Corticosterone response by Peromyscus mice to parasites, reproductive season, and age

A common response to parasite infestations is increased production of glucocorticoid hormones that regulate immune function. We examined relationships between ectoparasite infestations and fecal corticosterone metabolites (FCM) in deer mice (Peromyscus maniculatus). Furthermore, we experimentally removed fleas to determine if reductions in ectoparasites affected FCM production. Individuals were assigned to control (no flea removal) or treatment (anti-flea application, physical combing) groups and individuals were recaptured to assess changes in FCM concentrations. There was a significant and negative effect of number of anti-flea treatment applications on FCM concentrations of deer mice. However, models including host biology traits and environmental predictors had a better model fit compared to models containing ectoparasite predictors. In particular, there was a significant relationship of deer mouse FCM with date and host age, where glucocorticoid production decreased towards the end of the breeding season and increased with age. Overall, adverse events associated with reproduction and age class, rather than ectoparasites, may be more important to variation in glucocorticoids of deer mice.

Mechanisms driving postfire abundance of a generalist mammal

Changes in vertebrate abundance following disturbance are commonly attributed to shifts in food resources or predation pressure, but underlying mechanisms have rarely been tested. We examined four hypotheses for the commonly reported increase in abundance of deer mouse ( Peromyscus maniculatus (Wagner, 1845)) following forest fires: source–sink dynamics, decreased predation, increased food resources, and increased foraging efficiency. We found that reproduction of deer mouse was considerably higher in burned versus unburned forests and survival did not differ between habitats, indicating that burned forests were not sink habitats. Comparable survival also suggested that predation rates were similar between habitats. Increased reproduction in burned versus unburned forest suggested better resource conditions, but abundance of seeds and arthropods (the primary food resources for mice) either did not differ between habitats or were higher overall in unburned forest. Foraging experiments indicated that seed removal from depots was substantially higher in burned versus unburned forests after controlling for mouse density. Additionally, in both habitats, mice were captured more often in open microhabitats and the odds of individual insect removal increased with decreasing cover during certain sampling periods. Of the four hypotheses tested, greater foraging efficiency provided the best explanation for elevated populations of deer mouse. However, predation risk may have influenced foraging success.

A temporal dilution effect: hantavirus infection in deer mice and the intermittent presence of voles in Montana

The effect of intermittently occurring, non-reservoir host species on pathogen transmission and prevalence in a reservoir population is poorly understood. We investigated whether voles, Microtus spp., which occur intermittently, influenced estimated standing antibody prevalence (ESAP) to Sin Nombre hantavirus (SNV, Bunyaviridae: Hantavirus) among deer mice, Peromyscus maniculatus, whose populations are persistent. We used 14 years of data from central Montana to investigate whether ESAP among deer mice was related to vole presence or abundance while controlling for the relationship between deer mouse abundance and ESAP. We found a reduction in deer mouse ESAP associated with the presence of voles, independent of vole abundance. A number of studies have documented that geographic locations which support a higher host diversity can be associated with reductions in pathogen prevalence by a hypothesized dilution effect. We suggest a dilution effect may also occur in a temporal dimension at sites where host richness fluctuates. Preservation of host diversity and optimization of environmental conditions which promote occurrence of ephemeral species, such as voles, may result in a decreased ESAP to hantaviruses among reservoir hosts. Our results may extend to other zoonotic infectious diseases.

Deer mouse demography in burned and unburned forest: no evidence for source–sink dynamics

Deer mouse ( Peromyscus maniculatus (Wagner, 1845)) populations increase dramatically after wildfires. These increases are puzzling because there are no obvious food sources or vegetation cover in severely burned areas. We conducted a capture–mark–recapture study of deer mice in a mosaic of burned and unburned montane forests in western Montana to determine if their postfire increase could be explained by source–sink dynamics, with burned areas acting as a sink. When overall mouse densities were very low, the vast majority of the population was found in burned areas. Mice appeared regularly in unburned forest only when the densities were high. This pattern is precisely opposite to the expected results if the sink hypothesis were correct. Moreover, mice in burned areas did not show decreased body mass, reproductive performance, or survival when compared with mice in unburned areas. Age structure and sex ratio did not differ between burned and unburned sites. We conclude that burned areas do not function as population sinks; rather, they represent high-quality habitat for deer mice.

Use of Xanthorrhoea semiplana (grass-trees) for refuge by Rattus fuscipes (southern bush rat)

This study investigated the use of the grass-tree, Xanthorrhoea semiplana F.Muell. (family Xanthorrhoeaceae), for shelter by Rattus fuscipes (southern bush rat) in South Australia. Eight bush rats were radio-tracked for 4–8 days each. To identify the understorey shelters available to each animal, surveys were conducted using point-intercept sampling at 2-m intervals along transect lines. Grass-tree density was calculated in each area used by the radio-tracked animals, and canopy thickness of grass-trees selected for refuge was assigned a score; the availability of other potential shelters was also calculated. The results indicated that (1) R. fuscipes preferentially selected grass-trees over other understorey shelter; (2) the grass-trees chosen had thick canopy covers; (3) areas with high grass-tree densities were preferred for cover over areas with fewer grass-trees; and (4) grass-trees provided dense cover and, therefore, concealed burrows and nest sites.