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.

The balancing act of foraging: mammalian herbivores trade-off multiple risks when selecting food patches

Animals face multiple risks while foraging such as the risk of acquiring inadequate energy from food and the risk of predation. We evaluated how two sympatric rabbits (pygmy rabbits, Brachylagus idahoensis, and mountain cottontail rabbits, Sylvilagus nuttallii) that differ in size, use of burrows, and habitat specialization in the sagebrush-steppe of western North America respond to different types and levels of perceived risks (i.e., fitness cost × probability of occurrence), including fiber and toxins in food, exposure to predation, and distance from a refuge. We measured food intake by the rabbits at paired food patches that varied in these risks and used the method of paired comparisons to create a relative ranking of habitat cues, which revealed an animal's perceived risk on a single scale representing an integrated response to a variety of risks. Pygmy rabbits perceived exposure to predation risk and distance from a burrow as riskier than did cottontails, whereas cottontails perceived dietary toxin as riskier. Pygmy rabbits consumed lower quality food, containing higher fiber or toxins, thereby avoided feeding in exposed patches or traveling far from their burrow to forage. In contrast, cottontails fed in exposed patches and traveled farther from the burrow to obtain higher quality food. We have shown how risks can be integrated into a single model that allows animals to reveal their perceptions of risks on a single scale that can be used to create a spatially explicit landscape of risk.

Geographic variation in winter adaptations of snowshoe hares (Lepus americanus)

Understanding adaptations of nonhibernating northern endotherms to cope with extreme cold is important because climate-induced changes in winter temperatures and snow cover are predicted to impact these species the most. We compared winter pelage characteristics and heat production of snowshoe hares (Lepus americanus Erxleben, 1777) on the southern edge of their range, in Pennsylvania (USA), to a northern population, in the Yukon (Canada), to investigate how hares might respond to changing environmental conditions. We also investigated how hares in Pennsylvania altered movement rates and resting spot selection to cope with variable winter temperatures. Hares from Pennsylvania had shorter, less dense, and less white winter coats than their northern counterparts, suggesting lower coat insulation. Hares in the southern population had lower pelage temperatures, indicating that they produced less heat than those in the northern population. In addition, hares in Pennsylvania did not select for resting spots that offered thermal advantages, but selected locations offering visual obstruction from predators. Movement rates were associated with ambient temperature, with the smallest movements occurring at the lower and upper range of observed ambient temperatures. Our results indicate that snowshoe hares may be able to adapt to future climate conditions via changes in pelage characteristics, metabolism, and behavior.

Comparative analyses of basal rate of metabolism in mammals: data selection does matter

Basal rate of metabolism (BMR) is a physiological parameter that should be measured under strictly defined experimental conditions. In comparative analyses among mammals BMR is widely used as an index of the intensity of the metabolic machinery or as a proxy for energy expenditure. Many databases with BMR values for mammals are available, but the criteria used to select metabolic data as BMR estimates have often varied and the potential effect of this variability has rarely been questioned. We provide a new, expanded BMR database reflecting compliance with standard criteria (resting, postabsorptive state; thermal neutrality; adult, non-reproductive status for females) and examine potential effects of differential selectivity on the results of comparative analyses. The database includes 1739 different entries for 817 species of mammals, compiled from the original sources. It provides information permitting assessment of the validity of each estimate and presents the value closest to a proper BMR for each entry. Using different selection criteria, several alternative data sets were extracted and used in comparative analyses of (i) the scaling of BMR to body mass and (ii) the relationship between brain mass and BMR. It was expected that results would be especially dependent on selection criteria with small sample sizes and with relatively weak relationships. Phylogenetically informed regression (phylogenetic generalized least squares, PGLS) was applied to the alternative data sets for several different clades (Mammalia, Eutheria, Metatheria, or individual orders). For Mammalia, a 'subsampling procedure' was also applied, in which random subsamples of different sample sizes were taken from each original data set and successively analysed. In each case, two data sets with identical sample size and species, but comprising BMR data with different degrees of reliability, were compared. Selection criteria had minor effects on scaling equations computed for large clades (Mammalia, Eutheria, Metatheria), although less-reliable estimates of BMR were generally about 12-20% larger than more-reliable ones. Larger effects were found with more-limited clades, such as sciuromorph rodents. For the relationship between BMR and brain mass the results of comparative analyses were found to depend strongly on the data set used, especially with more-limited, order-level clades. In fact, with small sample sizes (e.g. <100) results often appeared erratic. Subsampling revealed that sample size has a non-linear effect on the probability of a zero slope for a given relationship. Depending on the species included, results could differ dramatically, especially with small sample sizes. Overall, our findings indicate a need for due diligence when selecting BMR estimates and caution regarding results (even if seemingly significant) with small sample sizes.

