Effects of Wildfire on the Thermal Ecology of Mojave Desert Tortoises (Gopherus agassizii)

Two tortoise species native to the American southwest have experienced significant habitat loss from development and are vulnerable to ongoing threats associated with continued development. Mojave desert tortoises Gopherus agassizii are listed as threatened under the US Endangered Species Act, and Sonoran desert tortoises G. morafkai are protected in Arizona (USA) and Mexico. Substantial habitat for both species occurs on multiple-use public lands, where development associated with traditional and renewable energy production, recreation, and other activities is likely to continue. Our goal was to quantify development to inform and evaluate actions implemented to protect and manage desert tortoise habitat. We quantified a landscape-level index of development across the Mojave and Sonoran desert tortoise ranges using models of potential habitat for each species (152485 total observations). We used 13 years of Mojave desert tortoise monitoring data (4732 observations) to inform the levels and spatial scales at which tortoises may be affected by development. Most (66-70%) desert tortoise habitat has some development within 1 km. Development levels on desert tortoise habitat are lower inside versus outside areas protected by actions at national, state, and local levels, suggesting that protection efforts may be having the desired effects and providing a needed baseline for future effectiveness evaluations. Of the relatively undeveloped desert tortoise habitat, 43% (74030 km2) occurs outside of existing protections. These lands are managed by multiple federal, state, and local entities and private landowners, and may provide opportunities for future land acquisition or protection, including as mitigation for energy development on public lands.

Mycoplasma agassizii is a common cause of upper respiratory tract disease in Mojave desert tortoises (Gopherus agassizii). So far, only two strains of this bacterium have been sequenced, and very little is known about its patterns of genetic diversity. Understanding genetic variability of this pathogen is essential to implement conservation programs for their threatened, long-lived hosts. We used next generation sequencing to explore the genomic diversity of 86 cultured samples of M. agassizii collected from mostly healthy Mojave and Sonoran desert tortoises in 2011 and 2012. All samples with enough sequencing coverage exhibited a higher similarity to M. agassizii strain PS6T (collected in Las Vegas Valley, Nevada) than to strain 723 (collected in Sanibel Island, Florida). All eight genomes with a sequencing coverage over 2x were subjected to multiple analyses to detect single-nucleotide polymorphisms (SNPs). Strikingly, even though we detected 1373 SNPs between strains PS6T and 723, we did not detect any SNP between PS6T and our eight samples. Our whole genome analyses reveal that M. agassizii strain PS6T may be present across a wide geographic extent in healthy Mojave and Sonoran desert tortoises.

We quantified aspects of hibernaculum use by desert tortoises (Gopherus agassizii) in the San Pedro Valley, Arizona. Tortoises hibernated primarily on steep south-facing slopes. Hibernacula included burrows in silt, silt with loose gravel, diatomite and/or diatomaceous marl, and beneath layers of well-lithified volcanic ash. Burrows were often also associated with live vegetation, dead and downed vegetation, and/or packrat (Neotoma albigula) nests. Male tortoises used longer hibernacula (x = 118.3 cm) than females (i = 24.4 cm). Maximum temperatures of hibernacula of females (x = 24.5 C) were consistently higher than maximum temperatures of hibernacula of males (k = 18.2 C), but the difference was not significant. Minimum temperatures of hibernacula of females (x = 4.3 C) were lower than minimum temperatures of hibernacula of males (x = 9.3 C). Temperatures in hibernacula of females fluctuated over a wider range than temperatures in hibernacula of males. Hibernacula used by males provided greater thermal buffering than those used by females. No tortoise (N = 8) used the same hibernaculum during both years of the study. Use of burrows and other shelters for thermoregulation and other purposes has been documented for all extant members of the genus Gopherus, and for other genera of land tortoises (Woodbury and Hardy, 1948; Mackay, 1964; Auffenberg, 1969; Rose and Judd, 1975; Douglas and Layne, 1978; Morafka, 1982; Geffen and Mendelssohn, 1989). Desert tortoises (Gopherus agassizii) use burrows throughout the year for thermoregulation, nesting, and protection from predators (Woodbury and Hardy, 1948; Auffenberg, 1969; Luckenbach, 1982; Vaughan, 1984; Barrett and Humphrey, 1986; Nagy and Medica, 1986; Barrett, 1990). Tortoises in the northern Mojave Desert remain below the surface in burrows approximately 98% of the year (Nagy and Medica, 1986). Although detailed data for Sonoran Desert populations are not yet available, those tortoises also spend considerable time within burrows (S. Bailey, unpubl. data; J. Snider, Arizona Game and Fish Dept., pers. comm.). Many reptiles hibernate and some species of turtles hibernate for more than half of their lives, but until recently this phenomenon remained poorly understood (Mayhew, 1965; Gregory, 1982; Ultsch, 1989). Little data concerning hibernation in desert tortoises is available, and quantitative data are generally limited to a few characteristics of hibernacula (e.g., dimensions) and duration of hibernation (Woodbury and Hardy, 1948; Auffenberg, 1969; Burge, 1977, 1978; Luckenbach, 1982; Vaughan, 1984; 2 Present Address: 3617 E. Third Street, Tucson, Arizona 85716, USA. Burge et al., 1989; Nagy and Medica, 1986; Lowe, 1990). During most years desert tortoises in the United States hibernate throughout the winter months (Nagy and Medica, 1986; pers. obs.). The majority of desert tortoises in the Mojave and Sonoran Deserts spend over 100 d (many over 150 and some over 200) per year in their hibernacula without exiting (Burge, 1977; Vaughan, 1984; Nagy and Medica, 1986). Male and female reptiles often select different thermal environments and differ in their use of habitat and shelter sites (Beauchat, 1986; Burger and Zappalorti, 1989; Sievert and Hutchison, 1989; Kaufmann, 1992; Brafia, 1993; Magnusson, 1993). In addition, some male and female reptiles are known to hibernate for different periods of time (Prestt, 1971; Gaffney and Fitzpatrick, 1973). Female desert tortoises in the Sonoran Desert seem to use shallow hibernacula more often than males and, thus, are observed more often in their shelters during the winter than males (C. Lowe and C. Schwalbe, unpubl. data). Our objectives were to quantify characteristics of hibernaculum use in a population of Sonoran desert tortoises to test the hypotheses that male and female desert tortoises differ in their use of overwintering sites and that thermal environments differ between hibernacula used by males and females. MATERIALS AND METHODS Study Area.-The study was conducted in the San Pedro River Valley (Lat. 32038'N, Long. 110?34'W), in Pinal County in southeastern Arizona. Vegetation was ecotonal between the Arizona Upland subdivision of the Sonoran DesThis content downloaded from 207.46.13.169 on Sat, 01 Oct 2016 05:31:35 UTC All use subject to http://about.jstor.org/terms

