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Reithrodontomys raviventrissalt marsh harvest mouse

By Francisco Veloz

Geographic Range

Salt-marsh harvest mice (Reithrodontomys raviventris), are limited to saltwater and brackish marshes bordering the San Francisco Bay area. Because these marshes have been altered by humans, throught draining, diking, and similar opperations, the distribution of this species is disjunct. The large Petaluma Marsh (in Sonoma County) supports a considerable amount of salt marsh harvest mice. The species is distributed eastward through Suisun Bay to the opening of the Sacramento River at Antioch Dunies. It reaches western regions through the marshes at the opening of Callinas Creek on the upper Marin Peninsula, and as far south to the San Mateo Bridge; including marshes in the San Francisco Bay National Wildlife Refuge. (Castronova and Beachman, 1994; Shellhammer, 1982)

Habitat

Salt-marsh harvest mice inhabit saline or brackish marshes. This species requires dense ground cover. R. raviventris prefers the cover of pickle weed, provided that it has non--submerged, salt-tolerant vegetation for escape during high tides. These mice rarely venture into the open. (Shellhammer, 1982)

Physical Description

R. raviventris has a dark brown dorsal area with a dark stripe extending vertically on its back, and pinkish cinnamon or tawny on its ventral side. There are often tufts of yellowish hairs near the anterior base of the ears. The ears themselves are dark in color. The tail is indistingly bicolored, with brownish hairs on the surface. Salt marsh harvest mice from the Southern San Francisco Bay area usually have a red belly. Toward the north, many members of the species have a white belly.

R. raviventris has a total length ranging from 118 to 175 mm. Tail length is reported at between 56 and 95 mm. The hindfoot length is 15 to 21 mm. These animals can weight between 7.6 and 14.5 g. (Ruff and Wilson, 1999; Shellhammer, 1982; Whitaker, October 1998)

  • Range mass
    7.6 to 14.5 g
    0.27 to 0.51 oz
  • Average mass
    11.01 g
    0.39 oz
  • Range length
    118 to 175 mm
    4.65 to 6.89 in

Reproduction

The mating system of salt marsh harvest mice has not been described. However, a congeneric mouse species, R. megalotis is reported to be polygynous. (Webster and Jones, 1982)

Females have a long breeding season that is from March to October or November, but reproductive prospective is low. Males are reproductively active from April to September. The average litter is approximately 4. Usually a female produces only one litter per year, although they are capable of bearing two to three litters per year. This makes these mice quite different from their congener, R. megalotis, which can produce litters just about every month, provided the weather is not too cold.

Gestation periods of the genus Reithrodontomys are 21-24 days. (Castronova and Beachman, 1994; Masicot, updated: 08/19/2001; Shellhammer, 1982; Webster and Jones, 1982)

Occurrance of maturation events, such as eruption of incisors, opening of ear pinnae and eyes, weaning and dispersal are not reported for R. raviventris. However, in R. megalotis, these events are well documented. In this latter species, neonates weigh between 1 and 1.5 g. They are born naked and helpless. They are only 7 to 8 mm in length. They grow hair and begin to crawl by the age of 5 days, and their manidublar incisors begin to emerge through the gums byt his time also. By day 11 or 12, the eyes and ears open. Baby R. megalotis are completely weaned by about 24 days of age, and disperse shortly thereafter. (Webster and Jones, 1982)

The timing of such events in R. megalotis may, or may not, be suggestive of the timing of such events in R. raviventris. Because salt marsh harvest mice are quite different from their congeners in their rate of reproduction, they may also be different in timing of developmental events.

  • Breeding interval
    These mice typically breed once annually.
  • Breeding season
    Breeding occurs from March to October or November.
  • Range number of offspring
    4 (high)

Reports of parental care in this species are not available. However, young of this genus are altricial, and require care from adults. In all mammals, females care for their young, providing them with milk, with shelter, and with protection until they reach the age of independence. It is not known if males of R. raviventris contribute to parental care. (Ruff and Wilson, 1999)

  • Parental Investment
  • no parental involvement
  • altricial
  • pre-fertilization
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

Most live less than 1 year. The longest it has been seen to live in the wild is about 18 months. (Masicot, updated: 08/19/2001)

  • Range lifespan
    Status: wild
    18 (high) months
  • Typical lifespan
    Status: wild
    12 (high) months

Behavior

R. raviventris is well suited for life in a salt marsh, where swimming is occasionally necessary. Because their fur does not become saturated quickly, they are quite buoyant, and can float well on the water when they must. These animals are not known to burrow. Instead, they build nests from balls of lose grasses on the ground. These nests of grass are about 150-175 mm in diameter. Salt marsh harvest mice do well in areas with glasswort, a succulent plant that recently has been increasing around San Francisco.

