Scaling and relations of morphology with locomotor kinematics in the sidewinder rattlesnake Crotalus cerastes.

The movement of limbless terrestrial animals differs fundamentally from that of limbed animals, yet few scaling studies of their locomotor kinematics and morphology are available. We examined scaling and relations of morphology and locomotion in sidewinder rattlesnakes (Crotalus cerastes). During sidewinding locomotion, a snake lifts sections of its body up and forward while other sections maintain static ground contact. We used high-speed video to quantify whole-animal speed and acceleration; the height to which body sections are lifted; and the frequency, wavelength, amplitude and skew angle (degree of tilting) of the body wave. Kinematic variables were not sexually dimorphic, and most did not deviate from isometry, except wave amplitude. Larger sidewinders were not faster, contrary to many results from limbed terrestrial animals. Free from the need to maintain dynamic similarity (because their locomotion is dominated by friction rather than inertia), limbless species may have greater freedom to modulate speed independently of body size. Path analysis supported: (1) a hypothesized relationship between body width and wavelength, indicating that stouter sidewinders form looser curves; (2) a strong relationship between cycle frequency and whole-animal speed; and (3) weaker effects of wavelength (positive) and amplitude (negative) on speed. We suggest that sidewinding snakes may face a limit on stride length (to which amplitude and wavelength both contribute), beyond which they sacrifice stability. Thus, increasing frequency may be the best way to increase speed. Finally, frequency and skew angle were correlated, a result that deserves future study from the standpoint of both kinematics and physiology.

Pit viper thermography: the pit organ used by crotaline snakes to detect thermal contrast has poor spatial resolution.

Pit vipers detect infrared radiation by means of temperature contrasts created on their pit organ membranes. Signals from pit organs integrate with visual signals in the optic tectum, leading to the conjecture that the facial pits operate as an extension of the visual system. Because similar mechanisms underlie thermal imaging technology, imagery from thermal cameras is often used to infer how pit vipers perceive their environment. However, pit organs lack a focusing mechanism, and biophysical models predict that pit organs should have poor spatial resolution compared with thermal imaging cameras. Nevertheless, behavioral studies occasionally suggest pits may have better resolution than predicted by biophysical models, indicating that processing in the central nervous system may improve imaging. To estimate the spatial resolution of the neural image informing behavior, we recorded snake responses evoked by targets moving across backgrounds composed of two contrasting temperatures with an average temperature equal to the target temperature. An unresolved background would appear uniform; thus, the target would be detectable only if the background pattern were resolved. Western rattlesnakes (Crotalus oreganus) displayed no statistically significant responses to targets presented in front of patterned backgrounds, regardless of the temperature contrasts or spatial frequencies within the background, but responded strongly to targets presented in front of homogeneous backgrounds. We found no evidence that the pit organ system can resolve spatial details subtending an angle of 9 deg or less. We discuss the implications of these results for understanding pit organ function in ecologically relevant habitats with thermal heterogeneity.

Crotalus atrox venom-induced cellular toxicity: Early wound progression involves reactive oxygen species.

Understanding the mechanisms that produce cellular cytotoxicity is fundamental in the field of toxicology. Cytotoxic stimuli can include organic toxins such as hemorrhagic snake venom, which can lead to secondary complications such as the development of necrotic tissue and profuse scarring. These clinical manifestations mimic cytotoxic responses induce by other organic compounds such as organic acids. We used hemorrhagic snake venom and human embryonic kidney cells (HEK 293T) as a model system to better understand the cellular responses involved in venom induced cytotoxicity. Cells stimulated with Crotalus atrox (CA) (western diamondback) venom for 4 or 10 h demonstrated significant cytotoxicity. Results from 2',7'-Dichlorodihydrofluorescein diacetate (H2 DCF-DA) assays determine CA venom stimulation induces a robust production of reactive oxygen species (ROS) over a 3-h time course. In contrast, pretreatment with polyethylene glycol (PEG)-catalase or N-acetyl cysteine (NAC) prior to CA venom stimulation significantly blunted H2 DCFDA fluorescence fold changes and showed greater cytoprotective effects than cells stimulated with CA venom alone. Pre- incubating HEK293T cells with the NADPH oxidase (NOX) pan-inhibitor VAS2870 prior venom stimulation significantly minimized the venom-induced oxidative burst at early timepoints (≤2 h). Collectively, our experiments show that pre-application of antioxidants reduces CA venom induce cellular toxicity. This result highlights the importance of ROS in the early stages of cytotoxicity and suggests muting ROS production in noxious injuries may increase positive clinical outcomes.