Excretion in fresh-water turtle (<i>Pseudemys scripta</i>) and desert tortoise (<i>Gopherus agassi</i>)

Predation on desert tortoises (Gopherus agassizii) by desert canids

A quantitative PCR method for assessing the presence of Pasteurella testudinis DNA in nasal lavage samples from the desert tortoise (Gopherus agassizii)

To elucidate ecological effects of variation in the temporal distribution of a limiting resource (water in the Mojave Desert), energetics of two free—living populations of desert tortoises (Gopherus [Xerobates] agassizii) were studied concurrently over 18 mo with use of doubly—labeled water. Field metabolic rates (FMR) and feeding rates (estimated from rates of water influx and rates of change in dry mass) were highly variable. This variability was manifested at several levels, including seasonal changes within populations, year—to—year differences within populations, and differences between populations. Underlying observed patterns and contrasts was considerable variation among individuals. Much of the variation in energetic variables was associated with a single climatic variable, rainfall. Seasonal, annual, and interpopulation differences in FMR and foraging rates corresponded to differences in availability of free—standing water from rainstorms. At least some of the differences among individuals were apparently due to differences in proclivity or ability to drink. Tortoises had very low FMRs relative to other reptiles, which allowed them to tolerate long periods of chronic energy shortage during a drought. Calculations suggested that tortoises experienced a net loss of energy on their spring diet of succulent annual plants. If so, tortoises require drier forage to accrue an energy profit, which emphasizes their reliance on drinking rainwater (which can be stored in the bladder and resorbed later to hydrate dry forage). Further, it suggests that growth (as protein deposition) and net acquisition of energy may be temporally decoupled in desert tortoises, which has potential consequences for geographic variation in life history traits. Energy acquisition and expenditure in desert tortoises are thus strongly constrained by the contingencies of rainfall, both indirectly through effects on availability and quality of food, and directly through reliance on free—standing water for drinking, which is apparently necessary for achieving a net annual energy profit.

