Last documented from the state in 1994, many were convinced that this state-listed (Endangered) species had actually been extirpated from its southwestern montane, riparian redoubt due to a combination of habitat loss from overgrazing and wildfire damage, as well as unsustainable predation from introduced invasive predators, especially bullfrogs and crayfish.
In June of this year, however, Doug Hotle and his crew of herpetologists from the Albuquerque BioPark Zoo discovered three survivors along the Gila River, where they are said to persist in zones of thick vegetation, feeding on tadpoles and minnows.
"This is a huge find for our team. We have spent nearly three years and thousands of man-hours looking for the northern Mexican gartersnake. Although many have written this species off for the state, we thought it was still here somewhere undetected. This discovery means there is still hope for the species and its habitat." - Doug Hotle, Curator of Reptiles and Amphibians, Albuquerque BioPark Zoo .
Hotle and the zoo hope to establish a breeding colony of the snakes with the ultimate goal of reintroducing some back into their natural habitat.
Amphibians and Reptiles of Texas, With Keys, Taxonomic Synopses, Bibliography, and Distribution Maps (Third Edition, Revised and Updated) by James R. Dixon, Photographs by Toby J. Hibbitts. Texas A&M University Press: College Station, 2013. W. L. Moody Jr. Natural History Series. Softcover. 460 pp. List: $39.95, Amazon: $26.89. ISBN 978-1603447348
Review by Tom Lott
This latest revision of what has become "The Bible" of advanced Texas herpers immediately catches the attention of owners of the previous editions in that it is obviously "new and improved." Delving into the statistics confirms that suspicion: it is exactly twice the weight of the previous edition, most likely due to the heavier, glossy paper on which it is printed, although it contains only twenty-eight more pages than its predecessor; it is physically larger than the second edition by almost an inch in length as well as width; it sports an improved binding, intermediate between a high-end paperback and a top-shelf hard cover edition, as well as the durable-appearing plasticized covers that have graced the rival University of Texas Press Natural History Guide series for some time now; and the cover is printed in color, providing a foretaste of what is perhaps the most significant alteration to this venerable series: the inclusion of color photographs of every herp species known to occur within the boundaries of the state.
Unfortunately, such improvements, even in this age of computerized typesetting and relatively frugal Chinese printing factories, invariably result in an increase in price as well. I can't remember what I paid for my now well-worn Second Edition (Dixon 2000), but I am certain that it was less than half of the $40 list price, and probably even less than half of Amazon's discounted price tag.
As mentioned above, the extensive inclusion of high quality color photographs in this version serves to set it apart from the two previous editions, and doubtless tends to make it a more valuable resource for casual users who, in my experience, lean more toward identifying specimens from photographs, rather than using the provided dichotomous keys, which unavoidably contain anatomical terms unfamiliar to the uninitiated. The photographs used are largely of excellent quality, with the vast majority having been taken by Toby J. Hibbitts, who is credited on the cover, along with a handful of other photographers who are recognized in the respective captions for their work. Locality to the county level is generally provided for each photograph, and it is obvious that a special effort was made to use photos of examples actually from Texas whenever possible. The photos appear to be printed with attention to their correct color registration and, in most cases, are rendered in a gratifyingly large format, frequently occupying as much as a third of a page. Considering that the previous edition contained only twenty-five black-and-white plates in total (out of 284 herp taxa recognized at the time), this represents a considerable concession to what is, by all accounts, a more visually-oriented population nowadays - and definitely makes for a more attractive upgrade to the series.
Still, as one rare negative reviewer of the second edition on Amazon noted, this book is not "intended for the use of the general public." At its academic heart, however, are its distribution maps, taxonomic comments, and especially its cumulative bibliography, none of which are of much interest to the casual user. To the serious student of herpetology, however, they are a treasure trove.
The distribution maps, which serve as the authority for countless range extensions and county records published in Herpetological Review each year, are now included within each species account rather than being isolated in their own section as before. They also retain the familiar county outline form, with a central dot indicating a confirmed record for a county. Gone, however, are the solid lines serving to indicate species and subspecies boundaries in the previous editions. Instead there are different-colored dots for each subspecies (by county), which suggest, rather than indicate, the boundaries of each taxon. For a very few well-documented subspecies, intergrade zones are depicted with intermediate-colored dots, but when darker colors are used, the maps must be viewed in very good light to distinguish between them. The maps appear to be current, at least through the year 2010, although in a cursory examination I did notice that a Guadalupe County record for Lampropeltis calligaster (Ferguson and Forstner 2005) was omitted, and a supposed Crockett County record for Eleutherodactylus cystignathoides actually represents an unremarkable locality for E. marnockii (Pope et al. 2008).
