Gordon W. Schuett

Evolution of rattlesnakes (Viperidae; Crotalus) in the warm deserts of western North America shaped by Neogene vicariance and Quaternary climate change

During Pleistocene, the Laurentide ice sheet rearranged and diversified biotic distributions in eastern North America, yet had minimal physical impact in western North America where lineage diversification is instead hypothesized to result from climatic changes. If Pleistocene climatic fluctuations impacted desert species, the latter would reflect patterns of restricted gene flow concomitant with indications of demographic bottlenecks. Accordingly, molecular evidence for refugia should be present within these distributions and for subsequent range expansions as conditions improved. We sought answers to these questions by evaluating mitochondrial DNA (mtDNA) sequences from four species of rattlesnakes [Crotalus mitchellii (speckled rattlesnake), Crotalus cerastes (sidewinder), Crotalus tigris (tiger rattlesnake), Crotalus ruber (red diamond rattlesnake)] with distributions restricted to desert regions of southwestern North America. We inferred relationships using parsimony and maximum likelihood, tested intraspecific clades for population expansions, applied an isolation‐with‐migration model to determine bi‐directional migration rates (m) among regions, and inferred divergence times for species and clades by applying a semiparametric penalized likelihood approach to our molecular data. Evidence for significant range expansion was present in two of eight regions in two species (Crotalus mitchellii pyrrhus, C. tigris region north). Two species (C. cerastes, C. mitchellii) showed a distribution concomitant with northward displacement of Baja California from mainland México, followed by vicariant separation into subclades. Effects of Pleistocene climate fluctuations were found in the distributions of all four species. Three regional diversification patterns were identified: (i) shallow genetic diversity that resulted from Pleistocene climatic events (C. tigris, C. ruber); (ii) deep Pleistocene divisions indicating allopatric segregation of subclades within refugia (C. mitchellii, C. cerastes); and (iii) lineage diversifications that extended to Pliocene or Late Miocene (C. mitchellii, C. cerastes). Clade‐diversifying and clade‐constraining effects impacted the four species of rattlesnakes unequally. We found relatively high levels of molecular diversification in the two most broadly distributed species (C. mitchellii, C. cerastes), and lower levels of genetic diversification in the two species (C. tigris, C. ruber) whose ranges are relatively more restricted. Furthermore, in several cases, the distributions of subspecies were not congruent with our molecular information. We suggest regional conservation perspectives for southwestern deserts cannot rely upon subspecies as biodiversity surrogates, but must instead employ a molecular and deep historical perspective as a primary mechanism to frame biodiversity reserves within this region.

Facultative parthenogenesis discovered in wild vertebrates

Facultative parthenogenesis (FP)—asexual reproduction by bisexual species—has been documented in a variety of multi-cellular organisms but only recently in snakes, varanid lizards, birds and sharks. Unlike the approximately 80 taxa of unisexual reptiles, amphibians and fishes that exist in nature, FP has yet to be documented in the wild. Based on captive documentation, it appears that FP is widespread in squamate reptiles (snakes, lizards and amphisbaenians), and its occurrence in nature seems inevitable, yet the task of detecting FP in wild individuals has been deemed formidable. Here we show, using microsatellite DNA genotyping and litter characteristics, the first cases of FP in wild-collected pregnant females and their offspring of two closely related species of North American pitviper snakes—the copperhead (Agkistrodon contortrix) and cottonmouth (Agkistrodon piscivorus). Our findings support the view that non-hybrid origins of parthenogenesis, such as FP, are more common in squamates than previously thought. With this confirmation, FP can no longer be viewed as a rare curiosity outside the mainstream of vertebrate evolution. Future research on FP in squamate reptiles related to proximate control of induction, reproductive competence of parthenogens and population genetics modelling is warranted.

Functional and numerical responses of predators: where do vipers fit in the traditional paradigms?

