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Global climate change is already contributing to the extirpation of numerous species worldwide, and sensitive species will continue to face challenges associated with rising temperatures throughout this century and beyond. It is especially important to evaluate the thermal ecology of endangered ectotherm species now so that mitigation measures can be taken as early as possible. A recent study of the thermal ecology of the federally endangered Blunt‐nosed Leopard Lizard (Gambelia sila) suggested that they face major activity restrictions due to thermal constraints in their desert habitat, but that large shade‐providing shrubs act as thermal buffers to allow them to maintain surface activity without overheating. We replicated this study and also included a population of G. sila with no access to large shrubs to facilitate comparison of the thermal ecology of G. sila populations in shrubless and shrubbed sites. We found that G. sila without access to shrubs spent more time sheltering inside rodent burrows than lizards with access to shrubs, especially during the hot summer months. Lizards from a shrubbed site had higher midday body temperatures and therefore poorer thermoregulatory accuracy than G. sila from a shrubless site, suggesting that greater surface activity may represent a thermoregulatory trade‐off for G. sila. Lizards at both sites are currently constrained from using open, sunny microhabitats for much of the day during their short active seasons, and our projections suggest that climate change will exacerbate these restrictions and force G. sila to use rodent burrows for shelter even more than they do now, especially at sites without access to shrubs. The continued management of shrubs and of burrowing rodents at G. sila sites is therefore essential to the survival of this endangered species., The Blunt‐Nosed Leopard Lizard is a federally endangered ectotherm adapted to California's San Joaquin Desert ecosystem. We compared a population of lizards who had access to shrubs to a population of lizards who had no access to shrubs in the Carrizo Plain National Monument, and found that lizards who do not have access to shrubs thermoregulated more accurately (unexpected), with a trade‐off of being forced underground into cooler burrows for more of the day compared with lizards who had access to shrubs.

Native-range thermal constraints may not reflect the geographical distributions of species introduced from native island ranges in part due to rapid physiological adaptation in species introduced to new environments. Correlative ecological niche models may thus underestimate potential invasive distributions of species from islands. The northern curly-tailed lizard (Leiocephalus carinatus) is established in Florida, including populations north of its native range. Competing hypotheses may explain this distribution: Thermal Matching (distribution reflects thermal conditions of the native range), Thermal Potential (species tolerates thermal extremes absent in the native range) and/or Thermal Flexibility (thermal tolerance reflects local thermal extremes). We rejected the Thermal Matching hypothesis by comparing ecological niche models developed from native vs. native plus invasive distributions; L. carinatus exists in areas of low suitability in Florida as predicted by the native-distribution model. We then compared critical thermal limits of L. carinatus from two non-native populations to evaluate the Thermal Potential and Flexibility hypotheses: one matching native range latitudes, and another 160 km north of the native range that experiences more frequent cold weather events. Critical thermal minima in the northern population were lower than in the south, supporting the Thermal Flexibility hypothesis, whereas critical thermal maxima did not differ.

Vertebrates harbour gut microbial communities containing hundreds of bacterial species, most of which have never been cultivated or isolated in the laboratory. The lack of cultured representatives from vertebrate gut microbiotas limits the description and experimental interrogation of these communities. Here, we show that representatives from >50% of the bacterial genera detected by culture-independent sequencing in the gut microbiotas of fence lizards, house mice, chimpanzees, and humans were recovered in mixed cultures from frozen faecal samples plated on a panel of nine media under a single growth condition. In addition, culturing captured >100 rare bacterial genera overlooked by culture-independent sequencing, more than doubling the total number of bacterial sequence variants detected. Our approach recovered representatives from 23 previously uncultured candidate bacterial genera, 12 of which were not detected by culture-independent sequencing. Results identified strategies for both indiscriminate and selective culturing of the gut microbiota that were reproducible across vertebrate species. Isolation followed by whole-genome sequencing of 161 bacterial colonies from wild chimpanzees enabled the discovery of candidate novel species closely related to the opportunistic pathogens of humans Clostridium difficile and Hungatella hathewayi. This study establishes culturing methods that improve inventories and facilitate isolation of gut microbiota constituents from a wide diversity of vertebrate species.

Most studies on how rising temperatures will impact terrestrial ectotherms have focused on single populations or multiple sympatric species. Addressing the thermal and energetic implications of climatic variation on multiple allopatric populations of a species will help us better understand how a species may be impacted by altered climates.We used eight years of thermal and behavioral data collected from four populations of Pacific rattlesnakes (Crotalus oreganus) living in climatically distinct habitat types (inland and coastal) to determine the field‐active and laboratory‐preferred body temperatures, thermoregulatory metrics, and maintenance energetic requirements of snakes from each population.Physical models showed that thermal quality was best at coastal sites, but inland snakes thermoregulated more accurately despite being in more thermally constrained environments. Projected increases of 1 and 2°C in ambient temperature result in an increase in overall thermal quality at both coastal and inland sites.Population differences in modeled standard metabolic rate estimates were driven by body size and not field‐active body temperature, with inland snakes requiring 1.6× more food annually than coastal snakes.All snakes thermoregulated with high accuracy, suggesting that small increases in ambient temperature are unlikely to impact the maintenance energetic requirements of individual snakes and that some species of large‐bodied reptiles may be robust to modest thermal perturbations under conservative climate change predictions. ​, We used long‐term thermal data collected from four populations of Pacific rattlesnakes (Crotalus oreganus) to determine and compare thermoregulatory metrics and maintenance energetic requirements of snakes living in climatically distinct habitats (coastal and inland). Thermal quality was best at coastal sites, but inland snakes thermoregulated more accurately despite being in more thermally constrained environments. Population differences in standard metabolic rate estimates were driven by body size rather than body temperature, with inland snakes requiring 1.6× more food annually than coastal snakes. ​

Understanding the mechanisms behind critical thermal maxima (CTmax; the high body temperature at which neuromuscular coordination is lost) of organisms is central to understanding ectotherm thermal tolerance. Body size is an often overlooked variable that may affect interpretation of CTmax, and consequently, how CTmax is used to evaluate mechanistic hypotheses of thermal tolerance. We tested the hypothesis that body size affects CTmax and its interpretation in two experimental contexts. First, in four Sceloporus species, we examined how inter- and intraspecific variation in body size affected CTmax at normoxic and experimentally induced hypoxic conditions, and cloacal heating rate under normoxic conditions. Negative relationships between body size and CTmax were exaggerated in larger species, and hypoxia-related reductions in CTmax were unaffected by body size. Smaller individuals had faster cloacal heating rates and higher CTmax, and variation in cloacal heating rate affected CTmax in the largest species. Second, we examined how body size interacted with the location of body temperature measurements (i.e., cloaca vs. brain) in Sceloporus occidentalis, then compared this in living and deceased lizards. Brain temperatures were consistently lower than cloacal temperatures. Smaller lizards had larger brain-cloacal temperature differences than larger lizards, due to a slower cloacal heating rate in large lizards. Both live and dead lizards had lower brain than cloacal temperatures, suggesting living lizards do not actively maintain lower brain temperatures when they cannot pant. Thermal inertia influences CTmax data in complex ways, and body size should therefore be considered in studies involving CTmax data on species with variable sizes.

