Sex-, morph- and size-specific susceptibility to stress measured by haematological variables in captive common wall lizard Podarcis muralis

Abstract

In polymorphic species of animals, colour morphs may show alternative physiological properties, and hence evolve or be maintained as an indirect response to selection exerted on these physiological attributes. In this study, we investigated if different colour morphs (white, red and yellow) of the polymorphic common wall lizard differed in their physiological responses to a long-term stress by determining variation between capture and release in leukocytes profiles, haemoparasite loads and body condition of male and females maintained in captivity throughout the breeding season. We found that most blood parameters of lizards varied significantly following captivity, and this variation was sex-, morph- and size-dependent. In particular, the heterophil:lymphocyte ratio (H:L), a sensitive measure of immunodepression and long-term stress, varied significantly among yellow females, larger individuals significantly increasing and smaller individuals decreasing their H:L ratio after captivity. This trend was reversed in red females, where smaller individuals presented raised H:L index at release. Our study indicated that response to long-term stressful conditions, such as those induced by captivity, differed among common wall lizard colour morphs, implying a sex-, size-(i.e. age) and morph-specific sensitivity to stress, and hence a different physiological profile of colour morphs, which may contribute to the maintenance of colour polymorphism in this species.

Introduction

Identifying the processes maintaining phenotypic/genetic variability in wild populations is a major concern in today's conservation and evolutionary biology, since phenotypic variation is at the heart of natural selection. A recently burgeoning literature (reviewed in Gray and McKinnon, 2007) has been devoted to understand the mechanisms that generate and maintain colour phenotypic variation in animals, i.e. colour polymorphism (hereafter CP), due to their implications in the evolution of reproductive isolation and sympatric speciation (Fisher, 1930, Van Valen, 1965, Van Valen and Grant, 1970). CP is defined as “the coexistence in one interbreeding population of two or more distinct and genetically determined colour forms, the least abundant of which is present in numbers too great to be due solely to recurrent mutation”, following the original definition by Huxley (1955).

Several mechanisms can contribute to CP development and maintenance (genetic drift and gene flow, disruptive selection, heterosis, apostatic selection, sexual selection, sensory bias, Galeotti et al., 2003, Roulin, 2004), but we know very little about which mechanism is the most important or how they might interact. To achieve these objectives it is crucial 1) to recognize what ecological factors promote CP, 2) to highlight the physiological and developmental mechanisms underlying morph differentiation, and 3) to elucidate how genotype interacts with environment to affect morph fitness. This is a complex goal to reach, since CP accomplishes a wide range of functions, such as conveying information on social and reproductive status or competitive ability of its bearer (Dawkins and Krebs, 1978, Cooper and Burns, 1987, Thompson and Moore, 1991a, Thompson and Moore, 1991b, Olsson, 1994, Weiss, 2002, Andrés et al., 2002) or supporting ecological functions such as antipredatory, thermoregulatory or habitat-use strategies (Cooper and Greenberg, 1992, Olsson and Madsen, 1998, Martin and Forsman, 1999, Galeotti et al., 2003, Galeotti and Rubolini, 2004, Lopez et al., 2004, Leal and Fleishman, 2004, Stuart-Fox et al., 2006). However, the close link between genotype and phenotype in polymorphic species may allow detailed studies of natural and sexual selection effects on evolutionary processes.

Reptiles offer a good model to investigate the evolution and maintenance of CP as they often show a high intraspecific variability of colour patterns (Cooper and Burns, 1987, Rand, 1992, Thompson et al., 1993, Forsman and Shine, 1995, Sinervo and Lively, 1996, Weiss, 2002, Vercken et al., 2007). In some species this polychromatism may be transient, since colourations are expressed only during the breeding season, or may change during ontogenesis (Rand, 1989, Rand, 1990, Martin and Forsman, 1999, Baird, 2004). In other species, particularly in lizards, individuals occur in discrete, stable, genetically based colour morphs, which coexist at different equilibrium levels within the same population (Thompson and Moore, 1991a, Thompson and Moore, 1991b, Carpenter, 1995, Sinervo and Lively, 1996, Sinervo and Zamudio, 2001, Sacchi et al., 2007b). Since CP is often expressed only ventrally in many species, rather than accomplishing antipredatory or thermoregulatory functions, it may be associated with alternative behavioural (territorial/reproductive) strategies in males as well as in females, which clearly underlie different physiological profiles due to different genotypes. For example, in the side-blotched lizard (Uta stansburiana), the three male morphs (orange, yellow and blue) are associated with territorial, sneaky and mate-guarding strategies respectively, depending on population density and morph-frequency (e.g., Sinervo and Lively, 1996, Sinervo and Zamudio, 2001, Svensson et al., 2001, Sirot et al., 2003, Zamudio and Sinervo, 2003, Svensson et al., 2005). Orange males had higher testosterone plasma levels and behaved more aggressively compared to the other morphs in this species (Sinervo et al., 2000a), as occurs also in tree lizards (Urosaurus ornatus, Hover, 1985, Knapp and Moore, 1996). Thus, colour morphs may signal genotype and the consequent physiological and behavioural adaptations to specific environmental conditions, which may be relevant for potential mates and rivals.

The common wall lizard, Podarcis muralis, exhibits a polymorphic colouration (Cheylan, 1988, Sacchi et al., 2007b), with both sexes showing three main discrete morphs differing in throat and belly colouration (i.e. red-, yellow- and white-throated, see Fig. 1). Throat and belly colourations develop starting from the second year of life in this species (Cheylan, 1988), and individuals maintain the same colour through their life (Cheylan, 1988; S. Scali and R. Sacchi, unpubl. data). Colour appears to be unimportant in determining contest outcome in intermale fighting (Sacchi et al., 2009), but a recent paper showed a morph-specific immune responsiveness in males of this species, since yellow morphs had a lower immune response compared to both red and white males, whose immune response was similarly high (Sacchi et al., 2007a). This result suggests that immune response could be involved in the processes maintaining colour polymorphism in this species. Thus, colour morphs may differ in their physiological traits as, for example, their haematological profile.

In this study, we aimed at identifying covariations between colour morph and physiological traits in the common wall lizard. To this aim, we determined the haematological profiles of colour morphs in order to 1) test for differences among morphs in body condition, blood parameters and blood parasitism upon capture, and 2) analyse how body condition, blood parameters and parasitism changed in relation to sex, morph and age after a captivity period involving reproduction and egg laying by females.