California Ground Squirrels Research Articles

Enhancing biodiversity: historical ecology and biogeography of the Santa Catalina Island ground squirrel, Otospermophilus beecheyi nesioticus.

People have influenced Earth's biodiversity for millennia, including numerous introductions of domestic and wild species to islands. Here, we explore the origins and ecology of the Santa Catalina Island ground squirrel (SCIGS; Otospermophilus beecheyi nesioticus), one of only five endemic terrestrial mammals found on California's Santa Catalina Island. We synthesized all records of archaeological/palaeontological SCIGS, conducted radiocarbon dating and stable isotope analysis of the potentially earliest SCIGS remains and performed genetic analysis of modern SCIGS. Squirrels were not identified in island palaeontological deposits, but at least 12 island archaeological sites contain SCIGS bones, including some that are butchered or burned. All directly dated SCIGS bones are Late Holocene in age and younger than approximately 1290 cal BP. The first mitochondrial genome for modern Otospermophilus beecheyi and 15 modern SCIGS mitogenomes document at least one introduction of squirrels. Stable isotope data indicate variable SCIGS diets and potential subsidies from marine environments to terrestrial plants consumed by some individuals. We cannot rule out a natural overwater dispersal, but the earliest SCIGS remains post-date the earliest evidence for people by several millennia and, along with other lines of evidence, support a human-assisted translocation of squirrels during the Late Holocene. These data illustrate the important role of Indigenous people in shaping and enhancing island biodiversity and ecology around the world.

Burrowing Owls Require Mutualist Species and Ample Interior Habitat Space

Mitigating habitat loss of western burrowing owls (Athene cunicularia hypugaea) often involves relocation from California ground squirrel (Otospermophilus beecheyi) burrows to offsite nest boxes. Naval Air Station Lemoore (NASL), Kings and Fresno counties, California, initiated this approach to displace a regionally important population from airfield grasslands. We examined monitoring data of burrowing owls and fossorial mammals at NASL to assess mitigation options. Occupied nests increased by 33 (61%), with 47 nest box installations in 1997–2001, peaked at 87 in 1999, then declined by 50 through 2013. Although ≥13 nest boxes were occupied in 2000, none were occupied in 2003–2013. Within a 43.1 ha isolated grassland monitored for 13 years, nest site reuse in ground squirrel burrows averaged only 17% between any 2 consecutive years. Compared to the average density across grassland study areas, ground squirrel burrow systems/ha numbered 43% higher within 60 m of occupied nests and non-breeding-season burrows. Vegetation clearing to restore kangaroo rat (Dipodomys n. nitratoides) habitat preceded a 7.4-fold increase in ground squirrel burrow systems and a 4-fold increase in occupied nests, but drought-induced extirpation of ground squirrels eliminated occupied nests from the 43.1 ha grassland study area. Ground cover near occupied nests averaged 58% of the mean vegetation height and 67% of the mean percentage of bare ground in the field. Both nest sites and non-breeding-season burrows occurred >60 m interior to field edges 1.4 times more than expected. Non-breeding-season burrows averaged 328 m from same-year nest sites, and only 7% of non-breeding-season burrows were also used as nest sites. Mitigating habitat loss should be made more effective by fostering natural burrow construction by fossorial mammals on patches of short-stature vegetation that is sufficiently expansive to support breeding colonies of ≥12 pairs averaging ≥60 m from the field’s edge and a separation between non-breeding-season burrows and nest burrows minimally equal to mean nearest-neighbor distances among nests.

Comparison of San Joaquin kit fox den and California ground squirrel burrow attributes