Research on the thermal ecology and physiology of free-living organisms is accelerating as scientists and managers recognize the urgency of the global biodiversity crisis brought on by climate change. As ectotherms, temperature fundamentally affects most aspects of the lives of amphibians and reptiles, making them excellent models for studying how animals are impacted by changing temperatures. As research on this group of organisms accelerates, it is essential to maintain consistent and optimal methodology so that results can be compared across groups and over time. This review addresses the utility of reptiles and amphibians as model organisms for thermal studies by reviewing the best practices for research on their thermal ecology and physiology, and by highlighting key studies that have advanced the field with new and improved methods. We end by presenting several areas where reptiles and amphibians show great promise for further advancing our understanding of how temperature relations between organisms and their environments are impacted by global climate change.

Telemetry has revolutionized studies in wildlife biology, ecology, physiology, and conservation. With the increased demand for telemetry, new technology has made great strides to enable long studies in harsh and remote areas on a wide variety of study species. As the climate crisis continues to impact animals, temperature-sensing telemetry has become a helpful technique for understanding the effects of climate change and how to protect wildlife from them. However, temperature-sensing telemetry and telemetry in general still pose technological challenges and accessibility issues for the researchers who use it. Currently available telemetry technology is expensive, too large and heavy for many study species, and cannot measure all variables researchers want to study. These technological improvements have especially been neglected for temperature-sensing telemetry, which may be underutilized given the current climate crisis. To understand why innovation has stalled, and where it should be directed going forward, we gathered opinions from researchers who use telemetry and from manufacturers that create and supply telemetry equipment. Our goal was to broadly describe the current technological landscape, compare it to what we envision for the future, and make suggestions for how to reach that future.

Venom is an integral feeding trait in many animal species. Although venom often varies ontogenetically, little is known about the proximate physiological mediators of venom variation within individuals. The glucocorticoid hormone corticosterone (CORT) can alter the transcription and activation of proteins, including homologues of snake venom components such as snake venom metalloproteinases (SVMPs) and phospholipase A2 (PLA2). CORT is endogenously produced by snakes, varies seasonally and also in response to stress, and is a candidate endogenous mediator of changes in venom composition and functional activity. Here, we tested the hypothesis that CORT induces changes in snake venom by sampling the venom of wild adult rattlesnakes before and after they were treated with either empty (control) or CORT-filled (treatment) Silastic implants. We measured longitudinal changes in whole-venom composition, whole-venom total protein content, and enzymatic activity of SVMP and PLA2 components of venom. We also assessed the within-individual repeatability of venom components. Despite successfully elevating plasma CORT in the treatment group, we found no effect of CORT treatment or average plasma CORT level on any venom variables measured. Except for total protein content, venom components were highly repeatable within individuals ([Formula: see text]). Our results indicate that the effects of CORT, a hormone commonly associated with stress and metabolic functions, in adult rattlesnake venom are negligible. Our findings bode well for venom researchers and biomedical applications that rely on the consistency of venoms produced from potentially stressed individuals and provide an experimental framework for future studies of proximate mediators of venom variation across an individual's life span.

It is well-known that women are underrepresented in science, technology, engineering and mathematics (STEM), but the extent of this underrepresentation varies among STEM fields. Analyzing gender demographics of publications within a field is an effective means of quantifying representation because of the importance of publications to scientists' careers and to the scientific community. We created a data set consisting of all publications accessed with a database search on each taxonomic order of herpetofauna (Anura, Caudata, Gymnophiona, Testudines, Crocodylia, Rhynchocephalia, and Squamata) as well as squamate suborders (Amphisbaenia, Lacertilia, Serpentes) from 2010 to 2019, and another data set with all publications on Lacertilia and Serpentes from 1970 to 2019, and used these data sets to estimate the genders of authors. During the past decade, our estimates show that male authors outnumbered female authors 2.24:1, with especially low levels of authorship by females in studies on Gymnophiona, Crocodylia, and Squamata. However, female authorship increased steadily during the decade. While male first authors also outnumbered females 1.95:1, male last authors outnumbered females 3.30:1, and male sole authors outnumbered females 5.29:1. Papers with female first authors or last authors were more likely to have female coauthors than were papers with male first authors or last authors. Papers with female first authors were more likely to be cited than papers with male first authors, and papers with female last authors were less likely to be cited than those with male last authors. Finally, qualitative analysis of authorship estimates in studies on lizards and snakes over the past 50 yr show that female authors represented about 10–15% of authors from 1970 to 2000, followed by a rapid rise in female authorship over the past 20 yr to current rates of >30% female authorship. Our data suggest that the gender gap in herpetology, which has traditionally appeared to be a male-dominated field, is slowly narrowing.

Conspecific animals living in multiple habitats may utilize different behaviours to survive and thrive in their environments. The Pacific rattlesnake (Crotalus oreganus), a generalist pit viper species, lives in a myriad of habitat types. We hypothesized that populations inhabiting hot, inland habitats and those in cool, coastal habitats would exhibit different behavioural strategies, especially those related to thermoregulation. Additionally, we determined whether environmental factors could be used as predictors of certain behaviours. We recorded environmental and behavioural data while radio-tracking adult, male rattlesnakes from two inland sites and two coastal sites throughout their active season. We found significant differences in thermoregulatory behaviour, with coastal snakes found more frequently above ground and in an active state than inland snakes. Additionally, wind, cloud cover, and air temperature were significant predictors of several snake behaviours. These results suggest that rattlesnakes exhibit plasticity in thermoregulatory behaviour.

