Melanism, body size, and sex ratio in snakes\u2014new data on the grass snake (Natrix natrix) and synthesis

Stanisław Bury,Katarzyna Kurek,Bartłomiej Najbar,Bartłomiej Zając,Tomasz D. Mazgajski

doi:10.1007/s00114-020-01678-x

Abstract

It is postulated that melanism in ectotherms is adaptive by enhancing thermoregulation, subsequent resource acquisition, and growth. Such effects may differ between the sexes as a result of the differential costs of self-maintenance and reproduction, but empirical support for the sex-specific consequences of melanism remains inconsistent. We studied the effects of melanism on body size and sex ratio in a population of the European grass snake (Natrix natrix) in SE Poland and also carried out a systematic review of the literature on the consequences of melanism in terrestrial snakes. Melanistic grass snakes of both sexes appeared to be smaller than the typical phenotype, which indicates higher predation pressure and minimal thermal benefits for black individuals. A female-biased sex ratio was observed in the typical phenotype, but not in melanistic snakes, suggesting that the costs for females and/or benefits for males are higher in melanistic individuals. In conjunction with earlier studies, our data indicate that the consequences of melanism may be related to the reproductive mode of species. In viviparous species, melanism tends to improve growth and/or body size and is more frequent in females, whereas the opposite holds for oviparous snakes. Further studies on melanism should examine a wider array of species with different reproductive strategies and traits beyond the usual thermal benefits.

Highlights

Body coloration is a multifunctional trait often characterized by sophisticated variation (Kemp et al 2005)
Melanism is an example of color polymorphism in which a phenotype is characterized by an overconcentration of melanin compared with the typical color morph (Trullas et al 2007)
The magnitude of size differences between phenotypes remains similar in both sexes, whereas sexspecific effects of the black phenotype appear to occur at the sex ratio level

Summary

Introduction

Body coloration is a multifunctional trait often characterized by sophisticated variation (Kemp et al 2005). The occurrence of melanism in ectotherms is interesting, owing to its predicted positive effect on thermoregulation, including increased heating rate and higher achievable body temperature (Forsman 1995b; Trullas et al 2007). Such thermoregulatory consequences are predicted to have a positive impact on the life-history traits of black individuals, including their body size—a prerequisite of survival and fecundity

Methods

Results

Conclusion