Endangered San Joaquin kit foxes (Vulpes macrotis mutica; SJKF) and California ground squirrels (Otospermophilus beecheyi; CAGS) occur sympatrically in many locations. CAGS can constitute a nuisance species and control strategies have included lethal measures administered within CAGS burrows. These measures could harm or kill a SJKF if mistakenly applied to an occupied SJKF den. To identify attributes to distinguish between SJKF dens and CAGS burrow, we assessed dimensions, penetration depths by 9.7-cm and 7.5-cm spheres, ejecta patterns, and the presence of various types of sign at 65 dens to which radiocollared SJKF were tracked and at 80 burrows that CAGS were observed to enter. Mean entrance height, width, and circumference all were significantly larger for SJKF den entrances. However, the ranges of values for all dimensions for the CAGS burrows completely encompassed the ranges for the SJKF dens. Penetration depths with the 7.5-cm sphere were similar for SJKF dens and CAGS burrows. The 9.7-cm sphere could not be inserted in over half of CAGS burrows and exceeded 1 m in depth in over half of the SJKF dens. Most SJKF dens had dirt berms but so did over half of the CAGS burrows. SJKF scats, prey remains, CAGS scats, and trash were observed at both SJKF dens and CAGS burrows although at different frequencies. CAGS tracks were only observed at seven CAGS burrows. None of the attributes we assessed provided unequivocal criteria for distinguishing between SJKF dens and CAGS burrows. Also, SJKF occasionally usurp CAGS burrows and CAGS occasionally move into vacant SJKF dens. Therefore, administering lethal control measures within burrows should be avoided as it presents too great a risk to SJKF as well as the many other species that also use CAGS burrows.

Otospermophilus douglasii (Rodentia: Sciuridae)

Abstract Otospermophilus douglasii (Richardson, 1829), the Douglas ground squirrel, was first described as a species and then relegated to a subspecies of the California ground squirrel, O. beecheyi (Richardson, 1829). However, genetic data recently were interpreted as indicative of returning O. douglasii back to valid species status. This semi-fossorial, social species is brown with small white spots, a dark dorsal patch, and silvery shoulders. Occupying diverse habitats and elevations, this native species is an important prey species and is most abundant in grasslands and open woodlands. It forages primarily on seeds and vegetation but is also in conflict with humans as a crop pest and disease vector that damages infrastructure through creating elaborate burrow systems. Currently, the International Union for Conservation of Nature has not provided a conservation assessment for O. douglasii.

Sensory integration of danger and safety cues may explain the fear of a quiet coyote.

Sensory integration theory predicts natural selection should favour adaptive responses of animals to multiple forms of information, yet empirical tests of this prediction are rare, particularly in free-living mammals. Studying indirect predator cues offers a salient opportunity to inquire about multimodal risk assessment and its potentially interactive effects on prey responses. Here we exposed California ground squirrels from two study sites (that differ in human and domestic dog activity) to acoustic and/or olfactory predator cues to reveal divergent patterns of signal dominance. Olfactory information most strongly predicted space use within the testing arena. That is, individuals, especially those at the human-impacted site, avoided coyote urine, a danger cue that may communicate the proximity of a coyote. By contrast, subjects allocated less time to risk-sensitive behaviours when exposed to acoustic cues. Specifically, although individuals were consistent in their behavioural responses across trials, 'quiet coyotes' (urine without calls) significantly increased the behavioural reactivity of prey, likely because coyotes rarely vocalize when hunting. More broadly, our findings highlight the need to consider the evolution of integrated fear responses and contribute to an emerging understanding of how animals integrate multiple forms of information to trade off between danger and safety cues in a changing world.

Effects of sample collection and storage methods on fecal bacterial diversity in California ground squirrels (Otospermophilus beecheyi)

Abstract Measures of fecal bacterial abundance and taxonomic composition are commonly used as proxies for gut microbial diversity in studies of free-living mammals. Because methods of sample collection and storage may affect measures of bacterial diversity, we evaluated the effects of several procedures on fecal bacterial diversity in a free-living population of California ground squirrels (Otospermophilus beecheyi). Replicate fecal samples from 12 adult female squirrels were collected either from the soil beneath traps in which individuals had been captured or from tubs placed under squirrels during handling. Samples were then either frozen immediately in liquid nitrogen or stored on ice for several hours before being transferred to a −80°C freezer. Sequencing of the bacterial 16s RNA gene revealed no differences among methods with regard to sequence read depth (number of sequences recovered per animal) or alpha (within-individual) diversity of bacterial taxa. While our collection-storage procedures had a significant effect on one of the metrics of beta (among-individual) bacterial diversity examined, this effect was small compared to that of individual identity. Date of sample collection affected alpha and beta diversity; samples collected only 1 week apart differed significantly in bacterial diversity. Overall, these findings suggest that the collection and storage methods employed yield generally comparable information and are capable of capturing potentially important patterns of fecal bacterial composition and diversity in free-living mammals.

