Development of an index of abundance for pygmy rabbit populations

Ideal conditions for increased trapping success of pygmy rabbits (Brachylagus idahoensis) across the Great Basin

Sagebrush-steppe ecosystems are one of the most imperiled ecosystems in North America and many of the species that rely on these habitats are of great conservation concern. Pygmy rabbits (Brachylagus idahoensis) are one of these species. They rely on sagebrush year-round for food and cover, and are understudied across their range in the intermountain west due in part to their recalcitrance to standard capture techniques. Identifying an efficient and minimally biased trapping method therefore is a critical first step in learning more about this species. We assessed how trap orientation and weather characteristics influenced trap success for Tomahawk traps placed in and around pygmy rabbit burrows by carrying out trapping surveys at 16 occupied pygmy rabbit sites across the Great Basin from 2016 to 2018. We found that pygmy rabbits had a greater probability of being captured in traps with the open end facing away from burrow entrances. Pygmy rabbits also were more likely to be captured on clear days (0–5% cloud cover) and during periods of cooler temperatures during summer months (June–August). We found no evidence that sex or age ratios differed, or that individuals differed meaningfully, in their preference for certain trap orientations. To increase trap success for pygmy rabbits, we suggest maximizing trapping effort during summer months, at dawn, and maximizing the proportion of Tomahawk traps facing away from burrow entrances. We anticipate that our monitoring protocol will enable more effective research into the ecology and conservation of this cryptic and potentially imperiled species.

Seasonal temperature acclimatization in a semi-fossorial mammal and the role of burrows as thermal refuges

Small mammals in habitats with strong seasonal variation in the thermal environment often exhibit physiological and behavioral adaptations for coping with thermal extremes and reducing thermoregulatory costs. Burrows are especially important for providing thermal refuge when above-ground temperatures require high regulatory costs (e.g., water or energy) or exceed the physiological tolerances of an organism. Our objective was to explore the role of burrows as thermal refuges for a small endotherm, the pygmy rabbit (Brachylagus idahoensis), during the summer and winter by quantifying energetic costs associated with resting above and below ground. We used indirect calorimetry to determine the relationship between energy expenditure and ambient temperature over a range of temperatures that pygmy rabbits experience in their natural habitat. We also measured the temperature of above- and below-ground rest sites used by pygmy rabbits in eastern Idaho, USA, during summer and winter and estimated the seasonal thermoregulatory costs of resting in the two microsites. Although pygmy rabbits demonstrated seasonal physiological acclimatization, the burrow was an important thermal refuge, especially in winter. Thermoregulatory costs were lower inside the burrow than in above-ground rest sites for more than 50% of the winter season. In contrast, thermal heterogeneity provided by above-ground rest sites during summer reduced the role of burrows as a thermal refuge during all but the hottest periods of the afternoon. Our findings contribute to an understanding of the ecology of small mammals in seasonal environments and demonstrate the importance of burrows as thermal refuge for pygmy rabbits.

Consequences for conservation: population density and genetic effects on reproduction of an endangered lagomorph

Understanding reproduction and mating systems is important for managers tasked with conserving vulnerable species. Genetic tools allow biologists to investigate reproduction and mating systems with high resolution and are particularly useful for species that are otherwise difficult to study in their natural environments. We conducted parentage analyses using 19 nuclear DNA microsatellite loci to assess the influence of population density, genetic diversity, and ancestry on reproduction, and to examine the mating system of pygmy rabbits (Brachylagus idahoensis) bred in large naturalized enclosures for the reintroduction and recovery of the endangered distinct population in central Washington, USA. Reproductive output for females and males decreased as population density and individual homozygosity increased. We identified an interaction indicating that male reproductive output decreased as genetic diversity declined at high population densities, but there was no effect at low densities. Males with high amounts (> 50%) of Washington ancestry had higher reproductive output than the other ancestry groups, while reproductive output was decreased for males with high northern Utah/Wyoming ancestry and females with high Oregon/Nevada ancestry. Females and males bred with an average of 3.8 and 3.6 mates per year, respectively, and we found no evidence of positive or negative assortative mating with regards to ancestry. Multiple paternity was confirmed in 81% of litters, and we report the first documented cases of juvenile breeding by pygmy rabbits. This study demonstrates how variation in population density, genetic diversity, and ancestry impact fitness for an endangered species being bred for conservation. Our results advance understanding of basic life history characteristics for a cryptic species that is difficult to study in the wild and provide lessons for managing populations of vulnerable species in captive and free-ranging populations.

Genetic Evidence Confirms the Presence of Pygmy Rabbits in Colorado

The pygmy rabbit Brachylagus idahoensis is a sagebrush-obligate species of conservation concern that occurs in the Great Basin and adjacent intermountain areas in the western United States. The species is not known to occur in Colorado, despite proximity to existing populations of pygmy rabbits in Wyoming. We provide the first documentation of the pygmy rabbit in Colorado. Fecal pellets diagnostic of pygmy rabbits were collected in the Vermillion Bluffs area of northwestern Colorado. Samples from 16 pellet clusters were collected for species identification via genetic analyses, and we were able to extract and amplify sufficient DNA from 7 of 16 pellet samples. All seven samples were identified as originating from pygmy rabbits based on a species-specific mitochondrial DNA fragment analysis test. To verify species identification, we also sequenced 225 base pairs of the mitochondrial DNA cytochrome b region from all seven pellet samples. Presence of pygmy rabbits was confirmed from three locations separated by 2.4–7.7 km and pellets represented both adult and juvenile rabbits. Based on the sparseness of burrows in the area, density of pygmy rabbits in the area likely is low; however, systematic surveys by trained observers are needed to delineate the range and density of this species in Colorado. Given the conservation concern for pygmy rabbits across their current range, the newly confirmed presence of this species suggests that assessment of their conservation status in Colorado is warranted.

Grazing in sagebrush rangelands in western North America: implications for habitat quality for a sagebrush specialist, the pygmy rabbit

Livestock grazing is one of the primary uses of sagebrush rangelands in western North America; therefore, an understanding of the ecological implications of grazing on habitat quality for sagebrush-dependent wildlife is needed to help land managers balance multiple objectives for land use. We studied effects of cattle grazing on components of habitat for an uncommon sagebrush habitat specialist, the pygmy rabbit (Brachylagus idahoensis), which has been petitioned for endangered or threatened status in the USA. We evaluated multiple components of habitat before and after grazing in replicated control and treatment plots in a mesic, high-elevation sagebrush-steppe environment in south-western Montana, USA. We predicted that grazing would decrease the biomass of herbaceous forage, alter security cover, and increase rate of collapse of rabbit burrows, and we expected that these effects would be more pronounced during summer than spring. As expected, cattle grazing reduced the biomass of perennial grasses available to pygmy rabbits after grazing that occurred during either spring or summer, and the biomass of forbs after spring grazing. In contrast, grazing did not markedly influence the functional properties of vegetation related to predation risk or the integrity of rabbit burrow systems. In the context of the stocking rate of the allotments in our study (7.3 acres/Animal Unit Month, 2.95 ha/Animal Unit Month), annual cattle grazing did not seem to markedly change habitat for pygmy rabbits in our study area; however, longer-term and higher intensity grazing might result in more pronounced habitat changes. Understanding the ecological implications of cattle grazing on habitat quality for pygmy rabbits and other sagebrush-dependent wildlife can guide conservation strategies for these species on sagebrush rangelands managed under multiple-use policies.