Baroreflex responses of decerebrate rattlesnakes (Crotalus durissus) are comparable to awake animals.

A decerebrate rattlesnake, Crotalus durissus, has previously been used as a model Squamate for cardiovascular studies. It enabled instrumentation for concomitant recordings of diverse variables that showed autonomic responses. However, to validate the preparation and its scope for use, it is necessary to assess how close its cardiovascular variables are to non-decerebrate snakes and the effectiveness of its autonomic responses. Similarly, it is important to analyze its recovery profile after instrumentation and observe if it maintains stability throughout the duration of experimental protocol. Here we have objectively assessed these points by comparing decerebrate preparations and non-decerebrate snakes, after the occlusive cannulation of the vertebral artery. We have assessed cardiovascular variables and the baroreflex to analyze the presence, magnitude and stability of complex autonomic-controlled parameters as indicators of autonomic nervous system (ANS) functionality. After instrumentation, mean heart rates were high but recovered to stable values within 24 h. Mean arterial pressure stabilized within 24 h in control snakes and 48 h in decerebrate preparations. After that, both parameters remained stable. The operational gain and effectiveness index of the baroreflex recovered within the first 6 h after instrumentation in both experimental groups. In addition, the baroreflex capacities and its limits were also equivalent between the groups. These experiments demonstrated that decerebrate preparations and inactive, non-decerebrate snakes showed comparable recovery profiles following anesthesia and cannulation, maintained similar values of cardiovascular variables during experimental manipulation and exhibited functional, ANS modulated reflexes. Accordingly, the present results attest the relevance of this decerebrate preparation for studies on cardiovascular modulation.

Comparative thermal ecology parameters of the mexican dusky rattlesnake (Crotalus triseriatus).

Montane habitats exhibit a high degree of thermal heterogeneity, and thus provide considerable thermoregulatory challenges for ectotherms. Comparative analyses provide an opportunity to understand how variation in abiotic factors (e.g., operative temperatures, thermal quality) can affect life history traits within species. We studied the thermal ecology of three populations of the rattlesnake Crotalus triseriatus inhabiting different volcanoes in the central region of Mexico using the Hertz et al. (1993) protocol. The average body temperature of dusky rattlesnakes from the three study sites was 22.4 °C; mean active body temperature was higher in site 2 than in sites 1 and 3, but no differences between females, males and juveniles nor an interaction among site and sex was found. The thermal quality was low in the three sites, particularly in sites 1 and 3. Thermoregulation accuracy statistically differed among populations: individuals from site 2 were more accurate thermoregulating, while individuals from site 1 were the least accurate. Compared to other snakes, dusky rattlesnakes can be considered as a eurythermic species, which can often be active at relatively low body temperatures.

When one phenotype is not enough: divergent evolutionary trajectories govern venom variation in a widespread rattlesnake species.

Understanding the origin and maintenance of phenotypic variation, particularly across a continuous spatial distribution, represents a key challenge in evolutionary biology. For this, animal venoms represent ideal study systems: they are complex, variable, yet easily quantifiable molecular phenotypes with a clear function. Rattlesnakes display tremendous variation in their venom composition, mostly through strongly dichotomous venom strategies, which may even coexist within a single species. Here, through dense, widespread population-level sampling of the Mojave rattlesnake, Crotalus scutulatus, we show that genomic structural variation at multiple loci underlies extreme geographical variation in venom composition, which is maintained despite extensive gene flow. Unexpectedly, neither diet composition nor neutral population structure explain venom variation. Instead, venom divergence is strongly correlated with environmental conditions. Individual toxin genes correlate with distinct environmental factors, suggesting that different selective pressures can act on individual loci independently of their co-expression patterns or genomic proximity. Our results challenge common assumptions about diet composition as the key selective driver of snake venom evolution and emphasize how the interplay between genomic architecture and local-scale spatial heterogeneity in selective pressures may facilitate the retention of adaptive functional polymorphisms across a continuous space.

Exposure to fluctuating temperatures leads to reduced immunity and to stress response in rattlesnakes.

