Aspects of Reproduction and Development in the Prairie Vole (microtus Ochrogaster)

Part 1

Produced by Chris Curnow, Tom Cosmas, Joseph Cooper and the Online Distributed Proofreading Team at http://www.pgdp.net

UNIVERSITY OF KANSAS PUBLICATIONS MUSEUM OF NATURAL HISTORY

Volume 10, No. 4, pp. 129-161, 8 figs. in text, 6 tables

December 19, 1957

Aspects of Reproduction and Development in the Prairie Vole (Microtus ochrogaster)

BY HENRY S. FITCH

UNIVERSITY OF KANSAS LAWRENCE 1957

UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

Editors: E. Raymond Hall, Chairman, Henry S. Fitch, Harrison B. Tordoff

Volume 10, No. 4, pp. 129-161, 8 figs. in text, 6 tables Published December 19, 1957

UNIVERSITY OF KANSAS Lawrence, Kansas

PRINTED IN THE STATE PRINTING PLANT TOPEKA, KANSAS 1957

26-7561

Aspects of Reproduction and Development in the Prairie Vole (Microtus ochrogaster)

HENRY S. FITCH

INTRODUCTION

The prairie vole is by far the most abundant mammal on the University of Kansas Natural History Reservation and on grassland areas throughout northeastern Kansas. This vole therefore affects the vegetation, perhaps more than any other native vertebrate, and it is an important food source for most of the vertebrate predators. Since the Reservation was established, in 1948, more data have been accumulated concerning this vole than for any other species of animal there. From February, 1950, to February, 1954, a grid of live-traps at 50-foot intervals was set for several days each month in a three-acre field inhabited by voles, and the population of marked individuals was studied throughout the four-year period. From November, 1953, to June, 1956, a half-acre trap grid with 20-foot interval was used on an area adjoining the three-acre field. Other trap lines in somewhat different habitats were maintained for shorter periods as a basis for comparison. By June, 1956, a total of some 3550 voles had been caught and recorded 14,750 times in all. The present report is a preliminary attempt to analyze, in part, these extensive data, and is concerned with certain phases of the species' reproduction and growth that have bearing on the observed population changes from month to month and from year to year on the Reservation.

Through the studies of Jameson (1947) and Martin (1956), both made in the same general area as my own, and several earlier studies, the life history and ecology of the prairie vole are already well known. The present report, with much larger amounts of data, further clarifies certain phases of the ecology; and by using types of data not available to Jameson and Martin I have dealt with some topics not included in their reports.

Previous studies of growth in Microtus have been based almost entirely on weights. However, the weight of an individual vole may fluctuate widely over a short period, depending on pregnancy and parturition, length of time in a trap without food, availability of moisture, and other factors. In the course of my study, in 1954 and 1955, and parts of 1953 and 1956, measurements of total length, in addition to weights, were recorded for most of the voles live-trapped.

To test the accuracy of measurements, successive readings were compared in individual voles that were already of large adult size and that presumably either had stopped growing or were growing so slowly that the gain was scarcely detectable in the relatively short periods involved. For 200 such readings 33 per cent were just the same as previous records for the same animals, 24 per cent deviated by 1 mm., 22 per cent deviated by 2 mm., 15 per cent by 3 mm., 4.5 per cent by 4 mm., .5 per cent by 5 mm., 1 per cent by 6 mm., and .5 per cent by 7 mm. On the average, successive measurements varied by 1.43 mm., somewhat less than one per cent of the adult vole's total length. Occasional errors of two to four per cent were easily eliminated because for the voles used for growth records, series of measurements were available, with clearly defined trends. The occasional readings that deviated from the general trend for the individual were discarded.

Measurements were recorded along with other data in the field at the point of capture. Obtaining a reasonably accurate measurement on a live and struggling vole required patience and practice. With the thumb and forefinger of the left hand, I grasped the vole by loose skin of the nape, and simultaneously grasped the tail at a point approximately three-fourths of the distance to the tip. Then, with gentle but steady pressure, I stretched the vole out to its full length, meanwhile manipulating a millimeter ruler with the free fingers, so that the vole was pressed against it, with the nose pad at the end of the ruler.

