Populations Of Prairie Voles Research Articles

Intraspecific variation in estrogen receptor alpha and the expression of male sociosexual behavior in two populations of prairie voles

Estrogen (E) regulates a variety of male sociosexual behaviors. We hypothesize that there is a relationship between the distribution of estrogen receptor alpha (ERα) and the degree of male social behavior. To test this hypothesis, ERα immunoreactivity (IR) was compared in prairie voles ( Microtus ochrogaster) from Illinois (IL), which are highly social, and Kansas (KN), which are less social. The expression of androgen receptors (AR) in males also was compared between populations. The expression of ERα and AR were compared in brains from KN and IL males and females using immunocytochemistry (ICC). There were significant intrapopulational differences, with males expressing less ERα-IR than females in the medial preoptic area, ventromedial nucleus, ventrolateral portion of the hypothalamus, and bed nucleus of the stria terminalis (BST). IL males also displayed less ERα-IR in the medial amygdala (MeA) than IL females. While IL males expressed significantly less ERα-IR in the BST and MeA than KN males, there was no difference in AR-IR. Differences in the pattern of ERα-IR between KN and IL males were behaviorally relevant, as low levels of testosterone (T) were more effective in restoring sexual activity in castrated KN males than IL males. The lack of difference in AR combined with lower expression of ERα-IR in IL males suggests that behavioral differences in response to T are associated with aromatization of T to E and that reduced sensitivity to E may facilitate prosocial behavior in males.

The Effect of Female Prairie Vole (Microtus ochrogaster) Immigrants on Space Use of Conspecific Female Residents

We tested the hypothesis that female prairie vole (Microtus ochrogaster) residents change their space use patterns in response to immigration pressure. We established prairie vole populations in four 32 × 32-m enclosures during 2000 and 2001. Four populations were supplemented with adult female immigrants and the other four served as control populations in which no females were introduced. Using radio-telemetry, we monitored and compared the home ranges and movement distances of residents vs. immigrants within the same enclosures and residents in enclosures with and without immigrants. Immigrants had significantly larger home ranges and moved greater distances during the evening hours than residents. Resident home ranges and hourly movements were unaffected by the presence of immigrants. The results suggested that resident females could successfully defend their territories without changes in their home ranges and movement distances. However, due to our experimental design, we could not exclude the possibility that female immigrants affected the space use of male residents.

The Influence of Habitat Quality on Dispersal, Demography, and Population Dynamics of Voles

