Thamnophis Sirtalis Research Articles

The effect of intracoelomic alfaxalone on righting reflex and response to Semmes-Weinstein monofilaments in the common garter snake (Thamnophis sirtalis)

Introduction: Loss (LRR) and return (RRR) of righting reflex and response to Semmes-Weinstein monofilaments (SWM) was evaluated in garter snakes receiving intracoelomic alfaxalone (IC).

Arginine Vasotocin and Neuropeptide Y Vary with Seasonal Life-History Transitions in Garter Snakes.

Transitions between life-history stages are often accompanied by dramatic behavioral switches that result from a shift in motivation to pursue one resource over another. While the neuroendocrine mechanisms that regulate such behavioral transitions are poorly understood, arginine vasotocin (AVT) and neuropeptide Y (NPY) are excellent candidates because they modulate reproductive and feeding behavior, respectively. We asked if seasonal changes in AVT and NPY are concomitant with the seasonal migration to and from the feeding grounds in red-sided garter snakes (Thamnophis sirtalis parietalis). Male and female snakes were collected in different migratory states during both the spring and fall. The total number of AVT- and NPY-immunoreactive (ir) cells was then quantified in each brain region of interest. To correct for potential variation in region volume related to sexually dimorphic body size in this species, we first determined that snout-vent length is an accurate proxy for regional brain volume. We then corrected each individual's ir cell number by its SVL to directly compare seasonal changes in AVT and NPY between males and females. Within the supraoptic nucleus, both males and females had more AVT-ir cells during the fall compared with spring. As predicted, males had significantly more AVT-ir cells during the spring mating season in the hypothalamus (HYP) and bed nucleus of the stria terminalis, brain regions important in regulating reproductive behavior. Females also had significantly more AVT-ir cells in the HYP during the spring, as well as a significantly higher number of hypothalamic AVT cells than males. During the fall, males had significantly more NPY-ir cells in the cortex and posterior HYP compared with spring, possibly reflecting increased feeding behavior during summer foraging. Females did not exhibit significant main effects of season on NPY-ir cell number in any region. Neither AVT- nor NPY-ir cell number varied significantly with migratory status, but we did observe significant changes related to seasonal transitions in reproductive state. Our results indicate that changes in brain AVT and NPY are associated with seasonal transitions in reproductive and foraging behaviors, and may be involved in mediating sex differences in the timing of life-history events.

Evolutionary GEM: The Evolutionary Arms Race of Garter Snakes and Newts

In evolutionary biology, predator-prey species pairs can be observed participating in evolutionary arms races between adaptations and counter-adaptations. For example, as a prey becomes more adept at avoiding capture, its predator becomes a more adept hunter. The rough-skinned newt (Taricha granulosa) produces a toxin that protects it from virtually all predators, except one. That one predator is the common garter snake (Thamnophis sirtalis), which has evolved resistance to this toxin. This predator-prey pair is seemingly engaged in a perpetual battle for higher toxicity and better resistance. While both adaptations come with costs, the coexistence of newt and garter snake imposes reciprocal selective pressure that drives this arms race.

Using whole-group metabolic rate and behaviour to assess the energetics of courtship in red-sided garter snakes

Reproductive effort is an important aspect of life history as reproductive success is arguably the most important component of fitness. Males tend to compete for access to females and, in the process, expend their energetic capital on mate searching, male–male competition and courtship rather than directly on offspring. Red-sided garter snakes, Thamnophis sirtalis parietalis , are an exceptional model for studying energetic costs of courtship and mating as they fast during the spring mating season, which segregates the cost of energy acquisition from the cost of courtship and mating. However, measuring an individual male's metabolic rate during courtship is complicated by the fact that male courtship behaviour in red-sided garter snakes is dependent on both the detection of a female sexual attractiveness pheromone and on facilitated courtship (i.e. vigorous courtship is only exhibited in the presence of other males). Thus, traditional techniques of placing a mask over the head of individuals would prevent male courtship behaviour, and single animals placed in a flow-through chamber would not yield ecologically realistic levels of courtship, which are only seen in the context of a mating aggregation in this species. Because of these difficulties, we placed groups of males in a flow-through metabolic chamber together with a single female whose respiratory gases were vented outside the chamber to yield a whole-group metabolic rate during competitive courtship. We also measured the standard metabolic rates (SMR) of the males individually for comparison with active metabolic rates. Conservative estimates of peak group metabolic rates during courtship are 10–20 times higher than resting group metabolic rate, which was 1.88 times higher than SMR. These measurements, coupled with the fact that these males are aphagous during the breeding, indicates that costs of courtship may be high for males and has implications for the male mating tactics in this system.