Short-term social dynamics following anthropogenic and natural disturbances in a free-living mammal

AbstractAnthropogenic disturbances are widely recognized for their far-reaching consequences on the survival and reproduction of wildlife, but we understand comparatively little about their effects on the social lives of group-living animals. Here we examined these short-term changes in affiliative behavior as part of a long-term study on a human-tolerant and socially flexible population of California ground squirrels (Otospermophilus beecheyi). We used social network analysis to examine short-term changes in affiliative behavior and individual consistency in response to disturbances by humans, domestic dogs, or a natural predator (the coyote). Overall, juveniles were more involved than adults in affiliative interactions, but the short-term directional effects of these acute disturbances on social cohesion varied by disturbance type. Human and dog presence reduced aboveground connectivity, particularly for juveniles, whereas disturbances by coyotes generally promoted it. Beyond these effects, we also detected non-random responses to disturbances, though individuals were not very consistent in their directional response to different disturbance types. Our results demonstrate the flexible changes in social behavior triggered by short-term disturbances imposed by humans and other threats. More generally, our findings elucidate the underappreciated sensitivity of animal social interactions to short-term ecological disturbances, raising key questions about their consequences on the social lives of animals.

Genetic characterization of potential venom resistance proteins in California ground squirrels (Otospermophilus beecheyi) using transcriptome analyses.

Understanding the molecular basis of adaptations in coevolving species requires identifying the genes that underlie reciprocally selected phenotypes, such as those involved in venom in snakes and resistance to the venom in their prey. In this regard, California ground squirrels (CGS; Otospermophilus beecheyi) are eaten by northern Pacific rattlesnakes (Crotalus oreganus oreganus), but individual squirrels may still show substantial resistance to venom and survive bites. A recent study using proteomics identified venom interactive proteins (VIPs) in the blood serum of CGS. These VIPs represent possible resistance proteins, but the sequences of genes encoding them are unknown despite the value of such data to molecular studies of coevolution. To address this issue, we analyzed a de novo assembled transcriptome from CGS liver tissue-where many plasma proteins are synthesized-and other tissues from this species. We then examined VIP sequences in terms of three characteristics that identify them as possible resistance proteins: evidence for positive selection, high liver expression, and nonsynonymous variation across CGS populations. Based on these characteristics, we identified five VIPs (i.e., α-2-macroglobulin, α-1-antitrypsin-like protein GS55-LT, apolipoprotein A-II, hibernation-associated plasma protein HP-20, and hibernation-associated plasma protein HP-27) as the most likely candidates for resistance proteins among VIPs identified to date. Four of these proteins have been previously implicated in conferring resistance to the venom in mammals, validating our approach. When combined with the detailed information available for rattlesnake venom proteins, these results set the stage for future work focused on understanding coevolutionary interactions at the molecular level between these species.

Estimating Reduction in Standing Crop Biomass from California Ground Squirrels in Central California Rangelands

California ground squirrels (Otospermophilus spp.) cause more economic damage to California rangelands than any other rodent. Damage comes in many forms, although forage loss is typically the greatest concern. These losses are believed to be significant for ranchers, particularly given the economically marginal environment in which they exist, yet our understanding of these economic losses is limited. Furthermore, current public opinion is often not supportive toward ground squirrel control on many public grazing lands. Information on the damage that ground squirrels cause to rangelands may be needed to justify management actions in the future. Therefore, we evaluated the amount of standing crop removed by California ground squirrels across 16 sites at four different ground squirrel density categories in central California rangelands from 2019 through 2020. We also included precipitation and livestock grazing intensity to help account for their potential effect on forage production. We found that ground squirrel abundance negatively affected standing crop biomass, with available forage reduced by 27.2 kg ha−1 per individual ground squirrel at the end of the growing season. Likewise, precipitation influenced standing crop, with each cm of precipitation yielding a 16.6 kg ha−1 increase in available forage. We did not observe any effect of livestock grazing intensity, an interaction between livestock grazing intensity and ground squirrel abundance, or an interaction between precipitation and ground squirrel abundance on residual standing crop. Collectively, this information will be useful to ranchers to help determine when control efforts may be needed for California ground squirrels given relative abundance of ground squirrels on their rangeland properties.

