Thamnophis Research Articles

Patterns of genetic differentiation in Thamnophis and Taricha from the Pacific Northwest

AbstractAim The co‐evolutionary interaction between the common garter snake, Thamnophis sirtalis, and the rough‐skinned newt, Taricha granulosa, takes place throughout much of the Pacific Northwest (North America). The biogeography of the Pacific Northwest has been heavily influenced by the last Pleistocene glaciation, which reached a maximum as late as 14,000 yr bp. We researched: (1) what type of population structure is present for garter snakes and newts, (2) whether the population structure of these species is consistent with a Pleistocene glaciation hypothesis, and (3) how population structure and migration possibly affect co‐evolution between these species.Location The Pacific Northwest of North America, specifically northern California, Oregon and Washington in the USA.Methods We sampled approximately 20 populations for each species from three different transects. Using microsatellite markers and tissue samples from both species, we quantified the population structure for both species. Individual‐based assignment tests were used to estimate contemporary migration rates.Results Both Th. sirtalis and Ta. granulosa exhibited little genetic differentiation among our study sites, even among those separated by large distances. Significant population structure was detected on multiple geographic scales. Differences in population structure were observed among transects and between garter snake and newt transects. Contemporary migration rate estimates indicate high levels of genetic exchange between populations.Main conclusions Prior to this study, little was known about the fine‐scale population structure of either species in this region. Patterns of population structure for garter snakes and newts reflect a shared biogeographical history affected by the Pleistocene glaciation in the Pacific Northwest. Both species apparently migrate frequently between populations, thus potentially retarding the process of adaptive co‐evolution. We find that populations from a northern coastal transect (Washington) are most likely to be locally adapted.

Experimental Infection of Common Garter Snakes(Thamnophis sirtalis)with West Nile Virus

The role of various reptilian species in the infectious cycle of several arboviruses is documented, but their role in that of West Nile virus (WNV) is uncertain. Common garter snakes (Thamnophis sirtalis) were infected subcutaneously with 10(5) plaque forming units (PFU) WNV-Isr 98, five of nine snakes became viremic, and five exhibited persistent low levels of neutralizing antibodies. Four of the parentally infected snakes died and high titers of virus were found in multiple organ samples. In contrast, orally infected garter snakes did not become viremic, but viral RNA was detected in cloacal swabs. Since oral infection of predator birds by WNV is known, their ingestion of infected snakes may also result in their becoming infected.

Sexual dimorphism of body and relative head sizes in neonatal common garter snakes

AbstractMorphological and behavioral differences between sexes are commonplace throughout the animal kingdom. Body size is one of the most obvious sex differences frequently found in snakes. However, the developmental origins of size differences in many species, including snakes, are not well known. We examined post‐natal variation in sexual size dimorphism in garter snakes Thamnophis sirtalis. The weights, body and tail lengths, and head sizes of male and female neonates born to mothers collected from ecologically dissimilar habitats on Beaver Island, Lake Michigan were compared. Sexual size dimorphism was prominent. Overall, males had significantly longer bodies and tails than females. Females were significantly heavier and had larger heads than male snakes. Maternal site affected head but not body measurements, perhaps due to differences in prey availability. The body condition of maternal females predicted neonatal body length. Significant litter variation suggests heritable variation in morphological traits possibly correlated with feeding success and survival.

Bald Eagle, <em>Haliaeetus leucocephalus</em>, Preying on Maritime Garter Snake, <em>Thamnophis sirtalis pallidulus</em>, on Cape Breton Island, Nova Scotia

At 1053 hours on 23 July 2006 a Bald Eagle, Haliaeetus leucocephalus, was observed in flight transporting in its talons a live Maritime Garter Snake, Thamnophis sirtalis pallidulus, at South Harbour (46°52'01.7"N, 60°26'45.8"W), Cape Breton Island, Victoria County, Nova Scotia.

