Betty McGuire

FACTORS INFLUENCING MOVEMENT DISTANCES OF TWO SPECIES OF SYMPATRIC VOLES

Abstract We studied factors influencing home-range size in fluctuating populations of Microtus ochrogaster and M. pennsylvanicus in alfalfa, bluegrass, and tallgrass habitats over a 25-year period in east-central Illinois. Preferred food availability for both species was greatest in alfalfa and least in tallgrass, lesser during winter than other seasons in all 3 habitats, and greater in alfalfa during winter than in the other 2 habitats. Vegetative cover was sparse, especially during winter, in alfalfa and dense year-round in bluegrass and tallgrass. Movement distances of M. ochrogaster tended to be smaller in alfalfa than in bluegrass and tallgrass; movement distances of neither vole species differed between bluegrass and tallgrass. Within alfalfa, movement distances of both species were smaller during winter, when cover was sparse and food availability low. No seasonal difference was found in movement distances of either species within bluegrass and tallgrass, where cover was dense year-round, but food availability was low during winter. Movement distances of M. ochrogaster were not affected by supplemental feeding in bluegrass and tallgrass; those of M. pennsylvanicus were smaller in supplementally fed tallgrass. We conclude that cover, as an indicator of risk of predation, influenced home-range sizes of both species more than did food availability.

Alternative male reproductive tactics in a natural population of prairie voles Microtus ochrogaster

Alternative reproductive tactics have been described in male mammals, but little information exists regarding fitness benefits and whether males change tactics. Adult male prairie voles Microtus ochrogaster (Wagner, 1842) display alternative tactics described as resident and wanderer. Enclosure studies provide conflicting data concerning the relative success of each tactic and whether males display one tactic throughout adulthood. To characterize further residents and wanderers in this species, we examined data collected during 5 years of monitoring a natural population in Illinois, USA. We found that during the breeding period, wandering males survived longer, moved longer distances, and were more likely than residents to have scrotal testes. During the nonbreeding period, wandering and resident males differed only in whether or not they established residency. Data on sources and fates of resident and wandering males revealed that a substantial proportion of males switched tactics. Our estimate of the reproductive contribution of wandering males to the population, which is based on the premise that wandering males typically mate with single females, suggests that wanderers contribute 34–38% of young recruited during March through October and 4–12% in November, when single females are less common. Parentage studies in natural populations are necessary to test our estimates.

VOLE POPULATION FLUCTUATIONS: FACTORS THAT INITIATE AND DETERMINE INTERVALS BETWEEN THEM IN MICROTUS OCHROGASTER

Abstract We studied factors associated with occurrence of high-amplitude population fluctuations of the prairie vole (Microtus ochrogaster) in alfalfa, bluegrass, and tallgrass habitats in east-central Illinois for 25 years. Increased survival was the most important factor associated with initiation of a population fluctuation during a given year. The proportion of reproductively active adult females was not associated with initiation of population fluctuations. The interval between fluctuations was not correlated with the previous peak density. We propose that population fluctuations in M. ochrogaster were initiated by the net effects of relaxation of predation pressure of multiple generalist predators, which occurred erratically across years.

Social dynamics and dispersal in free-living prairie voles (Microtus ochrogaster)

Abstract Following dispersal from 1 group, individuals may join other established social groups. Such intergroup transfer may increase access to potential mates and decrease mate competition. We used data from 402 individuals to examine patterns of intergroup transfer in prairie voles (Microtus ochrogaster). Nearly 32% of established social groups (single female units, male–female pairs, or communal groups of at least 2 adults of the same sex) were joined by 1 or more individuals. Most individuals (76%) that joined social groups were wanderers that were either unmarked, recently marked during grid trapping, or marked transients; 70% were males. Joining a group was not contingent upon recent disappearance of residents. Total number of residents positively affected the probability of a female joining a social group, whereas number of adult female residents and population density negatively affected it. Some individuals (24%) moved directly from one group to another without an intervening wandering stage; we refer to these instances of intergroup transfer as direct transfers. Most direct transferers moved into nearby groups, but not the closest group. Males were more likely than females to directly transfer into groups with potential mates and without potential competitors. Thus, males directly transferred in a manner consistent with maximizing reproductive opportunities. In contrast, 25% of females directly transferred into groups without potential mates and 96% into groups with at least 1 adult female. Females may be less constrained by group composition with respect to potential mates because wandering males, with which females can mate, are prevalent. All-male groups almost never occur in our population, so females probably cannot avoid joining groups with competitors.

Suckling behaviour in three species of voles

Summary Domestic mammals exhibit diversity in suckling behaviour, yet little is known about suckling in most wild species, including rodents with tenacious nipple attachment. This behaviour, whereby young cling tightly to nipples, has been interpreted as an adaptation to competition within and among litters for nipples and milk. Comparative studies of suckling behaviour of tenacious and non-tenacious species are lacking, however, as are studies placing patterns of suckling in a phylogenetic context. We compared suckling in prairie voles (Microtus ochrogaster; tenacious nipple attachment), woodland voles (= pine voles, M. pinetorum; tenacious nipple attachment) and meadow voles (M. pennsylvanicus; non-tenacious nipple attachment). We hypothesized that suckling behaviour of meadow voles differs from that of the other two species. We found that meadow vole pups display higher frequencies of nipple attachment, shorter durations of nipple attachment (significantly different from prairie voles only), more frequent nipple-switching, and no preference for nipple pairs (prairie vole and woodland vole young preferred the hindmost nipples). Mapped onto a phylogeny, our data suggest a suite of behavioural characters associated with suckling in prairie and woodland voles (tenacious nipple attachment, preference for hindmost nipples, and infrequent nippleswitching) and highlight diversity of suckling behaviour among closely related species.