We compared the effects of habitat quality on dispersal, demography, dynamics, and fitness of prairie vole (Microtus ochrogaster) and meadow vole (M. pennsylvanicus) populations by manipulating habitat patches in experimental landscapes. Four habitat patches in each of four replicate landscapes differed in availability of high-quality food and amount of vegetative cover in a 2 × 2 factorial design. High cover had a strong positive effect on basic habitat quality, as reflected by the performance of founders early in the season, but supplemental food had only a small effect. Population growth ceased for prairie voles after week 18 (mid-October) when densities had reached much higher levels in habitats with high cover (260 ± 27 voles/ha in high cover with either high or low food; mean ± 1 se) than in habitats with low cover (115 ± 38 voles/ha in high food, low cover and 60 ± 15 voles/ha in low food, low cover). Population growth had ceased in three habitat types for meadow voles by week 20 at much higher densities than for prairie voles in high cover (636 ± 101 voles/ha with high food, high cover; 556 ± 117 voles/ha with low food, high cover; 110 ± 74 voles/ha with high food, low cover; and 51 ± 16 voles/ha with low food, low cover). Correlates of fitness for prairie voles, particularly the number of young surviving to adulthood per female, indicated greater individual fitness in high cover than in low cover, which suggests ideal despotic habitat selection. The proportion of the total population found in habitat patches with high cover continued to increase until prairie vole populations stabilized late in the season. This result did not agree with expectations of either the ideal free model or the ideal despotic model of habitat selection. Fitness differed little for meadow voles in different habitat types, and the proportions of the populations in different habitats remained constant after the founders had settled and population growth began. Both of these patterns supported the ideal free model of habitat selection for meadow voles. Because positive net recruitment occurred in all habitats for both species of voles, source–sink dynamics could not occur in our experimental system. Two subpopulations of meadow voles in low-food, low-cover habitats did go extinct temporarily, but this habitat type did not appear to be a population sink because the losses occurred primarily from emigration rather than mortality. Emigration rates for both species of voles were inversely related to carrying capacity of the habitat (estimated as the density at which populations stabilized). This relationship and ideal free habitat selection are required by the balanced dispersal model, which produces equal numbers of dispersers between a pair of habitat types. Dispersal of meadow voles was balanced throughout the growing season, but dispersal of prairie voles was unbalanced, with net movement of individuals from low-quality to high-quality habitats until late in the season when populations stabilized. Unbalanced dispersal early in the season may have reflected a delayed response to habitat quality by prairie voles. Populations in high cover approached stable densities largely as a result of density-dependent in situ net recruitment, whereas populations in low cover stabilized more as a result of density-dependent net movement for both species of voles. Per capita emigration did not show a consistent positive relation to population growth in high cover, as would be expected if dispersal contributed to the tendency for prairie voles to display multi-annual population cycles in such habitats. In fact, emigration generally declined as density increased (inversely density dependent), which should have fostered greater population growth as densities increased. Nevertheless, population growth declined late in the season, which implied that emigration (dispersal) did not strongly affect the pattern of population dynamics in high-quality habitats. The overall results for the two species were remarkably similar, although prairie voles did differ from meadow voles in several respects. Meadow voles performed less well in low-cover habitats than did prairie voles, which probably reflected their greater sensitivity to predation risk. Prairie voles tended to move into habitats with supplemental food, and young prairie voles had greater body growth in habitats with supplemental food, whereas meadow voles showed neither response. Finally, although the results for meadow voles consistently indicated ideal free habitat selection throughout the growing season, those for prairie voles did not until late in the season. This difference between species likely reflected a more rapid response to habitat quality by the more vagile meadow voles.

EFFECTS OF FOOD SUPPLEMENTATION ON THE SOCIAL ORGANIZATION OF PRAIRIE VOLES (MICROTUS OCHROGASTER)

Mammalian social organization can vary over ecological time. We experimentally manipulated food resources in enclosed populations of prairie voles (Microtus ochrogaster) to test the hypothesis that food quality influences the potential for group formation. During each field season, populations were started by releasing 5 pairs of prairie voles into each of 8 0.1-ha enclosures. Populations were monitored for 18-19 weeks during each field season. One-half of the enclosures received supplemental food, and the other one-half were unsupplemented controls. Density of voles increased throughout each field season. There were significant increases through time in philopatry and number of groups in both the food-supplemented and unsupplemented treatments, but there were no differences between treatments. Groups formed early in the season, apparently before the need for thermoregulatory benefits. Results are consistent with the hypothesis that the social organization of prairie voles is not flexible in response to changes in food quality but that formation of groups might be a density-dependent response.

The winter immunoenhancement hypothesis: associations among immunity, density, and survival in prairie vole (Microtus ochrogaster) populations

Seasonal variations in photoperiod, temperature, and population density have been shown to modulate immune responsiveness of animals in laboratory studies. To examine these associations under natural conditions, we monitored 3 populations of prairie voles (Microtus ochrogaster) for temporal variations in selected immunological parameters, population density, and survival rate from winter 1996 to spring 1997. Spontaneous and cytokine-stimulated T-cell proliferative responsiveness of prairie voles peaked in winter and declined in spring. Relative organ mass, hemolytic-complement activity, and in vivo hypersensitivity responses varied temporally but showed no clear seasonal trend. The population density and survival rate of all 3 prairie vole populations varied temporally and correlated with measures of immunity. Multiple regression analysis indicated that the model containing relative spleen mass, cytokine-stimulated T-cell proliferation, and in vivo hypersensitivity explained a significant amount of variability in population density, while cytokine-stimulated T-cell proliferation and relative thymus mass explained a significant amount of variability in survival rate. The results suggest that seasonal environmental changes can enhance immune responsiveness of a host and may counteract the immunoenhancing effects of photoperiod in wild populations of prairie voles. Our results also suggest that there is an association between immune function and demography in wild populations.