A Novel Form of Behavioral Camouflage in Colubrid Snakes

Different types of animal camouflage ultimately have one function: avoiding detection. The most common form of camouflage, background matching, involves color patterns that match the surrounding environment while the individual remains motionless. However, in a dynamic environment, movement could also contribute to camouflage. For example, reverse crypsis or motion dazzle employs color patterns that can disrupt the pattern of motion to aid in concealing an individual, or by making it more difficult to track the individual's movements. Here I describe a new form of behavioral camouflage in colubrid snakes. The behavior consists of a series of small, laterally oscillating movements. Observed in three colubrids (Thamnophis sirtalis, Thamnophis sauritus, Opheodrys vernalis), I hypothesize that this behavior serves to blend the dorsal patterning of a snake with moving vegetation. Through 73 observations in T. sirtalis, the head-wobble was only observed being performed by small snakes in grassy/reedy areas whil...

Convergent adaptation to dangerous prey proceeds through the same first-step mutation in the garter snake Thamnophis sirtalis.

Convergent phenotypes often result from similar underlying genetics, but recent work suggests convergence may also occur in the historical order of substitutions en route to an adaptive outcome. We characterized convergence in the mutational steps to two independent outcomes of tetrodotoxin (TTX) resistance in separate geographic lineages of the common garter snake (Thamnophis sirtalis) that coevolved with toxic newts. Resistance is largely conferred by amino acid changes in the skeletal muscle sodium channel (NaV 1.4) that interfere with TTX-binding. We sampled variation in NaV 1.4 throughout western North America and found clear evidence that TTX-resistant changes in both lineages began with the same isoleucine-valine mutation (I1561V) within the outer pore of NaV 1.4. Other point mutations in the pore, shown to confer much greater resistance, accumulate later in the evolutionary progression and always occur together with the initial I1561V change. A gene tree of NaV 1.4 suggests the I1561V mutations in each lineage are not identical-by-decent, but rather they arose independently. Convergence in the evolution of channel resistance is likely the result of shared biases in the two lineages of T. sirtalis-only a few mutational routes can confer TTX resistance while maintaining the conserved function of voltage-gated sodium channels.

Age-related sex differences in body condition and telomere dynamics of red-sided garter snakes.

Life-history strategies vary dramatically between the sexes, which may drive divergence in sex-specific senescence and mortality rates. Telomeres are tandem nucleotide repeats that protect the ends of chromosomes from erosion during cell division. Telomeres have been implicated in senescence and mortality because they tend to shorten with stress, growth and age. We investigated age-specific telomere length in female and male red-sided garter snakes, Thamnophis sirtalis parietalis We hypothesized that age-specific telomere length would differ between males and females given their divergent reproductive strategies. Male garter snakes emerge from hibernation with high levels of corticosterone, which facilitates energy mobilization to fuel mate-searching, courtship and mating behaviours during a two to four week aphagous breeding period at the den site. Conversely, females remain at the dens for only about 4 days and seem to invest more energy in growth and cellular maintenance, as they usually reproduce biennially. As male investment in reproduction involves a yearly bout of physiologically stressful activities, while females prioritize self-maintenance, we predicted male snakes would experience more age-specific telomere loss than females. We investigated this prediction using skeletochronology to determine the ages of individuals and qPCR to determine telomere length in a cross-sectional study. For both sexes, telomere length was positively related to body condition. Telomere length decreased with age in male garter snakes, but remained stable in female snakes. There was no correlation between telomere length and growth in either sex, suggesting that our results are a consequence of divergent selection on life histories of males and females. Different selection on the sexes may be the physiological consequence of the sexual dimorphism and mating system dynamics displayed by this species.

To stress or not to stress: Physiological responses to tetrodotoxin in resistant gartersnakes vary by sex

