The Biogeography of the Salt
Mouse (Reithrodonomys raviventris)
BY GALINA GOLOVANOVA, STUDENT IN BIOGEOGRAPHY 316
Thank you for visiting our site. This web pages was written by a student in Geography 316: Biogeography and edited by the instructor, Barbara Holzman, PhD. All photos and maps are posted with specific copyright permission for the express use of education on these web pages. The students have tried to be as accurate as possible with the information provided and sources and references are cited at the end of each page.
Figure 1. Salt Marsh Harvest Mouse (Northern subspecies) Source: Dr. H. Shellhammer/courtesy of U.S.F.W.S. Pacific Region, 1998 (with permission)
DESCRIPTION OF SPECIES:
The scientific name Reithrodontomys
raviventris means “grooved-toothed mouse with a red belly"
Two species of Reithrodontomys
raviventris are recognized. The
subspecies of study, R. raviventris
halicoetes, the Northern Salt Marsh Harvest Mouse, is found in the marshes
of northern and central
Figure 2. Salt Marsh Harvest Mouse ( Northern subspecies) Source: B. "Moose" Peterson/courtesy of C.D.F.G., 2005 (with permission)
In the wild, on the average, the maximum age for the salt harvest mice, including R. raviventris halicoetes, is approximately twelve months, but most live less than eight months. Females have a low reproductive potential: they bear around 4 young per litter, and have only one litter per year. Reproductive activity, for females, ranges from March to November. Males are reproductively active from April to September. Adults comprise the majority of the population. The mice are density-dependent species: when the populations are too high, breeding is suppressed further into the spring. If local densities are too high, populations can be reduced to the point of local extinctions. During summer, when salinities of water and vegetation increase, the mice gain a competitive edge, since they can drink and survive on pure salt water (they can withstand high salinities in food and water intake). Notably, most of the northern subspecies can survive on sea water, but prefer having fresh water (Suisun Eco Workgroup, 2004).
Salt marsh harvest mice are cover-dependent species. That is, they only live under thick vegetation. They are dependent on thick cover of native halophytes (plants that thrive in salty environments) of the salt marsh environment, which is typified by salt marsh herbs, grasses and reeds. Salt marsh harvest mice use pickleweed (Salicornia virginica) as their primary/preferred habitat as long as they have non-submerged, salt-tolerant vegetation for escape during the highest tides (Fisler, 1965). They eat leaves and stems of halophytes. The mice prefer the deepest (60-75 cm tall), most dense pickleweed, which is intermixed with fat hen (Atriplex patula) and alkali heath (Frankenia grandifolia). The mice are non-intra-aggressive; therefore, short durations of populations’ densities are sustainable (for the high tide period). High tide’s refuge is taken in the upper zones of marshes, usually in the stands of fat hen and Australian salt brush (Atriplex semibaccata). Marshlands with low salinities and sparse pickleweed are not utilized by the mice (this is important, since most dikes marshes exist within the range of northern subspecies, within the Suisun Marsh, where less saline conditions are encouraged to optimize the habitat for waterfowl (Suisun Eco Workgroup, 2004).
The mice’s diet consists of seeds, grasses, forbs and insects. The northern and southern subspecies have longer intestines than their western counterpart, which is, primarily, a seed-eater. Salt marsh harvest mice are, generally, nocturnal species, but may be active during the day as well (Daiber, 1982). They are most active during the moonlit nights. Salt marsh harvest mice are mobile in diked salt marshes. The species is able to survive tidal or seasonal flooding due to their swimming, floating and climbing abilities. The mice are quick re-colonizers of flood-disturbed areas (Pomeroy and Wiegert, 1981). The mice exhibit long-distance dispersal abilities in young members. However, no dispersal occurs to bare or human-developed adjacent areas (these areas constitute a dispersal filter for mice). A narrow buffer zone of vegetation is probably needed for dispersal between adjacent preferable habitats, if they are isolated from each other (U.S.F.W.S., 1984).