The nature and frequency of social interactions among free-living prairie voles ( Microtus ochrogaster )

Hypotheses relating the behavior of voles to their population cycles often assume that the rate of social interaction increases with population density. To test this assumption, we examined the frequency of social interactions in a population of prairie voles (Microtus ochrogaster) over a 7-year period. In addition, we characterized space use by resident animals, patterns of visitation by nonresidents to nests, and participants in social interactions. Social groups within the population typically displayed little overlap in their use of space, even at high population densities. Nevertheless, nonresidents, particularly wandering males, were captured as visitors at nests. The number of visits per social group did not increase in a simple linear manner with population density and was particularly variable when there were fewer than 100 animals/ha. At such times, more single females and fewer pairs received visits from males than expected based on the frequency of occurrence of these groups in the population; a similar pattern was noted during periods of high population density (≥100 animals/ha) but the comparisons failed to reach statistical significance. Furthermore, at high population density, more communal groups received visits from females than expected. Patterns of visitation to communal groups were influenced by the number of adult male residents (winter only), but not by the number of adult female residents or presence of philopatric female offspring. These data indicate that the frequency of social contact in prairie voles does not increase linearly with population density and is influenced by the spacing and possible mate-guarding behavior of resident animals.

Intraspecific Variation in the Induction of Female Sexual Receptivity in Prairie Voles

ROBERTS, R. L., B. S. CUSHING AND C. S. CARTER. Intraspecific variation in the induction of female sexual receptivity in prairie voles. Physiol Behav 64(2) 209–212, 1998. Prairie voles ( Microtus ochrogaster) are monogamous New World rodents which show geographic variation in social behavior. In this study, parameters of female reproduction which might be related to mating system were compared in prairie voles from eastern Kansas (KAN) versus central Illinois (ILL). KAN females showed a more rapid onset of natural estrus following exposure to a male and were more likely to respond to injections of a low dose (0.5 μg) of exogenous estradiol benzoate than ILL females. Neither mating duration nor pregnancy success after mate removal differed in KAN versus ILL females. These results suggest that ILL voles are less sensitive than KAN voles to the estrus-inducing effects of either endogenous or exogenous estrogen, supporting the hypothesis that variations in reproductive strategy occur among geographically discrete populations of prairie voles.

Inbreeding avoidance in the prairie vole, Microtus ochrogaster

The prairie vole, Microtus ochrogaster forms communal groups by retention of philopatric offspring and the addition of unrelated individuals at malefemale and single female breeding units. Populations undergo high amplitude fluctuations in density with frequent low density bottle necks of 10-15 generations. We review the mating system and social behavior of the prairie vole with respect to potential for formation of genetically unique trait groups within the population as predicted by group selection theory. Hormonal-behavioral interactions reduce the opportunity for inbreeding within communal groups. Demographic traits, combined with dispersal and settlement behavior of those offspring that do disperse from the natal nest are such that breeding units would rarely include related animals. We predict that genetically distinct trait groups do not form in prairie vole populations, including during periods of low density bottlenecks.

Effects of Availability of Food and Interspecific Competition on Diets of Prairie Voles (Microtus ochrogaster)

Analysis of fecal samples from experimental populations of prairie voles ( Microtus ochrogaster ) and meadow voles ( Microtus pennsylvanicus ) revealed that their diets strongly overlapped. Both species preferred dicots, but meadow voles ate significantly more mono-cots than did prairie voles. To test the hypothesis that interspecific competition for food influenced the diet of prairie voles, we examined the diets of prairie voles with and without meadow voles present. We found significantly more monocots in the diet of prairie voles when meadow voles were present, but only in early autumn when availability of high-quality food items was lower and densities of voles were higher. We conclude that variability in diets of voles can occur as a response to changes in availability of food, quality of food, and interspecific competition, the latter particularly when availability of high-quality food is relatively low and densities of voles are relatively high.