Ectothermic organisms often experience considerable variation in their body temperature throughout the circadian cycle. However, studies focusing on the measurement of physiological traits are usually performed under constant temperature regimes. This mismatch between thermal exposure in the field and experimental conditions could act as a stressor agent, as physiological functions are strongly influenced by temperature. Herein, we asked the question whether constant thermal regimes would cause a stress response and impact the immunity of the South American rattlesnake (Crotalus durissus) when compared with a fluctuating thermal regime. We addressed this question by determining heterophil:lymphocyte (H:L) ratio, plasma bacteria-killing ability (BKA) and corticosterone (CORT) levels in snakes kept under a constant temperature regime at 30°C, and under a fluctuating regime that oscillated between 25°C at night and 35°C during the day. The experiments had a mirrored design, in which half of the snakes were subjected to a fluctuating-to-constant treatment, while the other half was exposed to a constant-to-fluctuating treatment. The shift from constant to fluctuating thermal regime was accompanied by an increase in plasma CORT levels, indicating the activation of a stress response. Exposure to a fluctuating thermal regime at the onset of the experiments induced a decrease in the BKA of rattlesnakes. H:L ratio was not affected by treatments and, therefore, the shift between thermal regimes seems to have acted as a low-intensity stressor. Our results suggest that removal from temperatures close to the snake's preferred body temperature triggers a stress response in rattlesnakes.

Respiratory sinus arrhythmia is a major component of heart rate variability in undisturbed, remotely monitored rattlesnakes, Crotalus durissus.

ECG recordings were obtained using an implanted telemetry device from the South American rattlesnake, Crotalus durissus, held under stable conditions without restraining cables or interaction with researchers. Mean heart rate (fH) recovered rapidly (<24 h) from anaesthesia and operative procedures. This preceded a more gradual development of heart rate variability (HRV), with instantaneous fH increasing during each lung ventilation cycle. Atropine injection increased mean fH and abolished HRV. Complete autonomic blockade revealed a cholinergic tonus on the heart of 55% and an adrenergic tonus of 37%. Power spectral analysis of HRV identified a peak at the same frequency as ventilation. This correlation was sustained after temperature changes and it was more evident, marked by a more prominent power spectrum peak, when ventilation is less episodic. This HRV component is homologous to that observed in mammals, termed respiratory sinus arrhythmia (RSA). Evidence for instantaneous control of fH indicated rapid conduction of activity in the cardiac efferent nervous supply, as supported by the description of myelinated fibres in the cardiac vagus. Establishment of HRV 10 days after surgical intervention seems a reliable indicator of the re-establishment of control of integrative functions by the autonomic nervous system. We suggest that this criterion could be applied to other animals exposed to natural or imposed trauma, thus improving protocols involving animal handling, including veterinarian procedures.

Thermal regime effects on the resting metabolic rate of rattlesnakes depend on temperature range.

While ectothermic organisms often experience considerable circadian variation in body temperature under natural conditions, the study of the effects of temperature on metabolic rates are traditionally based on subjecting animals to constant temperature regimes. Whether data resulting from constant-temperature experiments accurately predicts temperature effects under more natural fluctuating temperature regimes remains uncertain. To address such possibility, we measured the resting metabolic rates of the South American rattlesnakes (Crotalus durissus) under constant and circadian fluctuating thermal regimes in a range of temperatures. Metabolic rates measured at constant 20 °C and 25 °C did not differ from the rates measured at fluctuating regimes with corresponding mean temperatures. However, the difference between thermal regimes increased with temperature, with the metabolic rate measured at constant 30 °C being greater than that measured at the fluctuating thermal regime with corresponding mean temperature. Therefore, our results indicate that thermal regime effects on rattlesnakes' metabolism is dependent on temperature range. Broadly, our results highlight the importance of considering multi-factorial attributes of temperature variation in the exam of its effects over functional traits. Such approach provides a more solid support for inferences about temperature effects on the life history, ecology and conservation of ectothermic organisms.

In-Depth Venome of the Brazilian Rattlesnake Crotalus durissus terrificus: An Integrative Approach Combining Its Venom Gland Transcriptome and Venom Proteome.

Snake venoms are complex mixtures mainly composed of proteins and small peptides. Crotoxin is one of the most studied components from Crotalus venoms, but many other components are less known due to their low abundance. The venome of Crotalus durissus terrificus, the most lethal Brazilian snake, was investigated by combining its venom gland transcriptome and proteome to create a holistic database of venom compounds unraveling novel toxins. We constructed a cDNA library from C. d. terrificus venom gland using the Illumina platform and investigated its venom proteome through high resolution liquid chromotography-tandem mass spectrometry. After integrating data from both data sets, more than 30 venom components classes were identified by the transcriptomic analysis and 15 of them were detected in the venom proteome. However, few of them (PLA2, SVMP, SVSP, and VEGF) were relatively abundant. Furthermore, only seven expressed transcripts contributed to ∼82% and ∼73% of the abundance in the transcriptome and proteome, respectively. Additionally, novel venom proteins are reported, and we highlight the importance of using different databases to perform the data integration and discuss the structure of the venom components-related transcripts identified. Concluding, this research paves the way for novel investigations and discovery of future pharmacological agents or targets in the antivenom therapy.