There unfortunately appears to be little or no use of the various "citizen science" databases, such as the HERP database of the North American Field Herping Association (NAFHA), to further refine the distribution maps. Given the "austerity" trend currently prevalent among political funding entities, combined with the tendency of the ascending phylogenetics camp to denigrate traditional morphology-based museum collections (as well as the increasingly rare conventional taxonomists), it would appear that in the future tissue samples and photographic locality vouchers will increasingly become the norm. It seems somewhat ludicrous to waste space in peer reviewed publications for simple locality records that could be more easily posted to an online database (most of which appear to be serious about self-policing - probably the truest form of "peer review"); and it's not like some real "stinkers" haven't ever gotten past Herp Review's reviewers! Hopefully the failure to use these resources isn't merely due to "academic snobbery."
Taxonomically this latest edition remains characteristically conservative, with many of the recently-proposed generic names placed parenthetically between the more traditional genus and species names [e.g., Bufo (Anaxyrus) debilis]. Although this is the accepted form for indicating subgenera in taxonomic nomenclature, that does not appear to be the author's intent here, but rather to merely associate the novel and traditional names in hopes of alleviating confusion. Most of the more significant taxonomic changes (or proposals, as I prefer to think of them) at the genus level are addressed in a new beige-tinted text block to be found at the beginning of the treatment for each affected genus. Changes below the genus level continue to be discussed in the "Comments" section for each taxon; most of these are carried over with little change from the previous edition(s). Subspecies are retained much as they were in the previous editions, with little evidence of capitulation to the "evolutionary species" crowd, who would eliminate subspecies all together or, alternatively, promote all of them to full species.
For the really serious herpetologist (professional or avocational) the meat of the current volume lies in its third major section - the literature citations. For anyone who has ever had to teach "the" scientific method, regardless of which paradigm you use, there is that most important step typically placed between "Defining the Problem" and "Designing the Hypothesis," usually summarized as "Researching the Problem." In the biological sciences, "research" means tracking down virtually every reference to your study taxon ever published in the technical literature. Although the internet has softened this task somewhat, actually obtaining copies of the papers pertinent to one's research can still be daunting, barring ready access to a substantial university library. The present volume lightens that load to some extent, especially for those studying forms whose distributions are largely centered on Texas. There are 4234 literature citations in this third edition, a significant addition to the 1107 listed 41 years ago in the scholarly predecessor (Raun and Gehlbach 1972) to this current series. One should note, however, that the citations, numbered and listed alphabetically in the bibliography, have been renumbered (there are now no decimal points), whereas the first two editions of this work kept the same numbers as the Raun and Gehlbach volume by adding decimal points.
For the bottom line, anyone who is serious about the study of Texas herpetology will benefit from purchasing this book: for those without either of the previous two editions, it is indispensable; for those who already own one or both of the previous editions, there is enough new and updated material to make it worth your while, even at the elevated price (and even without the beautiful photographs). Congratulations to Drs. Dixon and Hibbitts for producing another outstanding addition to the growing bounty of Texas herpetology publications.
Dixon, J.R. 2000. Amphibians and reptiles of Texas. (2nd edition). College Station: Texas A&M Press, 421 pp.
Rio Grande Chirping Frog from Atascosa County, Texas
The Rio Grande Chirping Frog (Syrrhophus cystignathoides) has recently been found in Mobile County, Alabama, where it is occurring sympatrically (syntopically ?) with a similar, related West Indian form, the Greenhouse Frog (Eleutherodactylus planirostris).
Both species are exotics to the area, with the Rio Grande Chirper having traveled approximately 822 miles northeastward along the Gulf Coast from its presumed native environment in the Lower Rio Grande Valley of extreme south Texas. If we take the date of the first known extra-limital record of this species as the beginning of its peregrinations (1969), it would indicate that this diminutive frog has been expanding its range along the Gulf Coast at a rate of almost 19 miles per year.