MELANISTIC CALIFORNIA GROUND SQUIRRELS (OTOSPERMOPHILUS BEECHEYI)

Although melanism is common in some species of North American squirrels, its occurrence in the California ground squirrel (Otospermophilus beecheyi) was only anecdotal and reported in the early 20th century. Here we report on melanistic individuals in this species in northwestern Baja California in 2013 and 2020.

Behavioural correlations across multiple stages of the antipredator response: do animals that escape sooner hide longer?

A fundamental assumption in predator–prey ecology is that prey responses comprise two main stages: escape when attack occurs or appears imminent and avoid the threat by seeking refuge until it has passed. While numerous studies have examined either initial prey responses to an approaching predator (flight initiation distance, FID), or subsequent hiding behaviour (e.g. latency to resume activity), to our knowledge, no previous studies have repeatedly tested multiple individuals in nature to quantify whether initial escape tendencies, behaviour during the escape and latency to resume activity are repeatable, and whether these stages of the antipredator response are correlated. The goal of this study was to explore how consistent spatial differences in rates of human activity shape risk-sensitive behaviour throughout multiple steps of the antipredator response (to humans) in California ground squirrels, Otospermophilus beecheyi , tested in various group sizes and environmental contexts across time. Our study provides the first example showing that, as predicted: FIDs, latencies to resume activity and other post-FID aspects of prey responses were repeatable and positively correlated at the among-individual level. This correlation is ecologically important in that it provides an underlying mechanism for a trade-off involving not only the cost versus benefit of early versus late escape, or early versus late emergence from refuge, but for a trade-off based on variation in fearfulness expressed across stages. Furthermore, we found that human activity influenced some, but not all, stages of the antipredator response. • Prey face trade-offs across multiple stages of the antipredator response. • We explore repeatability of antipredator responses and correlations between stages. • Human activity influenced some, but not all, stages of the antipredator response. • Multiple stages of the response were repeatable and highly correlated. • Our study offers insights into links between animal behaviour, humans and ecology.

Breeding Density and Collision Mortality of Loggerhead Shrike (Lanius ludovicianus) in the Altamont Pass Wind Resource Area

Loggerhead shrike (Lanius ludovicianus) has declined across most or all of its geographic range. The species’ raptorial behavior requires maintenance of large territories, which means populations of breeding shrikes require large areas of habitat and are therefore sensitive to habitat loss and habitat fragmentation. We estimated breeding densities of loggerhead shrikes in the Altamont Pass Wind Resource Area (APWRA), California, where annual shrike mortality caused by wind turbine collisions was high until just before our study began in 2016. Based on surveys across an annual average 50 randomized sampling plots in 2016–2019, we estimated an average 129 breeding pairs/year across the 167.6-km2 APWRA. Relative to the size of the study area, density in the APWRA was relatively high compared to densities reported from other study sites across North America. It was higher than predicted by application of the Partners in Flight estimator, which was based on Breeding Bird Surveys along roads. We also found that loggerhead shrikes in the APWRA were limited by the availability of nest substrate and by California ground squirrels (Otospermophilus beecheyi) and their burrow complexes, which have keystone effects on vegetation and wildlife in the APWRA. To most effectively conserve loggerhead shrikes in the APWRA, wind turbine mortality should be minimized, ground squirrels conserved instead of eradicated as pests, and appropriate trees and shrubs cultivated where they are needed.

Phenotypic and functional variation in venom and venom resistance of two sympatric rattlesnakes and their prey.

