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The first distributional record of the least weasel, Mustela nivalis, in Northeastern Missouri.

Abstract: This paper reports a significant range extension for the least weasel (Mustela nivalis). Failure to capture least weasels during the previous 50-year period in which the Adair County area has been intensely trapped for small mammals suggests that this species is a recent immigrant into northeastern Missouri. The location of the capture sites near the break between the Missouri and Mississippi drainage systems does not support a riparian mode of dispersal. We speculate that changes in agricultural policies and practices that have reduced row-crop farming and increased meadows and USDA Conservation Reserve Program land are factors in the recent movement of M. nivalis into northeastern Missouri.

Key Words: Mustela nivalis, weasel, shrew, Missouri, dispersal.


The least weasel (Mustela nivalis), the smallest carnivore, shows Holarctic distribution. Southern limits of its reported North American range extend from Montana through the upper Midwest to western Pennsylvania then south along the Appalachians into North Carolina (Hall, 1981; Sheffield and King, 1994). Extensive trapping programs failed to show the least weasel in northeastern Missouri (Beckert, 1968; Crawford, 1968; Ellis, 1984). The nearest confirmed distributional records are from Nodaway County, Missouri and Ringgold County, Iowa to the northwest (Easterla, 1970), Story County, Iowa to the north (Polderboer et al., 1941), and Henry County, Iowa and McDonough County, Illinois to the northeast (Hall, 1951; Harty and Thom, 1978). Skeletal remains of M. nivalis found in a great horned owl (Bubo virginianus) pellet collected in Wapello County, Iowa, represent a tenuous closer record (Scott, 1939). Many papers describing marginal distributional records state or imply a recent southern movement for this spec ies (Hatt, 1940; Easterla, 1970; Harty and Thom, 1978). Most peripheral records are from locations along waterways suggesting a riparian mode of dispersal (Sheffield and King, 1994). Mustela nivalis ranged farther to the south in prehistoric times. Parmalee and Munyer (1966) report a least weasel lower mandible and skull fragments from an excavation in a limestone bluff just south of St. Louis, Missouri (Columbia, Monroe County, Illinois).

Least weasels inhabit areas based on prey species abundance and the presence of adequate cover. They prefer intermediate-sized, inactive mammals such as microtine rodents, but will take more active cricetid species if microtines are not available. Occasionally they eat young rabbits, nestling birds, moles, insects, small reptiles and amphibians (Sheffield and King, 1994). Shrews are captured less frequently than expected based on their relative abundance and are seldom eaten when taken (Korpimaki and Norrdahl, 1987). Captive least weasels can survive for extended periods on diets of mice, rats and pigeons (Lewellyn, 1942; Easterla, 1970). Two mustelid species can coexist when rodent populations are high. The larger weasels have a selective advantage when prey species numbers are decreasing. M. nivalis has a competitive edge in its ability to rapidly recolonize when rodent populations increase (Sheffield and King, 1994).

While odor production serves for intraspecific communication, a mustelid odor can be detrimental to predators when prey populations are low. Stoddart (1976) states that rodents selectively avoid sites that smell of weasel. Some reports mention a mustelid odor associated with M. nivalis (Llewellyn, 1942), but others imply that its scent is mild in comparison to sympatric mustelid species (Stubbe, 1972; Brinck et al., 1983).

Materials and Methods

We captured one M. nivalis at each of our trapping sites during a project to obtain shrews for laboratory use. Thirty kilometers separate the Schuyler County area located 5 km north-northwest of Greentop, Missouri, from the Adair County site situated 4 km east of Millard, Missouri. Each setting is typical old-field habitat, bordered by overgrown fencerows. Little row-crop agriculture occurs near either site. Most of the surrounding land is pasture, meadow or USDA Conservation Reserve Program (CRP) land. Highway US 63 generally follows the break between the Missouri and Mississippi drainage systems. Greentop and Millard lie along this highway. Thus, the site west of Greentop ultimately drains into the Missouri River and the site east of Millard is in the Mississippi drainage basin.

Standard (7.6 cm x 8.9 cm x 30.5 cm) Sherman traps (H.B. Sherman Traps, Inc., Tallahassee, FL, U.S.A.) with a fine treadle setting captured all animals described in this study. Traps baited with oatmeal soaked in peanut butter and corn oil were typically placed at alternate posts (8 m intervals) along bordering fence lines. Traps were set prior to dusk and checked every 2-4 h in the autumn and every 4-6 h in the spring and summer to assure shrew survival. Sampling occurred during the fall of 1996 at the Schuyler site and during the spring and summer of 1997 at both areas. Our animal use protocol required the release of all mammals captured other than shrews and vanguard species. Therefore, we did not routinely record rodent captures. The exception was the night of 17 April 1997 at the Adair site. We listed all species captured in 78 traps for that night. Animals were collected under Wildlife Collector's Permits issued by the Missouri Department of Conservation.

The captive weasels were maintained at the Kirksville College of Osteopathic Medicine, a facility with animal care and use programs accredited by American Association for the Accreditation of Laboratory Animal Care, International. All aspects of the study received full Institutional Animal Care and Use Committee approval, and were conducted in accordance with the Animal Welfare Act, the Guide for the Care and Use of Laboratory Animals (National Research Council, 1996) and Acceptable Field Methods in Mammalogy (ad hoc Committee, American Society of Mammalogists, 1987). The captive weasels were maintained in an isolation room under standard environmental conditions (22 + 2[degrees]C, 12/12-h light dark cycles, 10 fresh air changes/h). Housing consisted of 49 cm x 34 cm x 30 cm polycarbonate cages with ventilated tops, aspen shaving bedding (Northeastern Products Corp., Warrensburg, NY), and an opaque, 22 cm plastic square for cover. Animals received tap water ad libitum and two or more of the following food it ems were always available: fresh [CO.sub.2] euthanized laboratory animal carcasses, prairie voles (Microtus ochrogaster) or least shrews (Cryptotis parva); mealworms (Tenebric sp.); feline diet no. 5003 (Purina Mills Inc., St. Louis, MO); and a moistened shrew diet consisting of equal parts ground feline diet and Kozy Kitten Cat Food (Heinz Pet Products Co., Newport, KY).

Skins and skulls of the least weasels taken during this study were placed in the collection of the Truman State University museum. Carcasses were not preserved.


We captured two least weasels, a female at the Schuyler site on 10 October 1996, and a male at the Adair site on 23 August 1997. Both animals were collected along fence lines bordering state highways. Each site showed meadows of tall fescue (Festuca arundinacea); right-of-ways of smooth brome grass (Bromus inermis); and overgrown fencerows of lodged smooth brome grass, wild grape (Vitis sp.), poison ivy (Rhus radicans), and new growth trees. The saplings were black locust (Robinia pseudoacacia) at the Schuyler site and red cedar (Juniperus virginiana) at the Adair site. Students from the Truman State University mammalogy class concurrently trapped the Schuyler area during the autumn of 1996.

From our field observations and specimens classified by the students in the mammalogy class, the following species were listed for the Schuyler site: masked shrew (Sorex cinereus), southeastern shrew (S. longirostris), short-tailed shrew (Blarina brevicauda), western harvest mouse (Reithrodontomys megalotis), deer mouse (Peromyscus maniculatus), white-footed mouse (P. leucopus), meadow vole (Microtus pennsylvanicus), prairie vole (M. ochrogaster), woodland vole (M. pinetorum), southern bog lemming (Synaptomys cooperi), and house mouse (Mus musculus). On 17 April 1997 we listed all animals collected in 78 traps at the Adair site as follows: 50 Peromyscus (P. maniculatus, P. leucopus); 14 microtines (M. ochrogaster, M. pennsylvanicus, M. pinetorum, S. cooperi); and 6 B. brevicauda. These data are typical for mammalian abundance at each site. We routinely captured over twenty animals (80 to 100 traps) at each first post-dusk or post-dawn run; a total of 57 shrews (52 Blarina and 5 Sorex) were collected in 1054 trap-nights. The listed species for 17 April 1997, the weasels and one Norway rat (Rattus norvegicus) represent the species trapped at the Adair site. The female was taken near the highest point of the trapping area and the male was collected on a slope about 200 m from an extensive creek bottom. We were alerted to the capture of something other than a rodent or shrew by increased weight and activity within the trap. A faint musteline odor was noted upon sniffing the trap.

On 28 June 1997 we captured a long-tailed weasel (Mustela frenata) at the Adair site about 100 m down slope from the site where the male M. nivalis would be taken. The fence line was not overgrown in this area. Lodged brome grass provided the only cover. A strong musteline odor signaled the presence of a weasel as we approached the trap. We examined and then released the animal.

We reviewed the species lists at the Truman State University museum and from Missouri Department of Conservation's annual trapping reports. Higher vertebrate zoology or mammalogy classes at Truman State University have routinely collected mammals since 1948. Autumnal mammal census data have been gathered at selected northeastern Missouri riparian sites by the Missouri Department of Conservation since 1994. There were no reports of least weasel captures for either enterprise. Figure 1 shows the location of our trapping areas and previously confirmed capture sites. The Adair site is 150 km from the nearest listed site.

Captive weasels were not immediately euthanized. The female died during her first night in captivity. She consumed a least shrew carcass and some mealworms before her death, but failed to eat the feline diet or shrew food mixture. More detailed food preference studies were conducted during the four days that the male was housed. He preferred prairie vole to least shrew, but ate both, and refused all other food items. A musteline odor was not obvious in the well-ventilated animal cubicle.

The nonpregnant female from Schuyler County measured: weight, 35.3 g; total body length, 189 mm; tail length, 29 mm; hind foot, 18 mm; ear 5 mm. The male from Adair County measured: body length, 210 mm; tail length, 38 mm; hind foot, 24 mm; ear 7 mm. Both animals showed the typical summer pelage.


It is said that mammalian distribution maps reflect the distribution of mammalogists rather than the distribution of mammals. The previous marginal records for M. nivalis fit that pattern. Most capture sites shown in Figure 1 are near a major university. This thesis also supports the argument that M. nivalis is a recent immigrant to the northeastern Missouri region. The area around Kirksville has been intensely trapped for the past 50 years with no previous captures of least weasels.

