Body Size Evolution in Insular Speckled Rattlesnakes (Viperidae: Crotalus mitchellii)

Background Speckled rattlesnakes (Crotalus mitchellii) inhabit multiple islands off the coast of Baja California, Mexico. Two of the 14 known insular populations have been recognized as subspecies based primarily on body size divergence from putative mainland ancestral populations; however, a survey of body size variation from other islands occupied by these snakes has not been previously reported. We examined body size variation between island and mainland speckled rattlesnakes, and the relationship between body size and various island physical variables among 12 island populations. We also examined relative head size among giant, dwarfed, and mainland speckled rattlesnakes to determine whether allometric differences conformed to predictions of gape size (and indirectly body size) 2evolving in response to shifts in prey size. Methodology/Principal Findings Insular speckled rattlesnakes show considerable variation in body size when compared to mainland source subspecies. In addition to previously known instances of gigantism on Ángel de la Guarda and dwarfism on El Muerto, various degrees of body size decrease have occurred frequently in this taxon, with dwarfed rattlesnakes occurring mostly on small, recently isolated, land-bridge islands. Regression models using the Akaike information criterion (AIC) showed that mean SVL of insular populations was most strongly correlated with island area, suggesting the influence of selection for different body size optima for islands of different size. Allometric differences in head size of giant and dwarf rattlesnakes revealed patterns consistent with shifts to larger and smaller prey, respectively. Conclusions/Significance Our data provide the first example of a clear relationship between body size and island area in a squamate reptile species; among vertebrates this pattern has been previously documented in few insular mammals. This finding suggests that selection for body size is influenced by changes in community dynamics that are related to graded differences in area over what are otherwise similar bioclimatic conditions. We hypothesize that in this system shifts to larger prey, episodic saturation and depression of primary prey density, and predator release may have led to insular gigantism, and that shifts to smaller prey and increased reproductive efficiency in the presence of intense intraspecific competition may have led to insular dwarfism

Análise preliminar revela baixos níveis de diversidade na estrutura filogeográfica da cascavel mexicana Crotalus polystictus (Serpentes: Viperidae).

Se investigaron las relaciones matrilineales entre poblaciones de la serpiente de cascabel Mexicana cabeza de lanza  (Crotalus polystictus), esta especie se distribuye en valles que presentan elevaciones altas de la meseta del sur de México. Se analizó un fragmento mitocondrial de la ATPasa 8 y 6, los genes (589 pares de bases) revelaron bajos niveles de diversidad genética, con poco polimorfismo nucleótico entre la muestra geográfica analizada. La poca divergencia (1.0%) intraespecífica encontrada en los genes de la ATPasa 8 y 6 en C. polystictus contrastan con los fuertes porcentajes de divergencia (~1.0–14.1%) que han sido observados dentro de otros linajes de serpientes de cascabel mexicanas que habitan elevaciones altas. La variación intraespecífica es observadacomúnmente en especies de cascabel que se distribuyen en elevaciones menores y que presentan una distribución amplia (por ejemplo, C. tigris). Proponemos que la baja diversidad genética encontrada en C. polystictus comparada con la diversidad registrada en otras serpientes de cascabel que habitan elevaciones altas, se debe a diferencias ecológicas que han dado como resultado una respuesta evolutiva diferente en esta especie a los eventos climáticos del Pleistoceno. Nuestros resultados de una aparente baja diversidad genética en C. polystictus son un fuerte soporte para hacer un llamadoa la importancia de realizar iniciativas de conservación para proteger praderas con elevaciones altas en el centro de México.Foram investigadas as relações matrilineares entre as populações da cascavel mexicana Crotalus polystictus, uma espécie que habita vales de altitude do planalto do sul do México. Os genes (589 pares de bases) de um fragmento mitocondrial da ATPase 8 e 6 revelaram níveis relativamente baixos de diversidade genética, com poucos polimorfismos de nucleotídeos entre a amostra geográfica analisada. A baixa divergência da sequência intraespecífica (1.0%) encontrada nos genes da ATPase 8 e 6 de C. polystictus contrasta com asfortes divergências (~1.0–14.1%) observadas em outras linhagens de cascavel que habitam grandes altitudes, sendo que a variação intra-específica é observada comumente em espécies de cascavel que estão distribuídas em altitudes mais baixas e apresentam uma ampla distribuição (por exemplo, C.tigris). Propomos que a baixa diversidade genética encontrada em C. polystictus, comparada com a de outras cascavéis que habitam altitudes elevadas, pode refletir diferenças ecológicas que resultaram em uma resposta evolutiva diferente aos eventos climáticos do Pleistoceno. Nossos resultados deuma baixa diversidade genética aparente em C. polystictus destacam a importância de iniciativas de conservação para proteger os campos de altitude da região central do México.We investigated matrilineal relationships among populations of the Mexican lance-headed rattlesnake (Crotalus polystictus), a pitviper inhabiting high-elevation valleys of the densely populated southern Mexican Plateau. A fragment of the mitochondrial ATPase 8 and 6 genes (589 base pairs) revealed comparatively low levels of genetic diversity, with few nucleotide polymorphisms across the portion of the geographic distribution sampled. The shallow intraspecific sequence divergence (1.0%) in C. polystictus ATPase 8 and 6 genes contrasts with deepdivergences (~1.0–14.1%) observed within other montane rattlesnake lineages from the Mexican highlands, and is more typical of intraspecific variation observed in lowland rattlesnake species with similar distributional extents (e.g., C. tigris). We posit that the low genetic diversity in C. polystictus relative to that of other highland rattlesnakes may reflect ecological differences resulting in a different evolutionary response to Pleistocene climatic events. Our finding of apparently low genetic diversity in C. polystictus highlights the importance of conservation initiatives to protect high elevation grasslands in central Mexico

