just enough for them to get free, recover and reproduce in the future.