Saltmarsh harvest mouse (Reithrodontomys raviventris)

Also known as: red-bellied harvest mouse, salt marsh harvest mouse
GenusReithrodontomys (1)
SizeHead-body length: 5.6 - 8.4 cm (2)
Tail length: 5.6 - 9.5 cm (3)
Weight7.6 - 14.5 g (3)

Classified as Endangered (EN) on the IUCN Red List (1).

One of the smallest rodents in the United States (4), the saltmarsh harvest mouse is also notable for being one of the very few mammals able to drink sea water, an adaptation to its salty habitat (3) (5) (6) (7). Two subspecies are recognised, Reithrodontomys raviventris raviventris (red-bellied harvest mouse) and R. r. halicoetes (saltmarsh harvest mouse) (1) (3) (7) (8) (9), with the former generally distinguished by its reddish brown rather than whitish underparts (2) (3) (7) (10). Both subspecies are generally dark to cinnamon brown on the upperparts, darker on the ears and back, and have white or pale cream feet. The tail is relatively blunt-tipped, dark above and lighter (but not pure white) below, thinly haired, and usually longer than the head and body (2) (3) (5) (11). Juveniles may be more greyish in colour (5).

The Latin name of the saltmarsh harvest mouse means “grooved-toothed mouse with a red belly” (4) (7), and the grooved upper incisors, together with a more hairy tail, help to distinguish this species from the house mouse (Mus musculus), which it resembles (5). It is also easily confused with the western harvest mouse, Reithrodontomys megalotis, but has a slightly thicker, more uniformly coloured tail, a darker back and ears, and, at least in R. r. raviventris, more reddish underparts (2) (3) (7) (10).

The saltmarsh harvest mouse has a small and highly fragmented distribution around San Francisco Bay, California (1) (2) (3) (5) (8). R. r. halicoetes occurs in the northern part of the range, in saltmarshes around San Pablo and Suisun bays, and along the northern Contra Costa County coast, while R. r. raviventris occurs mostly in the southern parts of San Francisco Bay, as well as on the Marin Peninsula and near Point Richmond (1) (3) (7) (8).

As its name suggests, this species inhabits salt and brackish marshes, preferring areas with dense cover that are dominated by pickleweed (Salicornia) (1) (2) (3) (7) (9). It tends to avoid areas of cordgrass (Spartina) or alkali bulrush (Scirpus) (7), and seldom ventures into the open (9). The saltmarsh harvest mouse requires access to cover on high ground to escape high tides, and may spend some time in grasslands adjoining the marshes (1) (3) (7).

The saltmarsh harvest mouse is usually nocturnal (1) (5) (7) (9), and is active all year round (5). In addition to being an agile climber, it may also use runways along the ground, and is able to swim well, being buoyant and having fur that does not wet quickly (2) (3) (5) (7) (9). The diet consists mainly of green vegetation, including pickleweed, with a higher proportion of fresh green grasses consumed in winter, and some seeds and insects also taken (1) (3) (5) (7) (9). The species is one of the few land mammals able to tolerate quite salty food and water (2) (5) (6) (9), although, interestingly, the subspecies R. r. halicoetes appears able to survive on drinking salt water for long periods whereas R. r. raviventris does not (3) (4) (7).

The saltmarsh harvest mouse is believed to be solitary outside of the breeding season (9). Like other members of the genus (5), it builds ball-like nests of dry grasses and sedges, usually on the ground or in low vegetation. The nest is often built over an old bird’s nest, and is likely to be quickly rebuilt when wetted by high tides (1) (2) (3) (7). Subspecies R. r. raviventris, however, apparently does little nest building (3) (7), although it may use accumulations of vegetation on the ground (1). The saltmarsh harvest mouse breeds from March to November, the female usually giving birth to a single litter, averaging around four young. The breeding season may be slightly longer in R. r. raviventris than in R. r. halicoetes, and a second or third litter may be produced (1) (2) (3) (4) (7). The gestation period is likely to be around 21 to 24 days (5). The species is short-lived, with few individuals surviving longer than nine months to a year (4) (5).

