Stuart L. Iverson

Winter Weight Dynamics in Microtus Pennsylvanicus

A population of meadow voles (Microtus pennsylvanicus) 75 mi northeast of Winnipeg, Manitoba, Canada was live-trapped biweekly for two winters. From August to February of each year the mean population weight decreased by 30% to 40%. This decrease was not due to preferential death of the heavier animals because their survival was better than would have been expected if survival and weight were independent. Part of the weight loss was due to the immigration of young animals that failed to gain weight. However, much of the decrease in mean weight was due to that lost by individuals, because many samples of animals released and recaptured 2 wk later had lost significant amounts of weight. Four marked 8 8 trapped from August to February lost an average of 28.6% of their weights while 14 marked 9 9 lost 45.5% in the same time period. In February the mean weights of individuals and the population began to increase. It is suggested that weight loss is not simply a result of lack of food, but is an adaptive response cued by some factor such as day length.

THE CHARACTERIZATION OF VOLE POPULATION DYNAMICS

The population cycles of meadow voles (Microtus pennsylvanicus) were studied in southeastern Manitoba, Canada, from 1968 to 1978 on a 32-ha old field and in surrounding boreal forest. Vole density was monitored on mark-recapture live-trapping grids trapped at 2- to 4-wk intervals throughout the year. This was supplemented by a 3-d spring and a 30-d summer snap-trapping survey. Multiannual and annual fluctuations in density of Microtus were independent of a diversity of patterns seen in other small mammal species. Principal components analysis of demographic statistics was used to characterize changes in seasonal reproduction, survival, and dispersal that accounted for the observed numerical fluctuations. Conditions during a catastrophic winter population decline in 1970-1971 were compared with those in 1974-1975, when numbers recovered quickly after a modest spring decline. Voles were in good condition before and during both declines, as evidenced by normal body composition indices. Large body masses, characteristic of peak populations, preceded both population declines. The main conditions associated with the 1970-1971 decline were unusual patterns in social behavior and dis- persal. Changes in social behavior in the field were reflected in unprecedented numbers of multiple captures in Longworth traps before and during this decline. Field observations were supported by unusually high frequencies of amicable behavior in neutral arena bouts staged between males removed from a control population. Little dispersal was detected with conventional live-trapping during the winter period of chronic mortality in 1971. In contrast, a period of acute mortality, resulting in the near extinction of the population in March 1971, was associated with considerable dispersal. We propose two alternative hypotheses consistent with the unpredictable and episodic nature of vole population declines: (1) an immunological dysfunction exists in voles that makes them susceptible to naturally occurring microparasites or (2) opportunistic, pathogenic microparasites exploit the changes in dispersal and social contacts that occur as a normal part of the vole population cycle.

Winter Coexistence of Clethrionomys gapperi and Microtus pennsylvanicus in a Grassland Habitat

In October 1969 numerous red-backed voles (Clethrionomys gapperi) immigrated into a field normally occupied mainly by meadow voles (Microtus pennsylvanicus), established home ranges, and survived well until February 1970, when they began to disappear. Both species were at peak populations during the winter of 1969-70, in the Pinawa, Manitoba, area. The times of immigration and emigration corresponded with the end and onset of reproductive activity in the resident Microtus. It is suggested that Clethrionomys are normally excluded from grassland habitats by reproductively active and aggressive Microtus. In winter M. pennsylvanicus is less aggressive and the species can coexist. Winter colonization, allowed by the breakdown of habitat separation due to aggressive behavior, may explain the ubiquitous presence of forest animals in the isolated groves which are common in the aspen parkland. INTRODUCTION The mechanisms which preserve habitat separation between meadow voles (Microtus) and red-backed voles (Clethrionomys) have been the subject of several recent studies (Clough, 1964; Cameron, 1964; Morris, 1969; Grant, 1969, 1970a, 1970b), but are still incompletely understood (Clough, 1964; Morris, 1969). This paper describes a temporary breakdown of habitat separation, and discusses the implications of this event to an understanding of the mechanisms which normally preserve habitat separation.

UNDER-SNOW SHELTER FOR SMALL MAMMAL TRAPPING

The inexpensive, easily constructed shelter described allowed routine livetrapping of Microtus pennsylvanicus to continue successfully when temperatures fell as low as -35 C. It also provided sum- mer shade so that traps needed checking only once a day to maintain low trap mortality. Trap mor- tality rates are given. While making a long-term study of an old-field population of Microtus pennsyl- vanicus in southern Manitoba we needed an effective sampling method under winter conditions, where temperatures, during the December-March period, may reach -45 C. It is well known, however, that tempera- tures beneath several feet of snow remain within a few degrees below freezing. We therefore suspected that a Longworth trap, supplied with food and bedding material, placed in this subnivean space, the normal winter environment, should keep animals alive. Attempts to trap in this space proved fruitless, since the traps quickly fouled with snow, and were not readily found by the animals. We needed a shelter which would keep the traps snow-free and could be checked without disturbing runways. Two such shelters have been described in the literature. The system described by Fay

Seasonal changes in open-field behavior in wild male meadow voles (Microtus pennsylvanicus)

Open-field behavior of free-living meadow voles was measured in males held overnight in the laboratory. Movement variables were positively correlated, and had negative associations with grooming and freezing. Parameters including activity, freezing, urinating, and grooming showed annual fluctuations related to the reproductive season. Together with the results of a previous study showing castration of wild voles results in altered open-field behavior, these results emphasize the role of testicular hormones in influencing this behavior. Factor analysis identified an activity component accounting for 39% of the variation, but other parameters were little simplified by this procedure. Most factors cycled annually, and significant variation was found in all eight factors during the 4-year sample. Open-field behavior varied between different reproductive and age segments of the population, and may be related to population processes such as dispersal.

Postnatal Growth and Development of Captive Franklin's Ground Squirrels (Spermophilus franklinii)

Growth and development of Franklins ground squirrels (Spermophilus franklinii) were determined from 87 young raised by wild-caught females in the laboratory. Developmental stages appeared in the same sequence, but at a mean earlier age than in any other species of Spermophilus except S. undulatus. Lower incisors erupted on day 7, eyes and ears opened on day 20, and weaning occurred by day 28. Weight reached 25% of adult weight by day 34, 50% by day 44, 75% by day 54, and adult weight was reached by day 65. The instantaneous growth rate (IGR) was greatest in the 1st 10 days, with a maximum of 12.4% per day at day 3, decreasing to 1.2% by day 50. Hind foot length reached 75% of adult size by day 30, whereas total and tail lengths were less than 60% at the same age. Individual weight was negatively correlated with litter size, and there were indications of lower survival to weaning in numerically larger litters. Laboratory growth measurements cannot be directly extrapolated to field populations, since weaned laboratory young gained weight significantly more rapidly than field young. Data from similar studies on other hibernating sciurids suggest that birth dates and IGRs are related, and indicate a biological response to differing climates.