Defining ?high quality? food resources of herbivores: the case for meadow voles (Microtus pennsylvanicus)

Cascading effects of earthworm invasion increase graminoid density and rodent grazing intensities

Human-mediated dispersal of non-native earthworms can cause substantial changes to the functioning and composition of ecosystems previously earthworm-free. Some of these earthworm species have the potential to "geoengineer" soils and increase plant nitrogen (N) uptake. Yet the possible consequences of increased plant N concentrations on rodent grazing remains poorly understood. In this study, we present findings from a common garden experiment with two tundra communities, meadow (forb dominated) and heath (shrub dominated), half of them subjected to 4 years of earthworm presence (Lumbricus spp. and Aporrectodea spp.). Within four summers, our earthworm treatment changed plant community composition by increasing graminoid density by, on average, 94% in the heath vegetation and by 49% in the meadow. Rodent winter grazing was more intense on plants growing in soils with earthworms, an effect that coincided with higher N concentrations in plants, indicating a higher palatability. Even though earthworms reduced soil moisture, plant community productivity, as indicated by vegetation greenness (normalized difference vegetation index), was not negatively impacted. We conclude that earthworm-induced changes in plant composition and trophic interactions may fundamentally alter the functioning of tundra ecosystems.

The gut microbiome influences host diet selection behavior

The behavior of diet selection or diet choice can have wide-reaching implications, scaling from individual animals to ecological and evolutionary processes. Previous work in this area has largely ignored the potential for intestinal microbiota to modulate host foraging decisions. The notion that the gut microbiome may influence host foraging behavior has been highly speculated for years but has not yet been explicitly tested. Here, we show that germ-free mice colonized by differential microbiomes from wild rodents with varying natural feeding strategies exhibited significant differences in their voluntary dietary selection. Specifically, colonized mice differed in voluntary carbohydrate selection, and divergent feeding preferences were associated with differences in circulating essential amino acids, bacterial tryptophan metabolism, and intestinal morphology. Together, these results demonstrate a role for the microbiome in host nutritional physiology and foraging behavior.

Diet selection is a fundamental aspect of animal behavior with numerous ecological and evolutionary implications. While the underlying mechanisms are complex, the availability of essential dietary nutrients can strongly influence diet selection behavior. The gut microbiome has been shown to metabolize many of these same nutrients, leading to the untested hypothesis that intestinal microbiota may influence diet selection. Here, we show that germ-free mice colonized by gut microbiota from three rodent species with distinct foraging strategies differentially selected diets that varied in macronutrient composition. Specifically, we found that herbivore-conventionalized mice voluntarily selected a higher protein:carbohydrate (P:C) ratio diet, while omnivore- and carnivore-conventionalized mice selected a lower P:C ratio diet. In support of the long-standing hypothesis that tryptophan—the essential amino acid precursor of serotonin—serves as a peripheral signal regulating diet selection, bacterial genes involved in tryptophan metabolism and plasma tryptophan availability prior to the selection trial were significantly correlated with subsequent voluntary carbohydrate intake. Finally, herbivore-conventionalized mice exhibited larger intestinal compartments associated with microbial fermentation, broadly reflecting the intestinal morphology of their donor species. Together, these results demonstrate that gut microbiome can influence host diet selection behavior, perhaps by mediating the availability of essential amino acids, thereby revealing a mechanism by which the gut microbiota can influence host foraging behavior.

Rodent food quality and its relation to crops and other environmental and population parameters in an agricultural landscape

The diet, its quality and quantity considerably influence population parameters of rodents. In this study, we used NIRS methods for estimation of nitrogen content in stomachs of rodent populations. The study was carried out in diverse arable landscape in South Moravia, Czech Republic. Rodents were sampled in cultural crops (alfalfa, barley, wheat, sunflower, maize and rape) as well as in fallow habitats (herbal set-aside and old orchard). Influence of habitat, date, year, individual parameters (body length, sex, breeding and age), and relative abundance on quality of consumed food was studied. Under conditions of higher population density, dominant species [wood mouse (Apodemus sylvaticus) and common vole (Microtus arvalis)] consumed food richer in nitrogen. Also the strong effect of crop and date (season) was found in both species. There was no significant effect of the other parameters studied on food quality (N-content).

