Melanism in scorpions, reptiles and rodents inhabiting the volcanic fields of north-western Saudi Arabia

Body colouration is involved in multiple aspects of species ecology and behaviour. Melanism, a common colour polymorphism, has been associated with camouflage and thermoregulation, particularly in diverse, high-altitude habitats of arid regions. This study reports the first case of melanism in the lacertid Acanthodactylus boskianus , two scorpions, Leiurus haenggii and Compsobuthus manzonii , and the first records of brown-black melanistic colouration in three rodent species, Acomys dimidiatus , Meriones crassus , and Sekeetamys calurus , in the dark lava fields ( harrat ) of north-western Saudi Arabia. Additionally, it expands observations of melanism in the endemic colubrid Rhynchocalamus hejazicus and provides the first documented records of brown-black melanistic Acomys russatus for the region, consistent with earlier predictions. These observations suggest that colour polymorphism may serve a cryptic function through background matching in both diurnal and nocturnal species, but other roles of melanism should be further investigated. Despite their extent, lava fields in arid regions remain poorly studied due to their remoteness and limited accessibility, yet they offer unique opportunities to investigate phenotypic evolution in arid ecosystems. • First multi-taxon record of melanism in volcanic fields of Saudi Arabia. • Likely facilitates camouflage against a dark background. • Observed in both diurnal and nocturnal species. • Harrats emerge as natural laboratories for testing hypotheses of phenotypic evolution.