Habitat use and population density of five species of forest small mammals were monitored by annual spring snap-trap censuses at Pinawa, Manitoba, over 14 years. Population sizes were positively correlated among species and showed no evidence of density-dependent effects. Species were habitat selectors. Habitat use by species did not vary among years. Habitat separation between the dominant species was not correlated with environmental variables or with population size. We suggest that habitat selection and positive covariance among species abundances are the principal factors characterizing the dynamics of this community.

Demographie d'une population de C. Gapperi entre 1967 et 1982 dans le voisinage du Centre de Recherche nucleaire de Whiteshell (Canada, Manitoba)

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 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 dispersal. 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.

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.

A series of three experiments tested the hypothesis that castration would result in altered open-field behaviour and aggression in male meadow voles. The open-field behaviour of laboratory-reared voles was not affected, and that of wild-captured laboratory-housed animals showed only one significant difference. However, in free-ranging voles, recaptured at intervals for testing, castration resulted in significantly reduced intermale aggression, and significantly decreased activity and increased urination in the open field. Thus, only the results from the experiment using free-ranging voles indicate a significant castration effect, and we suggest the lack of effect in laboratory-housed animals may be due to changes in factors including photoperiod, social milieu, and season.The changes in aggression and open-field behaviour in the wild animals are in agreement with interpretation of previous field studies of intact male voles, which demonstrated changes in these two behaviours at the onset of the breeding season. Among free-ranging meadow voles, therefore, the decreased hormone levels resulting from castration led to changes in these behaviours opposite to those observed in the spring when hormone levels increase with seasonal reproductive maturity. These behaviours thus appear to be at least partially dependent upon circulating testosterone levels in wild voles.

Reproductive characteristics of Peromyscus maniculatus in Manitoba and Peromyscus leucopus in Ontario were compared in order to determine the extent to which breeding compensates for relatively short breeding seasons. Peromyscus maniculatus had a shorter breeding season than P. leucopus, but frequency of litters, litter size, and age of females at sexual maturity were the same for both species. A comparison of breeding characteristics among Peromyscus from different geographic areas indicated that basic reproductive characteristics do not vary in relation to length of the breeding season.In general, we have no evidence that the basic reproductive characteristics of Peromyscus vary in relation to length of the breeding season. Peromyscus with short breeding seasons, on average, must have a lower annual reproductive rate than Peromyscus with long breeding seasons.

Adrenal response was determined during a 13-month period in a population of southern Manitoba meadow voles (Microtus pennsylvanicus) as a preliminary part of a study to determine the effects of chronic ionizing radiation on free-living mammals. Livetrapped voles were subjected to behavioral tests, killed, and their isolated adrenals superfused and stimulated with ACTH. Superfusates were assayed fluorometrically for corticosterone. Peaks of adrenal response occurred during spring in both adult males and females, followed by a significant decline into early summer. Subadult males exhibited a peak response in early summer. Corticosterone levels of adults were generally higher than those of juveniles in the same season. Levels of nonpregnant adult females were higher than those of pregnant voles. Observed differences over the annual cycle were attributed to the behavioral and hormonal states of various age and sexual components of the population.

Growth and development of Franklin's 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.

Beginning in November 1973, numerous meadow voles (Microtus pennsylvanicus) moved onto a spruce forest grid occupied by red-backed voles (Clethrionomys gapperi). A resident meadow vole population resulted, the two species coexisting until April 1974, when most meadow voles disappeared from the grid during a relatively short period. Interspecific aggression levels, as determined from voles temporarily removed from the populations and tested in paired encounters in a laboratory arena, were low during the winter, but increased when males of both species entered reproductive condition in the spring. Microtus was generally dominant in early breeding period encounters, but this dominance declined concurrently with the meadow voles' disappearance from the forest. It is argued that meadow voles did not leave the forest to breed, or because the snow cover melted, since this species will live and reproduce in forest in the absence of Clethrionomys. The results are interpreted as support for an earlier hypothesis that competitive habitat exclusion varies seasonally with reproduction-related aggression. Thus, these species apparently may coexist in either of their preferred habitats when interspecific aggression is low (the nonbreeding season), but this relationship terminates when interspecific aggression levels increase with the resumption of breeding in the spring.

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 ♂ ♂ trapped from August to February lost an average of 28.6% of their weights while 14 marked ♀ ♀ 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.