The short-term impact of abundant fruit upon deer mouse (Peromyscus maniculatus), southern red-backed vole (Myodes gapperi), and woodland jumping mouse (Napaeozapus insignis) populations

Fruit has been identified as an important and potentially population-restricting food for southern red-backed voles (Myodes gapperi (Vigors, 1830)), deer mice (Peromyscus maniculatus (Wagner, 1845)), and woodland jumping mice (Napaeozapus insignis (Miller, 1891)). We added domestic dried strawberries (Fragaria × ananassa (Weston) Duchesne ex Rozier (pro sp.)) and European black currants (Ribes nigrum L.), which have native analogues and are preferred foods of these rodents, to white spruce (Picea glauca (Moench) Voss) plantations from May through August 2011 and 2012 to test fruit and fruit-based carbohydrate’s short-term (1–2 years) impact on these rodent populations. We used mark–recapture to estimate density, percentages of population that were juvenile and breeding female, mean home-range size, and body mass during spring and summer of both years, and fecundity via placental scars from euthanized females in summer 2012. Fruit enhancement had no apparent effect on our species’ fecundity, proportion of breeding females or juveniles during spring and summer of either year, nor were there differences among these metrics in spring 2012 following 2011 fruit additions. Overall, there were no impacts to the short-term adult population dynamics for any species during fruit addition. We are led to believe that short-term pulses of fruit and (or) fruit-based carbohydrate abundance do little to influence temperate forest small-mammal populations.

The response of rodents to scent marks: four broad hypotheses

This article is part of a Special Issue "Chemosignals and Reproduction". Many terrestrial mammals must be able to distinguish between the myriad of scent marks they encounter in order for them to facilitate or deter direct interactions with their scent donors. I review studies that examine how rodents, mainly meadow voles (Microtus pennsylvanicus), respond when they encounter the scent marks of conspecifics and heterospecifics, and how context, as well as the age and condition of senders and receivers, affect their responses. The review uses four broad hypotheses to discuss the response of rodents to scent marks. The four hypotheses are as follows: 1) Scent marks convey accurate information to the receiver about the sender's state and phenotype and genotype. 2) Scent marks are individually distinct. 3) The response of receivers to scent marks is flexible and would be modulated by the cognitive abilities of receivers. 4) Receivers respond to the information contained or conveyed by the scent mark in a manner that will increase their survival and fitness. The studies cited in this review show that scent marks signal accurate information about the sender's phenotype, genotype, and condition, which receivers use to distinguish among the scent marks of different conspecifics and heterospecifics, and by doing so, receivers tailor their response accordingly to increase their survival and fitness. Thus, the four broad hypotheses may serve as guide to increase our understanding of the response of receivers to scent marks and provide a conceptual framework for future research and the development of additional hypotheses.

Female meadow voles, Microtus pennsylvanicus, do not alter their over-marking in response to female conspecifics that were food deprived

Many terrestrial mammals will deposit scent marks and over-marks, the latter being the overlapping scent marks of two conspecifics. Studies have shown that male rodents that are exposed to the overlapping scent marks of two female conspecifics later spend more time investigating the mark of the top-scent female than that of the bottom-scent female. This suggests over-marking is a form of competition and that the top-scent female is more likely than the bottom-scent female to be chosen as a potential mate. Thus, females should over-mark the scents of neighboring females at a rate that will maximize their chances of attracting males. However, meadow voles live in areas that may contain patchy food availability, and residents may differ in their nutritional status. Females in a better nutritional state may be more likely than those in poorer nutritional states to indicate their presence in an area, signal possession of a territory, and to attract mates. Thus, we tested the prediction that female meadow voles, Microtus pennsylvanicus, that were not food deprived would deposit more over-marks than female voles that were food deprived for six hours. Food-deprived female voles and female voles that had continuous access to food deposited a similar number of scent marks and used a similar proportion of those marks as over-marks when they encountered the scent marks of female conspecifics. These findings suggest that the nutritional status of female voles does not affect their ability to signal their presence in an area marked by a female conspecific.

