|Species||Ondatra zibethicus is 40-65 cm in length and can weigh from 0.5 to over 2 kilograms. Females are similar or slightly smaller in size than males (McVey et al., 1993). This semi-aquatic mammal has a rudder like tail, short front legs and webbed back legs. The fur is soft and has a waterproof underlayer (McConnell and Powers, 1995).|
|Status in Estuaries||Breeder and year around resident in estuaries. The muskrat is specifically found in fresh and saltwater marshes, ponds, and rivers and prefers to build its lodge or den near slow moving water (McConnell and Powers, 1995). Muskrats are solitary or form pairs during the breeding season (McVey et al., 1993). The number of litters produced per year varies geographically, averaging from 2-6 (McConnell and Powers, 1995).|
|Abundance and Range||The muskrat is distributed throughout the U.S., except in the extreme southeast, central Texas, and most of California (McVey et al., 1993). It is common in Cape May marshes and areas of New Jersey, Delaware, and Pennsylvania (McConnell and Powers, 1995). From 1982-1983, approximately 7 million were trapped and their fur yielded a profit of 28 million dollars (McConnell and Powers, 1995).|
|Site Fidelity||The average home range is between 7 to 30 meters and rarely do muskrats explore distances greater than 150 meters from their lodge (McConnell and Powers, 1995).|
|Ease of Census||Difficult|
|Feeding Habits||Generalists. Both animal and plant matter is consumed, though aquatic plant material tends to dominate (McVey et al., 1993). Muskrats specifically feed on plants such as cattails, arrowhead, bulrush and occasionally animal matter such as clams, carp, crayfish, turtles, and snails (Willner et al., 1975). Feeding occurs in a nearby house separate from their main den (McVey et al., 1993).|
Muskrat Contaminant Exposure Data
A muskrat collected from the Gulf of Bothnia, Finland, contained 0.11 µg/g wet weight PCB and 0.08 µg/g DDE in muscle (Koivusaari, 1976). DDT was not detected in the sample of muscle.
From 1986 to 1988, muskrats were trapped at three locations in Virginia including the upper and lower Elizabeth River, and the Nansemond River (reference site) (Halbrook et al., 1993). Dieldrin, PCBs and DDE were detected in the carcasses at concentrations <1 µg/g dry weight.
No direct exposure data available
Trace Elements, Metals, and Metalloids
Muskrats living near a fly ash-contaminated pond in Lansing, New York contained concentrations of 2.8 µg/g dry weight Se in the liver, compared to 1.2 µg/g in muskrats living in a reference pond 16 km from the site (Gutenmann et al., 1976).
From 1972 to 1975, 14 muskrats trapped from the Wisconsin River were analyzed for Hg in the fur, liver, kidney, muscle, and brain (Sheffy and St. Amant, 1982). Mercury in the fur was detected at 0.06 µg/g wet weight. In all other tissues analyzed, Hg occurred at concentrations below the 0.02 µg/g detection limit.
In June and July 1974, muskrat samples were collected from four locations in Pennsylvania for metal analysis (Everett and Anthony, 1976). The following mean concentrations (µg/g) were detected at each site: Monocacy River, a relatively clean location—0.137 liver and 0.630 kidney Cd, 34.4 liver and 101.1 bone Zn, 4.12 liver and 2.29 kidney Cu, 0.0021 liver and 2.226 bone Pb, and 0.039 liver Hg; Saucon Creek, which receives much of its water from the New Jersey Zinc Company—0.316 liver and 1.071 kidney Cd, 81.2 liver and 305.0 bone Zn, 4.43 liver and 2.66 kidney Cu, 0.0332 liver and 1.430 bone Pb, and 0.053 liver Hg; Jordan Creek, which receives considerable discharge from agricultural areas—0.039 liver and 0.168 kidney Cd, 39.9 liver and 97.7 bone Zn, 4.62 liver and 2.11 kidney Cu, 0.0144 liver and 1.117 bone Pb, and 0.029 liver Hg; Tinicum Marsh, which receives industrial discharge—0.082 liver and 0.241 kidney Cd, 33.4 liver and 200.1 bone Zn, 2.46 liver and 1.48 kidney Cu, 0.1537 liver and 1.537 bone Pb, and 0.070 liver Hg.