Salt marsh harvest mice are able to survive, for extensive periods of time, on salinities near that of salt water. This constitutes a shared trait with desert-adapted animals (however, this is an adaptation based on salt tolerance, rather than on water conservation through the kidneys’ function, as seen in kangaroo rats). The northern subspecies, R. raviventris halicoetes, is able to survive for more than one year on sea water, with a reduction in weight for the period, since food and water consumption are curtailed while drinking sea water. The habitat for R. raviventris halicoetes experiences greater fluctuations in water salinities than that of R. raviventris raviventris (the southern subspecies) (Suisun Eco Workgroup, 2004).
The U.S. Fish and Wildlife
Service listed the salt marsh harvest mouse as endangered in 1970 (Suisun Eco Workgroup,
U.S.F.W.S. is responsible for the management/recovery, listing and law
enforcement/protection of the species. Department
of Defense, D.O.D., is responsible for the law enforcement/protection of the
species with applicable state and federal laws on public land under their
control (U.S.F.W.S. Recovery Plan, 1984). The
The Salt marsh harvest mice are endemic to the
DISTRIBUTION OF REITHRODONTOMYS RAVIVENTRIS HALICOETES (Northern Subspecies) & REITHRODONTOMYS RAVIVENTRIS RAVIVENTRIS (Southern Subspecies):
The marshes of Delta and the Bay began to be
diked off for salt-evaporating ponds as early as 1860.
By 1959, 581 square kilometers of marshlands and tidelands have been
diked off or filled. One of the most
severely reduced habitats of the
In addition to tidal marshes, non-tidal (diked)
marshes represent a second important wildlife habitat of the Bay.
Considerable difference exists between the diked marshes of the
until the last two hundred years, the salt marsh harvest mice were found in the most of the marshes throughout
Distribution and abundance of the salt marsh harvest mice are dependent on the availability of dense pickleweed salt marsh. Although this species makes some use of grassed and salt-tolerant forbs at the upper margins of salt and brackish marshes, it is closely tied to the cover of dense pickleweed, and it makes little use of pure alkali bulrush or Cordgrass stands (U.S.F.W.S., 1984). Salt marsh harvest mice are critically dependent on dense cover and their preferred habitat is pickleweed. Salt marsh harvest mice are seldom found in cordgrass or alkali bulrush. In marshes, with an upper zone of peripheral halophytes, mice use this vegetation to escape the higher tides, and may even spend a considerable portion of their lives there. Mice also move into the adjoining grasslands during the highest winter tides. Throughout much of the range of the salt marsh harvest mouse, however, subsidence and diking have eliminated the important peripheral halophyte zone. Few harvest mice survive in such marshes, even though other marsh conditions may be optimal, because there is little or no high tide escape. Studies have shown that the best type of pickleweed association for harvest mice has the following characteristics: 100% cover; a cover depth of 30-50 cm at summer maximum; a high percentage cover of pickleweed, i.e., 60% or more; complexity in the form of fat hen and alkali heath or other halophytes. The amount of salt grass, brass buttons, alkali bulrush, or other Scirpus or Typha species of plants, however, should be low. The latter species may be present, but not in large continuous stands, as pure stands of them are avoided by mice. Salt grass and brass buttons provide very poor habitat for the salt marsh harvest mice; they are low-growing, lack stratification and provide poor cover. Fat hen provides good cover for mice during the summer, but cannot be used year-round because it is annual (U.S.F.W.S., 1984).
The northern subspecies (Reithrodontomys raviventris halicoetes) of the salt marsh harvest
mouse are found in the marshes of
The western limit of the northern subspecies is
the marshes bordering the mouth of Gallinas Creek on the upper
Young members of the northern subspecies have shown an ability to disperse over great distances; however, their dispersal depends on provision of a buffer zone between the salt marsh habitat and the adjoining habitat. The mice, either northern or southern subspecies, are considered to be keystone species in tidal and brackish salt marsh habitats as the mice populations succeed best in complete, healthy ecosystems and decrease in numbers or are extirpated in human-altered marshes. The populations are negatively affected by factors such as the elimination of upland marsh habitat areas that provide refugia during high tides (Padgett-Flohr et al., 2003).