Communal nesting in prairie voles (Microtus ochrogaster): an evaluation of costs and benefits based on patterns of dispersal and settlement

We evaluated costs and benefits of group nesting in prairie voles (Microtus ochrogaster) by examining dispersal from communal nests. Nests at which dispersal occurred did not differ in size from those at which no dispersal was recorded, and most animals did not exhibit declines in body mass prior to dispersal. Of those animals which left communal groups that contained at least one potential mate, half left groups at which level of competition for mates was judged to be low as opposed to medium or high. Our data do not support competition for food or mates as important costs of group nesting. One probable cost to individuals living in family groups is lack of mating opportunity; about one-third of all dispersers were from groups composed solely of family members. Several of our findings suggest that energy conservation is not the primary reason for communal nesting in prairie voles. Reductions in winter group size typically did not precipitate dispersal, and most dispersers that settled into a nest in winter joined groups which were smaller than the groups they had left. Dispersal did not increase during snow cover. Benefits of group nesting in natural populations of prairie voles remain to be identified.

Reproductive activation of virgin female prairie voles ( Microtus ochrogaster) by paired and unpaired males

Virgin female Microtus ochrogaster were exposed to paired and unpaired unfamiliar adult males in semi-natural arenas. Although females were reproductively activated by both types of males, more than twice as many were activated by unpaired (51.6%) as by paired (18.2%) males. Our results suggest that, in natural populations of prairie voles, the proportion of philopatric females becoming reproductive is related to the abundance of unpaired males within the population.

Potential for social interaction in a natural population of prairie voles (Microtus ochrogaster)

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.

Effects of Food Availability and Predation on Prairie Vole Demography: A Field Experiment

Nutrition, spacing behavior, and predation have been proposed as major factors limiting the population density of microtine rodents (lemmings and voles). However, there is no general agreement as to the relative importance of these factors, nor have their interactions been carefully examined. This paper reports on a factorial experiment designed to test the general hypothesis that predation and the availability of high—quality food act simultaneously and additively to limit densities of microtine rodent populations. Our results showed that supplemental high—quality food significantly increased body growth rates, proportion of adults in the population, adult male body size, reproductive activity, recruitment, and density of prairie vole populations. Food availability had no effect on survival nor on sex ratio in any of the 3 yr of this study. Protection from predation significantly increased adult survival (somewhat sporadically), recruitment, and density, but it did not directly affect growth, population structure, or reproduction. Improved survival of young animals could not be detected, but increased recruitment suggested that it may have occurred, perhaps before the young reached trappable size. Food supplementation and protection from predation generally had additive and equal effects on density. The only significant food—predator interactions were for body growth rates and adult body size. The presence of predators appeared to inhibit the growth response to supplemental food, possibly because of more restricted movement and, therefore, less access to food.

The Effect of Sigmodon Hispidus on Spatial and Temporal Activity of Microtus Ochrogaster: Evidence for Competition

Spatial and temporal activities of cotton, rats Sigmodan hispidus, and prairie voles, Microtus ochrogaster, in an old—field habitat in northeast Kansas, USA were studied for 39 mo. During a major portion of the S. hispidus reproductive season the species were spatially segregated but co—occurred the rest of the year. Following a local extinction of cotton rats, prairie voles established a resident population in habitats formerly occupied by cotton rats that was greater than or equal to the vole population in areas which had never been occupied by cotton rats. Comparison of vole population size before and after the loss of cotton rats indicated that cotton rats produced a negative effect on vole population size during the period of spatial segregation but not during the period of spatial co—occurrence. Comparison of vole activity patterns indicated that, with the loss of cotton rats, voles shifted their activity during times when the species had co—occurred but not during periods when they were spatially segregated. Studies of behavioral interaction in a large (4 m x 4 m) arena with varying densities of vertical wires indicated that during single species tests voles and cotton rats preferred the densest areas. Their diel activity patterns were broadly overlapping. In the presence of nonreproductive cotton rats, voles continued to use the same areas as during the control tests but shifted their temporal activity reducing overlap with cotton rats. Interspecific encounters resulted in voles avoiding cotton rats even in the absence of overt aggression. In the presence of reproductive cotton rats, voles shifted their arena usage toward areas least used by cotton rats. Total nocturnal activity by voles was the same as during the control tests but was restricted to hours when cotton rats were least active. Reproductive cotton rats showed higher levels of interspecific aggression than did nonreproductive cotton rats. Voles in the presence of reproductive cotton rats showed increased frequency of serious wounds and deaths, and decreased mass when compared to voles in the presence of nonreproductive cotton rats. We conclude that cotton rats produce a negative effect on prairie vole populations but that the interaction is restricted to the cotton rats' reproductive season. Coexistence of the species appears to rely on the seasonality of the interaction, habitat heterogeneity, and the wider habitat tolerance of prairie voles.