Predator-prey interactions often lead to the coevolution of adaptations associated with avoiding predation and, for predators, overcoming those defences. Antagonistic coevolutionary relationships are often not simple interactions between a single predator and prey but rather a complex web of interactions between multiple coexisting species. Coevolution between venomous rattlesnakes and small mammals has led to physiological venom resistance in several mammalian taxa. In general, viperid venoms contain large quantities of snake venom metalloproteinase toxins (SVMPs), which are inactivated by SVMP inhibitors expressed in resistant mammals. We explored variation in venom chemistry, SVMP expression, and SVMP resistance across four co-distributed species (California Ground Squirrels, Bryant's Woodrats, Southern Pacific Rattlesnakes, and Red Diamond Rattlesnakes) collected from four different populations in Southern California. Our aim was to understand phenotypic and functional variation in venom and venom resistance in order to compare coevolutionary dynamics of a system involving two sympatric predator-prey pairs to past studies that have focused on single pairs. Proteomic analysis of venoms indicated that these rattlesnakes express different phenotypes when in sympatry, with Red Diamonds expressing more typical viperid venom (with a diversity of SVMPs) and Southern Pacifics expressing a more atypical venom with a broader range of non-enzymatic toxins. We also found that although blood sera from both mammals were generally able to inhibit SVMPs from both rattlesnake species, inhibition depended strongly on the snake population, with snakes from one geographic site expressing SVMPs to which few mammals were resistant. Additionally, we found that Red Diamond venom, rather than woodrat resistance, was locally adapted. Our findings highlight the complexity of coevolutionary relationships between multiple predators and prey that exhibit similar offensive and defensive strategies in sympatry.

Host adaptation to novel pathogen introduction: Predicting conditions that promote evolutionary rescue.

Novel pathogen introduction can have drastic consequences for naive host populations, and outcomes can be difficult to predict. Evolutionary rescue (ER) provides a foundation for understanding whether hosts are driven to extinction or survive via adaptation. Currently, patterns of host population dynamics alongside evidence of adaptation are used to infer ER. However, the gap between established ER theory and complexity inherent in natural systems makes interpreting empirical patterns difficult because they can be confounded with ecological drivers of survival under current theory. To bridge this gap, we expand ER theory to include biological selective agents, such as pathogens. We find birth processes to be more important than previously theorised in determining ER potential. We employ a novel framework evaluating ER potential within natural systems and gain ability to identify system characteristics that make ER possible. Identifying these characteristics allows a shift from retrospective observation to a predictive mindset, and our findings suggest that ER occurrence may be more limited than previously thought. We use the plague system of Yersinia pestis infecting Cynomys ludovicianus (black-tailed prairie dogs) and Spermophilus beecheyi (California ground squirrels) as a case study.

Conservation of threatened San Joaquin antelope squirrels: distribution surveys, habitat suitability, and conservation recommendations

The San Joaquin antelope squirrel (Ammospermophilus nelsoni: SJAS) is listed as Threatened pursuant to the California Endangered Species Act due to profound habitat loss throughout its range in the San Joaquin Desert in California. Habitat loss is still occurring and critical needs for SJAS include identifying occupied sites, quantifying optimal habitat conditions, and conserving habitat. Our objectives were to (1) conduct surveys to identify sites where SJAS were present, (2) assess habitat attributes on all survey sites, (3) generate a GIS-based model of SJAS habitat suitability, (4) use the model to determine the quantity and quality of remaining habitat, and (5) use these results to develop conservation recommendations. SJAS were detected on 160 of the 326 sites we surveyed using automated camera stations. Sites with SJAS typically were in arid upland shrub scrub communities where desert saltbush (Atriplex polycarpa) or jointfir (Ephedra californica) were the dominant shrubs, although shrubs need not be present for SJAS to be present. Sites with SJAS usually had relatively sparse ground cover with >10% bare ground and Arabian grass (Schismus arabicus) was the dominant grass. SJAS were more likely to occur on sites where kangaroo rats (Dipodomys spp.) were present and burrow abundance was greater, but SJAS were less likely to be present on sites with California ground squirrels (Otospermophilus beecheyi). Based on our habitat suitability model, an estimated 5,931 km2 of high or moderately high quality habitat and 4,753 km2 of lower quality habitat remain. To conserve SJAS, we recommend (1) conducting additional SJAS surveys on sites not surveyed but with suitable habitat, (2) conserving unprotected lands with suitable habitat, (3) managing vegetation on occupied sites if necessary, (4) restoring disturbed lands to increase suitability for SJAS, and (5) conducting translocations of SJAS to unoccupied sites with suitable habitat.