Corticosterone and the transition from courtship behavior to dispersal in male red-sided garter snakes ( Thamnophis sirtalis parietalis)

Seasonal modulation of baseline glucocorticoid concentrations as well as the sensitivity of the hypothalamic-pituitary-adrenal (HPA) axis plays an important role in supporting critical life-history events such as seasonal reproduction and migration. Despite numerous studies on adrenocortical modulation, little is known about the exact timing of this seasonal modulation with respect to critical life-history stages. We tested the hypothesis that seasonal modulation of the HPA axis during the spring mating season in male red-sided garter snakes ( Thamnophis sirtalis parietalis) is temporally linked to the mechanisms regulating dispersal. We compared hormonal responses to capture stress in courting male red-sided garter snakes collected from the den site and den perimeter to those of dispersing snakes collected 0.6 km from the den. We also investigated possible changes in steroid hormones during the spring mating season. These studies support previous findings that plasma androgen and corticosterone concentrations significantly decline over the mating season. Our results demonstrate that males 0.6 km into a 15–20 km route to the feeding grounds have lower baseline corticosterone concentrations than male snakes actively courting at the den. Dispersing males also exhibit a typical stress response marked by a significant increase in corticosterone while actively courting males do not. Capture stress did not significantly influence androgen concentrations of either courting or dispersing male red-sided garter snakes. There were no significant differences in body composition indices among male snakes collected from the den, den perimeter, or 0.6 km away from the den. However, we did observe a significant negative correlation between baseline corticosterone levels and body composition indices. These data suggest that breeding is a distinct stage accompanied by specific physiological parameters that differ from those during dispersal to the feeding grounds. Our results indicate that declining baseline corticosterone concentrations may play a role in the behavioral switch between actively courting and dispersing (i.e., feeding) in the late spring.

The energetic consequences of dietary specialization in populations of the garter snake, Thamnophis elegans

We investigated the intraspecific variation in digestive energetics between dietary specialist and generalist populations of the Western Terrestrial garter snake (Thamnophis elegans) in northern California. Coastal populations have a specialized diet of slugs and inland populations have a generalized diet of fish, anurans, mice and leeches. The difference in prey preference between the two populations is congenital, heritable and ontogenetically stable. To examine energetic specializations and trade-offs in these populations, we measured the net assimilation efficiency of each snake population on both slug (Ariolimax columbianus) and fish (Rhinichthys osculus) diets. The net assimilation efficiency was measured during digestion of a meal and continued until metabolic rate re-attained prefeeding levels. Coastal snakes were able to utilize 62% more of the ingested energy towards production from slug diets through both increased assimilation of nutrients and reduced digestive costs. For fish, assimilation and digestive costs were the same in both coastal and inland populations. These results support the hypothesis that snakes with specialized diets can evolve physiological traits to extract nutrients more efficiently. However, there was no apparent trade-off on the more generalized diet that was associated with this specialization.

Identification and Characterization of New Protein Chemoattractants in the Frog Skin Secretome

The vomeronasal organ is a chemosensory organ present in most vertebrates and involved in chemical communication. In the last decade, the deciphering of the signal transduction process of this organ has progressed. However, less is known about the vomeronasal organ ligands and their structure-function relationships. Snakes possess a highly developed vomeronasal system that is used in various behaviors such as mating, predator detection, or prey selection, making this group a suitable model for study of the vomeronasal chemoreception. In this work, we used a proteomics approach to identify and characterize proteins from frog cutaneous mucus proteome involved in prey recognition by snakes of the genus Thamnophis. Herein we report the purification and characterization of two proteins isolated from the frog skin secretome that elicit the vomeronasal organ-mediated predatory behavior of Thamnophis marcianus. These proteins are members of the parvalbumin family, which are calcium-binding proteins generally associated to muscular and nervous tissues. This is the first report that demonstrates parvalbumins are not strictly restricted to intracellular compartments and can also be isolated from exocrine secretions. Purified parvalbumins from frog muscle and mucus revealed identical chemoattractive properties for T. marcianus. Snake bioassay revealed the Ca(2+)/Mg(2+) dependence of the bioactivity of parvalbumins. So parvalbumins appear to be new candidate ligands of the vomeronasal organ.

