Endemic to the U.S., this mouse is one of the smallest rodents in this country. Another important characteristic of the Salt marsh harvest mouse is its ability of drinking sea water as a result of adaptation to its salty environment. The Latin name of this species means “grooved-toothed mouse with a red belly”, referring to the grooved upper incisors of the animal, which, along with its furred tail, distinguish the Salty mouse from the similar house mouse. In addition, this rodent is often mistaken for the Western harvest mouse, although differs from the latter by more reddish under parts, darker coloration of the back and ears as well as a thicker, more uniformly colored tail.
The Salt marsh harvest mice are found in tiny area around San Francisco Bay (California), where these animals occur in scattered populations. Preferred habitat of this rodent is salt and brackish marshes with an abundance of pickleweed (Salicornia). Meanwhile, the mouse usually avoids areas, dominated by cordgrass (Spartina) and alkali bulrush (Scirpus). The Saltmarsh harvest mice favor secluded places and are rarely found in open areas. In order to protect itself from high tides, this animal needs a cover on high ground, occasionally visiting grasslands, located nearby marshes.
This nocturnal rodent is active throughout the year. The Salt marsh harvest mouse is an amazingly agile creature both terrestrially and in trees. This animal is an exceptionally good climber. Moreover, it's an accomplished swimmer due to its buoyancy and the water resistant fur. When moving on the ground, the rodent uses special runways. The Salt marsh harvest mouse is generally a solitary creature, socializing only during the mating season. The globe-shaped nests of this species are constructed from dry grasses and sedges and located on the ground or among low vegetation. They prefer constructing their nests over abandoned bird nests. When a nest is destructed by high tides, the rodent will quickly rebuild it. There is no information on communication habits and behavior of these mice, but they are believed to rely on tactile cues as well as visual, acoustic and olfactory signals, like most of related mouse species.
The Salt marsh harvest mice are primary herbivores, they generally feed upon pickle weed and other salt marsh plants, supplementing this diet with occasional seeds and insects. The winter diet of this rodent generally consists of various grasses. In addition, this animal is capable of digesting saline water as a result of adaptation to its salty habitat.
There is no information on reproductive system of this species, although related mice are generally polygynous. The Salt marsh harvest mice breed between March and October or November. Gestation period lasts 21 - 24 days, yielding 4 babies per litter on average. Females are able to produce 2 - 3 litters a year, but they generally yield only a single one. Young are naked, helpless. At 5 days old, hairs appear on their skin, and the babies begin to crawl. They are born with closed ears and eyes, which open only at 11 - 12 days old. Weaning occurs at 24 days old, after which young mice disperse.
The most notable threats to the population of this species as a whole are: destruction of their natural habitat; degradation of wetland areas; and changes in local vegetation. In some areas of their range, the Salt marsh harvest mice suffer from groundwater pumping that decreases marsh size. Meanwhile, other marshes are polluted by sewage dumping. As a matter of fact, these rodents have lost much of their original marsh land range. On the other hand, nearly all marshes around the San Francisco Bay area are too small and too far from their range to be used by large populations of this species. And finally, the Salt marsh harvest mice currently face fragmentation of their range, which makes very difficult for them to breed and recolonize these lost territories.
No estimate of population size is available for the Salt marsh harvest mouse. Currently, this species is classified by the IUCN Red List as Endangered (EN) and its numbers are decreasing.