Habitat has changed unfavorably during the past 150 y for the desert tortoise Gopherus agassizii, a federally threatened species with declining populations in the Mojave Desert and western Sonoran Desert. To support recovery efforts, we synthesized published information on relationships of desert tortoises with three habitat features (cover sites, forage, and soil) and candidate management practices for improving these features for tortoises. In addition to their role in soil health and facilitating recruitment of annual forage plants, shrubs are used by desert tortoises for cover and as sites for burrows. Outplanting greenhouse-grown seedlings, protected from herbivory, has successfully restored (&amp;gt;50% survival) a variety of shrubs on disturbed desert soils. Additionally, salvaging and reapplying topsoil using effective techniques is among the more ecologically beneficial ways to initiate plant recovery after severe disturbance. Through differences in biochemical composition and digestibility, some plant species provide better-quality forage than others. Desert tortoises selectively forage on particular annual and herbaceous perennial species (e.g., legumes), and forage selection shifts during the year as different plants grow or mature. Nonnative grasses provide low-quality forage and contribute fuel to spreading wildfires, which damage or kill shrubs that tortoises use for cover. Maintaining a diverse “menu” of native annual forbs and decreasing nonnative grasses are priorities for restoring most desert tortoise habitats. Reducing herbivory by nonnative animals, carefully timing herbicide applications, and strategically augmenting annual forage plants via seeding show promise for improving tortoise forage quality. Roads, another disturbance, negatively affect habitat in numerous ways (e.g., compacting soil, altering hydrology). Techniques such as recontouring road berms to reestablish drainage patterns, vertical mulching (“planting” dead plant material), and creating barriers to prevent trespasses can assist natural recovery on decommissioned backcountry roads. Most habitat enhancement efforts to date have focused on only one factor at a time (e.g., providing fencing) and have not included proactive restoration activities (e.g., planting native species on disturbed soils). A research and management priority in recovering desert tortoise habitats is implementing an integrated set of restorative habitat enhancements (e.g., reducing nonnative plants, improving forage quality, augmenting native perennial plants, and ameliorating altered hydrology) and monitoring short- and long-term indicators of habitat condition and the responses of desert tortoises to habitat restoration.

Effects of Post-Hatching Maintenance Temperature on Desert Tortoise (Gopherus agassizii) Shell Morphology and Thermoregulatory Behavior

Noninvasive methods for measuring fat reserves in both captive and free-ranging animals are important for monitoring individual and population health, but chelonian anatomy and physiology present challenges to accurate measurements. Standard field-based methods for assessing body condition in Mojave desert tortoises (Gopherus agassizii) involve the qualitative body condition score, which relies on the apparent height of the temporalis muscle relative to the sagittal crest (in addition to other characteristics) and quantitative body condition indices that measure relative mass at size. However, it is unclear how these metrics relate to body fat reserves in this species. The aims of this study were to (1) describe the use of noninvasive computed tomography in measuring body fat volume of Mojave desert tortoises, (2) describe the location of fat reserves, (3) investigate relationships between fat reserves and body condition score and body condition index, and (4) explore whether relative temporalis muscle depth, measured via computed tomography, correlates with body condition score. Body condition scores were assessed for eight captive Mojave desert tortoises prior to euthanasia, and computed tomography was performed postmortem to quantify fat volume and measure temporalis muscle depth. At necropsy, the distribution of fat was documented. Fat volume calculated by computed tomography ranged from 2.83 to 145.38 cm3 (0.07-2.5% body volume). Neither qualitative body condition score nor quantitative body condition index was correlated with fat volume. Bladder content did not compromise body condition index. Body condition score was not correlated with relative temporalis muscle depth. Computed tomography is a noninvasive method for successfully identifying fat reserves and estimating total fat volume in Mojave desert tortoises. The lack of a relationship between computed tomography-determined metrics and commonly used body condition metrics indicates that computed tomography fills a critical gap in the health assessment tool kit for captive and free-ranging Mojave desert tortoises.

Using data from six wild Mojave Desert Tortoise (Gopherus agassizii (Cooper, 1861)) populations, we quantified seasonal differences in immune system measurements and microbial load in the respiratory tract, pertinent to this species’ susceptibility to upper respiratory tract disease. We quantified bacteria-killing activity of blood plasma and differential leukocyte counts to detect trends in temporal variation in immune function. We used centered log-ratio (clr) transformations of leukocyte counts and stress that such transformations are necessary for compositional data. We tested animals for the potential pathogen Pasteurella testudinis Snipes and Biberstein, 1982 with a newly created quantitative polymerase chain reaction (qPCR) assay, as well as for the known respiratory pathogens Mycoplasma agassizii Brown et al., 2001 and Mycoplasma testudineum Brown et al., 2004. We found very little disease and suggest that P. testudinis is a prevalent, commensal microbe in these Mojave Desert Tortoise populations, and its quantification may be a tool to study natural fluctuations in microbe levels in Mojave Desert Tortoise respiratory tracts. Our analyses showed that both the potential for inflammatory responses and microbe levels are highest in the spring for healthy Mojave Desert Tortoises, when lymphocyte levels are lowest. The genetic and statistical tools that we used are easily applicable to other wildlife systems and provide the necessary data to quantify species-wide trends in health and test hypotheses pertinent to host–microbe dynamics.