Potential impact of diphacinone application strategies on secondary exposure risk in a common rodent pest: implications for management of California ground squirrels

Anticoagulant rodenticides are a common tool used to manage rodents in agricultural systems, but they have received increased scrutiny given concerns about secondary exposure in non-target wildlife. Rodenticide application strategy is one factor that influences exposure risk. To understand the impact of application strategy, we tested residues of a first-generation anticoagulant (diphacinone) in liver tissue of radiotransmittered California ground squirrels (Otospermophilus beecheyi) following spot treatments, broadcast applications, and bait station applications in rangelands in central California during summer and autumn 2018–2019. We also documented the amount of bait applied, the mean time from bait application until death, and the proportion of ground squirrels that died belowground. We documented the greatest amount of bait applied via bait stations and the least by broadcast applications. We did not document a difference in diphacinone residues across any application strategy, although survivors had an order of magnitude lower concentration of diphacinone than mortalities, potentially lowering secondary exposure risk. We did not observe any difference among bait delivery methods in time from bait application to death, nor did we identify any impact of seasonality on any of the factors we tested. The vast majority of mortalities occurred belowground (82–91%), likely reducing secondary exposure. Secondary exposure could be further reduced by daily carcass searches. Results from this study better define risk associated with first-generation anticoagulant rodenticide applications, ultimately assisting in development of management programs that minimize non-target exposure.

Diverse Genotypes and Species of Cryptosporidium in Wild Rodent Species from the West Coast of the USA and Implications for Raw Produce Safety and Microbial Water Quality.

Cryptosporidium spp. are protozoan parasites that infect perhaps all vertebrate animals, with a subset of species and genotypes that function as food- and waterborne pathogens. The objective of this work was to collate the Cryptosporidium species and genotypes from common wild rodents on the west coast of the USA and update the information regarding the zoonotic potential of Cryptosporidium from these ubiquitous wild species. Representative sequences of the 18S rRNA gene for a unique set of Cryptosporidium isolates obtained from deer mice, house mice, mountain beavers, yellow-bellied marmot, long-tailed vole, California ground squirrels, Belding’s ground squirrels, and a golden-mantled ground squirrel in GenBank were selected for phylogenetic analysis. Phylogenetic and BLAST analysis indicated that 4 (18%) of the 22 unique Cryptosporidium sequences from these wild rodent species were 99.75% to 100% identical to known zoonotic species (C. parvum, C. ubiquitum, C. xiaoi), suggesting that a minority of these representative Cryptosporidium isolates could have a public health impact through food and waterborne routes of human exposure. These zoonotic isolates were shed by deer mice and a yellow-bellied marmot from California, and from a mountain beaver trapped in Oregon. In addition, the group of unique Cryptosporidium isolates from deer mice and ground dwelling squirrels exhibited considerable DNA diversity, with multiple isolates appearing to be either host-limited or distributed throughout the various clades within the phylogenetic tree representing the various Cryptosporidium species from host mammals. These results indicate that only a subset of the unique Cryptosporidium genotypes and species obtained from wild rodents on the US west coast are of public health concern; nevertheless, given the geographic ubiquity of many of these host species and often high density at critical locations like municipal watersheds or produce production fields, prudent pest control practices are warranted to minimize the risks of water- and foodborne transmission to humans.

Host traits, identity, and ecological conditions predict consistent flea abundance and prevalence on free-living California ground squirrels

Understanding why some individuals are more prone to carry parasites and spread diseases than others is a key question in biology. Although epidemiologists and disease ecologists increasingly recognize that individuals of the same species can vary tremendously in their relative contributions to the emergence of diseases, very few empirical studies systematically assess consistent individual differences in parasite loads within populations over time. Two species of fleas (Oropsylla montana and Hoplopsyllus anomalous) and their hosts, California ground squirrels (Otospermophilus beecheyi), form a major complex for amplifying epizootic plague in the western United States. Understanding its biology is primarily of major ecological importance and is also relevant to public health. Here, we capitalize on a long-term data set to explain flea incidence on California ground squirrels at Briones Regional Park in Contra Costa County, USA. In a 7 year study, we detected 42,358 fleas from 2,759 live trapping events involving 803 unique squirrels from two free-living populations that differed in the amount of human disturbance in those areas. In general, fleas were most abundant and prevalent on adult males, on heavy squirrels, and at the pristine site, but flea distributions varied among years, with seasonal conditions (e.g., temperature, rainfall, humidity), temporally within summers, and between flea species. Although on-host abundances of the two flea species were positively correlated, each flea species occupied a distinctive ecological niche. The common flea (O. montana) occurred primarily on adults in cool, moist conditions in early summer whereas the rare flea (H. anomalous) was mainly on juveniles in hot, dry conditions in late summer. Beyond this, we uncovered significantly repeatable and persistent effects of host individual identity on flea loads, finding consistent individual differences among hosts in all parasite measures. Taken together, we reveal multiple determinants of parasites on free-living mammals, including the underappreciated potential for host heterogeneity – within populations – to structure the emergence of zoonotic diseases such as bubonic plague.