Patterns of resource use, food quality, and health status of voles (Microtus pennsylvanicus) trapped from fluctuating populations

Recent studies suggest that diet quality is responsible for differential survivorship of vole cohorts (Boonstra and Boag 1987) and spacing behavior of females (Ims 1987). These phenomena have been related either to a lack of or a deterioration in the quality of the preferred food. To test this hypothesis, we compared foods habits, food quality and health status of meadow voles (Microtus pennsylvanicus) from high and low population density phases. In this study, seven plant species represented the main food items used on a regular basis and biomass values of preferred species decreased with browsing pressure. In addition, food quality of the main dietary items changed between phases forcing females to adjust their feeding strategy accordingly. Health of voles changed also opening the way to speculations on competition for high quality food resources and survivorship.

Re-feeding food-deprived male meadow voles affects the sperm allocation of their rival males

An individual's nutritional status affects the manner in which same- and opposite-sex conspecifics respond to that individual, which may affect their fitness. Male meadow voles, Microtus pennsylvanicus, increase their sperm allocation if they encounter the scent mark of an unfamiliar male that is not nutritionally challenged. If, however, the scent mark comes from a male that has been food deprived for 24 hours, stud male voles do not increase their sperm allocation. Food deprived males may be viewed as being lower quality and a reduced risk of sperm competition by rival males. We hypothesized that stud males in promiscuous mating systems tailor their sperm allocations depending on whether rival males have been food deprived and then re-fed. We predicted that newly re-fed males will be considered a strong risk of sperm competition because of the potentially high fitness and survival costs associated with food deprivation in males, and that they will cause stud males to increase their sperm allocation. Our results, however, showed that the recovery period from 24 hours of food deprivation was a relatively slow process. It took between 96 hours and 336 hours of re-feeding male scent donors that were food deprived for 24 hours to induce stud males to increase their sperm allocation to levels comparable to when scent donors were not food deprived. Stud male voles may be conserving the amount of sperm allocated until the male scent donors have recovered from food deprivation and subsequent re-feeding.

Food deprivation and restriction during late gestation affects the sexual behavior of postpartum female meadow voles, Microtus pennsylvanicus

In many species of small mammals, including meadow voles, Microtus pennsylvanicus, females come into postpartum estrus (PPE) within 12-24 h of giving birth, allowing them to mate and become pregnant while raising the current litter. PPE females show increases in attractivity, proceptivity, and receptivity, the three components of sexual behavior, relative to females not in PPE. Several studies have shown that food deprivation and restriction reduce attractivity, proceptivity, and receptivity of females not in PPE. We tested the hypothesis that food deprivation and restriction during late gestation causes deficits and decreases the attractivity, proceptivity, and receptivity of females when they enter PPE. Our data support the hypothesis. On day 1 of lactation, females that were food deprived and food restricted produced scent marks that were significantly less attractive as those produced by control PPE females. Food deprivation but not food restriction caused females to no longer display significant preferences for the scent marks of males over those of females (proceptivity). Food deprivation and food restriction were sufficient to induce females to become significantly less sexually receptive than control females. Eleven of 12 control PPE females mated, 4 of 12 food-restricted females mated, and 3 of 12 food-deprived females mated. Dams facing food deprivation or restriction during late gestation may have to balance the benefits of mating during PPE with the increased costs associated with getting pregnant while they are lactating.

Intraspecific Competition, Growth, Chemistry, and Susceptibility to Voles in Seedlings of Betula pendula

We studied the effects of the intensity of intraspecific competition, as indicated by seedling density, and competitive success within populations, as indicated by seedling size, on the secondary chemistry of the stems of silver birch seedlings and their palatability to field voles. We found that the size of seedlings and their total phenolic concentrations were inversely related to stand density. Voles, however, did not discriminate between seedlings grown at the densities studied. Variation in competitive success was associated with several phenomena. Seedling size was positively related to the concentrations of several secondary compounds and to vole preferences, but negatively related to nitrogen concentration. We suggest that the ecological consequences of variation in seedling size cannot be predicted without knowledge of the mechanism behind the variation.

