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We used intensive live trapping over a 1-y period to investigate the general ecology of a population of Norway rats (Rattus norvegicus) inhabiting a barn in east-central Illinois. At the start of our study in April 1986, the population contained 10 adult females and three adult males. The population increased and exhibited two peaks of about 100 individuals, one peak in late June and the other in late October 1986. Reproduction ceased during the late autumn and winter, and the population declined to only one adult male and one adult female by spring 1987. Increases in the number of rats represented young born at the barn, not adult rats moving into the population; decreases in population size likely resulted from predation rather than dispersal. Females first captured as adults persisted longer at the barn than did males and females first captured as juveniles or subadults; the few males first captured as adults persisted the shortest time of all age and sex classes. Young males gained body mass more rapidly than did young females. Wounding and parasitism by botflies occurred at relatively low levels. Our data indicate that a rat population with negligible immigration and seasonal breeding can exhibit dramatic changes in numbers, and that live-trapping at weekly intervals can yield high recapture rates useful for examining growth rates, survival and other basic life history characteristics.

We analyzed the costs and benefits of natal dispersal and philopatry in a free-living population of the prairie vole, Microtus ochrogaster, in which 70% of the males and 75% of the females were philopatric. Rather than settling into a nest, 40% of male dispersers continued to wander throughout the study area, while only 13% of female dispersers did so. Whereas males were more likely to wander in spring-early autumn during low density periods, females were more likely to wander during spring-early autumn, irrespective of population density. Of those animals that settled into a nest, females were more likely than males to settle as single individuals. Same-sex siblings that dispersed commonly joined the same social group. Although often settling within 5 m of each other, opposite- sex siblings that dispersed never joined the same social group or formed a male-female pair. Total length of life was longer for males and females that dispersed than for those that remained at the natal nest. Animals survived lon...

In microtine rodents, formation of communal groups often is associated with environmental stresses of winter or high population density. Social organization of the prairie vole ( Microtus ochrogaster ) consisted of communal groups formed by addition of philopatric young and unrelated adults to an original male-female pair or single-female breeding unit. Owing to high juvenile mortality during spring-early autumn, most social groups remained as male-female pairs or single females. Male-female pairs displayed behavioral traits associated with monogamy, including sharing a common nest and home range, mate guarding, dissolution of pairs by mortality, and low incidence of remating. Male-female pairs rarely involved related animals. Single-female breeding units were survivors of male-female pairs that had not formed a new pair; prevalence of single-female breeding units was not related to availability of unpaired males. When juvenile survival increased in late autumn, communal groups became the predominant social group. Formation of communal groups was unrelated to low temperatures and was not contingent upon the nonreproductive status of members. Although present at low population density, communal groups were the most common type of social group at high densities. We conclude that increased survival of juveniles in late autumn leads to the formation of communal groups and increases in population density.

Many hypotheses on the role of social behavior in population regulation of microtine rodents assume that social interactions are more frequent at high density than at low density. We evaluated this assumption by measuring potential for social contact in a population of prairie voles (Microtus ochrogaster). Our livetrapping regime monitored presence of resident and nonresident animals at the periphery of home ranges of social groups. Resident adults and young of both sexes were captured at home range boundaries. Potential for social interaction with nonresidents was greater at the periphery of home ranges than at nests. Intruders tended to be male adults from neighboring social groups. Presence of adult males in the resident group played an important role in deterring visits by nonresident males. During the spring, summer, and autumn reproductive period, number of captures of nonresident males, but not females, showed a strong positive relationship with population density. Despite relatively high densities during the winter nonreproductive period, there were few captures of residents and nonresidents at the periphery of home ranges. We conclude that the relationship between potential for social interaction and population density varied as a function of sex and season.