Physiological Stress Integrates Resistance to Rattlesnake Venom and the Onset of Risky Foraging in California Ground Squirrels.

Using venom for predation often leads to the evolution of resistance in prey. Understanding individual variation in venom resistance is key to unlocking basic mechanisms by which antagonistic coevolution can sustain variation in traits under selection. For prey, the opposing challenges of predator avoidance and resource acquisition often lead to correlated levels of risk and reward, which in turn can favor suites of integrated morphological, physiological and behavioral traits. We investigate the relationship between risk-sensitive behaviors, physiological resistance to rattlesnake venom, and stress in a population of California ground squirrels. For the same individuals, we quantified foraging decisions in the presence of snake predators, fecal corticosterone metabolites (a measure of “stress”), and blood serum inhibition of venom enzymatic activity (a measure of venom resistance). Individual responses to snakes were repeatable for three measures of risk-sensitive behavior, indicating that some individuals were consistently risk-averse whereas others were risk tolerant. Venom resistance was lower in squirrels with higher glucocorticoid levels and poorer body condition. Whereas resistance failed to predict proximity to and interactions with snake predators, individuals with higher glucocorticoid levels and in lower body condition waited the longest to feed when near a snake. We compared alternative structural equation models to evaluate alternative hypotheses for the relationships among stress, venom resistance, and behavior. We found support for stress as a shared physiological correlate that independently lowers venom resistance and leads to squirrels that wait longer to feed in the presence of a snake, whereas we did not find evidence that resistance directly facilitates latency to forage. Our findings suggest that stress may help less-resistant squirrels avoid a deadly snakebite, but also reduces feeding opportunities. The combined lethal and non-lethal effects of stressors in predator–prey interactions simultaneously impact multiple key traits in this system, making environmental stress a potential contributor to geographic variation in trait expression of toxic predators and resistant prey.

The molecular basis of venom resistance in a rattlesnake-squirrel predator-prey system.

Understanding how interspecific interactions mould the molecular basis of adaptations in coevolving species is a long-sought goal of evolutionary biology. Venom in predators and venom resistance proteins in prey are coevolving molecular phenotypes, and while venoms are highly complex mixtures it is unclear if prey respond with equally complex resistance traits. Here, we use a novel molecular methodology based on protein affinity columns to capture and identify candidate blood serum resistance proteins ("venom interactive proteins" [VIPs]) in California Ground Squirrels (Otospermophilus beecheyi) that interact with venom proteins from their main predator, Northern Pacific Rattlesnakes (Crotalus o. oreganus). This assay showed that serum-based resistance is both population- and species-specific, with serum proteins from ground squirrels showing higher binding affinities for venom proteins of local snakes compared to allopatric individuals. Venom protein specificity assays identified numerous and diverse candidate prey resistance VIPs but also potential targets of venom in prey tissues. Many specific VIPs bind to multiple snake venom proteins and, conversely, single venom proteins bind multiple VIPs, demonstrating that a portion of the squirrel blood serum "resistome" involves broad-based inhibition of nonself proteins and suggests that resistance involves a toxin scavenging mechanism. Analyses of rates of evolution of VIP protein homologues in related mammals show that most of these proteins evolve under purifying selection possibly due to molecular constraints that limit the evolutionary responses of prey to rapidly evolving snake venom proteins. Our method represents a general approach to identify specific proteins involved in co-evolutionary interactions between species at the molecular level.