Conversely, the Greenhouse Frog, based on genetic evidence, seems to have originated with Cuban stock perhaps incidentally transported to southern Florida along with tropical vegetation from its homeland. This species was first reported from Key West in 1863; by 1943 it had reached Jacksonville (507 miles in 80 years = ~ 6 mi/yr).
There is some evidence, however, that the Greenhouse Frog had been established on Key West long before it was "officially" reported, and some authorities maintain that it may have arrived naturally via over-water waif dispersal considerably before 1863 (Lazell 1989). The interval between its arrival at Jacksonville (1943) and at its current western terminus in Galveston County, Texas (1999) is 56 years, covering 890 miles (~ 16 mi/yr), a rate much closer to that of the Rio Grande Chirper and perhaps more informative.
Such calculations would be more meaningful if these tiny frogs were accomplishing their movements on their own, but they're not. Although I don't think anyone has actually proven it beyond a reasonable doubt, the currently accepted explanation for these two frogs' substantial range expansions seems to be that they (and/or their eggs) are being transported incidentally by the potted plant trade.
At any rate, the present occurrence of these two very similar, equally exotic frogs together in an expanse of Gulf Coast real estate extending from Mobile, Alabama to Galveston, Texas, raises the question of whether they will be able to coexist or will one out-compete the other?
Lazell, James D., Jr. 1989. Wildlife of the Florida Keys: A natural history. Washington, D.C.: Island Press, xvi + 253 pp.
Legendary primatologist Jane Goodall (78) joins the quasi-ignominious ranks of several other famous academics (e.g., Doris Kearns Goodwin and the late Stephen Ambrose) who stand accused of plagiarism (at worst) and/or careless writing (at best). The controversy arose in her new book Seeds of Hope, which was co-written with Gail Hudson.
Jerry Coyne examines the evidence in his blog, er, website.
Additionally, Dr. Coyne provides an account of a kerfuffle the renowned chimp researcher once had with the biologically literate Far Side cartoonist Gary Larson.
Book Review:Texas Amphibians: A Field Guide by Bob L. Tipton, Terry L. Hibbitts, Toby J. Hibbitts, Troy D. Hibbitts, and Travis J. LaDuc University of Texas Press: Austin, 2012. Softcover. 325 pp. $24.95. ISBN 978-0-292-73735-8
Review by Tom Lott
The most basic function of a field guide consists merely of aiding the reader in identifying organisms in question. In general, the wider the scope of any field guide, the less auxiliary information it will be able to include in a format intended to actually be carried into the field; a field guide restricted to only the amphibians of a single, albeit very large, state, however, should be able to provide more supplementary information than one encompassing all of the herps of the United States. The current volume does not disappoint in that respect.
This book is a collaboration between three "amateur" (in the sense that Lawrence Klauber was an amateur!) and two professional herpetologists, all of whom have vast experience with Texas herps. The lead author, Bob Tipton, sadly passed away in 2010 due to cancer, but he doubtless would have been pleased that the collective authors' efforts have finally reached fruition. It is the seventh volume in the University of Texas Press' Texas Natural History Guides and the third dealing with herpetological subjects (Dixon and Werler's 2005 Texas Snakes: A Field Guide and the late Andy Price's 2009 Venomous Snakes of Texas: A Field Guide are the others). One can hope that these are followed by similar works on the turtles and lizards.
In its physical dimensions, Texas Amphibians measures 7.5" X 4.8" (19 cm X 12.2 cm), the same as other works in the series, keeping with the notion that the book might occasionally actually be used in the field. Its covers are composed of a flexible, plastic-feeling material that appears to be quite durable, and its pages are actually sewn onto a cloth spine rather than being merely held in place by glue.
The previous work to which this new book will most likely be compared is the out-of-print Gulf Publishing Company's A Field Guide to Texas Reptiles and Amphibians, by the indefatigable R.D. and Patricia P. Bartlett (1999). That work covered 73 taxa of amphibians (including subspecies) to a similar level of textual detail, beyond mere identification (and also included turtles and lizards). The current work covers 72 species of amphibians (thanks mainly to the description of several new species of neotenic salamanders); subspecies, where they exist, are listed in the species account, but are not described, illustrated, nor shown in the range maps.