Intraspecific aggression of male Microtus pennsylvanicus drummondii was ob- served in 320 10-minute paired encounters in a neutral arena, during 17 months. Voles were drawn from a population live-trapped biweekly at Pinawa, Manitoba. The 17 months were divided into the reproductive periods: non-breeding, testes scrotal, and littering. There were two non-breeding, two testes scrotal periods, and a single littering period. Threats, vocalizing, and mutual uprights were significantly and positively associated in all but one of the periods. Together with fighting, these three acts formed the aggressive component of the behavior recorded. All aggressive acts increased in frequency as males became reproduc- tively active, and decreased as the breeding season ended. Latencies to aggressive acts were negatively related to the acts' frequencies. Less frequent acts (fighting, other, and submissives) also showed seasonal changes associated with breeding: fighting was com- mon throughout the breeding period, submissives most common early in the reproductive period, and grooming other most prevalent when juveniles were present in the population. Incidences of tail-wounding in the field population were significantly more common during the breeding season, and in males. A comprehensive Index of Aggression was devised by means of principal component analysis, based on the total frequencies of aggressive acts. Mean Indices of Aggression for the five periods showed that aggressive levels in the three breeding periods were significantly higher than in the two non-breeding periods. Re-tests of 53 voles showed scorn-. ii)m their two encounters to be positively correlated. Aggression mea,,:10 i n the encounters was related to population parameters derived from live-trapping results. Breeding season home ranges were larger than winter home ranges. Overwintered adults were more aggressive, had larger home ranges and survived longer than young of the year. Aggression and survival were positively related in adults but not in young. Resident voles were more aggressive than non-residents. Dominant voles were usually slightly heavier. Pairs of voles of similar weight exhibited more aggressive acts, but following was more frequent in dissimilar weight pairs. Aggressive levels did not vary consistently with population density. These observations demonstrate there is an annual cycle of intraspecific aggression syn- chronized with the breeding season in male M. pennsylvanicus. They support studies on Peromyscus, which also showed that aggression increased with spring breeding and declined as reproduction ceased in fall; thus, behavioral density regulation seems most likely to occur during the breeding period.

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.

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

Since physiological, morphological, and ecological differences have been found in two closely related populations of Perognathus parvus, this study was initiated to determine whether behavioral differences were also present. Sixteen animals from a high-altitude, cool, moist environment and 14 from a warm, dry environment, 1500 ft lower in elevation, were held in the laboratory for at least 6 months. Burrowing, washing, and sandbathing behavior were observed in an arena half filled with sandy soil. Behavior was recorded with a keyboard operating a multiple pen recorder and with a movie camera. Observations were submitted to frequency, duration, and sequence analysis. Slight but statistically significant differences were found in sandbathing behavior. Valley animals, living in a drier climate and in denser numbers, exhibited more belly rubs in sandbathing, whereas hillside animals, living in a wetter climate and in less dense numbers, showed more side rolls. These differences may be related to an olfactory function in communication and (or) to differences in substrate.

Thirteen taxa of small mammals were collected in the continuous forest, aspen groves, riparian woodland and prairie areas of the prairie-forest transition. Mammals captured were placed in three groups on the basis of habitat selection. Of the forest species, Peromyscus maniculatus gracilis was the most restricted and occurred only in or near conifer forest. P. leucopus noveboracensis was found in coniferous, deciduous and river-bottom forests and in large aspen groves. Clethrionomys gap peri had a similar distribution, but was also found in some small aspen groves with grassy ground cover. Tamias striatus was collected in the continuous forest and nearby large groves. Eutamias minimus and Synaptomys cooperi were collected only in the continuous forest. The prairie species, P. m. bairdii, Onychomys leucogaster and Microtus pennsylvanicus, were found essentially in grassland situations. P. m. bairdii also occurs in aspen groves and river-bottom forests where P. 1. noveboracensis is absent. M. pennsylvanicus appears to be a permanent resident in young groves with a grassy undercover. The nonselective species, Sorex cinereus, Blarina brevicauda and Zapus hudsonius, were found throughout the area in all habitats trapped. Sorex arcticus may also be present in moister habitats throughout the transition. The small-mammal fauna of the prairie-forest ecotone includes fragments of both prairie and forest faunas and reflects the state of flux in the plant associations.