Snakes typically are not considered top carnivores, yet in many ecosystems they are a major predatory influence. A literature search confirmed that terrestrial ectotherms such as snakes are largely absent in most discussions of predator‐prey dynamics. Here, we review classical functional and numerical responses of predator‐prey relationships and then assess whether these traditional views are consistent with what we know of one group of snakes (true vipers and pitvipers: Viperidae). Specifically, we compare behavioural and physiological characteristics of vipers with those of more commonly studied mammalian (endothermic) predators and discuss how functional and numerical responses of vipers are fundamentally different. Overall, when compared to similar‐sized endotherms, our analysis showed that vipers have: (i) lower functional responses owing primarily to longer prey handling times resulting from digestive limitations of consuming large prey and, for some adults, tolerance of fasting; (ii) stronger numerical responses resulting from higher efficiency of converting food into fitness currency (progeny), although this response often takes longer to be expressed; and (iii) reduced capacity for rapid numerical responses to short‐term changes in prey abundance. Given these factors, the potential for viperids to regulate prey populations would most likely occur when prey populations are low. We provide suggestions for future research on key issues in predator‐prey relationships of vipers, including their position within the classical paradigms of functional and numerical responses.

Sympatric rattlesnakes with contrasting mating systems show differences in seasonal patterns of plasma sex steroids

Long-term field studies conducted in Arizona show that two species of sympatric rattlesnakes differ in the structure of their mating systems, primarily in frequency and timing of mating seasons, despite exposure to identical environmental conditions. The western diamond-backed rattlesnake, Crotalus atrox, has two distinct mating seasons within a single annual spermatogenic cycle. The first mating season occurs from late August to early October. Following a hibernation period of 4 months, the second mating season occurs from mid-March to early May. Because there is a mating season in spring, long-term sperm storage by females during winter is facultative. In contrast, the black-tailed rattlesnake, Crotalus molossus, has a single mating season (mid-July to early September) within a single annual spermatogenic cycle. Due to the absence of a mating season in spring, long-term sperm storage by females during winter is obligatory. In both species, ovulation and fertilization occur in spring, and offspring are produced from mid-July to early September. Based on these robust data, we tested the hypotheses that seasonal patterns of plasma sex steroids (testosterone, 5α-dihydrotestosterone, and 17β-estradiol) differ between males in wild populations of C. atrox and C. molossus, and that peak levels would be coincident with the mating seasons. Specifically, we predicted that there would be two peaks of sex steroids in C. atrox and one peak in C. molossus, and that baseline levels would be detected outside the periods of mating and spermatogenesis. Our results supported these predictions. Furthermore, absolute concentrations of plasma testosterone and 5α-dihydrotestosterone, but not 17β-estradiol, were higher in C. atrox than in C. molossus. We discuss a possible scenario for the evolution of the different mating seasons in these sympatric rattlesnakes, and advocate that comparative approaches to address such questions should integrate proximate and ultimate causation to increase explanatory power.

Multiscale Influences of Landscape Composition and Configuration on the Spatial Ecology of Eastern Diamond-backed Rattlesnakes (Crotalus adamanteus)

Abstract Landscape composition and configuration affect ecological processes at the population and community levels, but few studies have demonstrated the effects of landscape pattern on individuals. Because heterogeneity influences abundance and distribution of critical resources, it is hypothesized that it indirectly affects home range size of individuals. To examine the spatial ecology of the declining Eastern Diamond-backed Rattlesnake (Crotalus adamanteus; EDB), we conducted a two-year study in southwestern Georgia. We obtained home range estimates via radio-telemetry, employed Euclidean distance analysis to examine habitat associations at two spatial scales, and used the software program FRAGSTATS to analyze landscape heterogeneity and examine its effect on home range size. Although no significant habitat associations were detected, there were trends for a positive association with pine habitat at the landscape scale and a negative association with agriculture within the home range. Home range size was negatively correlated with several landscape metrics representing heterogeneity in patch configuration, such that individuals in heterogeneous landscapes had small home ranges. This relationship was strongest at three spatial scales: the first was similar to mean home range size of EDBs; and the others were three and four times as large as the largest home range recorded. Together, these results suggest that management regimes to enhance population densities of EDBs emphasize the preservation of pine uplands, while maintaining a mosaic of other habitat types, and limit the conversion of forest to agriculture. Also, our results underscore the importance of using robust analytical tools and multiscale approaches in studies of spatial ecology.