Selection of food patches by sympatric herbivores in response to concealment and distance from a refuge

Small herbivores face risks of predation while foraging and are often forced to trade off food quality for safety. Life history, behaviour, and habitat of predator and prey can influence these trade-offs. We compared how two sympatric rabbits (pygmy rabbit, Brachylagus idahoensis; mountain cottontail, Sylvilagus nuttallii) that differ in size, use of burrows, and habitat specialization in the sagebrush-steppe of western North America respond to amount and orientation of concealment cover and proximity to burrow refuges when selecting food patches. We predicted that both rabbit species would prefer food patches that offered greater concealment and food patches that were closer to burrow refuges. However, because pygmy rabbits are small, obligate burrowers that are restricted to sagebrush habitats, we predicted that they would show stronger preferences for greater cover, orientation of concealment, and patches closer to burrow refuges. We offered two food patches to individuals of each species during three experiments that either varied in the amount of concealment cover, orientation of concealment cover, or distance from a burrow refuge. Both species preferred food patches that offered greater concealment, but pygmy rabbits generally preferred terrestrial and mountain cottontails preferred aerial concealment. Only pygmy rabbits preferred food patches closer to their burrow refuge. Different responses to concealment and proximity to burrow refuges by the two species likely reflect differences in perceived predation risks. Because terrestrial predators are able to dig for prey in burrows, animals like pygmy rabbits that rely on burrow refuges might select food patches based more on terrestrial concealment. In contrast, larger habitat generalists that do not rely on burrow refuges, like mountain cottontails, might trade off terrestrial concealment for visibility to detect approaching terrestrial predators. This study suggests that body size and evolutionary adaptations for using habitat, even in closely related species, might influence anti-predator behaviors in prey species.

Comparative anatomy and systematic implications of the turbinal skeleton in Lagomorpha (Mammalia)

In order to elucidate the systematic relevance of the turbinal skeleton in Lagomorpha the ethmoidal regions of 6 ochotonid, 21 leporid, and 2 outgroup species (Sciurus vulgaris, Tupaia sp.) species were investigated by high-resolution computed tomography (μCT). Number and shape of turbinals correspond to major clades and to several genera. All Lagomorpha under study have a deeply excavated nasoturbinal that is continuous with the lamina semicircularis; a feature likely to be an autapomorphy of lagomorphs. In particular, the olfactory turbinals (frontoturbinals, ethmoturbinals, and interturbinals) provide new systematic information. The plesiomorphic lagomorph pattern comprises two frontoturbinals, three ethmoturbinals, and one interturbinal between ethmoturbinal I and II. Ochotonidae are derived from the lagomorph goundplan by loss of ethmoturbinal III; an interturbinal between the two frontoturbinals is an autapomorphy of Leporidae. Pronolagus is apomorphic in having a very slender first ethmoturbinal, but shows a puzzling pattern in decreasing the number of turbinals. Pronolagus rupestris and Romerolagus diazi have independently reduced their turbinals to just two fronto- and two ethmoturbinals, which is the lowest number among the sampled lagomorphs. In contrast, the more derived leporid genera under study (Oryctolagus, Caprolagus, Sylvilagus, and Lepus) show a tendency to increase the number of turbinals, either by developing an ethmoturbinal IV (Caprolagus hispidus, Lepus arcticus) or by additional interturbinals. Intraspecific variation was investigated in Ochotona alpina, Oryctolagus cuniculus, and Lepus europaeus and is restricted to additional interturbinals in the frontoturbinal recess of the two leporids.

Plant protein and secondary metabolites influence diet selection in a mammalian specialist herbivore

For herbivores, nutrient intake is limited by the relatively low nutritional quality of plants and high concentrations of potentially toxic defensive compounds (plant secondary metabolites, PSMs) produced by many plants. In response to phytochemical challenges, some herbivores selectively forage on plants with higher nutrient and lower PSM concentrations relative to other plants. Pygmy rabbits (Brachylagus idahoensis) are dietary specialists that feed on sagebrush (Artemisia spp.) and forage on specific plants more than others within a foraging patch. We predicted that the plants with evidence of heavy foraging (browsed plants) would be of higher dietary quality than plants that were not browsed (unbrowsed). We used model selection to determine which phytochemical variables best explained the difference between browsed and unbrowsed plants. Higher crude protein increased the odds that plants would be browsed by pygmy rabbits and the opposite was the case for certain PSMs. Additionally, because pygmy rabbits can occupy foraging patches (burrows) for consecutive years, their browsing may influence the nutritional and PSM constituents of plants at the burrows. In a post hoc analysis, we did not find a significant relationship between phytochemical concentrations, browse status and burrow occupancy length. We concluded that pygmy rabbits use nutritional and chemical cues while making foraging decisions.