Another interesting thing about this mouse is that they move into higher grasslands during the highest winter tides. Since this species has a non-aggressive nature towards other members of its species, populations can be concentrated on high marsh levels during periods of tides. Studies have shown that R. raviventris can adapt to a variety of locations and plant diversity.

The species is largely nocturnal, and appears to be solitary when it is not in breeding season. (Castronova and Beachman, 1994; Ruff and Wilson, 1999)

Home Range

The home range size for these animals has not been reported.

Communication and Perception

Communication in this species has not been documented. However, like most mice, their communication probably involves some combination of visual, accoustic, olfactory, and tactile cues. (Ruff and Wilson, 1999)

Food Habits

The diet of salt marsh harvest mice seems to consist mainly of salt marsh plants, such as pickle weed. These mice eat a low ration of seeds and insects also. In winter, the diet switches mainly to grasses. In a unique adaptation to its habitat, this species is apparently tolerant of drinking saline water. (Shellhammer, 1982; Shellhammer, 1998)

  • Animal Foods
  • insects
  • Plant Foods
  • leaves
  • seeds, grains, and nuts

Predation

Members of the genus Reithrodontomys are eaten by many predaotrs, inclduing such as hawks, egrets, snakes, and skunks, but also by introduced rats, cats, and red foxes. (Ruff and Wilson, 1999)

Ecosystem Roles

These mice do not have high population densities or high rates of reproduction, so it is not likley that they are an important food source for any one species. It is unknown if they play a role in dispersal of seeds. (Shellhammer, 1982)

Economic Importance for Humans: Positive

These mice have little or no positive impact on people. However, they serve as food for animals which humans enjoy watching, such as hawks, owls, egrets, and foxes, so they can be considered to have an indirect positive entertainment value.

Economic Importance for Humans: Negative

R. raviventris is listed as an endangered species by the US Fish and Wildlife service. In order to protect this species, certain measures must be taken to protect its salt marsh habitat. This sort of protection probably interferes to some extent in human plans to develop, drain, dike, dam, and otherwise modify the salt marshes surrounding San Franscisco Bay. Although preservation of this habitat may not be considered a negative thing by most people, it probably affects some sections of the human economy negatively.

Conservation Status

Main reasons for the low population of salt marsh harvest mice is damage of wetlands, habitat destruction, and vegetation changes. Groundwater pumping has been diminishing marsh size in particular areas, while sewage dumping has contaminated others. Much of the marsh land habitat historically used by this species has been diked or drained, and almost all marshes around the San Francisco Bay area are too small and too far apart to support large populations. The extent of habitat fragmentation makes it difficult for this species to breed and recolonize habitat.

In 1972, The San Francisco Bay National Wildlife Refuge was founded, and has protected marshes in the South Bay. The largest of these marshes is Greco Island. California has obtained several areas that supply mouse habitat: Grizzly Island and Joyce Island Wildlife Areas, Hill Slough Wildlife Area, and Peytonia Slough Ecological Reserve. Also, another recovery plan was established in 1984. This plan has aimed to conserve the salt marsh harvest mouse through acquisition of larger marsh areas to combine with existing small isolated ones; altering upper edges of most marshes to provide three species of plants as refuges for mice when they get stressed by flooding. (Castronova and Beachman, 1994; Shellhammer, 1998)

Other Comments

R. raviventris can be distinguished from R. megalotis by its slender, more pointed and more unicolored tail. R. raviventris was once divided into two different species, but is now considered a single species with two subspecies: R. ravivnetris halicoetes and R. r. raviventris. (Castronova and Beachman, 1994; Ruff and Wilson, 1999)

Contributors

Nancy Shefferly (editor), Animal Diversity Web.

Francisco Veloz (author), Humboldt State University, Brian Arbogast (editor), Humboldt State University.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

granivore

an animal that mainly eats seeds

herbivore

An animal that eats mainly plants or parts of plants.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

marsh

marshes are wetland areas often dominated by grasses and reeds.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

Castronova, V., W. Beachman. 1994. Beachmans's guide to the endangered species of North America. Farmington Hills, MI: Beachman Publishing Inc..

Masicot, P. updated: 08/19/2001. "Animal Info-Saltmarsh Harvest Mouse" (On-line). Accessed November 1, 2001 at http://www.animalinfo.org/species/rodent/reitravi.htm.

Ruff, S., E. Wilson. 1999. The Smithsonian Book of North America. Washighton: Smithsonian Institution Press.

Shellhammer, H. 1998. A marsh is a marsh is a marsh...But not always to a salt marsh harvest mouse. Tideline, 18: 1-3. Accessed February 19, 2004 at http://desfbay.fws.gov/Archives/Salty/salty.htm.

Shellhammer, H. 1982. Reithrodontomys raviventris. Mammalian Species, 169: 1-3.

Webster, W., J. Jones. 1982. Reithrodontomys megalotis. Mammalian Species, 167: 1-5.

Whitaker, O. October 1998. Field Guide To Mammals. New York: Alfred A. Knopf.

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