Influence of income and capital on reproduction in a viviparous snake: direct and indirect effects

AbstractA central issue in life‐history studies is the extent to which organisms are ‘capital’ versus ‘income’ breeders (i.e. using stored resources vs. current food intake). Snakes are primarily capital breeders, but income during vitellogenesis can also contribute to reproductive output. In Manitoba, the garter snake Thamnophis sirtalis has a short active season, with mating occurring upon emergence from hibernation and vitellogenesis shortly thereafter. Given the brevity of this sequence, animals in these populations should be almost exclusively capital breeders. Consistent with this prediction, in this experiment the reproductive output of recently mated snakes was influenced by mass of the mother at the time of mating but not by her food intake shortly after mating. Food eaten during pregnancy contributed significantly to postpartum mass but not to litter mass. However, the initial mass of snakes influenced both postpartum and litter mass. For snakes given a high ration of food, a female's relative mass (i.e. adjusted for body length) significantly influenced the actual amount she consumed (relatively lighter snakes ate more). Perhaps most important, among pregnant snakes, those that were initially relatively heavier gave birth substantially and significantly earlier. Thus, snakes that are relatively heavy when they mate may gain further fitness benefits following pregnancy (e.g. more time to acquire resources and for offspring to grow before winter), which can also be interpreted as indirect effects on reproduction. These life‐history variations may be relevant to population dynamics.

Minimal overwintering temperatures of red-sided garter snakes (Thamnophis sirtalis parietalis): a possible cue for emergence?

Red-sided garter snakes ( Thamnophis sirtalis parietalis (Say in James, 1823)) in Manitoba, Canada, undergo 8 months of continuous winter dormancy prior to spring emergence. As in other ectothermic species, increases in ground temperature may be the cue for emergence from winter dormancy in these populations. To test this hypothesis, we measured body temperatures during winter dormancy by surgically implanting small temperature loggers into 32 female red-sided garter snakes before they entered their native hibernaculum. The following spring, we recaptured seven of the snakes implanted with temperature loggers. Body temperature declined gradually from mid-September (14.7 ± 0.24 °C, mean ± SE) to early April (1.1 ± 0.16 °C, mean ± SE) during winter dormancy, reaching minimal values approximately 1 month prior to spring emergence. Body temperatures of emerging snakes ranged from 0.5 °C during early spring to 6.3 °C during late spring (3.4 ± 0.84 °C, mean ± SE). These results do not support the hypothesis that an increase in ground temperature (and hence body temperature) is necessary for emergence from winter dormancy. We suggest that critically low temperatures (i.e., 0.5–1 °C) are a Zeitgeber entraining an endogenous circannual cycle that regulates snake emergence. These results offer new insight into the mechanisms regulating seasonal emergence from winter dormancy.

Female Snake Sex Pheromone Induces Membrane Responses in Vomeronasal Sensory Neurons of Male Snakes

The vomeronasal organ (VNO) is important for activating accessory olfactory pathways that are involved in sexually dimorphic mating behavior. The VNO of male garter snakes is critically important for detection of, and response to, female sex pheromones. In the present study, under voltage-clamp conditions, male snake VNO neurons were stimulated with female sexual attractiveness pheromone. Thirty-nine of 139 neurons exhibited inward current responses (reversal potential: -10.6 +/- 2.8 mV). The amplitude of the inward current was dose dependent, and the relationship could be fitted by the Hill equation. Under current-clamp conditions, application of pheromone produced membrane depolarizing responses and increases in firing frequency. These results suggest that the female pheromone directly affects male snake VNO neurons and results in opening of ion channels, thereby converting the pheromone signal to an electrical signal. The response to female pheromone is sexually dimorphic, that is, the pheromone does not evoke responses in VNO neurons of female snakes. An associated finding of the present study is that the female sex pheromone, which is insoluble in aqueous solutions, became soluble in the presence of Harderian gland homogenate.