The activation of the hypothalamic pituitary adrenal (HPA) axis is one of the most important physiological processes in coping with any deviation in an organism's homeostasis. This activation and the secretion of glucocorticoids, such as corticosterone, allow organisms to cope with perturbations and return to optimal physiological functioning as quickly as possible. In this study, we examined the HPA axis activation in common gartersnakes (Thamnophis sirtalis) as a response to a natural toxin, tetrodotoxin (TTX). This neurotoxin is found in high levels in the Rough-skinned Newt (Taricha granulosa), which is a prey item for these snakes. To consume this toxic prey, these snakes have evolved variable resistance. We hypothesized that the more resistant individuals would show a lower HPA axis response than less resistant individuals, as measured by corticosterone (CORT) and bactericidal ability, which is a functional downstream measurement of CORT's activity. We determined “resistance level” for tetrodotoxin from each individual snake by determining the dose which reduced race speed by 50%. Individuals were injected them with an increasing amount of tetrodotoxin (10, 25, and 50 MAMUs) to determine this value. Thirty minutes after every injection, we gathered blood samples from each snake. Our results show that, while there were no significant differences among individual CORT levels in a dose-dependent manner, female snakes did have a larger stress response when compared to both males and juveniles. Different life-histories could explain why females were able to mount a higher HPA axis response. However, TTX had no downstream effects on bactericidal ability, although juveniles had consistently lower values than adults. Our research shows a possible dichotomy between how each sex manages tetrodotoxin and gives way for a more comprehensive analysis of tetrodotoxin in an ecological context.

Patterns of stress responses shift during seasonal life-history transitions: An analysis comparing baseline, maximal and integrated corticosterone in female red-sided garter snakes (Thamnophis sirtalis parietalis)

Glucocorticoids often rise and fall with a variety of external and internal cues and frequently vary among life-history stages. This suggests that changing glucocorticoids may coordinate life-history transitions. To explore this hypothesis, we asked if the time-course of stress-induced glucocorticoid levels differ between two life-history transitions (i.e., spring and fall migration) in female red-sided garter snakes (Thamnophis sirtalis parietalis). We collected non-migratory females from a communal den and migratory females from a road along the migration route and treated them with 4h of capture stress; plasma corticosterone was measured before, during and after capture stress. During the spring, den-collected females exhibited a stress-induced peak in corticosterone at an earlier sampling time than migrating, road-collected females. Because the pattern of corticosterone responses varied with migratory state, negative feedback on and/or sensitivity of the hypothalamus-pituitaryadrenal (HPA) axis may be linked to spring migration. During the fall, capture stress elicited an increase in corticosterone in den-collected females but not in migrating, road-collected females. Baseline corticosterone was higher and both maximal and integrated corticosterone responses were lower during the fall compared to spring, indicating that stress responses are smaller when baseline corticosterone is elevated, perhaps due to a “ceiling effect”. These data suggest that HPA axis regulation changes during seasonal migration, possibly via altering negative feedback, HPA axis sensitivity, or some other mechanism. This study supports the hypothesis that glucocorticoids coordinate life-history events and suggests that examining a suite of stress response characteristics is most informative for understanding the function of HPA modulation.

Geographic variation and within-individual correlations of physiological stress markers in a widespread reptile, the common garter snake (Thamnophis sirtalis)

Characterizing the baseline and stress-induced hormonal, metabolite, and immune profiles of wild animals is important to assess the impacts of variable environments, including human-induced landscape changes, on organismal health. Additionally, the extent to which these profiles are coordinated across physiological systems within individuals remains an important question in understanding how stressors can differentially affect aspects of an individual's physiology. Here, we present data from wild populations of the common garter snake (Thamnophis sirtalis) on both baseline and stress-induced biomarkers: plasma corticosterone (CORT) concentration, plasma glucose concentration, and whole blood heterophil:lymphocyte ratio. Using a standardized restraint protocol with individuals from populations in disparate portions of this species' range – the Sierra Nevada Mountains of California and the plains of Iowa – we collected blood plasma samples at nine time points over three days. Both CORT and glucose response curves differed between georegions, with Iowa snakes attaining higher glucose concentration and maintaining elevated CORT and glucose levels for a longer duration. Additionally, both the total amount and proportional increases of CORT and glucose were lower in larger and therefore older snakes, suggesting ontogenetic shifts in stress perception or response. Within-individual correlation among the three physiological indicators was significant at the time of capture, absent after 3h in captivity, and partially restored after 3days in captivity, demonstrating the effect of stress on the relationships among these physiological systems. Together, these results provide further evidence for the great physiological flexibility of ectothermic tetrapods in maintaining homeostasis across a range of factors.