Behavioral strategies of mammal herbivores against plant secondary metabolites: the avoidance-tolerance continuum

We review the evidence for behavioral avoidance of plant secondary metabolites (PSMs) and identify how, and the circumstances under which it occurs. Behavioral strategies of avoidance of PSM can only be fully understood in relation to the underlying physiological processes or constraints. There is considerable evidence that animals learn to avoid PSMs on the basis of negative postingestive effects. The extent to which this process determines foraging choices is limited by the ability of animals to experience the consequences of their behaviors and associate particular cues in foods with their specific effects in the body. The proposed learning mechanisms require that animals must at least "sample" plants that contain PSMs. They do not completely avoid PSMs, but there is evidence that they restrict their ingestion to within limits that they are physiologically able to tolerate, and that the amounts of PSM ingested result from a balance between toxicological considerations and the nutrient content of the plant material. These limits are influenced by the kinetics of PSM elimination, which underlies patterns of bite and patch selection from plant parts to landscapes. We suggest that altering spatial location of feeding (to alternative food patches or alternative foods within patches, including plant parts), and temporal distribution of feeding activity, by either cessation of feeding or by continuing to feed, but on alternative foods, can both lead to reduction of the intake and toxic effects of PSMs. We propose that the strategy of avoidance or reduction of intake of PSMs coevolved with the animal's ability to physiologically tolerate their ingestion, and that avoidance and tolerance are inversely related (the avoidance-tolerance continuum). The animals' propensity and ability to seek alternatives also vary with the dispersion of their food resources. Further work is required to test these proposals and integrate temporal and spatial aspects of foraging behavior and its nutritional consequences in relation to PSMs.

Effects of food quality on growth and survival of juvenile Columbian ground squirrels (Spermophilus columbianus)

Within a cohort of Columbian ground squirrels (Spermophilus columbianus), most deaths during hibernation usually occur in the first winter. Over-winter survivorship of juvenile Columbian ground squirrels is positively related to prehibernation body mass, which is primarily determined by the growth rate after emergence from natal burrows. This study examined how food quality influences growth and survival of juvenile Columbian ground squirrels. Growth and survival of juveniles in three colonies was measured, and biomass, composition (forb versus grass content), gross energy, crude protein, and digestibility of the summer forage supply were assessed. Differences in biomass and gross energy among colonies did not explain differences in growth and survival of resident ground squirrels. Growth and survival of juveniles was enhanced on sites with higher forb content in the forage supply. Forbs contained 15-40% more protein and were 30-50% more digestible than grasses, but protein levels on all sites were probably in excess of ground squirrels' requirements. This suggests that digestibility was limiting growth and was a more important food-quality indicator for juvenile ground squirrels than either energy or protein.

Winter use of food by fenced voles (Microtus pennsylvanicus)

Meadow voles (Microtus pennsylvanicus) were housed in fenced plots over the winter to establish whether intense grazing in the summer could affect their winter diet and thereby reduce their chances of survival. Intense grazing in the summer was simulated by removing 14% of the fall biomass. In November 1992, voles were introduced into two clipped plots and two unclipped plots. Two other plots were used to follow changes in biomass not associated with intense grazing. In January 1993, population densities and chemical constituents of fecal samples were significantly reduced as a result of experimental and winter environmental conditions. Over time, the vole feces contained increasing proportions of phenolics and neutral detergent solubles, and their diet became more diversified. These effects were more evident in the clipped plots, suggesting that the removal of fall biomass caused a rapid decline in vole numbers and the increased use of low-quality food during winter.

Anemia at the onset of winter in the meadow vole (Microtus pennsylvanicus)

1. From 1981 to 1986, 6120 meadow voles (Microtus pennsylvanicus) were sampled for hematological indices in southeastern Manitoba, Canada. This survey revealed the sporadic occurrence of anemia in early winter at mean temperatures below about -5 degrees C. 2. Anemia was associated with leukocytosis and circulating normoblasts, suggesting a sudden, large blood loss. Individuals became anemic quickly, with no obvious predisposing factors. 3. Attempts were made to induce anemia by exposing voles in traps to various temperatures. Temperatures characteristic of most trapping sessions failed to induce anemia in both wild and laboratory-born voles. 4. Short-term exposure to more extreme temperatures (-20 to -30 degrees C) induced anemia. Voles lost blood through erosions of the epithelium of the glandular stomach, and developed other pathological lesions characteristic of hypothermia. 5. Although there was a strong association between cold weather and anemia, we could find no clear relationship between winter survival and winter weather. However, in 1984, extraordinarily cold temperatures were associated with anemia and a subsequent population decline. These events suggest a threshold mean daily temperature of about -15 degrees C, below which vole survival is grossly affected. 6. Deteriorating protein levels and energy reserves of small mammals in winter may make them particularly susceptible to cold stress. Hence, sporadic bouts of sustained cold may be responsible for some of the enigmatic winter declines seen in northern small mammals.