The saltmarshes inhabited by this small mouse have undergone widespread destruction and degradation, with many filled in for development or agriculture, diked, or used for commercial salt production. Groundwater pumping has also caused subsidence and increased flooding, reducing the extent of pickleweed, while predicted rising sea levels present a further flood risk (1) (2) (4) (8) (9). Most of the marshes have now been reduced to long, narrow strips along outboard dikes, lacking essential vegetation cover on higher ground. Pollution, management for wildfowl, invasive plants such as cordgrass (Spartina), and changes in water quality, such as an influx of fresh water from sewage treatment, have also changed the vegetation composition (1) (3) (4) (7) (8) (12). Finally, within altered marshes, the saltmarsh harvest mouse is at greater risk of predation by domestic cats and non-native red foxes (1) (4), and may suffer competition with the California vole (Microtus californicus) (13).

As a result of these many threats, around 84 percent of the original saltmarshes have now been lost, leaving the saltmarsh harvest mouse restricted to small, often degraded and severely fragmented remnants (1) (7) (8) (9) (14). In addition, the species may now number only a few thousand individuals at most (1) (5) (9), and its small, isolated populations may be at increased risk of inbreeding (1) (9). Subspecies R. r. raviventris is thought to be most at risk of extinction (1) (3) (5) (8) (14).

The saltmarsh harvest mouse is listed as Endangered by the U.S. Fish and Wildlife Service (11), and occurs in a number of protected areas (1) (4) (8) (11), although many may not be large enough or have features suitable to its long-term survival (1) (8). A recovery plan for the species was completed in 1984, with the aim of protecting, enhancing and restoring marsh habitat, and undertaking further research into the species and its requirements (10) (14), and a Tidal Marsh Ecosystem Recovery Plan, which will include this species, is currently under development (1) (7). Some efforts are underway to preserve saltmarsh habitat, for example through the removal of invasive Spartina (12), but a range of further measures will be needed, including the effective protection and management of remaining marshland, and more research into the saltmarsh harvest mouse, if this unique rodent is to survive in the long-term (1) (8).

To find out more about the saltmarsh harvest mouse and its conservation see:

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  1. IUCN Red List (January, 2010)
  2. Reid, F.A. (2006) A Field Guide to Mammals of North America. Houghton Mifflin Company, New York.
  3. Shellhammer, H. (1982) Reithrodontomys raviventris. Mammalian Species, 169: 1-3. Available at:
  4. Shellhammer, H. (1998) A marsh is a marsh is a marsh... but not always to a salt marsh harvest mouse. Tideline, 18(4): 1-3. Available at:
  5. Nowak, R.M. (1991) Walker's Mammals of the World. The Johns Hopkins University Press, Baltimore and London.
  6. Macdonald, D.W. (2006) The Encyclopedia of Mammals. Oxford University Press, Oxford.
  7. U.S. Fish and Wildlife Service, Sacramento Fish and Wildlife Office: Species Account - Salt Marsh Harvest Mouse, Reithrodontomys raviventris (January, 2010)
  8. Hafner, D.J., Yensen, E. and Kirkland Jr, G.L. (1998) North American Rodents: Status Survey and Conservation Action Plan. IUCN/SSC Rodent Specialist Group, IUCN, Gland, Switzerland and Cambridge, UK. Available at:
  9. UNEP-WCMC (January, 2010)
  10. U.S. Fish and Wildlife Service (1984) Salt Marsh Harvest Mouse and California Clapper Rail Recovery Plan. U.S. Fish and Wildlife Service, Portland, Oregon. Available at:
  11. U.S. Fish and Wildlife Service: Species Profile - Salt Marsh Harvest Mouse (Reithrodontomys raviventris) (January, 2010)
  12. San Francisco Estuary Invasive Spartina Project (January, 2010)
  13. Geissel, W. (1988) The ecology of the salt-marsh harvest mouse (Reithrodontomys raviventris) in a diked salt marsh. Journal of Mammalogy, 69(4): 696-703.
  14. Lidicker Jr, W.Z. (1989) Rodents: A World Survey of Species of Conservation Concern. IUCN/SSC Rodent Specialist Group, IUCN, Gland. Available at: