Natural History of the Prairie Vole (Mammalian Genus Microtus) Part 1

Natural History of the Prairie Vole (Mammalian Genus Microtus).

by E. W. Jameson.

INTRODUCTION

The prairie vole (_Microtus ochrogaster_) at Lawrence, Kansas, is approximately 5-1/2 inches in length, of which the tail comprises 1-1/4 inches, and weighs approximately 1-1/2 ounces. The color on the dorsum is dark gray with a grizzled appearance from the mixture of black and fulvous on the long hairs; the venter is paler, sometimes pale fulvous or cinnamon. The animal is compactly built much as are the other microtine rodents. The short legs and short tail, small eyes and partly hidden ears, and heavy and flattened head all suggest its semifossorial mode of life. The prairie vole spends most of its time in an elaborate system of tunnels (some entirely below the ground) and in almost hidden galleries in the dense gra.s.s.

_Microtus ochrogaster_ can be separated from other voles in its geographic range by a combination of several characters. The plantar tubercles usually number five, although a few individuals with six tubercles were found at Lawrence, Kansas. _Microtus pennsylvanicus_, normally with six plantar tubercles, as Bole and Moulthrop (1942:156) pointed out, sometimes has only five. Therefore, the number of plantar tubercles alone is not a certain means for separating _pennsylvanicus_ from _ochrogaster_. The color of the venter of _ochrogaster_ is usually fulvous or cinnamon instead of grayish as in _pennsylvanicus_, but there is variation in this respect too; some prairie voles also have a grayish venter. The shorter tail of _ochrogaster_ will a.s.sist in establis.h.i.+ng its ident.i.ty where it occurs with _pennsylvanicus_. The third upper molar has two closed triangles in _ochrogaster_ and usually three in _pennsylvanicus_. The pelage of _ochrogaster_ is coa.r.s.e whereas _pennsylvanicus_ has fine fur. Prairie voles may be separated from pine mice (_Pitymys nemoralis_ and _P. pinetorum_) with which they are sometimes found, by the larger eyes, less rusty color, and longer tail.

The Cooper lemming mouse (_Synaptomys cooperi_) differs from the prairie vole in having the upper incisors grooved, and in possessing a shorter tail which approximates the hind foot in length.

Of _Microtus ochrogaster_ from Lawrence, Douglas County, Kansas, average measurements of twenty-five adult males are: total length, 143 (121-167) mm.; tail, 32 (25-42) mm.; hind feet, 20 (17-22) mm.; weight, 43 (38-55) grams. Twenty-five adult females from the same place average: total length, 150 (131-170) mm.; tail, 33 (31-41) mm.; hind foot, 19 (17-21) mm.; weight, 45 (38-58) grams.

The prairie vole is found in suitable habitats in the central part of North America. It has been recorded from Edmonton, Alberta, in the northwest (Bailey, 1900:76), southeastward to Chesapeake, Ohio (Bole and Moulthrop, _op. cit._:156), and in the southwest as far as Ft. Reno, Oklahoma (Bailey, _op. cit._:74). See figure 1 showing the known range of _Microtus ochrogaster_. _Microtus ludovicia.n.u.s_, a close relative of _ochrogaster_, has been taken along the southern part of the boundary between Texas and Louisiana (Lowery, 1943:247).

The activities of voles, especially those of the genus _Microtus_, attracted the attention of naturalists even in early times. Aristotle (translated by Thompson, 1910) wrote: "The rate of propagation of field mice in country places, and the destruction that they cause, are all beyond telling. In many places their number is so incalculable that but very little of the corn-crop is left to the farmer; and so rapid is their mode of proceeding that sometimes a small farmer will one day observe that it is time for reaping, and on the following morning, when he takes his reapers afield, he finds his entire crop devoured. Their disappearance is unaccountable: in a few days not a mouse will be there to be seen."

[Ill.u.s.tration: FIGURE 1. Range of the Prairie Vole (_Microtus ochrogaster_).]

Several early naturalists in this country commented on the fluctuations in numbers of individuals, and on the breeding and feeding habits of voles. Kennicott (1857) in an agricultural report on the mammals of Illinois wrote about the breeding of the prairie vole. He described its stores of plants and commented on the behavior of some captives. Quick and Butler (1885) discussed the habits of _Microtus ochrogaster_ as well as those of _M. pennsylvanicus_, _Pitymys pinetorum_, and _Synaptomys cooperi_ in Indiana, and described the feeding and breeding habits of these species. Criddle (1926) gave an account of the feeding and breeding habits of _Microtus ochrogaster_ in Manitoba, and Fisher (1945) published a short description of the food and reproduction of the same species as he observed it in Missouri. Stone investigated the fauna in the nests of this vole in the same state, but has not yet, as of March, 1946, published his findings.