The most basic function of the field guide - identification - is first addressed by the inclusion of dichotomous keys that are somewhat modified versions (updated and with improved illustrations) of those included in James R. Dixon's (2000) work, and will generally take the user to the species level of classification for most specimens (Dixon's original keys included diagnosis to the subspecies level). At least nine of the endemic Edwards Plateau neotenic salamanders, however, are diagnosed in the keys by referring to their geographic origin, rather than to anatomical distinctions, a problem encountered more frequently in modern field guides due to the increasing tendency of taxonomists to describe new "cryptic" species based solely on genetic differences impossible to determine in the field (doubtless even the handful of experts on this group would be hard-pressed to visually identify some species in the absence of locality data). Also included is a brief key to the genera of most Texas amphibians that include a free-living larval stage in their development.
In most cases, however, I suspect the casual user of a field guide wishes to arrive at an identification primarily by consulting the illustrations. In this respect, Texas Amphibians is quite accommodating, at least in those situations where this is possible. There are typically two or three photographs in each of the species accounts, depicting variation in coloration, ontology, and geographic origin of the specimen (which is provided, by county, for each photo). The photographs are of consistently very high quality (presumably mostly taken by the authors) and are reproduced in a format that is gratifyingly larger than one finds in many such guides. There is also a very brief, but well-done, section in the introduction regarding techniques for photographing amphibians.
Each species account includes the following topics: size, description, voice (for anurans), similar species, distribution, natural history, reproduction, and comments and conservation. This last section is where the conservation status of each species is discussed, along with the authors' comments on the species within the state. If the taxon is not on the state Threatened & Endangered (T&E) list, its standing on either the "white" or "black" Texas Parks and Wildlife Department lists is given. Where major environmental threats are recognized for a particular species, they are mentioned in this section.
Taxonomic arrangements and common names used by the authors generally follow those endorsed by the combined national herp societies (Crother 2008) with three notable exceptions, which are explained in the introduction: 1) the genus Syrrhophus is conservatively retained for the chirping frogs (rather than lumping them with Eleutherodactylus) on the contention that they represent a distinct group more closely related to each other than to other members of the newly erected family Eleutherodactylidae; 2) citing reservations about the mitochondrial DNA data used in several recent studies that variously split up the Western Chorus Frog (Pseudacris triseriata) complex, Pseudacris feriarum is retained as a more conservative option for the Texas populations of the complex rather than accepting the newly described Cajun Chorus Frog (Pseudacris fouquettei); 3) considering the taxonomic status of the sirens of the Lower Rio Grande Valley to be currently unresolved, the authors chose to list them as merely Siren sp. Also, the recent splitting of several large, cosmopolitan genera such as Rana and Bufo are accommodated by placing the older, more familiar name within brackets (e.g., Anaxyrus [Bufo] speciosus for the Texas Toad).
The range maps are of the shaded area type and are rendered more detailed by their plotting over a county outline base map. In general, the maps tend to be somewhat less conservative than those in the snake field guide (Dixon and Werler 2005), but are lacking the surface detail of those in that work. The map for the Rio Grande Chirping Frog, for example, plots three northwesterly populations (two of which have not yet been published in the literature) as being isolated from the remainder of this invasive frog's distribution, which is depicted as contiguous. The distribution maps in this volume are, however, perhaps the most accurate ones currently available in their size and format, and comprise a vast improvement over those provided in Bartlett and Bartlett (1999).
A possible hybrid Pantherophis bairdi X P.o. lindheimeri from Bandera County, Texas.
Inevitably when the topic of hybrids/intergrades between Texas and Baird's ratsnakes comes up, someone always says something along the line of, "Well, let's just test their DNA and find out for sure." This line of reasoning sounds quite logical, but it has always presumed that protocols for such determinations were already out there, just waiting to be applied to this very problem. Now, however, such a technological solution appears to actually exist.
I perked up when I first saw the title of this new paper by Michael W. Vandewege and associates: "Evidence of Hybridization between Elaphe bairdi and Elaphe obsoleta lindheimeri Including Comparative Population Genetics Inferred from Microsatellites and Mitochondrial DNA." Dealing with a topic near and dear to my heart, I quickly flipped through the pages to the beginning of the article.
Unfortunately, though, there is very little in this typically jargon-dense phylogenetic publication that will be of use - or even decipherable - to the average herper in the field. To their credit, of course, this team has now established a protocol, using both mitochondrial and nuclear genes, that can be used in the laboratory to determine the genetic lineage of any questionable specimen of E. bairdi or E.o. lindheimeri one is likely to encounter in central or west Texas.