Soft ticks in the genus Ornithodoros occur throughout the Mojave Desert in southern Nevada, southeastern California, and parts of southwestern Utah and northwestern Arizona, USA, and are frequently observed parasitizing Mojave desert tortoises (Gopherus agassizii). However, limited research exists examining the relationship between ticks and desert tortoises. Mojave desert tortoises are listed as threatened by the US Fish and Wildlife Service, and as such, their populations are monitored and individual tortoise health is routinely assessed. These health assessments document the presence and abundance of ticks present on tortoises, but detailed examination of the relationship between ticks and tortoise health has been lacking. This study analyzed the relationship between tick presence and desert tortoise health assessments as a function of season, location, age (adult vs. juvenile), foraging behavior, evidence of clinical signs of disease, body condition score, and sex. Our results indicate that more ticks were found on tortoises in the summer than in any other season. Ticks were observed more frequently on captive tortoises versus wild tortoises, and more ticks were likely to be present on adult tortoises than on juveniles. Ticks were also more likely to be observed on tortoises that lacked evidence of foraging and on tortoises with observed clinical signs of disease. These findings provide valuable insights into the biology of ticks in relation to tortoises that may be useful for management of both captive and free-living threatened tortoise populations where ticks are detected. Our study also may improve understanding of potential tick-borne disease dynamics in the Mojave desert tortoise habitat, including Borrelia sp. carried by Ornithodoros ticks, which cause tick-borne relapsing fever in people.

Desert tortoise (Gopherus agassizii) populations have experienced precipitous declines resulting from the cumulative impact of habitat loss, and human and disease-related mortality. Evaluation of hematologic and biochemical responses of desert tortoises to physiologic and environmental factors can facilitate the assessment of stress and disease in tortoises and contribute to management decisions and population recovery. The goal of this study was to obtain and analyze clinical laboratory data from free-ranging desert tortoises at three sites in the Mojave Desert (California, USA) between October 1990 and October 1995, to establish reference intervals, and to develop guidelines for the interpretation of laboratory data under a variety of environmental and physiologic conditions. Body weight, carapace length, and venous blood samples for a complete blood count and clinical chemistry profile were obtained from 98 clinically healthy adult desert tortoises of both sexes at the Desert Tortoise Research Natural area (western Mojave), Goffs (eastern Mojave) and Ivanpah Valley (northeastern Mojave). Samples were obtained four times per year, in winter (February/March), spring (May/June), summer (July/August), and fall (October). Years of near-, above- and below-average rainfall were represented in the 5 yr period. Minimum, maximum and median values, and central 95 percentiles were used as reference intervals and measures of central tendency for tortoises at each site and/or season. Data were analyzed using repeated measures analysis of variance for significant (P < 0.01) variation on the basis of sex, site, season, and interactions between these variables. Significant sex differences were observed for packed cell volume, hemoglobin concentration, aspartate transaminase activity, and cholesterol, triglyceride, calcium, and phosphorus concentrations. Marked seasonal variation was observed in most parameters in conjunction with reproductive cycle, hibernation, or seasonal rainfall. Year-to-year differences and long-term alterations primarily reflected winter rainfall amounts. Site differences were minimal, and largely reflected geographic differences in precipitation patterns, such that results from these studies can be applied to other tortoise populations in environments with known rainfall and forage availability patterns.

Following field observations of wild Agassiz's desert tortoises (Gopherus agassizii) with oral lesions similar to those seen in captive tortoises with herpesvirus infection, we measured the prevalence of antibodies to Testudinid herpesvirus (TeHV) 3 in wild populations of desert tortoises in California. The survey revealed 30.9% antibody prevalence. In 2009 and 2010, two wild adult male desert tortoises, with gross lesions consistent with trauma and puncture wounds, respectively, were necropsied. Tortoise 1 was from the central Mojave Desert and tortoise 2 was from the northeastern Mojave Desert. We extracted DNA from the tongue of tortoise 1 and from the tongue and nasal mucosa of tortoise 2. Sequencing of polymerase chain reaction products of the herpesviral DNA-dependent DNA polymerase gene and the UL39 gene respectively showed 100% nucleotide identity with TeHV2, which was previously detected in an ill captive desert tortoise in California. Although several cases of herpesvirus infection have been described in captive desert tortoises, our findings represent the first conclusive molecular evidence of TeHV2 infection in wild desert tortoises. The serologic findings support cross-reactivity between TeHV2 and TeHV3. Further studies to determine the ecology, prevalence, and clinical significance of this virus in tortoise populations are needed.

The desert tortoise, Gopherus agassizii, is a threatened species native to the North American desert southwest and is recognized as having genetically distinct Mojave and Sonoran desert populations. The Mojave Desert population is federally listed as threatened under the Endangered Species Act and the Sonoran Desert population is fully protected under Mexican and United States state laws. We identified nine dinucleotide STR loci in the desert tortoise and tested their efficacy in 80 samples from both the Sonoran and Mojave deserts. One locus exhibited low allelic variation (4 alleles) while seven were highly variable (8–16 alleles). One locus exhibited a unique allele in congeners (G. flavomarginatus and G. berlandieri).