Maternal investment and delayed feeding in neonatal Lake Erie watersnakes: a life‐history strategy

AbstractNeonatal growth can have lasting consequences on survival and reproduction. In many taxa, larger, faster growing neonates experience higher survival, reach sexual maturity more rapidly, and achieve higher lifetime fecundity. In contrast to their closest relatives, Lake Erie watersnakes grow slower, mature later, and may delay feeding until after their first hibernation. To determine if Lake Erie watersnakes do indeed delay feeding until after their first hibernation, we compared age class 0 (between birth and first hibernation) and age class 1 (emergence from hibernation and through the first full season) snakes in the field and in the laboratory. In the field, only 0.6% of pre‐hibernation neonates were found to contain prey, while 11.9% of post‐hibernation neonates contained prey. During captive feeding experiments, the probability of eating was positively correlated with age (binary logistic regression, Wald = 25.354, P < 0.001). To clarify the underlying mechanism for delayed feeding, we compared neonatal yolk and fat reserves among species with delayed feeding (Lake Erie watersnakes, Graham's crayfish‐eating snakes), species that commence feeding immediately after birth (common gartersnakes, Dekay's brown snakes) and a species falling between these extremes (queen snakes). Like Lake Erie watersnakes, Graham's crayfish‐eating snakes contained large energy reserves (yolk + fat body mass to carcass mass ratio = 0.39 and 0.24, respectively), common gartersnakes and Dekay's brown snakes lacked measurable reserves (0.00), and queen snakes contained intermediate reserves (0.17). Taken together, this evidence suggests that neonatal Lake Erie watersnakes do delay feeding until after their first hibernation and contain large energy reserves at birth to facilitate this behavior. While most snake life‐history studies have focused on the trade‐off between offspring size and number, Lake Erie watersnake females invest large amounts of energy in offspring condition.

Cryptosporidium testudinis sp. n., Cryptosporidium ducismarci Traversa, 2010 and Cryptosporidium tortoise genotype III (Apicomplexa: Cryptosporidiidae) in tortoises.

Understanding of the diversity of species of Cryptosporidium Tyzzer, 1910 in tortoises remains incomplete due to the limited number of studies on these hosts. The aim of the present study was to characterise the genetic diversity and biology of cryptosporidia in tortoises of the family Testudinidae Batsch. Faecal samples were individually collected immediately after defecation and were screened for presence of cryptosporidia by microscopy using aniline-carbol-methyl violet staining, and by PCR amplification and sequence analysis targeting the small subunit rRNA (SSU), Cryptosporidium oocyst wall protein (COWP) and actin genes. Out of 387 faecal samples from 16 tortoise species belonging to 11 genera, 10 and 46 were positive for cryptosporidia by microscopy and PCR, respectively. All samples positive by microscopy were also PCR positive. Sequence analysis of amplified genes revealed the presence of the Cryptosporidium tortoise genotype I (n = 22), C. ducismarci Traversa, 2010 (n = 23) and tortoise genotype III (n = 1). Phylogenetic analyses of SSU, COWP and actin gene sequences revealed that Cryptosporidium tortoise genotype I and C. ducismarci are genetically distinct from previously described species of Cryptosporidium. Oocysts of Cryptosporidium tortoise genotype I, measuring 5.8-6.9 µm × 5.3-6.5 µm, are morphologically distinguishable from C. ducismarci, measuring 4.4-5.4 µm × 4.3-5.3 µm. Oocysts of Cryptosporidium tortoise genotype I and C. ducismarci obtained from naturally infected Russian tortoises (Testudo horsfieldii Gray) were infectious for the same tortoise but not for Reeve's turtles (Mauremys reevesii [Gray]), common garter snake (Thamnophis sirtalis [Linnaeus]), zebra finches (Taeniopygia guttata [Vieillot]) and SCID mice (Mus musculus Linnaeus). The prepatent period was 11 and 6 days post infection (DPI) for Cryptosporidium tortoise genotype I and C. ducismarci, respectively; the patent period was longer than 200 days for both cryptosporidia. Naturally or experimentally infected tortoises showed no clinical signs of disease. Our morphological, genetic, and biological data support the establishment of Cryptosporidium tortoise genotype I as a new species, Cryptosporidium testudinis sp. n., and confirm the validity of C. ducismarci as a separate species of the genus Cryptosporidium.