The southern subspecies, Reithrodontomys raviventris raviventris, are found in
Figure 3. Distribution of Salt Marsh Harvest Mouse (Northern & Southern subspecies) Source: C.D.F.G., 2005 (with permission)
Both of the subspecies occur with the closely-related, ubiquitous and abundant western harvest mouse, at upper edges of marshes, and in marginal areas. Both may be found in pickleweed, though Reithrodontomys raviventris halicoetes and Reithrodontomys raviventris raviventris exclude or replace Reithrodontomys megalotis (the western cousin) in denser stands (U.S.F.W.S., 1984)
The endangered status of Reithrodontomys
raviventris halicoetes, largely, results from commercial and residential
EVOLUTION OF REITHRODONTOMYS RAVIVENTRIS:
The earliest recorded mammals developed from a particular group of reptiles, the cynodonts, during the Triassic period, approximately 160 million years earlier. Known as tricodonts, these were small creatures which probably laid eggs but had developed the typical mammalian pattern of dentition (Alderton, 1996). In the subsequent Jurassic period, two new groups of mammals came into existence. Firstly, there were the multituberculates, which show remarkable similarities to rodents in the pattern of their dentition. They had characteristically sharp incisor teeth at the front of their mouth, with a gap behind. The premolar and molar teeth had broad surfaces with cusps, which were used to crush the vegetation that these animals ate. Their jaws moved up and down, rather than side to side (Alderton, 1996). The largest multituberculates were about the size of a contemporary beaver, whereas most remained no bigger than mice. It appears that this group of early mammals gradually died out, although some survived until the Eocene period, about 55 million years ago (Alderton, 1996).
The ancestors of today’s rodents evolved, about
60 million years ago during the Late Palaeocene epoch.
Their earliest fossilized remains have been unearthed in
During the Miocene period, temperature
across the globe continued to fall and this trend contributed to the decrease in
movement of animals through the Bering Land Bridge, which connects present day
generic, and, to a certain extent, the species composition of a present muroid
communities were established toward the early-middle Pleistocene (Ortiz et al.,
2000). The salt marsh harvest mice, both Reithrodontomys
raviventris halicoetis and Reithrodontomys
raviventris raviventris, the northern and southern subspecies, belong to the
Myomorpha group. Myomorphs have
successfully colonized almost every type of ecosystem, from the Arctic
to temperate woodlands, and deserts to tropical
rainforest. This suborder is split
into five families, of which the largest is Muridae.
Members of this group first developed in
5. Skull Comparisons: Harvest Mouse vs. House Mouse)
Salt marsh harvest mice
fall under the grouping of Cricetidae (Korth, 1994).
Cricetidae are small rodents. The
dental formula for cricetids is primitive for all.
The enamel surface of incisors was often ornamented by numerous ridges.
The primitive condition appears to have been numerous pinnate (radiating)
ridges present on the earliest known cricetid, Pappocricetodon,
Based on a study of patterns of karyotypic megaevolution ((karyotypic megaevolution is described as a radical reorganization of the karyotype in which normally stable G-band patterns are disrupted or rearranged to a point that it makes it difficult to observe normal patterns typically shared among closely related species (Baker & Bickham, 1980)) in Reithrodontomys, it was determined that:
Analysis of restriction enzyme digestion with EcoRI revealed that the 350 base pair monomer repeat in R. montanus and R. megalotis also was in R. zacatecae and R. raviventris (Bell et al., 2001)
|Figure 6. Diagrammatical interpretation of times-since-divergence values (approximately 1 million years) relative to patterns of chromosomal evolution for 7 species of Reithrodontomys. The tree branches read, from left to right: fulvescens, humulis, raviventris, montanus, sumichrasti, zacatecae and megalotis. Source: Bell et al., 2001|
The complete cytochrome b-gene (1,143 base pairs) was sequenced for seven species of Reithrodontomys. In all analyses, the two individuals representing R. megalotis, R. zacatecae, and R. raviventris formed sister taxa (Bell et al., 2001).