Seed ingestion and germination in rattlesnakes: overlooked agents of rescue and secondary dispersal.

Seed dispersal is a key evolutionary process and a central theme in the population ecology of terrestrial plants. The primary producers of most land-based ecosystems are propagated by and maintained through various mechanisms of seed dispersal that involve both abiotic and biotic modes of transportation. By far the most common biotic seed transport mechanism is zoochory, whereby seeds, or fruits containing them, are dispersed through the activities of animals. Rodents are one group of mammals that commonly prey on seeds (granivores) and play a critical, often destructive, role in primary dispersal and the dynamics of plant communities. In North America, geomyid, heteromyid and some sciurid rodents have specialized cheek pouches for transporting seeds from plant source to larder, where they are often eliminated from the pool of plant propagules by consumption. These seed-laden rodents are commonly consumed by snakes as they forage, but unlike raptors, coyotes, bobcats, and other endothermic predators which eat rodents and are known or implicated to be secondary seed dispersers, the role of snakes in seed dispersal remains unexplored. Here, using museum-preserved specimens, we show that in nature three desert-dwelling rattlesnake species consumed heteromyids with seeds in their cheek pouches. By examining the entire gut we discovered, furthermore, that secondarily ingested seeds can germinate in rattlesnake colons. In terms of secondary dispersal, rattlesnakes are best described as diplochorous. Because seed rescue and secondary dispersal in snakes has yet to be investigated, and because numerous other snake species consume granivorous and frugivorous birds and mammals, our observations offer direction for further empirical studies of this unusual but potentially important channel for seed dispersal.

Do free-ranging rattlesnakes use thermal cues to evaluate prey?

Rattlesnakes use infrared radiation to detect prey animals such as small mammals and lizards. Because ectotherm locomotor performance depends on temperature, rattlesnakes could use prey temperature to evaluate the potential of lizards to evade attacks. Here, we tested whether hunting rattlesnakes use infrared information to (1) detect and (2) evaluate prey before attack. We expected thermal contrast between prey and background to be the best predictor of predatory behaviour under the prey detection hypothesis, and absolute prey temperature under the prey evaluation hypothesis. We presented lizard carcasses of varying temperatures to free-ranging sidewinder rattlesnakes (Crotalus cerastes) and scored behavioural responses as a function of thermal contrast, absolute lizard temperature, and light level. Thermal contrast and light level were the most salient predictors of snake behaviour. Snakes were more likely to respond to lizards and/or respond at greater distances at night and when thermal contrast was high, supporting the known prey detection function of infrared sensing. Absolute lizard temperature was not an important predictor of snake behaviour; thus, we found no evidence for temperature-based prey evaluation. Infrared sensing is still poorly understood in ecologically relevant contexts; future research will test whether rattlesnakes learn to evaluate prey based on temperature with experience.

Cooler snakes respond more strongly to infrared stimuli, but we have no idea why.

The pit organ defining pit vipers (Crotalinae) contains a membrane covered with temperature receptors that detect thermal radiation from environmental surfaces. Temperature is both the environmental parameter being sensed and the mechanism by which the pit membrane detects the signal. As snakes are ectotherms, temperature also has a strong influence on neurological and locomotor responses to the signal. This study of Pacific rattlesnakes (Crotalus oreganus) systematically examined the effect of body, target and background temperatures on response to a moving target. We presented each snake with a moving pendulum bob regulated at a series of six temperatures against a uniform background regulated at one of three temperatures. Snake body temperatures varied from 18 to 36°C. As expected, we found stronger responses to positive contrasts (target warmer than background) than to negative contrasts, and stronger responses to greater contrasts. However, the effect of body temperature was contrary to expectations based on studies of the TRPA1 ion channel (believed to be the molecular basis for pit membrane temperature receptors) and typical thermal reaction norms for neural and motor performance. These predict (1) no response below the threshold where the TRPA1 channel opens, (2) response increasing as temperature increases, peaking near preferred body temperature, and (3) declining thereafter. Remarkably, this behavioral response decreased as body temperature increased from 18 to 36°C, with no threshold or peak in this range. We review various possible physiological mechanisms related to body temperature proposed in the literature, but find none that can satisfactorily explain this result.