The total length measurement is considered the best index to over-all size. The relative tail-length varies slightly between individuals, averaging approximately 22 per cent of the total length. Individuals having broken tails, or having the distal parts of their tails missing, were not included. The total length can be measured with greater accuracy than can either the head-and-body length or the tail-length separately.

GENERAL SOCIAL BEHAVIOR

As compared with other mammals, voles are tolerant and somewhat social. That individuals are not mutually exclusive (territorially) in areas occupied was demonstrated on many occasions when more than one individual was caught simultaneously in the same live-trap. Injury of a vole by a trap-mate was a rare occurrence.

Multiple captures often involved a female in oestrus and one or more males, or a female and her young, but other instances involved various combinations of sex and age groups. As many as five adults have been caught in a trap simultaneously at times when the population density was high. At such times, the meadow habitat is crossed by a maze of interconnecting surface runways and one runway may be traced continuously for 100 yards or more. Because each individual vole normally confines its activity to a small area, only a fraction of an acre, it is evident that individuals living at different places overlap in their home ranges, and also in the trailways followed in foraging. A high degree of tolerance is indicated. Where population is so sparse that the systems of surface runways comprise separate and isolated units, trapping experience has shown that one such system may harbor several or many individuals.

As direct observations on voles under natural conditions are rarely feasible, because of the animals' timidity, their utilization of concealing cover, and tendency to crepuscular habits, best evidence of social habits and underground life is based upon behavior of captive individuals. Many voles were kept in confinement for varying lengths of times, either singly or in association with others. Under such conditions there was sometimes sporadic fighting, but it was mainly defensive and serious injuries were rare. Two or more voles caught at a given spot regardless of whether they were found in the same trap simultaneously, or trapped separately within a short time, usually were completely tolerant of each other. When at rest in their container, such voles would huddle together in a corner or in a nest, if materials were provided, so that collectively they presented the minimum exposed surface. The intimacy and lack of antagonism displayed on such occasions, suggested that the voles were accustomed to living together amicably in the same nest chamber. In live-trapping, "double" captures in a single trap often involved the same two individuals. Such trap-mates were often male and female, and in many instances the female was not in breeding condition. That the voles are not monogamous in habits was demonstrated when the same female was often trapped in association with either of two males. Other trap associates taken together repeatedly often were two males, or two females. Voles that are nest mates or "neighbors" may tend to move about together in their foraging, or one confined in a trap may attract the other sufficiently to cause it to force an entrance by lifting the heavy door of a trap.

When a new vole, caught at a different location, is added to a container in which one or more are already confined, there is mutual circumspection between the original occupants and the newcomer. At first, each vole is intimidated by movements of the other, and as a result, the original occupants huddle in their established corner while the newcomer cowers in the most remote part of the container. Gradually the voles become less timid and one may approach another slowly and cautiously, to sniff at it. The vole approached may react with a show of hostility which is largely defensive. In the characteristic posture of threat for defense, the vole crouches, or rears back on its haunches, with snout elevated and incisors prominently displayed. If the warning posture is unheeded, or if the vole is made unusually aggressive by having young to defend, or for some other reason, it attacks with a sudden forward lunge, striking the adversary simultaneously with both forefeet and with the incisors. The lunge is so rapid that when I have observed it, I have been unable to discern whether the attacker bit its opponent. The attack serves to force back the other animal, throwing it off balance and intimidating it. The attacked animal may dodge nimbly to avoid the lunge, but whether or not it is actually struck, it usually retreats, avoiding or postponing further hostilities. Voles that have been kept in containers for periods of hours or days tend to be more hostile and aggressive toward a newcomer than are those newly introduced. After series of meetings resulting from the exploratory behavior of the newcomer and the curiosity or normal activity of those longer confined, hostility gradually subsides. Within a few hours a newcomer is usually accepted, and thenceforth he huddles with other members of the group when at rest, and hostility is rarely evident.

Continue reading Aspects of Reproduction and Development in the Prairie Vole (microtus Ochrogaster) free in the Wunder reader →