Contributions to Zoology, 76 (3) – 2007Juan M. Pleguezuelos; Juan R. Fernández-Cardenete; Santiago Honrubia; Mónica Feriche; Carmen Villafranca: Correlates between morphology, diet and foraging mode in the Ladder Snake Rhinechis scalaris (Schinz, 1822)

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Introduction

In animals, feeding and activity patterns determine not only the amount of energy acquired, but also other life history traits of the species (Stearns, 1992). For instance, in many vertebrates foraging mode (active versus sit-and-wait; Schoener, 1971) influences some natural history traits, such as the rate of food acquisition, or survival rate (Webb et al., 2003). Active foraging strategies normally imply an elevated rate of food acquisition with respect to sit-and-wait strategies (Schoener, 1971), but while the active forager searches for food, it expends energy and increases the risk of injuries from other predators or prey (Greene, 1997). Snakes are good models for investigating the influences of foraging mode in the ecology of organisms: the morphological simplicity of snakes facilitates the analysis of structural shifts (Shine, 2000), they feed almost exclusively on living organisms (Mushinsky, 1987) and they tend to exhibit fixed foraging tactics (Huey and Pianka, 1981). For this reason, many life-history studies seek to understand the relation of these prominent predators to their prey (Mushinsky, 1987; Holycross et al., 2002).

The Ladder Snake, Rhinechis scalaris, is a rather large, heavy-bodied colubrid that inhabits the Iberian Peninsula, south-eastern France and the westernmost part of Italy (Cheylan and Guillaume, 1993), being a quasi-Iberian endemic. This species is remarkably differentiated from other European rat snakes (Lenk et al., 2001) and there is evidence for a separate position of this taxon within the genus Elaphe sensu lato (s.l.), derived in the recent revalidation of the monotypic genus Rhinechis Michaeles, 1833 (Utiger et al., 2002). Most previous knowledge of the natural history of this species came from general studies (Valverde, 1967; Vericad and Escarré, 1976), involving small sample sizes. Specific studies on the natural history of R. scalaris dealt with diel and seasonal activity (Cheylan, 1986), ontogenetic shift in coloration pattern (Pleguezuelos et al., 1990), movement patterns (Blázquez, 1993) and reproduction (Blázquez, 1994; Pleguezuelos and Feriche, 2006; revision in Cheylan and Guillaume, 1993). Here we study a large sample of free-ranging animals and museum specimens from the south-eastern Iberian Peninsula. The goal is to seek for correlates between morphology, diet and foraging mode in this snake. At first we analysed certain morphological traits of the species, such as teeth number and tail breakage. Teeth features in snakes are closely related to diet (Cundall, 1987) and we tried to find any correlates between teeth number and feeding habits. The percentage of individuals with tail breakage would be an indirect evidence of predation pressure in snakes (Turner et al., 1982); in some snakes tail breakage provides an effective defence that favours escape from predators (Savage and Slowinsky, 1996); if R. scalaris is an active forager that moves widely, we would expect a high percentage of individuals with damaged tails compared to other species of the Iberian snake community. Secondly, we analysed the taxonomic composition of the diet, with patterns of dietary variation under ontogenetic and sexual cues. Apparently there is no sexual dimorphism in this species, at least not in body length and body mass (Pleguezuelos and Feriche, 2006), but lack of sexual difference in diet is expected (Shine, 1993). With respect to the ontogenetic cue, the specialised diet on endotherms of R. scalaris poses a question: how do neonates of this species face up to their first prey, small mammals and birds, being bulky prey for neonates of medium sized snakes? Lastly we studied size-dependent variation in its feeding habits. As R. scalaris only preys on relatively bulky prey (small mammals, birds; Valverde, 1967), we would expect the consumption of relatively small-sized prey with respect to its body size.