Density‐Dependent Foraging and Interference Competition by Common Gartersnakes are Temperature Dependent

AbstractThe ideal free distribution (IFD) predicts that optimal foragers will select foraging patches to maximize food rewards and that groups of foragers should thus be distributed between food patches in proportion to the availability of food in those patches. Because many of the underlying mechanisms of foraging are temperature dependent in ectotherms, the distribution of ectothermic foragers between food patches may similarly depend on temperature because the difference in fitness rewards between these patches may change with temperature. We tested the hypothesis that the distribution of Common Gartersnakes (Thamnophis sirtalis) between food patches can be explained by an IFD, but that conformance to an IFD weakens as temperature departs from the optimal temperature because fitness rewards, interference competition and the number of individuals foraging are highest at the optimal temperature. First, we determined the optimal temperature for foraging. Second, we examined group foraging at three temperatures and three density treatments. Search time was optimized at 27°C, handling time at 29°C and digestion time at 32°C. Gartersnakes did not match an IFD at any temperature, but their distribution did change with temperature: snakes at 20°C and at 30°C selected both food patches equally, while snakes at 25°C selected the low food patch more at low density and the high food patch more at high density. Food consumption and competition increased with temperature, and handling time decreased with temperature. Temperature therefore had a strong impact on foraging, but did not affect the IFD. Future work should examine temperature‐dependent foraging in ectotherms that are known to match an IFD.

Wild snakes harbor West Nile virus

West Nile virus (WNV) has a complex eco-epidemiology with birds acting as reservoirs and hosts for the virus. Less well understood is the role of reptiles, especially in wild populations. The goal of our study was to determine whether a wild population of snakes in Pennsylvania harbored WNV. Six species of snakes were orally sampled in the summer of 2013 and were tested for the presence of WNV viral RNA using RT-PCR. Two Eastern Garter Snakes, Thamnophis sirtalis sirtalis tested positive for viral RNA (2/123, 1.62%). These results indicate a possible role for snakes in the complex transmission cycle of WNV.

Sex‐obsessed or just sociable? Non‐copulatory displays in the hamerkop

Sex‐obsessed or just sociable? Non‐copulatory displays in the hamerkop

Cutaneous and Subcutaneous Soft Tissue Tumours in Snakes: A Retrospective Study of 33 Cases

Cutaneous and subcutaneous soft tissue tumours have been rarely described in detail in snakes. Several malignant entities show strikingly similar histological patterns and therefore the term soft tissue sarcoma (STS) has become a standard histopathological diagnosis. The present study characterizes soft tissue tumours in 33 snakes. Samples included 29 surgically excised masses and four carcasses. Additionally, six animals were humanely destroyed and submitted for necropsy examination following tumour recurrence. Benign neoplasms (n=8) were described as lipomas of varying differentiation. Recurrence was observed in two of five snakes in which the clinical course was recorded. Malignant neoplasms (n=25) were diagnosed as STS and graded according to a three-point system previously applied to canine STS. Five (20%) of the primary tumours were classified as grade 1, eleven (44%) as grade 2 and nine (36%) as grade 3 sarcomas. Clinically, recurrence of STS was observed in 11 of 17 cases with available follow-up information. Pathologically, multiple cutaneous metastases were found in one grass snake (Natrix natrix), while visceral metastases were observed in one carpet python (Morelia spilota) and two corn snakes (Pantherophis guttatus). Metastatic risk appears to increase with histological grade. Surgical excision generally represents the current therapy of choice for STS. This study includes the first reports of conventional lipomas in a ribbon snake (Thamnophis radix), angiolipomas in a black-headed python (Aspidites melanocephalus) and a corn snake as well as of STS in a Jamaican boa (Epicrates subflavus), emerald tree boa (Corallus caninus), grass snake (N. natrix), African house snake (Lamprophis fuliginosus), California kingsnake (Lampropeltis getula californiae) and common garter snake (Thamnophis sirtalis).

Effects of a mining‐altered environment on individual fitness of amphibians and reptiles

AbstractPollution can affect wildlife directly through toxicity and indirectly through changes in biotic and abiotic factors, however, how these mechanisms interact in affecting free‐ranging animals remains poorly understood. By examining effects on individual fitness proxies, we aimed to determine the mechanisms behind documented amphibian and reptile declines in barren landscapes where vegetation was nearly decimated due to atmospheric metal and sulfur deposition from smelting operations in Sudbury, Ontario, Canada. We examined individual characteristics of snakes (Thamnophis sirtalis, Storeria occipitomaculata) and frogs (Lithobates clamitans, L. pipiens, L. septentrionalis) in replicated barren and reference sites, including standard metabolic rates (in snakes), body condition (all species), female reproductive status (snakes), limb deformity (frogs), and fluctuating asymmetry (all species). Frogs in barren sites (except L. septentrionalis) were in poor body condition, and all but one case of limb deformities in frogs were found in barren sites. However, neither frogs nor snakes exhibited elevated frequency of fluctuating asymmetry in barren sites. Standard metabolic rates (SMR) were not elevated in either snake species, suggesting that metal toxicity does not present an energetic burden. Instead, female T. sirtalis from barren sites exhibited lower SMR and were in poor body condition, which is a predicted response to reduced food intake. By contrast, SMR and body condition of S. occipitomaculata were not different between barren and reference sites. While physiological responses differed between T. sirtalis and S. occipitomaculata, both species exhibited lower reproductive rates (proportion of pregnant females) in barren sites compared to reference sites. Thus, energy deficiency may explain reduced reproduction in T. sirtalis while reduced availability of suitable gestation microhabitat may be a limiting factor for S. occipitomaculata. Populations in smelting‐impacted habitats include individuals with potentially compromised fitness. The low values of our measured fitness proxies appear to have resulted from pollution‐induced changes in biotic and abiotic factors, in addition to (for frogs) or rather than (for snakes) from the direct toxicity. These results suggest that impact assessment based solely on toxicological data and species distributions can underestimate the full impacts of pollution, highlighting the need for integrated examination at multiple levels of biological organization.

