Melanism in polymorphic terrestrial snakes: A meta-analysis and systematic review

Background and aim

Color polymorphic populations constitute excellent model systems for testing ecological and evolutionary hypotheses, as alternate colorations are associated with differences in various biological, behavioral, and life-history traits. Melanism is the most common and most obvious form of polymorphism. In reptiles in general (and snakes in particular), thermal melanism has been a popular hypothesis used to explain the appearance of black individuals. It suggests that a trade-off exists between superior thermoregulation and reduced protection through the loss of crypsis. Surprisingly, despite the growing body of literature available, to date there are only two qualitative reviews on the evolutionary significance of melanism in reptiles and no quantitative synthesis. We conducted the first systematic review and meta-analysis summarizing the current knowledge on melanism in polymorphic terrestrial snakes and synthesized the evidence for an adaptive advantage of the melanistic morph.

Location

Northern Hemisphere 

Taxon

Polymorphic terrestrial snakes

Results

Mean prevalence of melanism is 31% with no significant differences between species. Annual precipitation was a significant moderator of melanism prevalence. Our results revealed no significant differences in odds ratios of melanism between sexes or in mean body size.

Main conclusions

Environmental plasticity can be considered the primary cause for melanism, possibly as a result of seasonal climatic variations, a result corroborated by the formal meta-analysis conducted. Conclusions from the meta-analysis are that melanism in snakes follows Gloger’s rule, as is the case in birds and mammals, and as opposed to the thermal melanism hypothesis. Further, our findings do not lend support for other predictions from the thermal melanism hypothesis, such as skewed frequency of melanistics in favor of males or females, or larger body sizes in black individuals. Our results hold implications for the future diversity of animal populations, as climate change is predicted to decrease the degree of color variation. --

Methods
We conducted an extensive literature search up to 30th of June 2023 and records were screened using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Page et al., 2021). We used Google Scholar and Web of Science databases, with no limit on the year of the publication. In Google Scholar we used the general search terms “melanism snake”, “melanistic snake”, "snake" AND "melanism" OR "melanistic" OR "melanic" and "snake" AND "melanism" OR "melanistic" OR "melanic", which amounted to 16295 results (Figure 1). In Web of Science, we used the search terms “melanism snake” and “melanistic snake”, giving us a further 116 results. We removed 31 records as duplicates while the rest were screened directly in the search window, therefore saving only the relevant articles. In the end our database contained 150 papers, from which we further excluded 41 which did not contain usable data (i.e. data used for the analyses was not part of the study), 26 in which data was not retrievable or not in a usable format and 1 record was inaccessible, leaving 77 papers to be used for the meta-analyses (Figure 1).

In our database we only included published studies, written in English, therefore excluding Ph.D. or M.Sc. theses and any type of report, without initial regard to sample size or the total number of studies for each species. We included only data pertaining to adult terrestrial snakes, as immature stages frequently possess standard coloration and melanism develops in adults (e.g.: Edgren (1957), Nilson and Andrén (1981), authors’ personal observations on Nikolsky’s viper, Vipera berus nikolskii) .