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

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.

Relationships between soil pollution by heavy metals and melanin‐dependent coloration of a fossorial amphisbaenian reptile

“…1). Being an underground, almost blind species, it is not plausible that this pattern of coloration could have any function in social interactions, thermoregulation or protection from UV radiation, as occurs in other -5reptiles (Clusella-Trullas et al 2009;Vroonen et al 2009;Bury et al 2020), although it might have a camouflage function if a predator dug or lifted-off a rock and the amphisbaenian became unintentionally exposed onto the surface. However, melanin can have many other physiological functions linked to its redox, metal chelating, or free radical scavenging properties (Solano 2014).…”

“…2013; Bury et al . 2020), although it might have a camouflage function if a predator dug or lifted off a rock and the amphisbaenian became unintentionally exposed onto the surface. However, melanin can have many other physiological functions linked to its redox, metal chelating, or free radical scavenging properties (Solano 2014).…”

An unusual morph of Naja naja (Linnaeus, 1758) (Squamata: Serpentes) from Goa, India

“…Pigmentation serves a protective role in many animals, including snakes, it functions in camouflage, warning, mimicry or thermoregulation (Bechtel 1978;Krecsák 2008). Body coloration is a multifunctional trait often characterized by sophisticated variation (Kemp et al 2005;Bury et al 2020). Therefore, discontinuous phenotypes are generally thought to bear fitness costs as a result of the primary functions of a given color variation having been lost (Bury et al 2020).…”

“…Body coloration is a multifunctional trait often characterized by sophisticated variation (Kemp et al 2005;Bury et al 2020). Therefore, discontinuous phenotypes are generally thought to bear fitness costs as a result of the primary functions of a given color variation having been lost (Bury et al 2020). The maintenance of such phenotypes within populations, i.e., color polymorphism, thus represents an interesting evolutionary phenomenon (Forsman 1995;Forsman et al 2008;Bury et al 2020).…”

“…Therefore, discontinuous phenotypes are generally thought to bear fitness costs as a result of the primary functions of a given color variation having been lost (Bury et al 2020). The maintenance of such phenotypes within populations, i.e., color polymorphism, thus represents an interesting evolutionary phenomenon (Forsman 1995;Forsman et al 2008;Bury et al 2020). Melanistic individuals exhibit an increased amount of dark pigmentation, a possible adaptive hypothesis for melanism in snakes is protection against sun damage (Lorioux et al 2008;Jablonski & Kautman 2017).…”