Comparing the Natural and Anthropogenic Sodium Channel Blockers Tetrodotoxin and Indoxacarb in Garter Snakes.

Synthetic chemicals, such as pesticides, are used in a variety of ways in the agricultural industry. Anthropogenic chemicals pose a unique challenge to organisms because of the lack of evolutionary history between the chemical and the organism. However, research has shown that some organisms have a resistance to these synthetic chemicals due to their evolved resistance to a natural compound with a similar structure or mode of action. Indoxacarb (INDOX) is a relatively new pesticide with a similar mode of action to that of tetrodotoxin (TTX). Tetrodotoxin is a naturally occurring toxin that is used as an antipredator defense in the rough-skinned newt (Taricha granulosa). Some populations of the common garter snake (Thamnophis sirtalis) have developed a resistance to tetrodotoxin. Here, we investigated the correlation between TTX and INDOX resistance in snakes. We hypothesized that INDOX would induce a much higher stress response than the naturally occurring TTX. We injected each snake with tetrodotoxin (1 mass-adjusted mouse unit). We did the same with mass-adjusted units of INDOX. We measured corticosterone, testosterone, and bactericidal ability. Our results show an acute stress response to INDOX, but not TTX through an elevate corticosterone and innate immune response, although there was no difference in testosterone concentration. These results suggest that, although INDOX may have a similar mechanism of action, garter snakes do not react in a similar manner as to TTX. This research gives a physiological perspective on the differences between naturally occurring compounds and synthetic compounds.

Evidence of Predation on Nestling Birds by Eastern Gartersnakes (<i>Thamnophis sirtalis sirtalis</i>)

Diet is an important aspect of the natural history of all animals, but diet can vary through space and time because of variations in prey availability. The diet of the Common Gartersnake (Thamnophis sirtalis) consists mainly of earthworms and frogs, but other prey items might be important when they are locally abundant. I report an observation of a female Eastern Gartersnake (Thamnophis sirtalis sirtalis) regurgitating 2 nestling birds in Ottawa, Ontario, Canada. Birds are seldom present in the diet of the Common Gartersnake. This rare food choice highlights the opportunistic nature of foraging by adult Common Gartersnakes and, further, demonstrates that diet depends not only on prey preference, but also on prey availability.

Differential fitness in field and forest explains density-independent habitat selection by gartersnakes.

The ideal free distribution concept predicts that organisms will distribute themselves between habitats in a density-dependent manner so that individuals, on average, achieve the same fitness in each habitat. In ectotherms, environmental temperature has a strong impact on fitness, but temperature is not depletable and thus not density dependent. Can density-dependent habitat selection occur in ectotherms when habitats differ in thermal quality? We used an observational study of habitat selection by small snakes in field and forest, followed by manipulative habitat selection and fitness experiments with common gartersnakes in enclosures in field and forest to test this hypothesis. Snakes were much more abundant in the field, the habitat with superior thermal quality, than in the forest. Gartersnakes in our controlled experiment only used the forest habitat when snake density was highest and when food was more abundant in the forest; habitat selection was largely density independent, although there was weak evidence of density dependence. No female gartersnake gave birth in the forest enclosures, whereas half of the females gave birth in the field enclosures. Growth rates of females were higher in field than in forest enclosures. Overall, our data indicate that temperature appears to be the most important factor driving the habitat selection of gartersnakes, likely because temperature was more limiting than food in our study system. Snakes, or at least temperate snakes, may naturally exist at population densities low enough that they do not exhibit density-dependent habitat selection.