Synaptic convergence of afferent inputs in primary infrared-sensitive nucleus (LTTD) neurons of rattlesnakes (Crotalinae) as the origin for sensory contrast enhancement.

Pitvipers have a specialized sensory system in the upper jaw to detect infrared (IR) radiation. The bilateral pit organs resemble simple pinhole cameras that map IR objects onto the sensory epithelium as blurred representations of the environment. Trigeminal afferents transmit information about changing temperature patterns as neuronal spike discharge in a topographic manner to the hindbrain nucleus of the lateral descending trigeminal tract (LTTD). A presumed, yet so far unknown neuronal connectivity within this central nucleus exerts a synaptic computation that constrains the relatively large receptive field of primary afferent fibers. Here, we used intracellular recordings of LTTD neurons in isolated rattlesnake brains to decipher the spatio-temporal pattern of excitatory and inhibitory responses following electrical stimulation of single and multiple peripheral pit organ-innervating nerve branches. The responses of individual neurons consisted of complex spike sequences that derived from spatially and temporally specific interactions between excitatory and inhibitory synaptic inputs from the same as well as from adjacent peripheral nerve terminal areas. This pattern complies with a central excitation that is flanked by a delayed lateral inhibition, thereby enhancing the contrast of IR sensory input, functionally reminiscent of the computations for contrast enhancement in the peripheral visual system.

Modulation of orthogonal body waves enables high maneuverability in sidewinding locomotion.

Many organisms move using traveling waves of body undulation, and most work has focused on single-plane undulations in fluids. Less attention has been paid to multiplane undulations, which are particularly important in terrestrial environments where vertical undulations can regulate substrate contact. A seemingly complex mode of snake locomotion, sidewinding, can be described by the superposition of two waves: horizontal and vertical body waves with a phase difference of ± 90°. We demonstrate that the high maneuverability displayed by sidewinder rattlesnakes (Crotalus cerastes) emerges from the animal's ability to independently modulate these waves. Sidewinder rattlesnakes used two distinct turning methods, which we term differential turning (26° change in orientation per wave cycle) and reversal turning (89°). Observations of the snakes suggested that during differential turning the animals imposed an amplitude modulation in the horizontal wave whereas in reversal turning they shifted the phase of the vertical wave by 180°. We tested these mechanisms using a multimodule snake robot as a physical model, successfully generating differential and reversal turning with performance comparable to that of the organisms. Further manipulations of the two-wave system revealed a third turning mode, frequency turning, not observed in biological snakes, which produced large (127°) in-place turns. The two-wave system thus functions as a template (a targeted motor pattern) that enables complex behaviors in a high-degree-of-freedom system to emerge from relatively simple modulations to a basic pattern. Our study reveals the utility of templates in understanding the control of biological movement as well as in developing control schemes for limbless robots.

THE FIRST REPORT OF A CROTALID ENVENOMATION IN A DOMESTICATED FERRET ( MUSTELA FURO) AND SUCCESSFUL TREATMENT WITH A NOVEL F(AB')2 ANTIVENOM.

A case report of a domesticated ferret ( Mustela furo) envenomated by a presumptive rattlesnake ( Crotalus sp.) treated successfully and safely with the novel Fab (2') North American Snake Antivenom (Veteria Labs). The ferret presented with clinical signs of depressed mentation and facial edema following a rattlesnake ( Crotalus sp.) bite. It developed hypotension, thrombocytopenia, and ecchymosis following the envenomation. It was treated with Fab (2') antivenom and given supportive care including crystalloid fluids and analgesia to resolution of clinical signs. This is the first documented case of rattlesnake envenomation in this species. This case supports the efficacy and short-term safety of this Fab (2') antivenom in this species without the use of antihistamines or glucocorticoids. This report also addresses the current standards of care with thorough review of the literature involving rattlesnake envenomation in zoological species.

Neuromuscular paralysis by the basic phospholipase A2 subunit of crotoxin from Crotalus durissus terrificus snake venom needs its acid chaperone to concurrently inhibit acetylcholine release and produce muscle blockage.