Ecology and Morphology of Thamnophis sirtalis pallidulus (Maritime Garter Snakes) on Georges Island, Nova Scotia

Georges Island supports an unusually dense and morphologically diverse population of Thamnophis sirtalis pallidulus. Between May and October 1993, a total of 391 garter snakes were marked. The total male:female ratio was 0.8:1.0; population density of snakes > 1 year old was minimally estimated at 120/ha. Thamnophis sirtalis pallidulus on Georges Island were docile: 50% of encounters showed no response or only a delayed anti-predator response to human handling. The population is unusual morphologically; it contains melanistic and partly melanistic individuals and an unusually high proportion (48%) of individuals with abnormal ventral and/or subcaudal scales. Adult garter snakes on Georges Island are slightly smaller than adults in mainland Nova Scotia populations. Despite a less diverse diet than is available to its mainland counterparts, the island population appears to thrive.

“As if Reviewing His Life”: Bull Lodge’s Narrative and the Mediation of Self-Representation

One night my Father had a vision in his sleep, he saw an old man standing at a distance on the horizon of a low hill. . . . Then the old man spoke, saying, “I came to tell you of my life, I give it to you, you will live until you die of old age, but before that time you will pass away in order that you may demonstrate the power which I am giving you. The power to arise after you have passed away.” —Garter Snake, in Fred P. Gone’s “Bull Lodge’s Life”

Temporal dynamics of emergence and dispersal of garter snakes from a communal den in Manitoba

Although garter snakes at communal overwintering dens on the Canadian prairies have attracted considerable behavioural ecology research, previous studies have relied upon sampling of active animals to describe broad patterns of distribution and abundance of snakes within the den population. We conducted a mark–recapture study to directly quantify temporal and spatial variation in the phenotypic traits (sex, size, body condition) of snakes at the den itself, and those dispersing through woodland 50 m away. Captures of 909 snakes on the days they emerged, and 6653 snakes as they dispersed, revealed massive spatiotemporal heterogeneity in phenotypic traits among samples. Day-to-day variation in weather conditions affected numbers and sex ratios of emerging and dispersing snakes; for example, small females dispersed in greater numbers after unusually cold nights, when harassment by courting males was reduced. Most snakes stayed at the den only briefly (<5 days) prior to dispersal, so that sampling at the den itself (the only evidence available from most previous studies) underestimates the number of animals in the population, as well as the proportions of females, of small adult males and of juvenile animals. Overall, the heterogeneous and temporally dynamic distributions of phenotypic traits (such as sex and size) among our samples are predictable on the basis of the central roles of male–male competition and sexual conflict in the mating system of these snakes. Surprisingly, however, many of the snakes that overwinter at this den play no part in den-based breeding aggregations

Immobility and supination in garter snakes (Thamnophis elegans) following handling by human predators.

A common response of animals to physical restraint is tonic immobility. The authors observed the behavior of garter snakes, Thamnophis elegans, in the field to determine the frequency of immobility as a response to handling. Most snakes fled after release, but the remainder remained immobile, sometimes on their backs (supination), for up to 10 min. Immobility was seen most often in pregnant snakes, which also were more likely than other snakes not to have moved before capture. Failure to move, either before or after capture, might be a consequence of the limited locomotory ability of gravid snakes. However, the lack of observations of interactions between snakes and their natural predators impedes researchers' understanding of the antipredator value, if any, of tonic immobility.

Comparison of erythrocyte osmotic fragility among amphibians, reptiles, birds and mammals