Vertebrates harbor trillions of microorganisms in the gut, collectively termed the gut microbiota, which affect a wide range of host functions. Recent experiments in lab-reared vertebrates have shown that changes in environmental temperature can induce shifts in the gut microbiota, and in some cases these shifts have been shown to affect host thermal physiology. However, there is a lack of information about the effects of temperature on the gut microbiota of wild-caught vertebrates. Moreover, in ectotherms, which are particularly vulnerable to changing temperature regimens, the extent to which microbiota composition is shaped by temperature and associated with host thermal tolerance has not been investigated. To address these issues, we monitored the gut microbiota composition of wild-caught western fence lizards (Sceloporus occidentalis) experimentally exposed to a cool-to-warm temperature transition. Comparing experimentally exposed and control lizards indicated that warm temperatures altered and destabilized the composition of the S. occidentalis gut microbiota. Warming drove a significant reduction in the relative abundances of a clade of Firmicutes, a significant increase in the rate of compositional turnover in the gut microbiota within individual lizards, and increases in the abundances of bacteria from predicted pathogenic clades. In addition, the composition of the microbiota was significantly associated with the thermal tolerance of lizards measured at the end of the experiment. These results suggest that temperature can alter the lizard gut microbiota, with potential implications for the physiological performance and fitness of natural populations. IMPORTANCE Gut microbial communities affect their animal hosts in numerous ways, motivating investigations of the factors that shape the gut microbiota and the consequences of gut microbiota variation for host traits. In this study, we tested the effects of increases in environmental temperatures on the gut microbiota of fence lizards, a vertebrate ectotherm threatened by warming climates. By monitoring lizards and their gut microbes during an experimental temperature treatment, we showed that the warming altered and destabilized the lizard gut microbiota. Moreover, measuring thermal performance of lizard hosts at the end of the experiment indicated that the composition of the gut microbiota was associated with host thermal tolerance. These results indicate that warming temperatures can alter the gut microbiota of vertebrate ectotherms and suggest relationships between variation in the gut microbiota and the thermal physiology of natural host populations.

Recognizing how climate change will impact populations can aid in making decisions about approaches for conservation of endangered species. The blunt-nosed leopard lizard (Gambelia sila) is a federally endangered species that, despite protection, remains in extremely arid, hot areas and may be at risk of extirpation due to climate change. We collected data on the field-active body temperatures, preferred body temperatures and upper thermal tolerance of G. sila. We then described available thermal habitat using biophysical models, which allowed us to (i) describe patterns in lizard body temperatures, microhabitat temperatures and lizard microhabitat use; (ii) quantify the lizards’ thermoregulatory accuracy; (iii) calculate the number of hours they are currently thermally restricted in microhabitat use; (iv) project how the number of restricted hours will change in the future as ambient temperatures rise; and (v) assess the importance of giant kangaroo rat burrows and shade-providing shrubs in the current and projected future thermal ecology of G. sila. Lizards maintained fairly consistent daytime body temperatures over the course of the active season, and use of burrows and shrubs increased as the season progressed and ambient temperatures rose. During the hottest part of the year, lizards shuttled among kangaroo rat burrows, shrubs, and open habitat to maintain body temperatures below their upper thermal tolerance, but, occasionally, higher than their preferred body temperature range. Lizards are restricted from staying in the open habitat for 75% of daylight hours and are forced to seek refuge under shrubs or burrows to avoid surpassing their upper thermal threshold. After applying climatic projections of 1 and 2°C increases to 2018 ambient temperatures, G. sila will lose additional hours of activity time that could compound stressors faced by this population, potentially leading to extirpation.

Thermal tolerance is an important variable in predictive models about the effects of global climate change on species distributions, yet the physiological mechanisms responsible for reduced performance at high temperatures in air-breathing vertebrates are not clear. We conducted an experiment to examine how oxygen affects three variables exhibited by ectotherms as they heat-gaping threshold, panting threshold, and loss of righting response (the latter indicating the critical thermal maximum)-in two lizard species along an elevational (and therefore environmental oxygen partial pressure) gradient. Oxygen partial pressure did not impact these variables in either species. We also exposed lizards at each elevation to severely hypoxic gas to evaluate their responses to hypoxia. Severely low oxygen partial pressure treatments significantly reduced the gaping threshold, panting threshold, and critical thermal maximum. Further, under these extreme hypoxic conditions, these variables were strongly and positively related to partial pressure of oxygen. In an elevation where both species overlapped, the thermal tolerance of the high elevation species was less affected by hypoxia than that of the low elevation species, suggesting the high elevation species may be adapted to lower oxygen partial pressures. In the high elevation species, female lizards had higher thermal tolerance than males. Our data suggest that oxygen impacts the thermal tolerance of lizards, but only under severely hypoxic conditions, possibly as a result of hypoxia-induced anapyrexia.

Human population and thus anthropogenic disturbance of natural systems are increasing at staggering rates. However, the impact of disturbance on natural systems likely varies with the extent of disturbance. While a considerable amount of research has been conducted on high-impact disturbances (e.g., environmental contamination, urban development), much less is known about the effects of less obtrusive activities such as ecotourism and outdoor recreation. Past research has documented behavioral and hormonal changes to non-consumptive wildlife activities; however, these studies rarely provide direct health and survival consequences, or examine impacts over a gradient of disturbances. In the current study, we quantified endocrine and immune responses and reproductive investment in six Galapagos marine iguana ( Amblyrhynchus cristatus ) populations located across a gradient of human disturbance stemming from the ecotourism industry, including urban development, boat traffic, and ecotourist activities. We regressed fitness-related physiological metrics to the intensity of human disturbance across these populations. As predicted, immunological capabilities decreased and oxidative stress increased with increasing intensity of human disturbance and development. Sexes also differed in their endocrine responses to disturbance intensity, with only males showing elevated baseline corticosterone at sites impacted by tourism. Testosterone and estradiol were both positively related to disturbance intensity. There was also a significant overall negative effect of the presence of human disturbance, but not its intensity, on reproductive investment. These results are the first to demonstrate that measures of physiological performance are sensitive to the intensity, and not merely the presence/absence of human disturbance, and that immunological measures in addition to more traditional methods can serve as important indicators of stress and potentially compromised fitness in natural populations.

