Geographic Variation in the Morphology of Crotalus Horridus (Serpentes: Viperidae)

Abstract

The Timber Rattlesnake (Crotalus horridus) occurs in discontinuous populations throughout the eastern and central United States. The species exhibits high levels of polymorphism in morphological traits, especially in coloration and pattern. Previous studies recognized either distinct northern and southern subspecies or three regional morphs (northern, southern, and western), but conflicting data sets and limited geographic sampling o f previous studies have left the relationships among those regional variants unclear. In this study, univariate and multivariate statistics, together with a geographic information system, were used to analyze geographic variation in 36 morphological characters recorded from 2,420 specimens of C. horridus across its range.Sexual dimorphism was strong in ventral and subcaudal scales, and weak to moderate in band length and band spacing. Univariate analyses detected substantial geographic variation in all meristic characters. Scutellation exhibited a general northsouth pattern of variation, and most scale counts averaged higher in southern regions. Pattern characters differentiated the northeastern, central-eastern, and north-central regions from the southern and western regions. Coloration displayed a pattern of strong clinal variation among three broad areas consisting of the combined northeastern, centraleastern, and southern Appalachian regions, the northwestern regions, and the southern Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. regions. Morphometric characters exhibited a general north-south pattern of geographic variation, with larger head and body sizes in southern regions. Principal component analysis indicated that band length was the most important variable for characterizing geographic variation. The northeastern regions remained moderately distinct in all multivariate cluster analyses. The northwestern regions appeared very distinct in most cluster analyses for females. However, the clusters in all models showed extensive geographic overlap. Percentage maps of clusters revealed two north-south patterns of clinal variation among the northeastern, northwestern, and southern regions. The discordant patterns of variation among individual characters, the overlapping patterns of coloration, and the extensive overlap among the multivariate clusters collectively indicate that zones of intergradation among subpopulations of C.horridus are much broader than previously thought. Furthermore, the general patterns of geographic variation are strongly clinal and support the conclusion that C. horridus is a single widespread, polymorphic species.Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission. V This dissertation is dedicated to my mother, Barbara Allsteadt, for her unconditional love and support.Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

Differential foraging success across a light level spectrum explains the maintenance and spatial structure of colour morphs in a polymorphic bird

“…). Ventral colouration is considered to decrease the conspicuousness of a predator against its background (Bartlotti ) which, in combination with ambient light condition (Endler ), appears to influence the ability of prey to visually detect predators (Green & Leberg ). This suggests that ventral plumage may play an important role in foraging success.…”

“…Clinal variation in colour morphs has been reported in a high proportion of polymorphic species, from insects (de Jong & Brakefield ), to reptiles (Alsteadt et al . ), mammals (Rounds ) and birds (Galeotti et al . ; Antoniazza et al .…”

“…Such local selective advantage may lead to disruptive selection (Mather 1955), with extreme phenotypes favoured under different prevailing conditions (Rueffler et al 2006). Disruptive selection has been invoked as the focal hypothesis proposed for the evolution and maintenance of polymorphism across a variety of species (Rueffler et al 2006;Antoniazza et al 2010), including insects (de Jong & Brakefield 1998), reptiles (Alsteadt et al 2006), mammals (Rounds 1987) and birds (Galeotti et al 2003;Antoniazza et al 2010;Amar et al 2014).…”

Diet and Foraging Behaviors of Timber Rattlesnakes,Crotalus horridus, in Eastern Virginia

“…In terms of large-bodied prey items, Clark (2002) found that C. horridus in southern populations consume more Eastern Cottontails, and we found frequent consumption of Eastern Gray Squirrels by the snakes of our population. Squirrels and rabbits are the largest prey items eaten by C. horridus, and their prevalence could be related to the larger head and body sizes and greater midbody dorsal scale counts of southern C. horridus when compared with northern populations (Allsteadt et al, 2006). Large body size could facilitate the specialization on large prey items (Bock, 1980) that may in turn be accompanied by shifts in foraging behavior; however, correlations between snake SVL and percent time spent either in open habitats (where densities of small prey are high) or in forested habitats (where larger but fewer prey are present) were not significant (P > 0.05 for both correlations).…”

“…Such variation may reflect interpopulation variation in morphology (Allsteadt et al, 2006), life-history traits (Brown, 1993), or prey communities across the species' range. Foraging behavior also may be mediated through ambush site selection in response to previous conspecific success.…”