The analysis in which transitions & transversions were equally weighted resulted in a single most parsimonious tree. The topology of that tree depicted two clades. In the 1st clade, R. megalotis and R. zacatecae formed a sister taxon relationship and were then joined to R. sumichrasti. R. montanus and R. raviventris formed a 2nd clade. Those two clades were joined together, and R. humulis and R. fulvescens were added in a stepwise manner (Bell et al., 2001).
All analyses depicted R. raviventris and R. montanus as sister taxa (Bell et al., 2001).
R. megalotis, R. montanus, R. raviventris, R. humulis, R. sumichrasti and R. zacatecae have experienced substantial chromosomal evolution involving different chromosomal rearrangements (Bell et al., 2001).
R. raviventris and R. montanus have undergone karyotypic megaevolution and have returned to a mode of chromosomal stasis (Bell et al., 2001).
In general, studies done by Baker and Bickham (1980) led to the recognition of a high diploid, mostly arcocentric group (R. creeper, R. fulvescens, R. mexicanus, R. tenuirostris, and R. humulis) and a low diploid, entirely biarmed group (R. montanus, R. raviventris, R. megalotis, R. sumichrasti, and R. zacatecae). These studies have documented extensive chromosomal evolution within these five species and provided evidence that radical eurochromatic rearrangements have occurred (Baker and Bickham, 1980).
Despite several attempts to determined systematic relationships among species of Reithrodontomys complete congruency among data sets has been rare (Arnold et al., 1983).
R. raviventris has a highly restricted geographic range and perhaps originated as a result of geographic isolation due to the formation of salt marshes in the San Francisco Bay region (Fisler, 1965). This information could be used to support the statement by the U.S.F.W.S. that the salt marsh harvest mice evolved with the creation of the S.F. Bay some 8,000-25,000 years ago (U.S.F.W.S., 2005). Before 1984, the specific status of R. raviventris was based on the assumption that its closest living relative was R. megalotis and that it was sympatric with R. megalotis (Fisler, 1965). With the development of the hypothesis that R. raviventris was sister to R. montanus, the question became whether R. raviventris was an isolated subspecies of R. montanus or was it specifically distinct from R. montanus. Analyses of karyotypic data suggested that R. raviventris was a species distinct from R. montanus and was related more closely to this taxon than to R. megalotis (Bell et al., 2001).
Given the restricted distribution of R. raviventris to the highly populated San Francisco Bay region and the ever-increasing threat of loss of habitat, Bell et al. data indicates that this taxon is unique and contributes to the biodiversity of the genus. Conservation of this taxon is paramount (Bell et al., 2001).
For more information, visit:
California Department of Fish and Game www.dfg.ca.gov/
California Department of Pesticide Regulation www.cdpr.ca.gov/
Goals Project 1999 http://www.sfei.org/sfbaygoals/docs/goals1999/final031799/pdf
Sacramento Fish and Wildlife Office http://sacramento.fws.gov/
U.S. Fish and Wildlife Service http://www.fws.gov/
Alderton, David. 1996. Rodents of the World. New York, N.Y.: Facts on File, Inc.