Thirty muskrats were collected at various distances from ore smelters at Flin Flon, Manitoba, Canada (Radvanyi and Shaw, 1981). Metals were measured in the kidney, liver, duodenum, and muscle, respectively, at the following concentrations (µg/g dry weight): 12.8, 11.7, 7.69, 3.38 Cu; 1.33, 0.306, 1.11, 0.044 Cd, 0.836, 0.540, 2.27, 0.282 Pb, and 75.2, 81.7, 107, 65.4 Zn. Concentrations of Mn, Na, Fe, and Mg showed significant location differences, but the trend was neither unidirectional or consistent.
From September through October 1975, 126 muskrats were collected from four locations within a marsh area in Tinicum National Environmental Center near Philadelphia (Erickson and Lindzey, 1983). Lead ranged from 3.71 to 5.23 µg/g in the liver and from 2.63 to 4.25 µg/g in the kidney. Cadmium ranged from 0.042 to 0.064 µg/g in the liver and from 0.110 to 0.157 µg/g in the kidney.
From 1981-1982, 28 muskrat samples were collected from three sites in southeast Missouri: Big River, which receives runoff from old mine tailings, Black River, located in an area which is currently mined, and Irondale, a relatively clean location upstream from the mine tailings (Niethammer et al., 1985). At Irondale, metal concentrations were <0.32 µg/g wet weight Pb and 19.6-39.6 µg/g Zn in liver, and <0.65 µg/g Cd in kidney. Further downstream at Big River, values ranged from 0.29-1.60 µg/g Pb and 23.0-35.2 µg/g Zn in liver, and 0.13-5.30 µg/g Cd in kidney. At three locations along the Black River (Upper River, Clearwater Lake, and Lower River), concentrations were <0.53 µg/g Pb and 19.7-33.1 µg/g Zn in liver, and <1.30 µg/g Cd in kidney.
From 1986 to 1988, metal concentrations were measured in the kidneys of muskrats collected from the Elizabeth and Nansemond Rivers (Halbrook, 1993). Mean levels of Al (3.19 µg/g dry weight), Cd (3.08 µg/g), Cu (12.85 µg/g), Ni (0.50 µg/g), and Zn (88.83 µg/g) were higher in muskrats located along the lower Elizabeth River than those located along the upper portion of the Elizabeth and Nansemond Rivers. Selenium was detected at its highest mean concentration, 5.31 µg/g, at the Upper Region of the Elizabeth River. Mean Mn (6.60 µg/g) and V (1.10 µg/g) concentrations were highest in muskrats from the Nansemond River.
Hair samples were taken from the inguinal and lumbar regions of 58 muskrats from four sites in the Oak Ridge Reservation, Tennessee (Stevens et al., 1997). Mercury was detected in 75% of all hair samples, 90% of adult samples, and 45% of juvenile samples. Mean concentrations in adults were 3.87, 0.24, and 0.24 µg/g dry weight in contaminated sites and 0.13 µg/g at reference sites. In juveniles, means were 1.55 and 0.13 at contaminated sites and 0.09 at reference sites. There was no difference in mean Hg concentration between males and females.
Sixty-three percent of the muskrat carcasses collected along the Elizabeth and Nansemond Rivers contained low levels (range 0.03-0.15 µg/g dry weight) of polynuclear aromatic hydrocarbons (PAHs) (Halbrook et al., 1993). The most common PAHs detected were phenanthrene, anthracene, fluorene, pyrene, and chrysene.
In 1960 and 1961, four muskrats were collected from the Oak Ridge National Laboratory settling basin, which receives waste from a wastewater treatment plant and from operations at the laboratory (Kaye and Dunaway, 1963). Cesium-137 ranged from 0.122 to 0.976 µCi in the GI tract and from 0.312 to 3.710 µCi in the carcass. Cobalt-60 was elevated in the GI tract of one muskrat (67.6 µCu) when compared to concentrations detected in the other muskrats. Concentrations of Ru-106, Zn-65, and Ce-144 were found to be <0.560 µCi in all muskrats. Strontium-90 reached a high of 12.4 µCi in the GI tract and 13.8 µCi in the carcass.