Limitations of Climatic Data for Inferring Species Boundaries: Insights from Speckled Rattlesnakes

Phenotypes, DNA, and measures of ecological differences are widely used in species delimitation. Although rarely defined in such studies, ecological divergence is almost always approximated using multivariate climatic data associated with sets of specimens (i.e., the "climatic niche"); the justification for this approach is that species-specific climatic envelopes act as surrogates for physiological tolerances. Using identical statistical procedures, we evaluated the usefulness and validity of the climate-as-proxy assumption by comparing performance of genetic (nDNA SNPs and mitochondrial DNA), phenotypic, and climatic data for objective species delimitation in the speckled rattlesnake (Crotalus mitchellii) complex. Ordination and clustering patterns were largely congruent among intrinsic (heritable) traits (nDNA, mtDNA, phenotype), and discordance is explained by biological processes (e.g., ontogeny, hybridization). In contrast, climatic data did not produce biologically meaningful clusters that were congruent with any intrinsic dataset, but rather corresponded to regional differences in atmospheric circulation and climate, indicating an absence of inherent taxonomic signal in these data. Surrogating climate for physiological tolerances adds artificial weight to evidence of species boundaries, as these data are irrelevant for that purpose. Based on the evidence from congruent clustering of intrinsic datasets, we recommend that three subspecies of C. mitchellii be recognized as species: C. angelensis, C. mitchellii, and C. Pyrrhus

Factors Influencing Activity and Detection of Species in a Cross Timbers Snake Assemblage

The Cross Timbers of North America is an ecotone between the eastern deciduous forests and central grasslands and exhibits a diverse composite snake fauna. Over a 3-yr period, we performed repeated nocturnal road surveys of snakes across three transects in Wise County, Texas, and evaluated species composition, relative abundances, vehicular mortality, detection probability, and the influence of environmental variables on snake activity. Sixteen taxa and 406 observations were recorded over 250 surveys, and the three most frequently encountered species had detection probabilities of approximately 0.20: Pantherophis obsoletus, Agkistrodon laticinctus, and Nerodia rhombifer. Only the two species of Agkistrodon present at our study area showed significant differences in count frequencies across the three transects. Covariate effects on overall snake activity were subtle, with barometric pressure, ordinal date, and ambient temperature better explaining variation in combined snake counts than humidity, wind speed, or lunar phase. Furthermore, peak snake activity corresponded closely to the timing of peak warm-season precipitation in the study area, suggesting that snake activity coincides with rainfall periodicity in the Cross Timbers region. However, results of site-occupancy models indicated that covariate effects were different across species, and that several common species did not show clear responses to any of the survey covariates

Allometry of sexual size dimorphism in turtles: a comparison of mass and length data