Global temperature increases over the next century are predicted to contribute to the extinction of a number of taxa, including up to 40% of all lizard species. Lizards adapted to living in lowland tropical areas are especially vulnerable because of their dependence on specific microhabitats, low vagility, and a reduced capacity to physiologically adjust to environmental change. To assess the potential effects of climate change on lizards in the lowland tropics, we measured the critical thermal maximum (CTmax) of ten species from La Selva, Costa Rica. We also examined how well body size, microhabitat type, and species predicted the CTmax. We used current temperature data along with projected temperature increases for 2080 to predict which species may be at the greatest risk at La Selva. Of the ten species sampled, four are at serious risk of lowland extirpation and three others might also be at risk under the highest predicted temperature-increase models. Forest floor lizards at La Selva have already experienced significant population declines over the past 40 years, and we found that each of the forest floor species we studied is at serious risk of local extirpation. We also found that microhabitat type is the strongest predictor of CTmax, demonstrating the profound impact habitat specialization has on the thermal limits of tropical lizards.

Pythons are model organisms for investigating physiological responses to food intake. While systemic growth in response to food consumption is well documented, what occurs in the brain is currently unexplored. In this study, male ball pythons (Python regius) were used to test the hypothesis that food consumption stimulates cell proliferation in the brain. We used 5-bromo-12’-deoxyuridine as a cell-birth marker to quantify and compare cell proliferation in the brain of fasted snakes and those at two and six days after a meal. Throughout the telencephalon, cell proliferation was significantly increased in the six-day group, with no difference between the two-day group and controls. Systemic postprandial plasticity occurs quickly after a meal is ingested, during the period of active digestion; however, the brain displays a surge of cell proliferation after most digestion and absorption is complete.

Acute stressors can be costly, often requiring alteration of normal physiological processes to mitigate their effects. Animal translocation may be a very stressful event and result in a reduced ability to maintain homeostasis. The impacts of translocation on the thermal ecology of ectothermic vertebrates, which may rely on preferred habitats for thermoregulation, are currently unknown. In this study, 22 adult male Northern Pacific rattlesnakes (Crotalus oreganus oreganus) were implanted with automated temperature loggers and radio-tracked. Snakes were assigned to one of three treatments: translocation, handling control, and undisturbed control. Short-distance translocation (SDT) and handling treatments were applied weekly for 6 weeks. Hourly body temperature (Tb ) was recorded during the course of the study. Mean Tb was impacted in a time-dependent fashion, where translocated snakes had lower mean Tb than handled controls during the first week of the study only, especially the first 24 hr after translocation. Separating the dataset into day and night revealed that this effect was localized to Tb variation during the day only. Variance in temperature was not impacted by translocation or handling. Snake body mass and time of year were the major factors influencing the thermal profiles of these rattlesnakes. Thermal ecology in male rattlesnakes is resilient to SDT, suggesting that they quickly resume normal behaviors following repeated bouts of acute capture stress and disturbance of their spatial ecology. This study provides support for SDT as a safe measure for mitigating human-snake interactions and facilitating conservation practices regarding male snakes, which are the most frequently encountered sex.

Translocation, a management and conservation strategy used commonly in which animals are moved from their sites of origin to other localities, has proven controversial. We examined the physiological and behavioral impacts of repeated handling and short-distance translocation on rattlesnakes, which are often translocated from areas of human use because of a perceived threat to people. Northern Pacific Rattlesnakes (Crotalus oreganus oreganus) were radiotracked for 2 months, during which time one of three treatments was imposed weekly: translocation, walk and release at that day's capture site (handling control), and undisturbed control. At both the beginning (spring) and the end (summer) of the study, blood samples were obtained before and after an acute handling stressor, and plasma concentrations of corticosterone (CORT) and testosterone (T) were determined. All rattlesnakes showed a CORT stress response, but baseline and stressed concentrations of neither hormone were affected by either translo...

Field endocrine studies providing new comparisons for inference into the evolutionary and ecological factors shaping organismal physiology are important, often yielding novel physiological insights. Here, we explored factors associated with the sex steroid hormone concentrations and adrenocortical response to capture stress in Hog Island boas (Boa constrictor imperator) in the Cayos Cochinos archipelago of Honduras to generate comparative field hormone data from a tropical reptile and test the island tameness hypothesis. Baseline concentrations of testosterone, corticosterone, estradiol, and progesterone were measured during the wet and dry seasons, and an acute stressor of 1 h in a cloth bag was used to assess the stress response. Plasma steroid concentrations in these snakes were generally low in comparison to other taxa. Higher testosterone concentrations in males and higher estradiol and corticosterone concentrations in females were observed during the wet season compared to the dry season, which may be indicative of mating activities and vitellogenesis during this period. Snakes displayed a 15-fold increase in corticosterone concentrations in response to capture stress, a rise that was not impacted by whether a snake had been captured during previous years. The adrenocortical stress response was greater in males and positively related to body temperature. We suggest that this system merits future inquiries into the physiology and behavior of B. c. imperator, particularly as a model for studying insular impacts on diverse life history characters.

Metachrosis, or color change, in reptiles is used for thermoregulation, crypsis, and many other purposes. The mechanism and function of metachrosis remain unknown for many species, however...

The spatial ecology of animals is influenced by a combination of factors. Spatial ecology parameters are commonly calculated for rattlesnakes of the genus Crotalus, but they are highly variable within and among populations and species. Rattlesnake movements can be influenced by sex, body size, season, and abiotic factors, but the relative importance of each still remains unknown. We quantified aspects of the spatial ecology of a population of Northern Pacific Rattlesnake (Crotalus oreganus oreganus) in the southern part of its range in order to determine the factors that influence its spatial ecology. We found that males had larger home range sizes than females for the entire 14-month study. However, sex differences in movements were present during the spring mating season but not the summer post-mating season, suggesting that male mate searching causes males to have larger overall home ranges than females. We also found that body size was not related to home range size, but this effect was sensitive to t...

In the face of global change, free-ranging organisms are expected to experience more unpredictable stressors. An understanding of how organisms with different life history strategies will respond to such changes is an integral part of biodiversity conservation. Corticosterone (CORT) levels are often used as metrics to assess the population health of wild vertebrates, despite the fact that the stress response and its effects on organismal function are highly variable. Our understanding of the stress response is primarily derived from studies on endotherms, leading to some contention on the effects of chronic stress across and within taxa. We assessed the behavioral and hormonal responses to experimentally elevated stress hormone levels in a free-ranging, arid-adapted ectotherm, the Southern Pacific rattlesnake (Crotalus helleri). Plasma CORT was significantly elevated in CORT-implanted snakes 15days after implantation. Implantation with CORT did not affect testosterone (T) levels or defensive behavior. Interestingly, we observed increased defensive behavior in snakes with more stable daily body temperatures and in snakes with higher plasma T during handling (tubing). Regardless of treatment group, those individuals with lower baseline CORT levels and higher body temperatures tended to exhibit greater increases in CORT levels following a standardized stressor. These results suggest that CORT may not mediate physiological and behavioral trait expression in arid-adapted ectotherms such as rattlesnakes.