We tested experimentally the hypothesis that failure to acquire a new mate by monogamously paired female Microtus ochrogaster that lose their mate represents pair-bonding behavior, not a lack of available males. Males were removed from reproductive male–female pairs and the females provided an opportunity to pair with unfamiliar sexually inexperienced or experienced males in a semi-natural arena. Only three of 22 (13.6%) females formed a new pair, two with inexperienced males and one with an experienced male. Three other females spent more time with an experienced male than with the inexperienced male, but did not form a pair or co-nest with that male. The results confirm field observations that availability of males is not a factor in the failure of female prairie voles to form a new pair following loss of their mate. This study concludes failure to form a new pair is associated with pair-bonding behavior.

We used intensive livetrapping to examine natal dispersal and philopatry in prairie voles (Microtus ochrogaster). The majority of male (70.0%) and female prairie voles (75.1 %) remained at the natal nest until death. Those males and females that did disperse left home at about the same age (45–55 days) and moved similar distances (28–33 m). Dispersal was more common (1) from small natal groups than from large natal groups, (2) following disappearance of parents, (3) during the breeding period than during the nonbreeding period, and (4) at low population densities than at high densities. Dispersal was not associated with level of competition for mates within natal groups, and dispersers did not differ from nondispersers in body weight. Our data do not support competition for mates or resources as important factors influencing natal dispersal in prairie voles. The absence of sex differences in dispersal tendency or distance, and our fording that more than half of dispersers had become reproductive before leaving the natal nest, lead us to suggest that inbreeding avoidance is not a primary function of dispersal in this species. Dispersal was, however, more common when potential mates within the natal group were relatives than when they were nonrelatives. Although not tested here, if family members avoid mating with one another through patterns of mate choice, then some animals may leave home in search of mates. The precise benefits associated with philopatry in prairie voles remain to be identified.

A communal nesting group, formed by additions to a monogamous pair breeding unit, is the basic year-round social unit of the prairie vole, Microtus ochrogaster. Before communal groups form, presumed monogamous pairs display traits normally associated with monogamy including cohabitation of a common nest, a shared home range, and dissolution of the pair primarily by mortality of one or both members. When juvenile nest mortality was reduced during late autumn-winter, extended-family groups formed. There was no difference in natal dispersal between summer and autumn-winter. Approximately 70% of the animals added to monogamous-pair breeding units were philopatric young. Unrelated adults (30% of the additions) joined extended-family groups in equal proportions in summer and late autumn-winter. Approximately half of the unrelated adults were reproductive when they joined an extended family group. During periods of winter reproduction most winter communal groups included at least one reproductive male and female, approximately half included two or more reproductive males and females (half of these were unrelated adults).

Spacing behavior appears to play a role in population regulation of microtine rodents. We used information gathered from livetrapping at nests and the periphery of home ranges of prairie voles (Microtus ochrogaster) to continue our examination of the role of spacing behavior in population regulation. Resident and nonresident adult males captured at the outskirts of home ranges were reproductive rather than nonreproductive and did not differ in body weight. Although the majority of adult females were reproductive, the pattern was more pronounced for residents. The presence of adult males in a resident group deterred visits by nonresident males. By excluding nonresident males from the home range, resident males appeared to influence the reproductive condition of their daughters; if females became reproductive while living at home, they did so at an earlier age when living in groups where resident males were absent than where resident males were present. From these results we suggest that adult males play a role in setting breeding density in natural populations of prairie voles.

L'influence des prelevements sanguins dans le sinus suborbital sur la survie a ete mesuree a l'aide du taux de recapture du campagnol M. ochrogaster

Seasonal differences in breeding units of Microtus ochrogaster were studied in E-central Illinois. Average breeding-unit size was 1.9 during spring-early autumn and 4.8 during late autumn and winter. Increased nest survival of juveniles (0.7 and 1.5/litter, spring-early autumn and late autumn-winter, respectively), combined with high philopatry at all seasons, appears to be the primary factor involved in formation of larger breeding units during late autumn and winter. Additions to the basic breeding unit included 71.1% young born into the family group, and 28.9% unrelated adults that joined the group. Large snakes are the only known predators on voles in this region that can enter undisturbed underground nests and are not active during late autumn-winter. Breeding unit size and composition in an exclosure from which snakes, but not other predators, were assumed to have been excluded were nearly identical in August and September to those of breeding units in the open population during late autumn and winter. We conclude that the basic breeding unit of M. ochrogaster is an extended-family, communal-nesting group which also includes 1-2 unrelated adults. Because of high juvenile nest mortality from snakes during spring-early autumn, such communal groups are observed primarily during late autumn and winter.