Erythrocyte osmotic fragility (EOF) is a measure of erythrocyte strength and its ability to withstand varying osmotic gradients. Erythrocyte osmotic fragility was compared among a variety of terrestrial ectotherms, aquatic/semiaquatic ectotherms and endotherms. We hypothesized that EOF should be lowest in those animals that are aquatic and whose red blood cells would likely be exposed to hypotonic conditions (i.e., some amphibians and reptiles). The greatest erythrocyte osmotic resistance (i.e., least fragile erythrocytes) was found in the bullfrog (Rana catesbeiana), followed in decreasing order by the larval tiger salamander (Ambystoma tigrinum), marine toad (Bufo marinus), ornate box turtle (Terrepene ornata ornata), painted turtle (Chrysemys picta bellii), red-eared slider (Trachemys scripta elegans), red-sided garter snake (Thamnophis sirtalis parietalis), domestic chicken (Gallus gallus), cotton rat (Sigmodon hispidus), Sprague-Dawley rat (Rattus norvegicus), and eastern woodrat (Neotoma floridana). A significantly higher erythrocyte osmotic resistance was detected in the aquatic/semiaquatic amphibians (R. catesbeiana and A. tigrinum) than in the terrestrial amphibian (B. marinus). However, among reptilian species, no significant differences were noted between terrestrial and aquatic/semiaquatic species. The erythrocytes of ectotherms possessed a greater osmotic resistance than those of endotherms. Such differences might be necessary because of exposure to wide temperature ranges in ectotherms and/or the long erythrocyte life span of ectothermic red blood cells relative to those of endotherms. Nucleated erythrocytes were more osmotically resistant than were non-nucleated erythrocytes. Additionally, erythrocyte osmotic resistance was correlated with erythrocyte mean cell volume. Larger erythrocytes were more osmotically resistant than smaller erythrocytes. Polynomial regression equations computed to approximate these data showed the cubic polynomial to be an excellent fit when data for the larval tiger salamander were excluded. Our data suggest that erythrocyte volume may play a significant role in erythrocyte osmotic resistance.

Population Structure of the Giant Garter Snake, Thamnophis gigas

The giant garter snake, Thamnophis gigas, is a threatened species endemic to California’s Central Valley. We tested the hypothesis that current watershed boundaries have caused genetic differentiation among populations of T. gigas. We sampled 14 populations throughout the current geographic range of T. gigas and amplified 859 bp from the mitochondrial gene ND4 and one nuclear microsatellite locus. DNA sequence variation from the mitochondrial gene indicates there is some genetic structuring of the populations, with high FST values and unique haplotypes occurring at high frequency in several populations. We found that clustering populations by watershed boundary results in significant between-region genetic variance for mtDNA. However, analysis of allele frequencies at the microsatellite locus NSU3 reveals very low FST values and little between-region variation in allele frequencies. The discordance found between mitochondrial and microsatellite data may be explained by aspects of molecular evolution and/or T. gigas life history characteristics. Differences in effective population size between mitochondrial and nuclear DNA, or male-biased gene flow, result in a lower migration rate of mitochondrial haplotypes relative to nuclear alleles. However, we cannot exclude homoplasy as one explanation for homogeneity found for the single microsatellite locus. The mitochondrial nucleotide sequence data supports conservation practices that identify separate management units for T. gigas.

Flexible mate choice: a male snake's preference for larger females is modified by the sizes of females encountered

Why do males exert strong mate choice in some taxa but not others? Theory suggests that mate discrimination will enhance male fitness when encounter rates with potential mates are high, when those potential mates vary in the fitness consequences likely to accrue from an attempted insemination, and when courting one female reduces the male’s opportunity to court other females. One widespread form of mate choice involves a trend for males of many ectothermic species to court larger (and thus, more fecund) females. To test whether such preferences are dynamically adjusted to local conditions, we studied male preference for larger females in red-sided garter snakes, Thamnophis sirtalis parietalis, near a communal den in Manitoba, Canada. Courting a small female imposes a high opportunity cost for a male in the centre of the den, because many large and easily located females are nearby. In the surrounding woodland, in contrast, a male that neglects a small female is unlikely to encounter a larger substitute partner. In arena trials, male snakes from the den selected larger females more than did males from the surrounding woodland. Manipulating a den male’s exposure to females (none, large, small) for 60 min led males to adjust their criteria for courtship depending upon the sizes of females encountered. Hence, the local environment can modify courtship criteria, with male garter snakes adjusting their mate choice selectivity based upon spatial and temporal factors that affect the opportunity costs of courtship.