We tested the oxygen limitation hypothesis, which states that animals decline in performance and reach the upper limits of their thermal tolerance when the metabolic demand for oxygen at high temperatures exceeds the circulatory system's ability to supply adequate oxygen, in air-breathing lizards exposed to air with different oxygen concentrations. Lizards exposed to hypoxic air (6% O2) gaped, panted, and lost their righting response at significantly lower temperatures than lizards exposed to normoxic (21% O2) or hyperoxic (35% O2) air. A greater proportion of lizards in the hyperoxic treatment were able to withstand body temperatures above 44°C than in the normoxic treatment. We also found that female lizards had a higher panting threshold than male lizards, while sex had no effect on gaping threshold and loss of righting response. Body size affected the temperature at which lizards lost the righting response, with larger lizards losing the response at lower temperatures than smaller lizards when exposed to hypoxic conditions. These data suggest that oxygen limitation plays a mechanistic role in the thermal tolerance of lizards.

We describe the reproductive cycle of Northern Pacific rattlesnakes (Crotalus oreganus) by quantifying steroid hormone concentrations and observing reproductive behaviors in free-ranging individuals. Additionally, we examined reproductive tissues from museum specimens. Plasma steroid hormone concentrations were quantified for both male and female snakes throughout the active season (March–October). We measured testosterone (T), 5a-dihydrotestosterone (DHT), and corticosterone (B) concentrations in both sexes and 17b-estradiol (E2) and progesterone (P) in females only. We observed reproductive behaviors (e.g., consortship, courtship, and copulation) in the field and measured testis and follicle size in male and female snakes from museum collections to relate steroid hormone concentrations to the timing of reproductive events. Our study revealed that C. oreganus in central California exhibits a bimodal pattern of breeding, with most mating behavior occurring in the spring and some incidences of mating behavior observed in late summer/fall. Each breeding period corresponded with elevated androgen (T or DHT) levels in males. Testes were regressed in the spring when the majority of reproductive behavior was observed in this population, and they reached peak volume in August and September during spermatogenesis. Although we did not detect seasonal variation in female hormone concentrations, some females had high E2 in the spring and fall, coincident with mating and with increased follicle size (indicating vitellogenesis) in museum specimens. Females with high E2 concentrations also had high T and DHT concentrations. Corticosterone concentrations in males and females were not related either to time of year or to concentrations of any other hormones quantified. Progesterone concentrations in females also did not vary seasonally, but this likely reflected sampling bias as females tended to be underground, and thus unobtainable, in summer months when P would be expected to be elevated during gestation. In females, P was positively correlated with T and DHT, and E2 was positively correlated with T.

Translocation is an increasingly common conservation tool used to augment declining populations or to remove nuisance animals from areas of human conflict. Studies show that venomous snakes translocated long distances may wander and experience increased mortality. However, potential sub-lethal physiological effects on translocated snakes remain unknown. We conducted an experimental study on free-ranging rattlesnakes to test the hypothesis that long distance translocation is stressful. The glucocorticoid response to translocation was variable among snakes. There was some evidence that translocation may be stressful, as baseline corticosterone levels in most snakes rose following translocation, whereas levels remained consistent in control snakes. Interestingly, testosterone levels rose dramatically following translocation, possibly reflecting effects of interaction with new environmental cues and/or resident snakes, or effects of navigation in a new environment. Corticosterone and testosterone were positively correlated. Our study shows that long distance translocation can affect steroid hormone concentrations in rattlesnakes, a result that should be taken into consideration when managing nuisance snakes or repatriating animals to the wild.

Numerous factors impact the dynamics of host-parasite relationships, such as host sex, hormonal state, reproductive condition, host health, and behavior. In particular, males from a variety of taxa frequently carry heavier parasite burdens than females, particularly during breeding season when testosterone concentrations are elevated. Using western fence lizards (Sceloporus occidentalis), we tested the hypothesis that high circulating testosterone concentrations in male lizards induce high tick and mite loads. We implanted male lizards with either testosterone or control implants in the field during the spring, when tick and mite loads are highest. One month later, testosterone-implanted males had significantly higher tick loads, but lower mite loads, than control males. These results suggest that testosterone differentially impacts ectoparasitic acarine burdens. Testosterone may modulate aspects of lizard physiology and behavior that enhance or diminish parasitism by certain acarines during periods of peak reproductive effort. J. Exp. Zool. 00A:1-9, 2012. © 2012 Wiley Periodicals, Inc.

Parasitism is one of the most notable forms of symbiosis in the biological world, with nearly all organisms hosting parasites. In many vertebrates, males have higher ectoparasite burdens than females, especially when testosterone concentrations are elevated. Furthermore, reproductive females may have higher ectoparasite burdens than non-reproductive females. It is possible that testosterone-stimulated behaviors in males and offspring investment by females incur energetic costs that inhibit immune function. If questing ticks can sense host sex or reproductive condition prior to attachment, they could potentially choose hosts with the poorest immune function, thereby leading to improved feeding success and decreased feeding duration. In this study, we examined the host-parasite relationship between western fence lizards (Sceloporus occidentalis) and the western black-legged tick (Ixodes pacificus) to test the following hypotheses: (1) ticks prefer male lizards to female lizards. (2) Ticks prefer male lizards with higher testosterone. (3) Ticks prefer reproductive female lizards to non-reproductive female lizards. (4) Ticks feed to repletion more rapidly (decreased feeding duration) on reproductive females and males with higher testosterone. In all three experiments, ticks failed to show a preference for one group over another as demonstrated by similar attachment rates between groups. This suggests that observed differences in ectoparasite loads in free-ranging lizards is due to some other factor than host choice. However, tick feeding duration on female lizards was shorter when hosts were reproductive, suggesting that host reproductive condition alters tick feeding, possibly due to a decreased immune response. Interestingly, ticks fed more slowly on male lizards with elevated testosterone, suggesting that testosterone may actually improve immune function against ectoparasites.