METHODS

The information in the present account was obtained by observing animals in the field, and by examining trapped animals that were brought into the laboratory. Five hundred individuals were caught in snap-traps, and forty additional voles that were marked were captured a total of 157 times. More than 90 per cent of the specimens were trapped at Lawrence, Douglas County, Kansas, but voles were examined also in Ellsworth, Atchison, and Jefferson counties, Kansas, and in Douglas County, Illinois. My data pertain to _Microtus ochrogaster_ in the above named areas from October, 1945, until August, 1946. The findings may not be typical of this species in other areas and in other years.

The museum special traps were used both with and without bait. The bait consisted of a piece of walnut meat on the treadle. By placing the trap crosswise in the runway, voles were captured whether or not the treadle was baited. Immediately upon removal from the trap, each vole was placed in a white flannel sack, one sack sufficing for several voles when necessary. In this way the loss of ectoparasites was kept to a minimum.

The fleas were counted, and the numbers of lice and mites were estimated; some specimens of ectoparasites were saved for identification.

The voles taken in live traps were marked and released. The marking was done by cutting off one or more toes in such a manner that the vole could later be identified. From left to right, the toes were a.s.signed numbers from one to five on the left hind foot, and by tens from ten to fifty on the right hind foot. Number 33, therefore, was a.s.signed to the one vole of which the middle toe of each hind foot had been cut off.

Each time an animal was captured alive, it was weighed, specimens of fleas, lice and mites were preserved, and the external appearance of the reproductive organs was noted. The extent of the molt line, if the vole was molting, was recorded. Corresponding data were kept for each dead vole caught in a snap trap.

a.s.sistance is acknowledged from Professors E. Raymond Hall, A. Byron Leonard, Worthie H. Horr, and Donald F. Hoffmeister; and I have had also much helpful advice from Professors W. J. Hamilton, Jr., and P. C.

Stone.

MOLT

The skins of 44 molting prairie voles were pinned out flat. The flesh sides clearly show the areas of molt. Various stages in the molt process were observed also in animals caught in live traps. The molt begins when the animal is three or four weeks old; at this time the juvenal pelage is replaced by the subadult pelage. The second molt occurs when the prairie vole is between eight and twelve weeks old, and is the means by which the adult pelage replaces the subadult pelage. These same two molts were found by Hatfield (1935) to occur in captive _Microtus californicus_. Molting voles of the species ochrogaster were trapped in each month of the year.

The molting processes of juveniles and subadults follow the same pattern. The first area of molting is in the pectoral region. The molt patch extends caudad toward the tail and cephalad toward the chin. New pelage separates this area of active molt into two strips on the fourth or fifth day. By this time each strip has spread laterad to the legs and sides, and is 10 to 20 mm. wide. Ultimately each strip unites with its opposite, usually at the center of the dorsum. This area of molt then spreads cephalad and caudad. Fourteen to fifteen days after the beginning of the molt, the entire dorsum is in process of being covered with new pelage. Shortly before the completion of the molt, the dorsal area of molt divides into two patches, one on the rump and one on the nape. The areas last to be covered with new pelage are the crown and that between the ears and the eyes. A slight variation in the above process occurred in some specimens in which the lateral strips joined immediately cephalad of the tail instead of at the center of the dorsum.

The entire process takes approximately three weeks.

Large voles (45 grams or more) grow hair in irregular patches that measured 5 to 15 mm. In these large voles the molt is accomplished slowly and does not cover large areas of the body at any one time. The small areas of molt are visible for 7 to 10 days, and were found on approximately three quarters of the large voles examined.

FOOD AND HABITAT

The diet of the prairie vole reflects both its environment and its choice of food. The plants eaten are usually green and succulent, but some dry, hard seeds and small stems of woody plants are also eaten. The vegetation, which supplies the food for the vole, is important as cover or nesting material. For this reason food and habitat are discussed together.

TYPES OF COVER

Prairie voles inhabit areas where the dominant plants in summer are clover or gra.s.ses or both. The lawn on the campus at the University of Kansas consists mostly of several kinds of gra.s.ses, but in some places alfalfa (_Medicago sativa_) replaces clover (_Trifolium_ sp.), and in other places sedges (_Scirpus_ spp.) are found in addition to the gra.s.ses. The gra.s.s is short; it is mowed to a length of 4 to 6 inches.

Bluegra.s.s (_Poa pratensis_) and crabgra.s.s (_Digitaria ischaemum_) form most of the sod. Bluejoint (_Andropogon furcatus_) is common in a spa.r.s.ely wooded part of the campus, an area which has many voles.

