RESUMO
Crotalus scutulatus (Mohave rattlesnake) is a clinically significant pit viper broadly distributed across much of the arid southwestern United States and mainland Mexico. Identification of C scutulatus is a concern among emergency medical service and emergency department personnel owing to its reputation for severe envenomations and difficulty in visually differentiating between C scutulatus and other species, primarily Crotalus atrox (western diamond-backed rattlesnake). We contrast distinctive characteristics of C scutulatus, C atrox, and 3 other sympatric species: Crotalus molossus, Crotalus ornatus, and Crotalus viridis (western and eastern black-tailed rattlesnakes and prairie rattlesnake, respectively). Greenish coloration eliminates C atrox but does not confirm C scutulatus. Obvious coarse and fine speckling of the dorsal pattern and a pale postocular stripe intersecting the mouth characterize C atrox. Dorsal speckling is insignificant or absent in the other species, whereas the pale postocular stripe passes above the mouth in C scutulatus and C viridis and is absent in C molossus and C ornatus. Tails boldly ringed with alternating black and white or contrasting shades of gray are shared by C atrox and C scutulatus, respectively, but a lack of boldly ringed tails characterizes the other species. The proximal rattle segment is yellow and black, or entirely yellow, in C scutulatus but black in the others. The most reliable visual identifications are based on evaluations of multiple traits, all of which are variable to some extent. Traits such as tail ring width and the size and number of crown scales have frequently been overemphasized in the past.
Assuntos
Venenos de Crotalídeos , Crotalus , Animais , MéxicoRESUMO
The secretive behavior and life history of snakes makes studying their biology, distribution, and the epidemiology of venomous snakebite challenging. One of the most useful, most versatile, and easiest to collect types of biological data are photographs, particularly those that are connected with geographic location and date-time metadata. Photos verify occurrence records, provide data on phenotypes and ecology, and are often used to illustrate new species descriptions, field guides and identification keys, as well as in training humans and computer vision algorithms to identify snakes. We scoured eleven online and two offline sources of snake photos in an attempt to collect as many photos of as many snake species as possible, and attempt to explain some of the inter-species variation in photograph quantity among global regions and taxonomic groups, and with regard to medical importance, human population density, and range size. We collected a total of 725,565 photos—between 1 and 48,696 photos of 3098 of the world's 3879 snake species (79.9%), leaving 781 “most wanted” species with no photos (20.1% of all currently-described species as of the December 2020 release of The Reptile Database). We provide a list of most wanted species sortable by family, continent, authority, and medical importance, and encourage snake photographers worldwide to submit photos and associated metadata, particularly of “missing” species, to the most permanent and useful online archives: The Reptile Database, iNaturalist, and HerpMapper.
RESUMO
The uplift and final connection of the Central American land bridge is considered the major event that allowed biotic exchange between vertebrate lineages of northern and southern origin in the New World. However, given the complex tectonics that shaped Middle America, there is still substantial controversy over details of this geographical reconnection, and its role in determining biogeographic patterns in the region. Here, we examine the phylogeography of Bothrops asper, a widely distributed pitviper in Middle America and northwestern South America, in an attempt to evaluate how the final Isthmian uplift and other biogeographical boundaries in the region influenced genealogical lineage divergence in this species. We examined sequence data from two mitochondrial genes (MT-CYB and MT-ND4) from 111 specimens of B. asper, representing 70 localities throughout the species' distribution. We reconstructed phylogeographic patterns using maximum likelihood and Bayesian methods and estimated divergence time using the Bayesian relaxed clock method. Within the nominal species, an early split led to two divergent lineages of B. asper: one includes five phylogroups distributed in Caribbean Middle America and southwestern Ecuador, and the other comprises five other groups scattered in the Pacific slope of Isthmian Central America and northwestern South America. Our results provide evidence of a complex transition that involves at least two dispersal events into Middle America during the final closure of the Isthmus.
Assuntos
Bothrops/classificação , Filogeografia , Animais , Sequência de Bases , Teorema de Bayes , Bothrops/genética , América Central , DNA Mitocondrial/genética , Demografia , Variação Genética , Geografia , Filogenia , Análise de Sequência de DNA , Fatores de TempoRESUMO
Venomous snakes often display extensive variation in venom composition both between and within species. However, the mechanisms underlying the distribution of different toxins and venom types among populations and taxa remain insufficiently known. Rattlesnakes (Crotalus, Sistrurus) display extreme inter- and intraspecific variation in venom composition, centered particularly on the presence or absence of presynaptically neurotoxic phospholipases A2 such as Mojave toxin (MTX). Interspecific hybridization has been invoked as a mechanism to explain the distribution of these toxins across rattlesnakes, with the implicit assumption that they are adaptively advantageous. Here, we test the potential of adaptive hybridization as a mechanism for venom evolution by assessing the distribution of genes encoding the acidic and basic subunits of Mojave toxin across a hybrid zone between MTX-positive Crotalus scutulatus and MTX-negative C. viridis in southwestern New Mexico, USA. Analyses of morphology, mitochondrial and single copy-nuclear genes document extensive admixture within a narrow hybrid zone. The genes encoding the two MTX subunits are strictly linked, and found in most hybrids and backcrossed individuals, but not in C. viridis away from the hybrid zone. Presence of the genes is invariably associated with presence of the corresponding toxin in the venom. We conclude that introgression of highly lethal neurotoxins through hybridization is not necessarily favored by natural selection in rattlesnakes, and that even extensive hybridization may not lead to introgression of these genes into another species.
Assuntos
Venenos de Crotalídeos/química , Crotalus/genética , Evolução Molecular , Hibridização Genética , Neurotoxinas/química , Animais , Venenos de Crotalídeos/genética , Crotalus/classificação , DNA Mitocondrial/genética , NADH Desidrogenase/genética , Neurotoxinas/genética , New Mexico , Análise de Componente Principal , Característica Quantitativa HerdávelAssuntos
Crotalus/genética , Demografia , Ecossistema , Filogenia , Animais , Brasil , Crotalus/fisiologia , Geografia , Dinâmica PopulacionalRESUMO
Abstract Pleistocene fragmentation of the Amazonian rainforest has been hypothesized to be a major cause of Neotropical speciation and diversity. However, the role and even the reality of Pleistocene forest refugia have attracted much scepticism. In Amazonia, previous phylogeographical studies have focused mostly on organisms found in the forests themselves, and generally found speciation events to have predated the Pleistocene. However, molecular studies of open-formation taxa found both north and south of the Amazonian forests, probably because of vicariance resulting from expansion of the rainforests, may provide novel insights into the age of continuous forest cover across the Amazon basin. Here, we analyse three mitochondrial genes to infer the phylogeography of one such trans-Amazonian vicariant, the Neotropical rattlesnake (Crotalus durissus), which occupies primarily seasonal formations from Mexico to Argentina, but avoids the rainforests of Central and tropical South America. The phylogeographical pattern is consistent with gradual dispersal along the Central American Isthmus, followed by more rapid dispersal into and across South America after the uplift of the Isthmus of Panama. Low sequence divergence between populations from north and south of the Amazon rainforest is consistent with mid-Pleistocene divergence, approximately 1.1 million years ago (Ma). This suggests that the Amazonian rainforests must have become fragmented or at least shrunk considerably during that period, lending support to the Pleistocene refugia theory as an important cause of distribution patterns, if not necessarily speciation, in Amazonian forest organisms. These results highlight the potential of nonforest species to contribute to an understanding of the history of the Amazonian rainforests themselves.