Seasonal variability in intensities of ectoparasites of western fence lizards (Sceloporus occidentalis) was quantified in spring, summer, and autumn 2008. Lizards were suspended over pans of water in the laboratory to enumerate replete western black-legged ticks (Ixodes pacificus) and mites. Intensity of ectoparasites was greatest in spring. On average, mites comprised the greatest proportion of ectoparasites, followed by larval and nymphal ticks, respectively. Mites were on lizards year-round. Overall intensity of ticks was greatest in spring, but low during summer and autumn. Males were more heavily parasitized by ticks than females in spring, whereas females had greater intensities of mites in autumn. There was a slight, positive relationship between size of lizard and intensity of ticks, but not with intensity of mites. We report greater intensities of ectoparasites on S. occidentalis than several other studies, which may be due to differences in habitats and methods.

Differences in resource availability and quality along environmental gradients are important influences contributing to intraspecific variation in body size, which influences numerous life-history traits. Here, we examined variation in body size and sexual size dimorphism (SSD) in relation to temperature, seasonality, and precipitation among 10 populations located throughout Arizona of the western diamond-backed rattlesnake (Crotalus atrox). Specifically, in our analyses we addressed the following questions: (i) Are adult males larger in cooler, wetter areas? (ii) Does female body size respond differently to environmental variation? (iii) Is seasonality a better predictor of body size variation? (iv) Is SSD positively correlated with increased resources? We demonstrate that male and female C. atrox are larger in body size in cooler (i.e., lower average annual maximum, minimum, and mean temperature) and wetter areas (i.e., higher average annual precipitation, more variable precipitation, and available surface water). Although SSD in C. atrox appeared to be more pronounced in cooler, wetter areas, this relationship did not achieve statistical significance.

Accurately assessing free-ranging animals’ patterns of surface activity and refuge use is critical, yet fundamentally challenging for biologists and wildlife managers. We evaluate the accuracy of an automated technique—temperature-based activity estimation (TBAE)—in estimating surface activity and refuge use patterns of two sympatric reptiles, the western diamond-backed rattlesnake (Crotalus atrox) and the Gila monster (Heloderma suspectum) in the Sonoran Desert. TBAE derived from a comparison of body temperature to shaded air temperature was effective in estimating the overall percent surface activity for both rattlesnakes (observed surface activity 51.8%, TBAE estimated surface activity 48.2%) and Gila monsters (observed 22.3%, TBAE 24.5%). There was, however, considerable interspecific difference in the effectiveness of TBAE in predicting surface activity at specific time points; TBAE was far more accurate for Gila monsters than for rattlesnakes (96% vs. 66% time point-specific accuracy, respectively). We assert that, when validated, TBAE can be used to yield concurrent and accurate body temperatures and activity estimates for multiple free-ranging animals, particularly in arid environments, which improves our understanding of animal biology and can be used to inform management decisions. r 2008 Elsevier Ltd. All rights reserved.

Synopsis Sexual dimorphism in size (sexual size dimorphism; SSD) is nearly ubiquitous, but the relative importance of genetic versus environmental control of SSD is not known for most species. We investigated proximate determinants of SSD in several species of squamate reptiles, including three species of Sceloporus lizards and the diamond-backed rattlesnake (Crotalus atrox). In natural populations of these species, SSD is caused by sexual differences in age-specific growth. Males and females, however, may often share similar potentials for growth: growth is strongly responsive to the availability of food, and sexual differences in growth can be greatly suppressed or completely absent under common environmental conditions in the laboratory. Sexually divergent growth is expressed in natural environments because of inherent ecological differences between males and females and because of potential epigenetic effects of sex-specific growth regulators. In field-active Sceloporus, sexual differences in growth rate are associated with sexual divergence in plasma testosterone. Experiments confirm that testosterone inhibits growth in species in which females are larger (for example, S. undulatus and S. virgatus) and stimulates growth in those in which males are larger (for example, S. jarrovii). Interestingly, however, sexual divergence in plasma testosterone is not accompanied by divergence in growth in S. jarrovii or in male-larger C. atrox in the laboratory. Furthermore, experimental effects of castration and testosterone replacement on growth are not evident in captive S. jarrovii, possibly because growth effects of testosterone are superseded by an abundant, high-quality diet. In female-larger S. undulatus, growth may be traded-off against testosterone-induced reproductive costs of activity. In male-larger species, costs of reproduction in terms of growth are suggested by supplemental feeding of reproductive female C. atrox in their natural environment and by experimental manipulation of reproductive cost in female S. jarrovii. Growth costs of reproduction, however, do not contribute substantially to the development of SSD in male-larger S. jarrovii. We conclude that the energetic costs of testosteroneinduced, male reproductive behavior may contribute substantially to the development of SSD in some female-larger species. However, despite strong evidence that reproductive investment exacts a substantial cost in growth, we do not support the reproductive cost hypothesis as a general explanation of SSD in male-larger species.

The western black-legged tick (Ixodes pacificus) is a common tick species throughout the western USA and is the major vector for Borrelia burgdorferi, the Lyme disease causing bacterium. Western fence lizards (Sceloporus occidentalis) are a major host for juvenile I. pacificus, but are incompetent hosts for B. burgdorferi, which makes this host–parasite relationship of particular interest. In order to shed further light on this complex host–parasite relationship, we investigated the effects of temperature on feeding duration (number of days to repletion), success (number feeding to repletion), and efficiency (replete tick mass) of larval I. pacificus. Western fence lizards were experimentally infested with larval ticks and exposed to three constant temperatures (21, 27, 33 °C). Larvae feeding at 21 °C took approximately twice as long as larvae at 27 and 33 °C. Effects of temperature on feeding duration are likely mediated through effects on host blood circulation and functionality of tick salivary proteins. Our results here suggest temperature is another important factor influencing the feeding dynamics of I. pacificus, and likely other tick species. Future research is needed to clarify the exact mechanisms behind temperature effects on tick feeding.