From spring 1985 to fall 1986, 81 muskrats were collected from the Sudbury-North Bay and Timagami areas of the Great Lakes-St. Lawrence Region of Canada (Mirka et al., 1996). Muskrats were collected from study sites located in Quirke Lake, and from two control sites which did not receive mine tailing drainage: one local and one distant. No age or sex related difference in accumulation of Ra-226 was noted so groups were pooled. Geometric mean concentrations of Ra-226 were 345 mBq/g dry weight the bone of muskrats in the study site with the highest levels of Ra-226 in the water, 80.3 mBq/g in the study area with lower levels in the water, 79.1 mBq/g in the local control site, and 11.5 mBq/g in the distant control site.
Muskrat Contaminant Response Data
DNA adducts were detected by P-32 postlabeling methods in liver tissue of muskrats collected in 1989 from the Elizabeth and Nansemond Rivers in Virginia (Halbrook et al., 1992). Though PAH sediment concentrations were significantly higher in the Elizabeth River, DNA adduct concentrations were similar between sites, ranging from 15-236 nmol adduct per mol normal nucleotide. Two muskrats dosed with 10 mg benzo[a]pyrene contained DNA adduct concentrations of 219 and 97 nmol per mole, compared to 37 nmol per mol in a muskrat dosed only with corn oil.
No response data available
Trace Elements, Metals, and Metalloids
Muskrats were studied along the Elizabeth River from July-August, 1986 and May 1987-August 1988 (Halbrook et al., 1993). Samples were collected from the lower Elizabeth River, which tended to have higher metal concentrations, the upper Elizabeth River, and a relatively clean site, the Nansemond River. Body weight, carcass weights, spleen weight, and fat index were lowest in muskrats from the lower portion of the Elizabeth River and adrenal weight was higher. Histological examination revealed multifocal coagulative necrotic lesions on one muskrat from the upper and one from the lower Elizabeth River. One muskrat from the lower Elizabeth River had multifocal granulomas. Seven muskrats from the lower, 2 from the upper, and 3 from the Nansemond location has portal perivascular mononuclear infiltration. Tapeworm cysts were detected in 27% of muskrats examined from the lower Elizabeth River and 3% from the Upper Elizabeth and Nansemond Rivers.
Muskrats were collected from the lower section of Virginia’s Elizabeth River, an area with heavily contaminated with metals and PAH’s, in 1987 and 1988 (Halbrook et al., 1993). Pentobarbital sleeping times averaged 38.6 min in this region. Sleeping times of muskrats were 60.1 minutes from the upper section of the Elizabeth River and 63.1 minutes from the Nansemond River. The lowered sleeping times are indicative of enzyme induction.
Laboratory studies indicated that as the oil concentrations within a controlled tank increase from 0 to 4 cc of oil, muskrats had increasing trouble staying afloat (Wragg, 1954).
In 1945, in order to control mosquitos, a section of a game preserve near Hamilton, Ontario was sprayed with a 3% DDT solution mixed with fuel oil and kerosene, applied at a concentration of 1 gallon/acre (Wragg, 1954). A massive decline in the muskrat population was observed the following year, attributed to fuel oil toxicity.
In 1972, when muskrats were experimentally oiled, it was found that about 80% of the oil present adhered to the fur (McEwan et al., 1974). When muskrats were placed in a tank with between 2-49 g of oil, the following behaviors were noted: the muskrat tries to escape, preens, and begins to shiver. Wet fur and loss of buoyancy occurred when 25.6 to 42.3 grams of oil were applied to the fur.
Following the 1988 oil spill in the Gasconade River in Missouri, population studies were completed to determine the effect on the number of muskrats living downstream of the spill (Heatherly, 1993). When downstream and upstream populations were compared, no difference in the numbers of adult and juveniles were found beginning 18 months after the spill.
In 1988, 15 oiled dead muskrats were found downstream from a 863,000 gallon spill of crude oil on the Gasconade River in Central Missouri (Heatherly, 1993). Three weeks after the spill only six dead muskrats, which had been eaten by scavengers, were found.