The reproductive biology of male cottonmouths (Agkistrodon piscivorus): Do plasma steroid hormones predict the mating season?

To better understand the proximate causation of the two major types of mating seasons described for North American pitvipers, we conducted a field study of the cottonmouth (Agkistrodon piscivorus) in Georgia from September 2003 to May 2005 that included an extensive observational regime and collection of tissues for behavioral, anatomical, histological, and hormone analysis. Enzyme immunoassays (EIA) of plasma samples and standard histological procedures were conducted on reproductive tissues. Evidence from the annual testosterone (T) and sexual segment of the kidney (SSK) cycle and their relationship to the spermatogenic cycle provide correlative evidence of a unimodal mating pattern in this species of pitviper, as these variables consistently predict the mating season in all snake species previously examined under natural conditions. In most reptiles studied to date, high plasma levels of T and corticosterone (CORT) coincide during the mating period, making the cottonmouth an exception to this trend; we suggest two possible explanations for increased CORT during spring (regulation of a spring basking period), and decreased CORT during summer (avoiding reproductive behavioral inhibition), in this species.

Conservation phylogenetics of helodermatid lizards using multiple molecular markers and a supertree approach

We analyzed both mitochondrial (mt-) and nuclear (n) DNAs in a conservation phylogenetic framework to examine deep and shallow histories of the Beaded Lizard (Heloderma horridum) and Gila Monster (H. suspectum) throughout their geographic ranges in North and Central America. Both mtDNA and intron markers clearly partitioned each species. One intron and mtDNA further subdivided H. horridum into its four recognized subspecies (H. n. alvarezi, charlesbogerti,exasperatum, and horridum). However, the two subspecies of H. suspectum (H. s. suspectum and H. s. cinctum) were undefined. A supertree approach sustained these relationships. Overall, the Helodermatidae is reaffirmed as an ancient and conserved group. Its most recent common ancestor (MRCA) was Lower Eocene [35.4 million years ago (mya)], with a approximately 25 my period of stasis before the MRCA of H. horridum diversified in Lower Miocene. Another approximately 5 my passed before H. h. exasperatum and H. h. horridum diverged, followed by approximately 1.5 my before H. h. alvarezi and H. h. charlesbogerti separated. Heloderma suspectum reflects an even longer period of stasis (approximately 30 my) before diversifying from its MRCA. Both H. suspectum (México) and H. h. alvarezi also revealed evidence of historic range expansion following a recent bottleneck. Our conservation phylogenetic approach emphasizes the origin and diversification of this group, yields information on the manner by which past environmental variance may have impacted its populations and, in turn, allows us to disentangle historic from contemporary impacts that might threaten its long-term persistence. The value of helodermatid conservation resides in natural services and medicinal products, particularly venom constituents, and these are only now being realized.

Consecutive Virgin Births in the New World Boid Snake, the Colombian Rainbow Boa, Epicrates maurus

Until recently, facultative automictic parthenogenesis within the squamate reptiles exhibiting ZZ:ZW genetic sex determination has resulted in single reproductive events producing male (ZZ) or female (ZW) offspring. With the recent discovery of viable parthenogenetically produced female (WW) Boa constrictors, the existence of further parthenogenetic events resulting in WW females was questioned. Here, we provide genetic evidence for consecutive virgin births by a female Colombian rainbow boa (Epicrates maurus), resulting in the production of WW females likely through terminal fusion automixis. Samples were screened at 22 microsatellite loci with 12 amplifying unambiguous products. Of these, maternal heterozygosity was observed in 4, with the offspring differentially homozygous at each locus. This study documents the first record of parthenogenesis within the genus Epicrates, a second within the serpent lineage Boidae, and the third genetically confirmed case of consecutive virgin births of viable offspring within any vertebrate lineage. Unlike the recent record in Boa constrictors, the female described here was isolated from conspecifics from birth, demonstrating that males are not required to stimulate parthenogenetic reproduction in this species and possibly other Boas.