Vandewege et al. also managed to confirm the suspicions of earlier workers (e.g., Lawson and Lieb, 1990) that F1 hybrids between these two species are fertile and that backcrossing does occur, meaning that not every hybrid found will be a 50%-50% mixture of each parental species; the relative percentages of bairdi and lindheimeri in any given admixture can vary significantly. They also determined that the issue of hybridization is limited to the zone of sympatry, consisting primarily of the south-central portion of the Edwards Plateau, and apparently does not affect populations of either species occurring outside of that zone. Additionally, they have determined that whatever hybridization events are occurring are most likely the result of secondary contact and that these two lineages have been evolving independently and, therefore, are best regarded as full species, even though each is the other's closest relative.
All of the genetically identified hybrids detected in this study came from the Junction area of the South Llano River drainage in Kimble County (most drainages in the south-central portion of the Edwards Plateau appear to also harbor hybrids [see Lawson and Lieb, 1990, which remains the best morphological assessment of hybridization between these two species]. The authors report, however, that only five (8.9%) of their total sample of 56 specimens (most of which were collected in wide-ranging areas of central and west Texas, where hybrids would not be expected) showed genetic evidence of hybridization and they consequently conclude that such events (even in the zone of sympatry - the Junction area in this study) are "infrequent." Since the authors found no hybrids outside of the zone of sympatry, however, it would seem to follow that the frequency of hybrids within the zone of sympatry must be >8.9%, although we are not provided with the total number of specimens from the hybrid zone.
As indicated in the earlier study of Lawson and Lieb (1990), morphological characters can be quite useful in detecting many cases of apparent hybridization between these two species, with the number of dorsal body blotches (mainly in juveniles) being absolutely diagnostic, where apparent (lindheimeri = 27-37; bairdi = 44-61).
Vandewege et al. sought to initially classify each of their specimens according to morphological characters provided by Olson (1977) for bairdi and by Burbrink (2001) for lindheimeri, although some of Olson's characters (e.g., # of supralabials, subdivision of genials, presence of stripes, etc.) were later shown to be of little diagnostic value by Lawson and Lieb (1990). Based on morphology alone, three of the authors' 56 specimens were considered to be possible hybrids. When the molecular data was analyzed, however, it actually showed that five specimens were genetic hybrids. Significantly, the five genetic hybrids included the three that were detected morphologically.
Perhaps the most pragmatically refreshing aspect of this paper is the authors' insistence on retaining their "classically accepted" taxon names (i.e., Elaphe bairdi for the Baird's Ratsnake and Elaphe obsoleta lindheimeri for the Texas Ratsnake). Note also that they chose to retain the subspecies name "lindheimeri," rather than dumping all populations to the west of the Mississippi into "Pantherophis obsoletus," as has become the fad among the majority of phylogeneticists nowadays. This is especially so considering that the paper was published in the SSAR's Journal of Herpetology, which has a reputation for enforcing its own vision of standardized nomenclature. Perhaps it was the authors' innocent yet bold statement that, since they consider the nomenclature of this genus to be "unresolved," - and, by implication, do not regard Burbrink (2001) to necessarily represent the final word on its phylogeny - it is appropriate to retain the more traditional taxonomic arrangement.
Literature Cited [unfortunately, I cannot find PDFs for any these papers]
Lawson, R., and C.S. Lieb. 1990. Variation and hybridization in Elaphe bairdi (Serpemtes: Colubridae). J. Herpetol. 24(3): 280-292.
Olson, R.E. 1977. Evidence for the species status of Baird’s ratsnake. Tex. J. Sci. 29(1): 79-84.
Vandeweege, M.W., Rodriguez, D., Weaver, J.P., Hibbetts, T.D., Forstner, M.R.J., and L.D. Densmore, III. 2012. Evidence of hybridization between Elaphe bairdi and Elaphe obsoleta lindheimeri including comparative population genetics inferred from microsatellites and mitochondrial DNA. J. Herpetol. 46(1): 56-63.
A Mottled Rock Rattlesnake (Crotalus l. lepidus) from the Davis Mountains of west Texas.