Our report does not support a riparian mode of dispersal. The capture sites are located near the break between the Missouri and Mississippi drainage systems, at a considerable distance from major rivers that extend into the previously reported range of M. nivalis. Changes in agricultural policies and practices are the probable reason for the recent immigration. The farms in the two northern tiers of Missouri counties averaged 26.5 percent of their land in the CRP (1992 Census of Agriculture). By any standards the numbers of mammals that we captured, particularly during the spring, are exceptional. These high small-mammal populations could support sympatric weasel species with minimal competition for prey. Most studies of Mustela sympatry have involved M. nivalis and M. erminea. It would be worthwhile to determine if least weasels interact with long-tailed weasels in manners similar to their described interactions with ermines. The lack of the strong musteline odor in M. nivalis could give it a competitive ad vantage in prey capture over M. frenata. Certainly to the human nose, M. nivalis is stealthier. It is unfortunate that carcasses were not fixed. Much information concerning scent production, reproductive status and anal gland morphology could have been gathered.

Our captive weasels show no aversion to Cryptotis carcasses as a food source. Previous reports of food avoidance concerned Sorex sp. (Korpimaki and Norrdahl, 1987). One would expect a difference in odor and taste between the genera but there are no reports that indicate that Cryptotis is less repugnant. It is likely that M. nivalis prefers shrews to many of the items listed as occasional food items by Sheffield and King (1994).


This research was supported in part by EPA Grant CR823734010 awarded to OBM. We thank Steven Sheffield for his helpful suggestion on this project, and Brian Root for his review of the northeastern Missouri autumnal mammalian census data.

Literature Cited

ad hoc Committee for Acceptable Field Methods in Mammalogy. 1987. Acceptable field methods in mammalogy: preliminary guidelines approved by the American Society of Mammalogists. Journal of Mammalogy, 68(4), Supplement: 1-18.

Beckert, J.E. 1968. A field study of mammal distribution in Scotland County, Missouri. Masters Thesis No. 414. Truman State University, Kirksville, Missouri, 56pp.

Brinck, C., S. Erlinge, and M. Sandell. 1983. Anal sac secretion in mustelids: a comparison. Journal of Chemical Ecology, 9:727-745.

Crawford, R.D. 1968. The distribution and abundance of mammals of Adair County, Missouri. Masters Thesis No. 435. Truman State University, Kirksville, Missouri, 64pp..

1992 Census of Agriculture. 1994. Volume 1. Geographic Area Series, Part 25. Missouri State and County Data. Bureau of the Census, Washington, D.C., 593pp. +A-E app.

Easterla, D.A. 1970. First records of least weasel, Mustela nivalis, from Missouri and southwestern Iowa. Journal of Mammalogy, 5 1:333-340.

Ellis, L.S. 1984. Small mammal studies in northeast Missouri: use of railroad right-of-ways and survey of ten sites. Report submitted to Missouri Department of Conservation, Jefferson City, Missouri, 40pp.

Hall, E.R. 1981. The Mammals of North American. Second edition. John Wiley & Sons, New York, 2:601-118 1 + 90.

Hall, E.R. 1951. American weasels. University of Kansas Publications, Museum of Natural History, 4:1-466.

Harty, F.M., and R.H. Thom. 1978. Distribution of the least weasel (Mustela nivalis) in Illinois. Transactions, Illinois State Academy of Science, 71:81-87.

Hatt, R.T. 1940. The least weasel in Michigan. Journal of Mammalogy, 21:412-416.

Korpinmaki, E., and K. Norrdahl. 1987. Low proportion of shrews in the diets of small mustelids in western Finland. Zeitschrift fur Saugetierkunde, 52:257-260.

Lewellyn, L.M. 1942. Notes on the Alleghenian least weasel in Virginia. Journal of Mammalogy, 23:439-441.

National Research Council, 1996. Guide for the care and use of laboratory animals. National Academy Press, Washington, D.C., l25pp.

Parmalee, P.W., and E.A. Munyer. 1966. Range extension of the least weasel and pigmy shrew in Illinois. Transactions, Illinois State Academy of Science, 59:81-82.

Polderboer, Emmett B., Kuhn, Lee W., and Henrickson, George 0. 1941. Winter and spring habits of weasels in central Iowa. Journal of Wildlife Management, 5:115-119.

Scott, Thomas G. 1939. Noteworthy additions to the collection of mammals from Iowa in 1938. Iowa State College Journal of Science, 13: 239-241.

Sheffield, S.R. and C.M. King. 1994. Mustela nivalis. Mammalian Species, 454:1-10.

Stoddart, D.M. 1976. Effect of the odour of weasels (Mustela nivalis L.) on trapped samples of their prey. Oecologia (Berlin), 22:439-441.

Stubbe, M. 1972. Die analen markierungsor gane der Mustela arten. Zoologische Garten (Neue Folge), Leipzig, 42:176-188.
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Author:Easterla, David A.
Publication:Transactions of the Missouri Academy of Science
Geographic Code:1U4MO
Date:Jan 1, 2001
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