BACKGROUND AND PURPOSE: Crotoxin (CTX), a heterodimeric phospholipase A2 (PLA2) neurotoxin from Crotalus durissus terrificus snake venom, promotes irreversible blockade of neuromuscular transmission. Indirect electrophysiological evidence suggests that CTX exerts a primary inhibitory action on transmitter exocytosis, yet contribution of a postsynaptic action of the toxin resulting from nicotinic receptor desensitization cannot be excluded. Here, we examined the blocking effect of CTX on nerve-evoked transmitter release measured directly using radioisotope neurochemistry and video microscopy with the FM4-64 fluorescent dye. EXPERIMENTAL APPROACH: Experiments were conducted using mice phrenic-diaphragm preparations. Real-time fluorescence video microscopy and liquid scintillation spectrometry techniques were used to detect transmitter exocytosis and nerve-evoked [3H]-acetylcholine ([3H]ACh) release, respectively. Nerve-evoked myographic recordings were also carried out for comparison purposes. KEY RESULTS: Both CTX (5µg/mL) and its basic PLA2 subunit (CB, 20µg/mL) had biphasic effects on nerve-evoked transmitter exocytosis characterized by a transient initial facilitation followed by a sustained decay. CTX and CB reduced nerve-evoked [3H]ACh release by 60% and 69%, respectively, but only the heterodimer, CTX, decreased the amplitude of nerve-evoked muscle twitches. CONCLUSION AND IMPLICATIONS: Data show that CTX exerts a presynaptic inhibitory action on ACh release that is highly dependent on its intrinsic PLA2 activity. Given the high safety margin of the neuromuscular transmission, one may argue that the presynaptic block caused by the toxin is not enough to produce muscle paralysis unless a concurrent postsynaptic inhibitory action is also exerted by the CTX heterodimer.

When less means more: dehydration improves innate immunity in rattlesnakes.

Immune function can vary based on availability of resources, and most studies of such influences have focused on the co-investment of energy into immune and other physiological functions. When energy resources are limited, trade-offs exist, which can compromise immunity for other functions. As with energy, water limitation can also alter various physiological processes, yet water has received little consideration for its possible role in modulating immune functions. We examined the relationship between immunocompetence and hydration state using the western diamond-backed rattlesnake (Crotalus atrox). This species is known to undergo substantial seasonal fluctuations in water availability with extreme limitations during the hot-dry season. We collected blood samples from free-ranging C. atrox to compare osmolality and innate immune function (lysis, agglutination and bacterial growth inhibition) during the milder and relatively moister early spring season, the hot-dry season and the hot-wet season. To isolate effects of dehydration from other possible seasonal influences, we complemented this field study with a laboratory study in which we withheld food and water from individually housed adult C. atrox for up to 16 weeks. We collected blood samples from each snake as it dehydrated and collected a final sample after the snake was given water ad libitum at the end of the experiment. Our results demonstrate that C. atrox experience significant dehydration during the hot-dry season, and that, in general, innate immune function is highly correlated with osmolality, whether natural or artificially manipulated.

Physiological and behavioral effects of exogenous corticosterone in a free-ranging ectotherm.

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.

Behavioral thermal tolerances of free-ranging rattlesnakes (Crotalus oreganus) during the summer foraging season.

Increasing temperature due to climate change is one of the greatest challenges for wildlife worldwide. Behavioral data on free-ranging individuals is necessary to determine at what temperatures animals modify activity as this would determine their capacity to continue to move, forage, and mate under altered thermal regimes. In particular, high temperatures could limit available surface activity time and time spent on fitness-related activities. Conversely, performance, such as feeding rate, can increase with temperature potentially having positive fitness effects. Here, we examine how the hunting behaviors of free-ranging Northern Pacific Rattlesnakes (Crotalus oreganus) associate with air temperature and body temperature. We continuously recorded snakes in the field using videography, capturing behaviors rarely considered in past studies such as movements in and out of refuge and strikes on prey. We found that as mean daily air temperature increased, hunting activity and the likelihood of hunting at night decreased, while the number of movements and distance moved per day increased. Snakes typically retreated to refuge before body temperatures reached 31°C. Body temperatures of snakes hunting on the surface were lower compared to temperatures of non-hunting snakes in refuge in the morning, while this relationship was inverted in the afternoon. Snake body size influenced the disparity of these temperatures. Finally, strike initiation and success occurred across a wide range of body temperatures, indicating hunting performance may not be strongly constrained by temperature. These results on the temperatures at which free-ranging rattlesnakes exhibit fitness-related behaviors could be valuable for understanding their vulnerabilities to future climates.