References for Muskrats
Erickson, D.W. and J.S. Lindzey. 1983. Lead and cadmium in muskrat and cattail tissues. J. Wildl. Manage. 47:550-555.
Everett, J.J. and R.G. Anthony. 1976. Heavy metal accumulation in muskrats in relation to water quality. Trans. Northeast Sect. Wildl. Soc. 33:105-118.
Gutenmann, W.H., C.A. Bache, W.D. Youngs, and D.J. Lisk. 1976. Selenium in fly ash. Science 191:966-967.
Halbrook, R.S., R.L. Kirkpatrick, D.R. Bevan, and B.R. Dunn. 1992. DNA adducts detected in muskrats by 32P-Postlabeling analysis. Environ. Toxicol. Chem. 11:1605-1613.
Halbrook, R.S., R.L. Kirkpatrick, P.F. Scanlon, M.R. Vaughan, and H.P. Veit. 1993. Muskrat populations in Virginia's Elizabeth River: physiological condition and accumulation of environmental contaminants. Arch. Environ. Contam. Toxicol 25:438-445.
Heatherly, W.G. 1993. Demographic characteristics of riverine muskrats after an oil spill. M.S. Thesis Univ. Mo.
Kaye, S.V. and P.B. Dunaway. 1963. Estimation of dose rate and equilibrium state from bioaccumulation of radionucleotides by mammals. In V. Schultz and A.W. Klement, Jr., Radioecology. Reinhold, NY. pp.107-111.
Koivusaari, J. 1976. Chlorinated hydrocarbons and total mercury in the prey of the white-tailed eagle (Haliaeetus albicilla L.) in the Quarken Straits of the Gulf of Bothnia, Finland. Bull. Environ. Contam. Toxicol. 15:235-241.
McEwan, E.H. N. Aitchison, and P.E. Whitehead. 1974. Energy metabolism of oiled muskrats. Canad. J. Zool. 52:1057-1062.
McConnell, P.A. and J.L. Powers. 1995. Muskrat. In L.E. Dove and R.M. Nyman, eds., Living Resources of the Delaware Estuary. The Delaware Estuary Program. pp. 507-513.
McVey, M., K. Hall, P. Trenham, A. Soast, L. Frymier, and A. Hirst. 1993. Wildlife Exposure Factors Handbook, Volume I. U.S. Environmental Protection Agency, Washington D.C., EPA/600/R-93/187a.
Mirka, MA., F.V. Clulow, N.K. Dave, and T.P. Lim. 1996. Radium-226 in cattails, Typha latifolia, and bone of muskrat, Ondatra zibethica (L.), from a watershed with uranium tailings near the city of Elliot Lake, Canada. Environ. Pollut. 91:41-51.
Niethammer, K.R., R.D. Atkinson, R.D., T.S. Baskett, and F.B. Samson. 1985. Metals in riparian wildlife of the lead mining district of southeastern Missouri. Arch. Environ. Contam. Toxicol. 14:213-223.
Radvanyi, A., and G.G. Shaw. 1981. Heavy metal contamination of foods and tissues of muskrats in northern Manitoba. In J.A. Chapman and D. Purslet, eds., Proceedings of the Worldwide Furbearer Conference, Frostburg, Maryland. pp. 1691-1697.
Sheffy, T.B. and J.R. St. Amant. 1982. Mercury burdens in furbearers in Wisconsin. J. Wildl. Manage. 46:1117-1120.
Stevens, R.T., T.L. Ashwood, and J.M. Sleeman. 1997. Mercury in hair of muskrats (Ondatra zibethicus) and mink (Mustela vison) from the U.S. Department of Energy Oak Ridge Reservation. Bull. Environ. Contam. Toxicol. 58:720-725.
Willner, G.R., J.A. Chapman, and J.R. Goldsberry. 1975. A study and review of muskrat food habits with special reference to Maryland. Public. in Wildl. Ecol. No.1. Maryland Wildl. Admin.
Wragg, L.E. 1954. The effect of DDT and oil on muskrats. Canad. Field-Nat. 68:11-13.
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