Arnold, L. J., Robbins, W., Chesser R. K., and Patton J.C. 1983. Phylogenetic relationships among six species of Reithrodontomys. Journal of Mammalogy 64:128-132
Baker R. J., and Bickham, J. W. 1980. Karyotypic evolution in bats: evidence of extensive and conservative chromosomal evolution in closely related taxa. Systematic Zoology 33:339-341
Bell, D. M., Hamilton M. J., Edwards C. W., Wiggins L. E., Martinez R. M., Strauss R. E., Bradley R. D., and Baker R. J. 2001. Patterns Of Karyotypic Megaevolution in Reithrodontomys: Evidence From A Cytochrome-b Phylogenetic Hypothesis. Journal of Mammalogy 82(1):81-91
California Department of Fish and Game, California Department of Pesticide Regulation, Endangered Species Project. 2005. Salt Marsh Harvest Mouse Field I.D. Card. California Department of Fish and Game, California Department of Pesticide Regulation, Endangered Species Project. 2005. Salt Marsh Harvest Mouse Field I.D. Card. [Online] Available: http://www.cdpr.ca.gov/docs/es/espdfs/smhm1.pdf ( Accessed April, 2005)
California E.P.A., Department of Pesticide Regulation. 2005. Salt Marsh Harvest Mouse (Reithrodontomys raviventris). [Online] Available: http://www.cdpr.ca.gov/docs/es/espdfs/smhmall.pdf ( Accessed April, 2005)
Daiber, F. C. 1982. Animals of the Tidal Marsh. New York, N.Y: Van Nostrand Reinhold Company
Dedrick, K. 1993. San Francisco Bay tidal marshland acreages: recent and historic values. In: O.T. Magoon, ed. Proceedings of the Sixth Symposium on Coastal and Ocean Management (Coastal Zone '89). Charleston, South Carolina, July 11-14, 1989. Published by the American Society of Civil Engineers.
Fisler, G.F. 1965. Adaptations and speciation in harvest mice of the marshes of San Francisco Bay. University of California Publications in Zoology, Volume 77. Berkeley, C.A.: University of California Press
Harvey and Stanley Associates, Inc., 1980. Status of the salt marsh harvest mouse (Reithrodontomys raviventris) in the Suisun Marsh. Report to Water and Power Resources Service. Sacramento, California
Korth, William W. 1994. The
Tertiary Record of Rodents in
Padgett-Flohr, G. E., and Isakson, L. 2003. A Random Sampling of Salt Marsh Harvest Mice in a Muted Tidal Marsh. Journal of Wildlife Management 67(3):646-653
Pomeroy L. R., and Wiegert R. G. 1981. The Ecology of the Salt Marsh. New York, N.Y.: Springer-Verlag
Sacramento Fish and Wildlife Service Office. 2004. Salt Marsh Harvest Mouse (Reithrodontomys raviventris). Species Account. [Online] Available: http://sacramento.fws.gov/es/animal_spp_acct/salt_marsh_harvest_mouse.htm ( Accessed April, 2005)
Shellhammer, H. S., Jackson R., Davilla W., Gilroy A. M., Harvey H. T., and Simons L. 1982. Habitat preferences of salt marsh harvest mice (Reithrodontomys raviventris). The Wasmann Journal of Biology 40(1-2):102-114
Suisun Eco Workgroup. 2004. Wildlife of the Suisun Marsh, Salt Marsh Harvest Mouse. [Online] Available: http://www.iep.water.ca.gov/suisun_eco_workgroup/workplan/report/wildlife/shmouse.html (Accessed April, 2005)
Thelander, C. ed. 1994. Life on the edge: a guide to California's endangered natural resources. Santa Cruz, C.A.: Biosystems Books. p 80-81
U.S. Fish and Wildlife Service. 1984. Salt Marsh Harvest Mouse and California Clapper Rail Recovery Plan. Portland, Oregon. [Online] Available: http://ecos.fws.gov/docs/recovery_plans/1984/841116.pdf ( Accessed April, 2005)
Vrba, E. S. 1992. Mammals As A Key To Evolutionary Theory. Keynote Address, Presented at the 70th Annual Meeting of The American Society of Mammalogists, Frostburg, MD, June 1990. Journal of Mammalogy 73(1):1-28
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