We conducted an experiment to examine the effects of sex and food intake on growth, mass gain, and attainment of sexual maturity in Western Diamond-backed Rattlesnakes (Crotalus atrox). We also measured testosterone levels to determine whether testosterone might be involved in the male-biased sexual size dimorphism observed in this species. We collected neonate rattlesnakes and raised them in the laboratory for 2 years on either a high-intake diet (fed one mouse per week) or a low-intake diet (fed one mouse every 3 weeks). High-intake snakes grew and gained mass more rapidly than low-intake snakes, but males did not grow or gain mass more rapidly than females in either treatment group. High-intake snakes attained reproductive maturity earlier than low-intake snakes, indicating that size, not age, is the critical determinant of reproductive maturity. Males had higher levels of testosterone than females but did not grow more quickly, suggesting that testosterone may not affect growth in this species and may therefore not be the proximate determinant of sexual size dimorphism. J. Exp. Zool. 303A:598-607, 2005. r 2005 Wiley-Liss, Inc. Darwin (1871) recognized that sexual size dimorphism (SSD) was prevalent among animal species and varied in both direction and magni- tude. Mammals and birds tend most often to show male-biased SSD where males are larger than females (Andersson, '94). Reptiles are variable but typically show female-biased SSD where females are larger than males (Fitch, '81). Numerous ultimate explanations of SSD have been proposed, but Darwin's original hypotheses are still the most widely discussed: in species where females are larger, size may provide a fecundity advantage

We report seasonal variation in steroid hormone levels in blood samples from free-ranging Western Diamond-backed Rattlesnakes (Crotalus atrox), and the relationship between these hormones and events in the reproductive cycle. At a field site in the Sonoran Desert of south-central Arizona, we collected monthly blood samples over the course of two active seasons from 17 radiotelemetered females, and over three active seasons from 103 randomly encountered males. We used radioimmunoassay to measure plasma levels of 17beta-estradiol, progesterone, corticosterone, and testosterone in samples from females, and corticosterone and testosterone in samples from males. Non-reproductive females have consistently low levels of circulating 17beta-estradiol, progesterone, and testosterone throughout the year. In reproductive females, 17beta-estradiol levels increase dramatically and testosterone levels increase modestly during vitellogenesis in April and May, while progesterone levels increase dramatically at ovulation in June and then steadily decline until parturition in August. Corticosterone levels appear relatively constant in non-reproductive females, whereas reproductive females show increased levels at the end of gestation. Plasma testosterone levels in males are low in early summer and are elevated during spring and late summer, corresponding to the two mating periods of C. atrox. Plasma corticosterone levels in males did not vary seasonally and were not related to testosterone levels.

We used intracoelomically implanted temperature dataloggers to obtain semi-continuous body temperature data and establish monthly thermal profiles for free-ranging rattlesnakes. We mimicked random and non-random point-sampling methods by selecting a single daily data point from all values or from restricted times of day to reflect common point-sampling constraints. Thermal profiles generated from point-sampling differed from those generated from semi-continuous sampling, and this difference was more apparent when point-sampling was non-random. We conclude that semi-continuous sampling provides a better estimate of thermal profiles, and that point-sampling methods are highly sensitive to deviations from true randomness.

Acute stressors can be costly, often requiring alteration of normal physiological processes to mitigate their effects. Animal translocation may be a very stressful event and result in a reduced ability to maintain homeostasis. The impacts of translocation on the thermal ecology of ectothermic vertebrates, which may rely on preferred habitats for thermoregulation, are currently unknown. In this study, 22 adult male Northern Pacific rattlesnakes (Crotalus oreganus oreganus) were implanted with automated temperature loggers and radio-tracked. Snakes were assigned to one of three treatments: translocation, handling control, and undisturbed control. Short-distance translocation (SDT) and handling treatments were applied weekly for 6 weeks. Hourly body temperature (Tb ) was recorded during the course of the study. Mean Tb was impacted in a time-dependent fashion, where translocated snakes had lower mean Tb than handled controls during the first week of the study only, especially the first 24 hr after translocation. Separating the dataset into day and night revealed that this effect was localized to Tb variation during the day only. Variance in temperature was not impacted by translocation or handling. Snake body mass and time of year were the major factors influencing the thermal profiles of these rattlesnakes. Thermal ecology in male rattlesnakes is resilient to SDT, suggesting that they quickly resume normal behaviors following repeated bouts of acute capture stress and disturbance of their spatial ecology. This study provides support for SDT as a safe measure for mitigating human-snake interactions and facilitating conservation practices regarding male snakes, which are the most frequently encountered sex.

Although ticks are known to exhibit preferences among host species, there is little evidence that ticks select hosts within a species based on physiological condition. It may be beneficial for ticks to choose hosts that are easier to feed upon if the ticks can perceive indicative chemical or other signals from the host. For example, if ticks can detect host hematocrit they may choose hosts with high hematocrit, facilitating a faster blood meal. It may similarly be adaptive for ticks to avoid anemic hosts because it may be difficult for them to obtain an adequate meal and feeding duration may be extended. We tested the hypothesis that larval western black-legged ticks (Ixodes pacificus) detect host hematocrit using external cues and choose healthy over anemic hosts, allowing them to feed more quickly. We presented groups of larval ticks with pairs of healthy and anemic male western fence lizards (Sceloporus occidentalis), allowed them to select a host, and measured the feeding duration of the ticks. We found that the ticks did not exhibit a statistically significant preference for healthy over anemic lizards, but that the ticks fed to repletion significantly faster on healthy hosts than on anemic hosts. Larval ticks may not be able to detect external cues indicating the health of the host, at least not in terms of their hematocrit. The extended feeding duration likely reflects the extra time needed for the ticks to concentrate the blood meal of their anemic hosts.

Numerous factors impact the dynamics of host-parasite relationships, such as host sex, hormonal state, reproductive condition, host health, and behavior. In particular, males from a variety of taxa frequently carry heavier parasite burdens than females, particularly during breeding season when testosterone concentrations are elevated. Using western fence lizards (Sceloporus occidentalis), we tested the hypothesis that high circulating testosterone concentrations in male lizards induce high tick and mite loads. We implanted male lizards with either testosterone or control implants in the field during the spring, when tick and mite loads are highest. One month later, testosterone-implanted males had significantly higher tick loads, but lower mite loads, than control males. These results suggest that testosterone differentially impacts ectoparasitic acarine burdens. Testosterone may modulate aspects of lizard physiology and behavior that enhance or diminish parasitism by certain acarines during periods of peak reproductive effort.

