Jerry O. Wolff

Of Mice and Mast Ecological connections in eastern deciduous forests

popular characterization of ecological systems is that everything is connected to everything else. Although in some metaphysical sense it may be true that influences experienced by a particular species are transmitted to all other species in the ecosystem, such pervasive connections have not been demonstrated scientifically. Instead, the focus of community ecology has been on pairwise interactions between species. The paradigm has been that the determinants of community structure can be elucidated by studies of direct effects of populations on one or a few other populations (Kareiva 1994). To identify the role of predation or competition in structuring ecological communities, typically a predator or competitor is removed, and the responses of prey or other competitors are monitored (Connell 1983, Menge and Sutherland 1976, Schoener 1983, Sih et al. 1985). These studies reveal the importance of direct effects in ecological communities. However, such experiments often produce unexpected results, In oak forests, both

Population Fluctuations of Mast-Eating Rodents Are Correlated with Production of Acorns

I used a 14-year dataset on dynamics of mast-consuming rodents from an eastern deciduous forest to determine the relationship between population fluctuations of white-footed mice ( Peromyscus leucopus ), deer mice ( P. maniculatus ), and eastern chipmunks ( Tamias stria-tus ) and production of acorn mast. Mast (acorn) production was episodic with four excellent crops produced in 14 years. Densities of rodents in summer ranged from 3 to 103 animals/ha and correlated positively with production of mast the previous autumn (all r 2 > 0.56). During years of high production of mast, stores of acorn lasted throughout winter, whereas in most years, acorns were gone by January. During years of high production of mast, mice bred all winter, which resulted in high densities the following summer. Episodic production of mast and resulting fluctuations in consumers of mast have implications for the predator-satiation hypothesis and other community processes.

Why are female small mammals territorial

I question the widely accepted view that female small mammals are territorial to defend food, and suggest that theoretical and empirical evidence are more compatible with a pup-defense hypothesis to protect young from infanticide. The fact that females are territorial during the time of greatest food abundance and not during food limitation (such as winter) contradicts a food-defense hypothesis. Energy conservation through huddling does not appear sufficient to explain shared use of space during nonbreeding seasons. Aggression and territorial defense are most intense during lactation and are directed toward other females, those most likely to commit infanticide, and not toward males and other food competitors

COUNTER-STRATEGIES TO INFANTICIDE IN MAMMALS : COSTS AND CONSEQUENCES

Infanticide, the killing of nonspecific young, has been documented in numerous species of mammals and is considered an adaptive behavioral strategy to enhance the reproductive success of the perpetrator. The potential benefits of committing infanticide for males are obtaining nutritional gain and mating partners, and for females are acquiring access to resources such as food and nest sites. Some costs are associated with committing infanticide such as additional energy expenditure, risk of injury, and exposure to predation. However, the major costs associated with infanticide are borne by the victim female and the sire male in loss of fitness. In response to this selection, males and females use a variety of counter-strategies to protect their young from infanticide. We summarize the published accounts and theory associated with infanticide and the occurrence of counter-strategies in a variety of mammalian groups in order to explain how infanticide may influence individual behavior as well as the social systems of mammals. We focus on the behavioral strategies used, primarily by females, to deter major losses in reproductive success. These strategies include aggression, female choice of dominant males, and promiscuity to confuse paternity as defense against males, and territoriality, association with kin, reproductive suppression, and reproductive synchrony as defenses against females. Male counter-strategies are less well known, but intrasexual territoriality may in part function as defense against infanticide. The costs associated with the different male and female counter-strategies are likely to vary, but may include increased energy expenditure, exposure to predators and injury for both sexes, as well as increased competition for resources, limited mate choice, and postponed reproduction for females. We propose that the occurrence of infanticide does not only have the potential to affect the behavior of individuals (e.g. aggression, spacing and mate choice), but may also have consequences for the shaping of mammalian mating systems.

Variation in neural V1aR predicts sexual fidelity and space use among male prairie voles in semi-natural settings

Although prairie voles (Microtus ochrogaster) are socially monogamous, males vary in both sexual and spatial fidelity. Most males form pairbonds, cohabit with one female, and defend territories. Wandering males, in contrast, have expansive home ranges that overlap many males and females. In the laboratory, pairing is regulated by arginine vasopressin and its predominant CNS receptor, vasopressin 1a receptor (V1aR). We investigated individual differences in forebrain V1aR expression of male prairie voles in mixed-sex seminatural enclosures. Individual differences in V1aR were compared with space use measured by radio telemetry and paternity determined with microsatellite markers. Animals engaging in extra-pair fertilizations (EPFs) as either wanderers or paired residents overlapped significantly more in same- and opposite-sex home ranges. Surprisingly, neither social fidelity measured by space use nor sexual fidelity measured by paternity was associated with V1aR expression in the ventral pallidum (VPall) or lateral septum, areas causally related to pairbond formation. In contrast, V1aR expression in the posterior cingulate/retrosplenial cortex (PCing) and laterodorsal thalamus (LDThal), areas implicated in spatial memory, strongly covaried with space use and paternity. Animals engaging in EPFs either as wanderers or paired residents exhibited low levels of LDThal and PCing V1aR expression. Individual differences in brain and behavior parallel differences between prairie voles and promiscuous congeners. The concordance among space use, paternity, and V1aR in spatial circuits suggests a common link between the mechanisms of spatial behaviors and success at EPF. The combined data demonstrate how organismal biology can inform our understanding of individual and species differences in behavioral mechanisms.