Nutrition and Population Dynamics of the Prairie Vole, Microtus ochrogaster, in Central Illinois

SUMMARY (1) Nutrition and demographic characteristics of three populations of prairie voles were compared in adjacent, but distinctly different, habitats in central Illinois. (2) Live trappings from 1972 to 1976 provided data for two cyclic fluctuations in density. Densities reached higher levels in all habitats during the first cycle (prairie-38/ha, bluegrass-128/ha and alfalfa-244/ha) than during the second cycle (10/ha, 60/ha and 102/ha, respectively). (3) Voles had greater reproduction and survival and higher body weights in the alfalfa than in the bluegrass or prairie habitats. Data obtained by snap-trapping indicated that litter sizes and fat reserves were also greater in the alfalfa habitat. (4) Examination of stomach contents showed that dicotyledons comprised a major portion of the vole diet in all habitats even though grasses dominated the vegetation in the bluegrass and prairie habitats. Voles in the latter habitats supplemented their diet with seeds and insects. (5) Analysis of nutrients in three common food items (alfalfa, clover and bluegrass) revealed that legumes not only contained more digestible energy than bluegrass but also higher levels of crude protein, calcium, phosphorus and sodium. Moreover, voles grew more rapidly, bred earlier and produced more young when fed alfalfa than when fed dandelions and bluegrass. (6) When released in enclosures, voles grew faster and reproduced sooner in the alfalfa than in the bluegrass or prairie habitats. (7) We conclude that the quality of available food can exert a strong influence on the density of herbivores. Higher forage quality accounted for greater peak densities of vole populations, but it did not prevent population declines.

Population Biology of a "Noncycling" Population of Prairie Voles and a Hypothesis on the Role of Migration in Regulating Microtine Cycles

As part of the US/International Program Grassland Biome study in northeastern Colorado, six 1—ha plots of shortgrass prairie were manipulated for 6 yrs (1971—1976) through the addition of N, water, or both. Two additional 1—ha plots served as controls. Microtus ochrigaster invaded the 2 N + water treatment plots in 1971, and increased in annual peak densities (occurring in late summer or fall each year) linearly through the completion of this study in 1976. These vole populations exhibited no multiyear cycles in population density usually common in microtines, but they did undergo exaggerated annual fluctuations in density. Patterns in reproduction, mortality, sex ratios, body weight, annual population fluctuations, and dispersal were analyzed and compared with those of populations reported in the literature to exhibit 2—4 year cycles in population density. Only small differences in these parameters were found between the Pawnee populations and multiyear cyclic populations. The N + water plots represented "islands" of suitable vole habitat in a "sea of shortgrass prairie," which is apparently inhospitable for voles. Thus, these semi—isolated populations of noncyclic (or annually cyclic) prairie voles allowed us to speculate on several aspects of the Chitty/Krebs theory of self—regulation in microtine populations. This theory (as modified by Krebs) implicates emigration as an important determinant of 2—4 year cycles. We propose that the importance of emigration is in regulating population size while immigration is probably responsible for the existence of the cycle.

Parental care and mating system of the prairie vole, Microtus ochrogaster

1. The mating system of prairie voles (Microtus ochrogaster) was investigated by experimentally manipulating the sexual composition of vole societies, thereby providing each vole with a variety of mating choices. 2. In male-female pairs, both parents appeared to contribute equally to the care of young, with the exception of lactation. Paternal activities included nest and runway construction, food caching, and grooming, retrieving, and brooding the young. 3. Voles in 26 of 27 societies mated monogamously; those in the other group mated polygynously. 4. Nonreproductive voles in the experimental societies generally lived in communal groups. Communal groups are known to occur in field populations of prairie voles, but it is not known if they exist during periods of reproductive activity. 5. We address the questions of the adaptive significance of voles living communally and we consider the relationship between communal voles and dispersing voles in nature. The results of this study were interpreted relative to four mating systems that may be associated with paternal care in mammals. We conclude that M. ochrogaster is basically monogamous; a verbal model is presented for the evolution of monogamy in this species.