The hippocampus of birds and mammals plays a crucial role in spatial memory and navigation. The hippocampus exhibits plasticity in adulthood in response to diverse environmental factors associated with spatial demands placed on an animal. The medial and dorsal cortices of the telencephalon of squamate reptiles have been implicated as functional homologues to the hippocampus. This study sought to experimentally manipulate the navigational demands placed on free-ranging northern Pacific rattlesnakes (Crotalus o. oreganus) to provide direct evidence of the relationship between spatial demands and neuroplasticity in the cortical telencephalon of the squamate brain. Adult male rattlesnakes were radio-tracked for 2 months, during which time 1 of 3 treatments was imposed weekly, namely 225-meter translocation in a random direction, 225-meter walk and release at that day’s capture site (handling control) or undisturbed (control). Snakes were then sacrificed and the brains were removed and processed for histological analysis of cortical features. The activity range was larger in the translocated (Tr) group compared to the handled (Hd) and undisturbed control (Cn) groups when measured via 95% minimum convex polygon (MCP). At the 100% MCP level, Tr snakes had larger activity ranges than the Cn snakes only. The volume of the medial cortex (MC) was larger in the Tr group compared to the Cn group. The MC of Hd snakes was not significantly different from that of either of the other groups. No differences in dorsal cortex (DC) or lateral cortex volumes were detected among the groups. Numbers of 5-bromo-2′-deoxyuridine (BrdU)-labeled cells in the MC and DC 3 weeks after BrdU injection were not affected by treatment. This study establishes a causal relationship between navigational demands and greater MC volume in a free-ranging reptile.

Publisher Summary This chapter focuses on current knowledge of the hormonal regulation of reproduction in snakes. Studies conducted have disproportionately examined temperate zone viperid and colubrid snakes, especially the redsided garter snake (Thamnophis sirtalis parietalis). Indeed, extensive observational and experimental studies on T. s. parietalis form the basis for our understanding of the hormonal regulation of reproduction in snakes. This review focuses on seasonal hormone concentrations in snakes in relation to events in the reproductive cycle; the limited data available on hypothalamic hormones, gonadotropins (GTHs), hormone receptors, and binding globulins; the neuroendocrinology of reproduction; and the environmental, social, physiological, and embryonic aspects of hormonal regulation of reproduction in snakes. Suggestions for future research are also provided, including studies of a more diverse ecological and taxonomic representation of snakes, experimental studies on the effects of hormones on reproductive tissues and behaviors, and further research into the neuroendocrinology of reproduction in this highly diverse group of animal. This chapter attempts to provide a broad overview of the effects of hormones on reproduction in snakes, focusing on seminal studies.

The Baja California rattlesnake, Crotalus enyo, is a medium-sized rattlesnake restricted to the southern two-thirds of the Baja California peninsula. Very little is known about the natural history of this species. The goal of this study was to document the diet of C. enyo. I dissected the stomachs and hindguts of 113 preserved museum specimens of C. enyo and identified the ingested prey items. The diet of C. enyo consists of small rodents, lizards, and centipedes. Crotalus enyo of all sizes eat rodents and lizards, in contrast to many other rattlesnakes that eat almost exclusively lizards as juveniles and mammals as adults. However, C. enyo exhibits a modest ontogenetic shift in diet: small snakes eat lizards more often than do large snakes, and large snakes eat mammals more often than do small snakes. Adult C. enyo also eat large centipedes of the genus Scolopendra. Larger snakes tended to contain multiple prey items more often than smaller snakes, but this trend was not significant. Crotalus en...

We describe the reproductive cycle of Northern Pacific rattlesnakes (Crotalus oreganus) by quantifying steroid hormone concentrations and observing reproductive behaviors in free-ranging individuals. Additionally, we examined reproductive tissues from museum specimens. Plasma steroid hormone concentrations were quantified for both male and female snakes throughout the active season (March-October). We measured testosterone (T), 5alpha-dihydrotestosterone (DHT), and corticosterone (B) concentrations in both sexes and 17beta-estradiol (E2) and progesterone (P) in females only. We observed reproductive behaviors (e.g., consortship, courtship, and copulation) in the field and measured testis and follicle size in male and female snakes from museum collections to relate steroid hormone concentrations to the timing of reproductive events. Our study revealed that C. oreganus in central California exhibits a bimodal pattern of breeding, with most mating behavior occurring in the spring and some incidences of mating behavior observed in late summer/fall. Each breeding period corresponded with elevated androgen (T or DHT) levels in males. Testes were regressed in the spring when the majority of reproductive behavior was observed in this population, and they reached peak volume in August and September during spermatogenesis. Although we did not detect seasonal variation in female hormone concentrations, some females had high E2 in the spring and fall, coincident with mating and with increased follicle size (indicating vitellogenesis) in museum specimens. Females with high E2 concentrations also had high T and DHT concentrations. Corticosterone concentrations in males and females were not related either to time of year or to concentrations of any other hormones quantified. Progesterone concentrations in females also did not vary seasonally, but this likely reflected sampling bias as females tended to be underground, and thus unobtainable, in summer months when P would be expected to be elevated during gestation. In females, P was positively correlated with T and DHT, and E2 was positively correlated with T.

Recent field studies on the reproductive ecology of western diamond-backed rattlesnakes ( Crotalus atrox ) from populations in southern Arizona showed significant differences in the concentration of plasma sex steroids (testosterone, T; 5α-dihydrotestosterone, DHT; and 17β-estradiol, E2) throughout the active season (March–October), and peak levels were coincident with the two mating periods (late summer and early spring). There is, however, no information on levels of sex steroids during winter. Similar to most snakes, hibernating individuals of C. atrox are typically inaccessible, but in southern Arizona, where environmental conditions are typically mild during winter, adult males frequently bask at or near the entrances of communal dens. Basking activity, therefore, offers a unique logistical opportunity to assess the complete annual profile of plasma sex steroid levels in males of a temperate reptile in nature. From November to February, we measured levels of plasma T, DHT, and E2 in adult male C. atrox that were located basking at communal dens. Additionally, cloacal, core body, and ambient air temperatures were obtained to investigate potential relationships between body temperatures and levels of sex steroids. Mean levels of T, DHT, and E2 were relatively high, and the concentration hierarchy was T > DHT > E2. Mean levels of T, DHT, and E2 showed no significant variation across the four months of sampling; however, E2 levels decreased progressively. In the annul cycle, sex steroid levels during winter were not basal when compared to values obtained during the active season. Mean cloacal temperatures of basking males were significantly higher than core body temperatures of non-basking males (inside dens) from November–December, and in February, which suggests that one function of winter basking is to elevate body temperatures. Steroid levels, nonetheless, were not significantly correlated with cloacal temperatures. We suggest that future field studies of male C. atrox should: (a) investigate sex steroid levels in non-basking individuals and (b) test whether elevated levels of sex steroids during winter facilitate the large increases that occur in early spring, which are coincident with the second mating season. Our findings on the reproductive biology of C. atrox and other viperids are discussed in the context of the associated–dissociated model of reproduction.