Testing alternative models of climate-mediated extirpations

Biotic responses to climate change will vary among taxa and across latitudes, elevational gradients, and degrees of insularity. However, due to factors such as phenotypic plasticity, ecotypic variation, and evolved tolerance to thermal stress, it remains poorly understood whether losses should be greatest in populations experiencing the greatest climatic change or living in places where the prevailing climate is closest to the edge of the species' bioclimatic envelope (e.g., at the hottest, driest sites). Research on American pikas (Ochotona princeps) in montane areas of the Great Basin during 1994-1999 suggested that 20th-century population extirpations were predicted by a combination of biogeographic, anthropogenic, and especially climatic factors. Surveys during 2005-2007 documented additional extirpations and within-site shifts of pika distributions at remaining sites. To evaluate the evidence in support of alternative hypotheses involving effects of thermal stress on pikas, we placed temperature sensors at 156 locations within pika habitats in the vicinity of 25 sites with historical records of pikas in the Basin. We related these time series of sensor data to data on ambient temperature from weather stations within the Historical Climate Network. We then used these highly correlated relationships, combined with long-term data from the same weather stations, to hindcast temperatures within pika habitats from 1945 through 2006. To explain patterns of loss, we posited three alternative classes of direct thermal stress: (1) acute cold stress (number of days below a threshold temperature); (2) acute heat stress (number of days above a threshold temperature); and (3) chronic heat stress (average summer temperature). Climate change was defined as change in our thermal metrics between two 31-yr periods: 1945-1975 and 1976-2006. We found that patterns of persistence were well predicted by metrics of climate. Our best models suggest some effects of climate change; however, recent and long-term metrics of chronic heat stress and acute cold stress, neither previously recognized as sources of stress for pikas, were some of the best predictors of pika persistence. Results illustrate that extremely rapid distributional shifts can be explained by climatic influences and have implications for conservation topics such as reintroductions and early-warning indicators.

Nutritional Requirements and Diet Choices of the Pygmy Rabbit (Brachylagus idahoensis): A Sagebrush Specialist

Sagebrush (Artemisia tridentata) comprises up to 99% of the winter and 50% of the summer diets of pygmy rabbits (Brachylagus idahoensis). Few animals specialize on such plants as sagebrush, which contain high levels of plant chemicals that can be toxic. We investigated the nutritional requirements of pygmy rabbits and their ability and propensity to consume sagebrush alone and as part of a mixed diet. We compared diet choices of pygmy rabbits with that of a generalist forager, the eastern cottontail (Sylvilagus floridanus). Pygmy rabbits had a moderately low nitrogen requirement (306.5 mg N/kg(0.75)/d), but a relatively high energy requirement, needing 750.8 kJ digestible energy/kg(0.75)/d to maintain their body mass while residing in small cages. They digested fiber in pelleted diets similarly to other small hindgut fermenters, but both cottontails and pygmy rabbits digested the fiber in sagebrush better than expected based on its indigestible acid detergent lignin content. Pygmy rabbits consumed more sagebrush than cottontails, regardless of the amount and nutritional quality of supplemental pellets provided. When consuming sagebrush alone, they ate barely enough to meet their energy requirements, whereas cottontails ate only enough sagebrush to meet 67% of theirs. Both rabbit species ate more sagebrush as the quality and quantity of supplemental pellets declined, and more greenhouse-grown sagebrush than sagebrush grown outside. Urine was more acidic when consuming sagebrush than when consuming pellets, indicating detoxification by the liver. Pygmy rabbits do not require sagebrush to survive, but seem to tolerate terpenes and other plant chemicals in sagebrush better than cottontails do.

The influence of climate on the basal metabolic rate of small mammals: a slow-fast metabolic continuum

The influence of climate (mean annual rainfall, rainfall variability, ambient temperature, T(a)) on the basal metabolic rate (BMR) of 267 small mammals (<1 kg) from six zoogeographical zones was investigated using conventional and phylogenetically independent data (linear contrasts). All climate variables varied between zones, as did BMR and body temperature ( T(b)), but not thermal conductance. Holarctic zones were more seasonal and colder, but rainfall was less variable, than non-Holarctic zones. In general, the BMR was most strongly influenced by body mass, followed by T(a) and the rainfall variables. However, there was significant variation in the strength of these relationships between zones. BMR and T(b) increased with latitude, and mass-independent BMR and T(b) were positively correlated. The latter relationship offers evidence of a slow-fast metabolic continuum in small mammals. The fast end of the continuum (high BMR) is associated with the highest latitudes where BMR is most strongly influenced by T(a) and mean annual rainfall (i.e. mean productivity). The slow end of the continuum (low BMR) is associated with the semi-tropics, low productivity zones, and climatically unpredictable zones, such as deserts. Here rainfall variability has the strongest influence on BMR after body size. The implications of a slow-fast metabolic continuum are discussed in terms of various models associated with the evolution of BMR, such as the aerobic capacity models and the "energetic definition of fitness" models.