The Reproductive Cycle of Microtus ochrogaster in Eastern Kansas

About 800 Microtus ochrogaster were live—trapped at biweekly intervals from May 1971 through March 1973 in 3 grassland study areas in eastern Kansas, USA. Details of reproduction were determined by autopsy. Population density increased through the first winter, reached a peak in April 1972, then declined sharply during that summer and beyond. Body length, but not body mass, tended to be positively related to density. Both sexes matured at about the same weight; development was somewhat prolonged during the winter months, especially in the winter preceding the population peak. Pregnancy rates were high, approaching maximal iteroparity, throughout the study, with intervals of nonbreeding in both Julys, and in August and December of the population decline. Embryo counts increased significantly during the months of peak density but did not diminish in the period of the population decline. There was no association between either maternal weight or parity and number of embryos. Corpora counts were not higher in heavy or multiparious ♀ ♀, nor did the level of prenatal mortality increase during the period of greatest density. Except for a few individuals taken in July and August 1971, ♂ ♂ with a mass 30 g or more were judged to be fertile, as were a majority of the 20 to 29—g ♂ ♂. Body weight and testes weight were significantly correlated in fertile ♂ ♂ but not in nonfertile ♂ ♂. These reproductive findings were applied to a model of population regulation of microtine cycles. The pattern of reproduction of Kansas and Indiana prairie voles was compared. Three concordant lines of evidence seem to indicate that Kansas and possibly other geographical populations of prairie voles have adjusted the breeding schedule to accommodate both hot and dry summers and cold, snowy winters. Pregnancy rate, litter size, and adjusted testes weight all show a significant increase in March–April and September–October and a decrease in midsummer and midwinter. Thus the pattern of reproduction of Kansas prairie voles may represent a composite of the patterns shown by the dry—adapted species, such as Microtus californicus, and the cold—adapted microtines, such as Microtus pennsylvanicus. Based on the observations of this study, Kansas prairie voles seem to have adjusted to both harsh seasons by breeding more or less continuously, with the midsummer depression possibly reflecting the Great Plains origin of the species, and the winter depression being a facultative response to weather, density, or quality of habitat.

Population Ecology of the Prairie Vole, Microtus ochrogaster, in the Western Mixed Prairie of Nebraska

Populations of prairie voles were studied on two grids of 4.1 and 3.7 acres in the western mixed prairie of Scotts Bluff National Monument, Nebraska. This xeric prairie, with a mean annual precipitation of 36.6 cm, is dominated by mid and short grasses. Over three seasons, 1968-69, live-trapping revealed population densities of one to 14 voles per acre. The mean trap-revealed home range was .22 acres during the summer of 1968. Correlation of trapping results with known distribution of plants found in fecal samples suggests home ranges as large as .34 acres for some individuals. In general, population densities for the prairie vole in western Nebraska are considerably lower than reported in the mesic tall-grass prairie of eastern Kansas and somewhat lower than reported in the mesic mixed prairie of west-central Kansas. Home ranges in western Nebraska are considerably larger than those reported in the tall-grass prairie. Lower population densities may result from lower precipitation and, indirectly, from the sparser cover and decreased productivity characteristic of the western mixed prairie. Previous studies have documented the importance of precipitation in affecting available food supply and reproduction of voles. Year-round reproduction is supported by indirect evidence, and the populations showed a decline in numbers of juveniles and an increase in mean body weight through late summer, winter and early spring. Fecal analysis indicates that voles eat green succulent grasses and sedges, and show seasonal preferences dependent on the characteristic growth patterns of the most abundant plants. The food in winter is similar to that of summer, indicating a dependence on cached food, while that of spring is most different, reflecting the shifts in preference to various rapidly growing grasses and sedges. Stations with good cover had a significantly larger number of captures and individuals per station on one plot but not the other. This may have been due to the greater uniformity of the latter plot. There was no evident correlation between the location of specific grasses important for food and the distribution of captures for voles.