Social but not genetic monogamy is associated with greater breeding success in prairie voles

Much attention has focused on distinguishing between social and genetic monogamy in avian taxa. However, surprisingly few studies have directly investigated this distinction among mammals. We investigated the genetic mating system of the prairie vole, Microtus ochrogaster, a popular model for mammalian monogamy and human attachment. We used space use patterns to define paired and single animals and assessed paternity using microsatellite loci. Prairie voles in this study engaged in significantly more extrapair fertilizations than predicted under genetic monogamy but fewer than predicted under random mating, demonstrating social but not genetic monogamy. Furthermore, we found that paired individuals were more likely to produce offspring than were unpaired individuals of either sex. This finding was true for both sexes and was attributable to differences in fertilization rates rather than litter sizes. Among mated individuals, however, faithful animals were no more successful than those that mated outside a pair. Taken together, our data demonstrate that paired prairie voles have greater breeding success than single voles, but such success is not contingent on mating exclusively with a social partner. If this species is to serve as a model for human love, our findings emphasize the need to distinguish between mammalian social attachment and sexual fidelity.

CACHE MANAGEMENT BY SMALL MAMMALS: EXPERIMENTAL EVIDENCE FOR THE SIGNIFICANCE OF ACORN-EMBRYO EXCISION

Abstract We conducted 2 field experiments to assess relative importance of acorn-embryo excision in the caching decisions of small mammals. In the 1st, we selectively provisioned small mammals with metal-tagged acorns of red oak (Quercus rubra) and white oak (Q. alba) at 40 point locations in 8 sites in an oak forest in northeastern Pennsylvania. We then followed the fate of cached seeds by relocating acorns with metal detectors soon after they were cached and again in spring after seeds began to germinate. At least 1 species of small mammal excised embryos of >70% of the cached acorns of white oak and <4% of those of red oak. Animals also were observed to revisit caches in spring and excise embryos of germinating acorns. More excised acorns of white oak were found intact in spring than those of red oak, indicating that the behavior is important for long-term storage of these seeds. In a 2nd experiment, we presented free-ranging Mexican gray squirrels (Sciurus aureogaster) with pairs of acorns of 5 native white oak and 5 native red oak species and recorded caching events and whether or not cached seeds had their embryos removed. Squirrels cached significantly more acorns of white oak species, frequently excised embryos of these seeds, and only excised embryos of red oaks when they were germinating. These results support our previous hypothesis that the behavior of embryo excision is geographically widespread and has important implications for cache-management strategies of some diurnal tree squirrels and their effect on dispersal of oaks.

Effects of acorn production and mouse abundance on abundance and Borrelia burgdorferi infection prevalence of nymphal Ixodes scapularis ticks.

Risk of exposure to Lyme disease is a function of the local abundance of nymphal Ixodes ticks that are infected with the etiological agent, the spirochete Borrelia burgdorferi. We monitored abundance of white-footed mice (the principal B. burgdorferi reservoir in the eastern and central United States) and acorns (a critical food resource for mice), and Ixodes scapularis ticks, as well as ambient temperature (cumulative growing degree days) and growing season precipitation, in a forested landscape of southeastern New York State from 1994 to 2000. We found that acorn production in autumn strongly influenced abundance of white-footed mice the following summer and that abundance of mice in summer, when larval ticks are active, influenced the abundance of infected nymphs the following year. Consequently, the abundance of infected nymphal ticks can be predicted from acorn production 1.75 years earlier. Monitoring of natural fluctuations in acorn production thus supports results of prior acorn addition experiments that were conducted at small spatial scales. Growing degree days and precipitation either had no significant effect on density of nymphs or marginally increased the explanatory power of models that included acorns or mouse density as independent variables. We conclude that, at our study site in New York, the risk of human exposure to Lyme disease is affected by mouse density in the prior year and by acorn production 2 years previously.

Response of Gray-Tailed Voles to Odours of a Mustelid Predator: A Field Test

We conducted a field study to test the hypothesis that gray-tailed voles Microtus canicaudus would move from preferred tall-grass habitat into open, short-grass habitat after exposure to the odours of a mustelid predator in their preferred tall-grass habitat. We also tested the hypothesis that if voles did not avoid the odours of a mustelid predator, they would exhibit suppressed reproduction, delayed sexual maturation, and decreased activity. The experiment was conducted from May to August 1996 in 0.2-ha enclosures in which one-half of each enclosure contained tall grass and in the other one-half the grass was mowed to a height of 5-20 cm. The proportion of voles living in the preferred tall-grass habitat averaged > 0.9 and did not differ significantly before, during, or after voles were exposed to faeces and urine of mink Mustela vison (a mustelid predator), or rabbit Oryctolagus cumiculus (a nonpredator, control). We observed no significant differences between treatments in reproductive rates, time to sexual maturation, juvenile recruitment, or activity. We conclude that in a field situation, gray-tailed voles do not exhibit a behavioural or demographic response to simulated predation risk by a mustelid predator (odours of faeces and urine). The disagreement between our results and those obtained from laboratory studies is discussed.

Laboratory Studies with Rodents: Facts or Artifacts?

Abstract Rodents have been studied extensively in the laboratory as model species to address and, in some cases, develop paradigms in mammalian behavioral biology. However, the laboratory environment presents obvious limitations that can compromise results, inferences, and application to evolutionary theory and the species’ natural history. Here I revisit several research areas that have been developed in the laboratory that either have never been tested in the field or, when they were tested, did not support laboratory results. Some of these studies include the Bruce effect, scent marking, mate choice, artificial selection, predator-induced reproductive suppression, and other behavioral anomalies. Whether laboratory results for these and other studies have produced facts or artifacts is equivocal, but they warrant critical evaluation. Rodents are excellent model systems for testing hypotheses in behavioral ecology. However, to improve our confidence in results from laboratory studies, the behavior being studied should be documented in the field, subjected to alternative hypothesis testing, applied to evolutionary theory and the species’ natural history, and field-validated.