Background The macroevolutionary pattern of Rensch’s Rule (positive allometry of sexual size dimorphism) has had mixed support in turtles. Using the largest carapace length dataset and only large-scale body mass dataset assembled for this group, we determine (a) whether turtles conform to Rensch’s Rule at the order, suborder, and family levels, and (b) whether inferences regarding allometry of sexual size dimorphism differ based on choice of body size metric used for analyses. Methods We compiled databases of mean body mass and carapace length for males and females for as many populations and species of turtles as possible. We then determined scaling relationships between males and females for average body mass and straight carapace length using traditional and phylogenetic comparative methods. We also used regression analyses to evalutate sex-specific differences in the variance explained by carapace length on body mass. Results Using traditional (non-phylogenetic) analyses, body mass supports Rensch’s Rule, whereas straight carapace length supports isometry. Using phylogenetic independent contrasts, both body mass and straight carapace length support Rensch’s Rule with strong congruence between metrics. At the family level, support for Rensch’s Rule is more frequent when mass is used and in phylogenetic comparative analyses. Turtles do not differ in slopes of sex-specific mass-to-length regressions and more variance in body size within each sex is explained by mass than by carapace length. Discussion Turtles display Rensch’s Rule overall and within families of Cryptodires, but not within Pleurodire families. Mass and length are strongly congruent with respect to Rensch’s Rule across turtles, and discrepancies are observed mostly at the family level (the level where Rensch’s Rule is most often evaluated). At macroevolutionary scales, the purported advantages of length measurements over weight are not supported in turtles

Use of scalation landmarks in geometric morphometrics of squamate reptiles: a comment on homology

Meik, Jesse M., Lawing, A. Michelle, Watson, Jessica A. (2020): Use of scalation landmarks in geometric morphometrics of squamate reptiles: a comment on homology. Zootaxa 4816 (3): 397-400, DOI: https://doi.org/10.11646/zootaxa.4816.3.1

FIGURE 1 in Use of scalation landmarks in geometric morphometrics of squamate reptiles: a comment on homology

FIGURE 1. Diagrams of specimens of the Crotalus viridis complex with 14 (A) and 16 (B) supralabial scales, respectively (supralabial scale rows are shaded in grey). For (A), the red dot marks a fixed landmark at the center of the series, seven scales from the first and seven scales from the last supralabial scale. Placing a corresponding fixed landmark on specimen B requires that one choose between seven scales from the first supralabial, seven scales from the last supralabial, or the suture at the center of the series, providing three mutually exclusive options (red dots). Selecting any of these options changes the topological position of the landmark relative to the first, the last, or both of these scales in the series. Fixed landmarks applied to scale rows variable in count renders secondary homology intractable, regardless of whether scales are counted from a consistent anchor point.Published as part of Meik, Jesse M., Lawing, A. Michelle & Watson, Jessica A., 2020, Use of scalation landmarks in geometric morphometrics of squamate reptiles: a comment on homology, pp. 397-400 in Zootaxa 4816 (3) on page 399, DOI: 10.11646/zootaxa.4816.3.12, http://zenodo.org/record/439674

Data from: Sexual differences in head form and diet in a population of Mexican Lance-headed Rattlesnakes, Crotalus polystictus

Sexual dimorphism of phenotypic traits associated with resource use is common in animals, and may result from niche divergence between sexes. Snakes have become widely used in studies of the ecological basis of sexual dimorphism because they are gape-limited predators and their head morphology is likely to be a direct indicator of the size and shape of prey consumed. We examined sexual dimorphism of body size and head morphology, and sexual differences in diet in a population of Mexican lance-headed rattlesnakes, Crotalus polystictus, from the State of México, Mexico. Maximum snout–vent length of males was greater than that of females by 21%. Males had relatively larger heads, and differed from females in head shape after removing effects of head size. In addition, male rattlesnakes showed positive allometry in head shape: head width was amplified while snout length was truncated with increased head size. In contrast, our data did not provide clear evidence of allometry in head shape of females. Adults of both males and females ate predominately mice and voles; however, males also consumed a greater proportion of larger mammalian species, and fewer small prey species. The differences in diet correspond with dimorphism in head morphology, and provide evidence of intersexual niche divergence in our study population. However, because the sexes overlapped greatly in diet, we hypothesize that diet and head dimorphisms in C. polystictus are likely related to different selection pressures in each sex arising from preexisting body size differences rather than from character displacement for reducing intersexual competition