Vole population dynamics: factors affecting peak densities and amplitudes of annual population fluctuations ofMicrotus pennsylvanicus

We studied factors affecting peak densities and amplitudes of fluctuation during 20 annual population fluctuations ofMicrotus pennsylvanicus Ord, 1815 in alfalfa and bluegrass habitats over a 25-year period. Survival was correlated with population density over the 25 years and was the most consistent variable associated with stoppage of population growth. Although not correlated with population density over the 25 years, a decline in the proportion of reproductively active adult females contributed to cessation of growth of population fluctuations that peaked in late autumn-winter, and to cessation of growth of eight of eleven population fluctuations that peaked during summer-early autumn. We conclude variation in survival to be the primary factor affecting peak densities and amplitudes of population fluctuation ofM. pennsylvanicus.

VOLE POPULATION FLUCTUATIONS: FACTORS THAT INITIATE AND DETERMINE INTERVALS BETWEEN THEM IN MICROTUS PENNSYLVANICUS

Abstract Factors influencing initiation of population fluctuations of Microtus pennsylvanicus were studied in alfalfa and bluegrass habitats for 25 years. Increased survival during spring and summer appeared to be the most important factor associated with initiation of a population fluctuation. The proportion of reproductively active adult females did not influence initiation of population fluctuations. The interval between fluctuations was not correlated with density of the previous population fluctuation. We propose that population fluctuations were initiated by the net effects of relaxation of predation pressure of multiple generalist predators, which occurred erratically across years.

Effects of conspecific and heterospecific residents on patterns of immigration in two species of voles

Studies with birds have shown that presence and density of resident conspecifics and heterospecifics can influence patterns of habitat selection. There have been few studies on the effects of social cues on rates of immigration in mammals. We report results from a long-term live trapping study of immigration in two species of voles, Microtus ochrogaster Wagner, 1842 and Microtus pennsylvanicus Ord, 1815, in bluegrass habitat in east-central Illinois, USA. We compare immigration into control sites from which no individuals of either species were removed with immigration into experimental sites from which either conspecifics or heterospecifics were removed. We focus on characteristics of immigrants and rates of immigration in relation to density in destination habitats. Within each species, immigrants into control and removal sites were similar with respect to body mass and reproductive condition, indicating no major differences in the physical condition of immigrants into sites with established populations and sites without established populations. For both species, density of conspecifics at a site positively influenced rate of immigration at that site. Density of heterospecifics at destination sites did not significantly influence rate of immigration for either species. These results suggest that site selection by dispersing M. ochrogaster and M. pennsylvanicus is characterized by conspecific attraction.

Multiple captures provide evidence of small mammal social behavior

Social behavior of small mammals living under natural conditions often is inferred from live-trapping data, particularly from incidents in which two or more individuals are captured together in a trap. We examined whether multiple-capture data from a long-term study of prairie voles (Microtus ochrogaster) and meadow voles (Microtus pennsylvanicus) were consistent with well-known species differences in social behavior (whereas prairie voles are highly social and display monogamy, meadow voles are less social and promiscuous). When possible, we also examined multiple captures of two nontarget species, northern short-tailed shrews (Blarina brevicauda) and western harvest mice (Reithrodontomys megalotis). Percent of total captures that were multiple captures and percent of total adult captures that were male–female captures were highest for prairie voles and lowest for meadow voles; values for harvest mice and shrews were in between those of the vole species, but more similar to values for meadow voles. Repeat captures of the same male–female pair occurred most commonly in prairie voles, and multiple captures of this species typically involved individuals from the same social group. Multiple captures of adults and juveniles were more common in prairie voles than meadow voles, except for captures of at least one adult male and at least one juvenile, which did not differ between the two vole species. Multiple capture data for prairie voles and meadow voles were largely consistent with established species differences in social behavior, suggesting that such data can provide an accurate indication of social and mating systems of small mammals.

Demography of fluctuating prairie vole populations: comparison of demographic variables among phases of fluctuations

We tested for differences in the proportion of reproductively active males and females, proportion of the population composed of young and immigrants, and monthly survival (total, adult, young) among phases (trough, increase, and decline) and among habitats (alfalfa, bluegrass, and tallgrass) of 30 population fluctuations ofMicrotus ochrogaster Wagner, 1842 over 25 years in east-central Illinois USA. Total population survival and survival of adults and young were greatest during the increase phase, among fluctuations, irrespective of habitat. The proportion of reproductively active adult males and females was lowest during the decline phase, an effect of lower reproduction during the winter. These results suggest that phase-specific changes in survival were the primary demographic factor driving population fluctuations ofM. ochrogaster in our study sites. We conclude that small-scale spatially different population fluctuations may be explained by the same mechanisms that explain fluctuations within a population.