DOI: 10.5061/dryad.n5tb2rc55
title: Data from: Melanism in polymorphic terrestrial snakes: A meta-analysis and systematic review

description: 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. --
description: We conducted an extensive literature search up to
 30<sup>th</sup> 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,
 <em>Vipera berus nikolskii</em>) .
description: # Melanism in polymorphic terrestrial snakes: A meta-analysis and
 systematic review
 [https://doi.org/10.5061/dryad.n5tb2rc55](https://doi.org/10.5061/dryad.n5tb2rc55) ## Description of the data and file structure The zip archive contains the data and R script used for the meta-analyses and meta-regressions regarding polymorphic terrestrial snakes. The Excel file contains three sheets, the first represing data for frequency of melanistics and non-melanistcs, the second sheet contains data on frequency of melanistic individuals for each sex, while the last sheet contains body size data for menalistics and standard-colored individuals. The sheet used for the meta-analysis of mean body (lnRR_size) contains some NA cells for standard deviations. During the analysis these were replaced with standard deviatons generated using ImputeSD. For detailed explanations please read the article. Measurement units for body sizes were not recorded during the data extraction phase. The Word document (Appendix S1) contains the references used for extracting data for the meta-analyses, a detailed systematic review of the literature, as well as various tables and plots that were not presented in the main article. ### Files and variables #### File: data\_and\_script.zip **Description:** data and R script used for the meta-analyses and meta-regressions ## Code/software R software with the appropriate packages loaded for running the R script Microsoft Excel for opening the database file.