Foxtail (_Setaria lutescens_ and _S. viridis_) and prairie threeawn (_Aristida oligantha_) are also common on the lawn, but these become dry in late summer, and at that time supply neither food nor cover for the voles. The voles make well-beaten depressions in the sod, and the gra.s.s arches over them to form canopies.

In the winter, when the snow flattened the gra.s.s on the campus so that there were no longer protective canopies of blades over the runways of the voles, they migrated into areas of j.a.panese honeysuckle (_Lonicera j.a.ponica_). At this season the honeysuckle was their main food. In areas where this vine was not available, the voles abandoned their surface runways and remained below the ground, coming to the surface only under the protection of a blanket of snow. The voles returned to the gra.s.s and clover habitat in March and April in 1946.

One pure stand of Ladino clover in Jefferson County, Kansas, was studied in late November and early December of 1945. The clover was 2 to 4 inches high, and although it was the sole food of the voles, it furnishes but little cover. They were common here; 300 traps yielded 111 voles in two nights.

CUTTINGS

The voles seek particularly the tender heads of gra.s.ses and the terminal leaves of sweet clover (_Melilotus alba_). To obtain these parts, the voles begin by cutting through the base of the plant. The surrounding plants are often near enough to support the freshly cut piece in an upright position. The vole makes successive cuttings, 40 or 50 millimeters from the ground, until the desired parts of the plant are within reach. The cuttings that have acc.u.mulated at the base of the plant may be eaten, but frequently they remain as evidence of the vole's feeding activity.

On May 12, 1946, an a.n.a.lysis was made of the cuttings found in an area of alfalfa, gra.s.ses, and weeds. From table 1 it may be seen that quackgra.s.s, alfalfa, wild lettuce, and cleavers were common. In three nights 70 traps caught 8 prairie voles and 3 deer mice; no pine mice or cotton rats were caught on the area. The stomachs of the voles and the deer mice were examined, and only the stomachs of the voles contained green material. a.n.a.lysis of the cuttings (see table 2) indicates that alfalfa was eaten in greater quant.i.ty than any other plant; it made up almost three quarters of the cuttings although but one quarter of the cover. All other plants occurred less commonly in the piles of cuttings than they did in the estimated composition of the cover. Gra.s.ses and wild lettuce were next to alfalfa in importance.

TABLE 1.--_The Relative Abundance of Plants in an Area of Alfalfa, Gra.s.ses, and Weeds_[A]

Percentage by number Species of plants

Quackgra.s.s (_Agropyron repens_) 30 Speargra.s.s (_Poa annua_) 1 California brome (_Bromus carinatus_) 1 Smooth brome (_Bromus inermis_) 1 Alfalfa (_Medicago sativa_) 25 Peppergra.s.s (_Lepidium densiflorum_) 2 Cleavers (_Galium aparine_) 15 Wild lettuce (_Lactuca scariola_) 25

TABLE 2.--_Composition of Ten Piles of Cuttings_[B]

Frequency of Species Ten piles of cuttings occurrence

_Agropyron repens_ 1 0 0 2 0 0 6 19 0 4 13 _Poa annua_ 0 0 0 0 0 0 0 0 0 0 00 _Bromus carinatus_ 0 0 0 0 0 10 0 0 0 0 04 _Bromus inermis_ 0 0 0 0 0 0 0 0 0 0 00 _Medicago sativa_ 40 14 30 30 31 5 0 0 21 4 73 _Lepidium densiflorum_ 0 0 0 0 0 0 0 0 0 0 00 _Galium aparine_ 0 0 0 0 1 0 1 0 0 0 01 _Lactuca scariola_ 6 2 1 2 5 0 0 0 2 4 09

[Footnote A: a.n.a.lysis made on May 12, 1946, on an area 20 80 yards, at Lawrence, Kansas.]

[Footnote B: Each of the first ten vertical columns gives the composition of one pile of cuttings. The last column gives the percentage of occurrence in the piles of cuttings of each species of plant in the area. Place and date for data in table 2 same as for table 1.]

Approximately one out of every ten voles caught in snap traps had a piece of plant in its mouth. Occasionally a vole took a piece of food into a live trap. Evidently the food is not always eaten where it is procured. Gra.s.ses of the genus _Poa_ are the kinds most frequently found in the mouths of dead voles. _Bromus carinatus_, _B. inermis_ and sweet clover (_Melilotus alba_) were found in the runways. The pulpy fruit of the horse nettle (_Solanum carolinense_) was found partly eaten, especially near the entrances to underground pa.s.sages.

FOOD CACHES