Body coloration in reptiles results from a complex interaction between pigmentary and structural components. It serves diverse functions, including crypsis, aposematism, social signalling, and thermoregulation (Bogert 1949; Cuthill et al. 2017). Among chromatic anomalies in squamates, melanism is the most frequently reported and is considered particularly ecologically significant. Melanism is defined as the excessive deposition of dark eumelanin-based pigments. In reptiles, melanism occurs across many lineages and has been linked to thermoregulation, camouflage against dark substrates, protection from ultraviolet radiation, and predator-prey interactions, so that dark morphs provide useful natural models for testing ecological and evolutionary hypotheses about color variation (Clusella-Trullas et al. 2007; Cuthill et al. 2017). Two main adaptive hypotheses have been proposed to account for its geographic distribution. The thermal melanism hypothesis was formalised by Clusella-Trullas, et al. (2007) following Bogert (1949). It proposes that darker individuals are favoured in cooler climates because they heat more rapidly and attain higher equilibrium body temperatures than lighter individuals under equivalent solar radiation. By contrast, Gloger’s rule predicts that darker pigmentation should predominate in warm, humid environments (Rensch 1929; Delhey 2019). However, whether this thermal advantage is sufficient to drive melanism evolution in specific lineages remains unresolved.

Melanism in snakes has frequently been used as a model system for studying color polymorphism, owing to its broad taxonomic occurrence and pronounced geographic variation (Sahlean et al. 2025). Field and intraspecific studies indicate that melanistic individuals may exhibit higher growth rates and improved body condition. They may also have longer activity periods and enhanced locomotor performance compared to normally colored individuals (Andrén and Nilson 1981; Clusella-Trullas et al. 2007; Castella et al. 2013). At the macroevolutionary level, Martínez-Freiría et al. (2020) provided the first phylogenetic comparative evidence supporting the thermal melanism hypothesis in Eurasian vipers. Storniolo et al. (2025) reported similar findings for elapids. However, the evidence remains inconsistent. A recent meta-analysis of polymorphic terrestrial snakes found that the prevalence of melanism correlated more strongly with annual precipitation than with temperature. This suggests that Gloger’s rule may better explain the distribution of dark morphs than thermal advantage alone (Sahlean et al. 2025). Predation pressure adds further complexity, as dark snakes lacking species-typical patterns may be more conspicuous to visually oriented predators (Martínez-Freiría et al. 2020). The adaptive significance of melanism in snakes remains debated. Melanism has been recorded in several snake genera such as Vipera (Martínez-Freiría et al. 2020; Schmid et al. 2023), Dolichophis (Göçmen et al. 2015), Hierophis (Zuffi 2008; Rato et al. 2009), Natrix (Mebert 2011; Habiboğlu et al. 2016; Yenmiş et al. 2022), Coronella (Barbadillo et al. 1997; Reading and Pernetta 2009), and Zamenis (Colla et al. 2025).

Asia Minor Dwarf Racer, Eirenis modestus (Martin, 1838), is a small to medium-sized colubrid. It rarely exceeds 600 mm in total length. Its range includes Armenia, Azerbaijan, eastern Georgia, and southern Russia, as well as north-western Iran and several Greek islands in the nominal subspecies E. m. modestus, E. m. semimaculatus (Boettger, 1876) in central and western Anatolia, and E. m. cilicius (Schmidtler, 1993) in southern Türkiye (Mahlow et al. 2013). This species is typically found in dry, rocky habitats with sparse vegetation, such as maquis, steppe, fallow fields, and ruins. It shelters under flat stones during the day and emerges at twilight to forage. Its diet consists primarily of terrestrial arthropods, particularly beetles, orthopterans, and centipedes. Occasionally, it preys on small lizards (Çiçek and Mermer 2007; Baran et al. 2021). The dorsum is generally uniform, light brown to greyish, often with a darker collar and a characteristic dark pileus pattern that fades with age. Populations from southern Anatolia and parts of the Aegean exhibit variable rows of dark blotches on the anterior body, especially within E. m. semimaculatus (Mahlow et al. 2013).

Set against a backdrop of generally light coloration, melanistic individuals in E. modestus are a rarely documented phenomenon. The earliest published account is by Baran (1986), who described island populations from Yassıca and Sarıot in the Muğla Province of south-western Türkiye that included dark individuals. Three decades later, Kalaentzis et al. (2018) reported two melanistic specimens – a juvenile and an adult – from the Greek island of Kastellorizo (Megisti) in the south-eastern Aegean Sea. More recently, Searle-Webb and Kuttalam (2025) documented a single melanistic adult from Symi, also in the Dodecanese. This individual had a dark grey dorsum that contrasted only faintly with the head pattern. Despite these scattered records, melanism in E. modestus remains largely unstudied. There is no published assessment of its frequency within populations, nor information on its genetic basis, and no formal evaluation of whether it follows the thermoregulatory or moisture-related patterns observed in other snakes (Kalaentzis et al. 2018). Continued documentation of additional cases, especially from the Anatolian mainland where the species is most abundant, is essential to clarify the geographic distribution of dark morphs and to support future ecological and evolutionary analyses.