Patterns of parental care have been investigated in several species of voles within the genus Microtus that seem to have different types of social organization (Gruder-Adams and Getz, 1985; Hartung and Dewsbury, 1979; McGuire and Novak, 1984, 1986; Oliveras and Novak, 1986; Thomas and Birney, 1979; Wilson, 1982). In presumably monogamous species such as M. ochrogaster and M. pinetorum, a male and female share a nest and males display paternal care. This pattern of nesting and parental care has been observed under all experimental conditions. In seemingly promiscuous species such as M. montanus and M. pennsylvanicus, males nest separately and exhibit no paternal behavior when observed in large enclosures. In small cages, however, M. montanus and M. pennsylvanicus males display appreciable paternal behavior, though the amount of care is still less than that provided by males of monogamous species under identical conditions. Little is known about parental behavior or social organization within the genus Lagurus. The North American form curtatus (subgenus Lemmiscus; Thomas, 1912) differs from Old World species Lagurus lagurus and L. luteus in several morphological features (Carleton, 1981; Davis, 1939) and recent classifications elevated Lemmiscus to generic status (Carleton and Musser, 1984). Field and laboratory observations of Lemmiscus curtatus provide limited and conflicting information on social behavior. James and Booth (1954) reported that captive male sagebrush voles were excluded from nests of females and killed pups if nests could not be defended. Maser et al. (1974) observed captive females to share nests and to nurse each other's young. Although L. curtatus was described as colonial on the basis of early field studies (James and Booth, 1954; Maser et al., 1974), a recent study indicated that sagebrush voles are solitary or live in pairs (Mullican and Keller, 1986). Given the conflicting results of previous observations on behavior and social organization, the present study was undertaken to record parental behavior in sagebrush voles. We observed seven pairs of voles in a seminatural laboratory environment in which the male could nest either separately or with the female and pups. Patterns of care displayed by Lemmiscus are compared with those of Microtus species recorded under similar conditions in previous studies (McGuire and Novak, 1984, 1986). Animals used in this study were laboratory-reared descendants of stock originally captured in Utah in 1981. All seven pairs had reared at least one previous litter to weaning. Before their use in this study, breeding pairs were housed in 25by 45-cm plastic cages containing a peat and wood-chip substrate and an 8-cm thick layer of straw. Sunflower seeds, oats, cracked corn, Purina rabbit chow, and water were provided ad lib.; a reversed 15L:9D light cycle was maintained. A breeding pair was placed in an observation pen when the female was nearing parturition, as determined by date of birth of the last litter and abdominal palpation. Immediately before introduction of a pair into a pen, the ventral fur of the male was dyed black with Nyanzol D for identification purposes. Generally, breeding pairs spent 3-7 days in the pen before birth of their young. Pens measured 1.3 by 2.6 m; the sides of each pen were lined with sheets of aluminum, and the bottom consisted of a sheet of transparent Plexiglas. Each pen contained a 3-cm-thick substrate of wood chips over which straw was scattered. To provide the opportunity for separate nesting by the adults, pens were divided into three sections with 20-cm-high wooden partitions; openings (3-cm diameter) in partitions allowed voles to move between sections. Pens were suspended 60 cm above the floor so that activities of voles could be observed from below (like other species of voles, sagebrush voles kept the bottom of their nests clear of wood chips). Food and photoperiod were identical to those described for the breeding colony. Data collection began with birth of a litter and continued for 25 days. There were two 45-min observation periods per day. Most observations were conducted between 0800 and 1400 h (lights off: 1100 h) and between 1800 and 2200 h (lights on: 2000 h) because sagebrush voles are crepuscular (Maser et al., 1974).