It has generally been assumed that small rattlesnakes, in which lizards comprise a significant portion of the diet, employ a different technique for subduing such prey; that is, that they strike and hold onto lizards (which pose little danger to the attacking snake), as opposed to the striking, envenomating, tracking behavior they use against small rodents, which are capable of defending themselves by biting.
Biologists Vicente Mata-Silva, Jerry D. Johnson, and Arturo Rocha of the University of Texas at El Paso were following a radio-transmitter-equipped adult male Mottled Rock Rattlesnake (Crotalus l. lepidus) at the university's Indio Mountain Research Station in Hudspeth County, Texas when they found the snake near a dying Greater Earless Lizard (Cophosaurus texanus). That the lizard had been struck and envenomated was surmised from two bloody puncture wounds on its dorsal surface. Shortly thereafter the snake began what was described as "frenzied" tongue-flicking searching behavior. Locating the lizard, the snake grasped it by the head, dragged it beneath a nearby bush, and commenced the swallowing process.
Although C. lepidus undoubtedly frequently retains its hold on struck lizards, as conventional wisdom has long maintained, this observation indicates that they are equally capable of tracking down lizards that have been struck and released.
Mata-Silva, V., Johnson, J.D., and A. Rocha. 2011. Crotalus lepidus: Feeding behavior. Herpetol. Rev. 42(3): 439.
A recent phylogenetic study, by Adam D. Leache of the University of California at Davis, of 53 out of the 90+ species in the lizard genus Sceloporus employs four nuclear genes, which were contrasted against a new mitochondrial genealogy based on six genes, revealed more conflicting than concordant relationships among the 21 defined species groups.
Perhaps the most effective means of conveying these results is to simply reproduce Figure 4 from the paper, which clearly shows the level of disagreement between the two methodologies via a multitude of skewed dotted lines. The tree derived from nuclear data is on the left, that from mitochondrial data to the right. Bear in mind that this graphic is comparing species groups, not individual species (e.g., the Sceloporus graciosus species group is comprised of the three species: S. arenicolus, S. vandenburgianus, as well as S. graciosus itself).
[Click on the image to enlarge] The author concludes:
"The phylogenetic relationships inferred from the nuclear and mtDNA data are in strong disagreement (Fig. 4). Conflicts are not restricted to weakly supported or unresolved nodes, but include relationships that receive strong support in the separate analyses . . . . Furthermore, conflicts are found across different levels of the phylogeny and involve alternative placements for species groups and individual species. . . .
"Incongruence is not restricted to weak or unresolved nodes as might be expected under a scenario of rapid diversification . . ., but extends to conflicts involving clades receiving strong support (Fig. 4). This latter type of incongruence indicates that the nuclear genes are tracking a species history that is distinctly different from that of the mtDNA genome. . . .
"Tapping into the nuclear genome to assemble data sets containing hundreds of independent markers offers greater potential for elucidating difficult phylogenetic relationships . . ., such as those presented by Sceloporus, than does continued sequencing of the remaining genes of the mtDNA locus."
Some of the problems encountered when attempting to establish a phylogeny of Sceloporus are doubtless related to the hypothesized explosive radiation in its recent evolutionary history, which has resulted in an extensive and rapid diversification. One remarkable mechanism which could play a role in rapid diversification among such groups involves the acquisition of differing numbers of chromosomes, which would of course instantly render these groups reproductively isolated from each other:
"Variation in chromosomes numbers is a particularly interesting feature of Sceloporus, because chromosomal changes can contribute to species formation . . . . It is uncommon for members of a species group to have overlapping distributions; however, when communities of Sceloporus do form, they are generally composed of species with different chromosome numbers . . . . This pattern suggests that chromosomal rearrangements may play a key role during lineage formation by establishing genetic incompatibilities between species . . . . Whether the chromosomal changes observed in Sceloporus are adaptive is an open question, and the mechanism(s) responsible for increasing the rate of chromosome evolution in Sceloporus remain unknown."
The author additionally questions the wisdom of combining data obtained from nuclear and mitochondrial DNA for analysis, as most current studies utilizing nuclear genes are presently doing.