The melanistic specimen was collected on 9 May 2024 during fieldwork, between Çağman and Demre, Antalya Province, Türkiye (Fig. 1). The specimen was discovered beneath a rock at approximately 17:30 h. The exact locality of the specimen was determined using a GPS. The specimen was anesthetised with ether, fixed with a 96% ethanol injection, and deposited in 96% ethanol in the Zoology Lab. of the Department of Biology at the Faculty of Science, Dokuz Eylül University. The specimen was photographed using a Nikon D80 DSLR camera in order to determine color and pattern characteristics. Mensural, meristic, and qualitative data were recorded following the system of Başoğlu and Baran (1980) and Mahlow et al. (2013). Meristic (scale) counts and pholidosis were examined under a Leica stereomicroscope, whereas mensural (metric) measurements were taken with digital callipers. The ventral plates were counted according to Dowling (1951).

Fieldwork in the area yielded no additional individuals of E. modestus, and the findings reported here are therefore based on this single specimen. The individual had a snout-vent length of 155 mm and a tail length of 47.3 mm, with 17 dorsal scale rows and 161 ventral scales. Pileus length and width measured 7.55 mm and 4.27 mm, respectively; frontal length and width were 2.66 mm and 1.63 mm. The preocular and postocular counts of 1 and 2, respectively, fall within the range documented for the species (Mahlow et al. 2013). Supralabialia were 7 on both sides (Fig. 2). This configuration is consistent with the typical count of 7 reported for the species and the melanistic individual from Yassıcaada (Baran 1986). Sublabialia numbered 8 on both sides, while the usual count was 8, rarely 9, reported for E. modestus. Subcaudalia totalled 72, consistent with the species range of 51–86. Taken together, these pholidotic characters (17 dorsal scale rows at midbody, a single preocular and two postoculars, seven supralabials, eight sublabials, 161 ventrals and 72 subcaudals), together with the characteristic dark pileus and nuchal-collar pattern, identify the specimen as E. modestus and distinguish it from other small Eirenis species occurring in southern Anatolia (Mahlow et al. 2013).

The dorsum was entirely black from the head to the tip of the tail (Fig. 3). In this comparatively young individual, greyish-white scales were scattered across the neck region, fading rapidly toward midbody, with white scales gradually increasing from the lateral surfaces toward the ventrals. The ventral surface was greyish-white, becoming progressively darker toward the tail.

The meta-analysis by Sahlean et al. (2025) suggests that melanistic snakes are more frequently encountered in coastal regions and on islands than in mainland populations. Therefore, this mainland record represents a noteworthy finding for E. modestus. As in previous records (Baran 1986; Kalaentzis et al. 2018; Searle-Webb and Kuttalam 2025), melanistic individuals of this species have been documented exclusively from islands. This may be attributable to adaptation to coastal environments or to the constraints associated with island living. Melanistic individuals generally inhabit cooler regions than their lighter-colored counterparts (Clusella-Trullas et al. 2007; Martínez-Freiría et al. 2020; Schmid et al. 2023). On the other hand, according to Gloger's rule, animals in warmer and more humid environments tend to deposit greater amounts of melanin (Delhey 2019). Çağman-Demre and the previously reported island localities are all characterised by relatively high humidity. Such environments harbour denser vegetation and soils with higher organic matter content. In these settings, background coloration is generally darker and light conditions are reduced, which favours darker pigmentation for camouflage. Our single observation is therefore more consistent with the pattern predicted by Gloger's rule than with thermal melanism, although a single record cannot by itself test either hypothesis. Because this finding is based on one individual, its dark phenotype could equally reflect a rare, non-adaptive mutation rather than a locally adaptive trait: if melanism were adaptive in this region, a substantial proportion of the local E. modestus would be expected to be dark, whereas the typical pale morph in fact predominates throughout southern Anatolia (Mahlow et al. 2013; Baran et al. 2021).

Further research in Anatolia and nearby islands is needed. This will help determine whether melanism in E. modestus and other snake species is truly limited to humid coastal and island habitats. Also, studying the molecular mechanisms of melanism, along with broad geographic and climate data, may clarify the evolutionary advantages and disadvantages of this trait, and the relative importance of thermal melanism and Gloger's rule.