Mating systems, reproductive success, and sexual selection in secretive species: a case study of the western diamond-backed rattlesnake, Crotalus atrox.

Long-term studies of individual animals in nature contribute disproportionately to our understanding of the principles of ecology and evolution. Such field studies can benefit greatly from integrating the methods of molecular genetics with traditional approaches. Even though molecular genetic tools are particularly valuable for species that are difficult to observe directly, they have not been widely adopted. Here, we used molecular genetic techniques in a 10-year radio-telemetric investigation of the western diamond-backed rattlesnake (Crotalus atrox) for an analysis of its mating system and to measure sexual selection. Specifically, we used microsatellite markers to genotype 299 individuals, including neonates from litters of focal females to ascertain parentage using full-pedigree likelihood methods. We detected high levels of multiple paternity within litters, yet found little concordance between paternity and observations of courtship and mating behavior. Larger males did not father significantly more offspring, but we found evidence for size-specific male-mating strategies, with larger males guarding females for longer periods in the mating seasons. Moreover, the spatial proximity of males to mothers was significantly associated with reproductive success. Overall, our field observations alone would have been insufficient to quantitatively measure the mating system of this population of C. atrox, and we thus urge more widespread adoption of molecular tools by field researchers studying the mating systems and sexual selection of snakes and other secretive taxa.

Genealogical Concordance between Mitochondrial and Nuclear DNAs Supports Species Recognition of the Panamint Rattlesnake (Crotalus mitchellii stephensi)

Abstract The Speckled Rattlesnake (Crotalus mitchellii) is a polytypic taxon presently composed of five subspecies that range across southwestern North America, including the Baja Peninsula and islands in the Pacific Ocean and Sea of Cortés. The principles of genealogical concordance were employed to test the taxonomic status of three of the five subspecies (C. m. mitchellii, C. m. pyrrhus, and C. m. stephensi). We used two molecular marker systems: mitochondrial (mt) DNA ATPase 8 and 6 genes (675 base pairs, bp), and introns 5 and 6 of the nuclear (n) DNA ribosomal protein (RP) gene (449 bp). These markers were evaluated across 104 individuals of C. mitchellii: C. m. mitchellii (n  =  3), C. m. pyrrhus (n  =  83), C. m. stephensi (n  =  18), with Sistrurus c. catenatus as the distant outgroup. Deep phylogenetic splits were detected in the subspecies of C. mitchellii, with 5.0–6.4% mtDNA sequence divergence (SD) separating C. m. mitchellii and C. m. pyrrhus, while C. m. mitchellii and C. m. stephensi had SD values of 6.4–7.3%. Similarly, C. m. pyrrhus and C. m. stephensi had SD values of 5.2–6.7%. In addition, C. m. mitchellii and C. m. pyrrhus were identical in all 449 intron bp, but C. m. stephensi differed from both at a single nucleotide polymorphism. Our molecular results diagnose C. m. stephensi as sister to mainland subspecies of the C. mitchellii complex, a result consistent with certain head scalation characters and its northernmost geographic distribution in this complex. Furthermore, four morphological synapomorphies (supraocular scales prominently ridged and/or creased, contact between rostral and prenasal scales, ground coloration of tail congruent with that of body, and black rings in the distal 15% of the tail) also diagnose C. m. stephensi from all other subspecies of C. mitchellii. We hypothesize that the northern distribution of C. m. stephensi likely resulted from two vicariant events: Pliocene expansion of the Sea of Cortés as the Salton Trough, and Pliocene development of the lacustrine Bouse Embayment along the Colorado River drainage. Despite earlier conclusions based on morphology, our molecular results showed no evidence of intergradation between C. m. pyrrhus and C. m. stephensi. Based on the principles of genealogical concordance, we advocate that C. m. stephensi be elevated to a full species, which renders a minimum of two species within the C. mitchellii clades we examined.