Leache, A.D. 2010. Species trees for spiny lizards (Genus Sceloporus): Identifying points of concordance and conflict between nuclear and mitochondrial data. Mol. Phylogent. Evol. 54(2010): 162-171. [PDF]
Skeptics of the currently en vogue mitochondrial DNA sequencing techniques for generating phylogenetic trees have occasionally joked that even such an "obviously" homogeneous taxon as the Western Diamond-backed Rattlesnake (Crotalus atrox) would doubtless be split into several "cryptic" species if ever subjected to mtDNA-based study. Imagine my surprise to discover that not only have a couple of such a studies been done, but that the results support the traditional notion of atrox as a relatively homogeneous species despite its broad range.
Castoe, Spencer, and Parkinson (2007) sampled tissues of C. atrox from across its range (including Mexico but excluding peripheral populations in Nevada, Oklahoma, and Arkansas) for mtDNA sequencing. Although numerous haplotypes were discerned during the study, most samples resolved into two clades ("Eastern" and "Western"), which are hypothesized to have diverged slightly from each other as a result of Pleistocene climatic cooling events pushing populations further to the south into refugia about 1.36 million years ago. Prior to this, the Eastern and Western clades were apparently in contact along the Cochise filter barrier (roughly between the Animas and Chiricahua mountains in NM and AZ, respectively) and this contact has resumed in post-Pleistocene times. This study also indicates that the Eastern clade experienced a much greater contraction during the Pleistocene than was experienced by the Western clade, but the Eastern clade currently occupies a substantially larger range than the Western one, implying a dynamic post-Pleistocene expansion in the east.
Carol L. Spencer (2008), who was one of the investigators in the above mtDNA study, also conducted a complementary morphological study of C. atrox as part of her doctoral requirements at the University of Texas - Arlington. Originally, Spencer collected measurements and counts of 68 morphological characters from 922 museum specimens of all age groups from throughout the range of the species. The number of characters was then reduced to 37 by eliminating those that were repetitive or that showed no variation across specimens. Counts and measurements of the juveniles were also eliminated because of purported high rates of variation among subadults and the fact that the proportions of various measurements may change from the juvenile to the adult stage. This data winnowing left Spencer with information from 673 adults, which were then subjected to multivariate and univariate statistical programs for analysis.
The results of this analysis failed to show any appreciable patterns for most of the morphological characters investigated, although a number of east-west clines were detected, notably in a few scale counts, and a north-south cline in body size (which seems to differ from the conventional wisdom and which the author attributes to an adherence to Bergman's Rule). Regardless of these clinal variations, the author found more variation within the three groups she investigated (Eastern, Central, and Western) than between them, effectively discounting the possibility that differentiation occurs along the most probable zone (the aforementioned Cochise Filter Barrier, which extends along the Continental Divide in SE AZ and SW NM). This conclusion agreed with the previous mtDNA study (Castoe et al., above and op cit), which showed an intergradation of mitochondrial genes in that region, indicating that although different haplotypes had developed during their more southerly Pleistocene sequestration, both clades had been freely interbreeding since their post-Pleistocene reintroduction to each other.
These two studies offer up an example of what we should see more of in herpetological taxonomy: the application of various, complementary methodologies to a problem instead of making a quixotic decision to disturb taxonomic stability based upon a single study involving a technique (mtDNA sequencing) that is becoming increasingly suspect when compared to results from other methods. The chauvinism exhibited by molecular taxonomists towards other (especially morphological) methods tends toward an unscientific incuriosity in many cases. All morphological characters are, after all, determined by nuclear DNA, not by mitochondrial DNA, and it is becoming increasingly evident that mitochondrial DNA sequencing is inadequate (by itself) to detect many intraspecific populations and subspecies that are "obvious" in a cursory morphological inspection. Additionally, I am becoming increasingly suspicious that the "phylogenetic history" recorded within the two respective genomes (mitochondrial and nuclear) will eventually prove to be significantly different for most species.
Castoe, T.A., Spencer, C.L., and C.L. Parkinson. 2007. Phylogeographic structure and historical demography of the western diamondback rattlesnake (Crotalus atrox): A perspective on North American desert biogeography. Molecular Phylogenetics and Evolution 42(2007): 193-212. [PDF]
Spencer, Carol L. 2008. Geographic variation in Western Diamond-backed Rattlesnake (Crotalus atrox) morphology. Pp. 55-78. InHayes, W.K., Cardwell, M.D., Beaman, K.R., and S.P. Bush (eds.). 2008. The Biology of Rattlesnakes. Loma Linda, CA: Loma Linda Univ. Press, xvi + 606 pp., 20 color plates. [PDF]