Mate location tactics in garter snakes: effects of rival males, interrupted trails and non‐pheromonal cues

Summary The ability to follow substrate‐deposited scent trails using sophisticated vomeronasal abilities is a key feature of snake biology. However, previous research on this topic has derived mostly from a highly artificial test situation: captive snakes following continuous trails over structurally simple homogeneous substrates, in the absence of any other cues (e.g. visual or olfactory) either from the target of that search or from other snakes. Our field experimental trials with Red‐Sided Garter Snakes (Thamnophis sirtalis parietalis) at a communal den in Manitoba explored wider issues in mate location. Males followed a female's trail less successfully if she had recently been followed by rival males, suggesting that trailing males obscured the cues left by the female. Males that lost contact with a female's trail exhibited stereotyped, spatially extensive and prolonged circling behaviour centred on the last point at which the trail was detectable; in nature, this behaviour would maximize the male's ability to relocate a discontinuous trail. Trail‐following males switched from chemical to visual cues as soon as they were close to a female, enhancing speed but decreasing the accuracy of mate location (because males could distinguish between females and other males based on either visual or airborne olfactory cues). Incorporating additional cues and broader spatial scales can clarify aspects of trail‐following behaviour not evident from studies in small, highly simplified laboratory situations.

DOES LARGE BODY SIZE IN MALES EVOLVE TO FACILITATE FORCIBLE INSEMINATION? A STUDY ON GARTER SNAKES

A trend for larger males to obtain a disproportionately high number of matings, as occurs in many animal populations, typically is attributed either to female choice or success in male-male rivalry; an alternative mechanism, that larger males are better able to coercively inseminate females, has received much less attention. For example, previous studies on garter snakes (Thamnophis sirtalis parietalis) at communal dens in Manitoba have shown that the mating benefit to larger body size in males is due to size-dependent advantages in male-male rivalry. However, this previous work ignored the possibility that larger males may obtain more matings because of male-female interactions. In staged trials within outdoor arenas, larger body size enhanced male mating success regardless of whether a rival male was present. The mechanism involved was coercion rather than female choice, because mating occurred most often (and soonest) in females that were least able to resist courtship-induced hypoxic stress. Males do physically displace rivals from optimal positions in the mating ball, and larger males are better able to resist such displacement. Nonetheless, larger body size enhances male mating success even in the absence of such male-male interactions. Thus, even in mating systems where males compete physically and where larger body size confers a significant advantage in male-male competition, the actual selective force for larger body size in males may relate to forcible insemination of unreceptive females. Experimental studies are needed to determine whether the same situation occurs in other organisms in which body-size advantages have been attributed to male-male rather than male-female interactions.

Population genetic analysis identifies source–sink dynamics for two sympatric garter snake species (Thamnophis elegans and Thamnophis sirtalis)

Population genetic structure can be shaped by multiple ecological and evolutionary factors, but the genetic consequences of these factors for multiple species inhabiting the same environment remain unexplored. We used microsatellite markers to examine the population structures of two coexisting species of garter snake, Thamnophis elegans and Thamnophis sirtalis, to determine if shared landscape and biology imposed similar population genetic structures. These snakes inhabit a series of ponds, lakes and flooded meadows in northern California and tend to converge on prey type wherever they coexist. Both garter snakes had comparable effective population sizes and bidirectional migration rates (estimated using a maximum-likelihood method based on the coalescent) with low but significant levels of genetic differentiation (F(ST) = 0.024 for T. elegans and 0.035 for T. sirtalis). Asymmetrical gene flow revealed large source populations for both species as well as potential sinks, suggesting frequent extinction-recolonization and metapopulation dynamics. In addition, we found a significant correlation between their genetic structures based on both pairwise F(ST)s for shared populations (P = 0.009) and for bidirectional migration rates (P = 0.024). Possible ecological and evolutionary factors influencing similarities and differences in genetic structure for the two species are discussed. Genetic measures of effective population size and migration rates obtained in this study are also compared with estimates obtained from mark-recapture data.