Tobago's Melanistic Reptile Fauna

Melanism is the increase of the black pigment melanin, in the skin of animals. Melanin plays a role in several physiological processes including the synthesis of vitamin D; the protection of cells from harmful ultraviolet radiation; and it may be involved in the prervention of autoimmune diseases in humans. In cats, melanism may provide resistance to viral infections. And, at least one study suggests recessive-gene melanism is linked to disease resistance rather than altitude in cats.

However, despite extensive research, the function and adaptive significance of melanism remain controversial in reptiles. It is generally though that melanistic lizards and snakes, enjoy a thermal advantage compared with normal-colored individuals due to superior thermoregulatory capabilities. The often cited hypothesis is that melanistic individuals have a thermal advantage that lengthens daily and seasonal active allowing melanistic individuals to obtain more food, and a subsequent higher growth rate and larger body size. But, experimental evidence and observations suggests this is not the entire story (White et al. 2002; Tanaka 2009).

Tobago has several melanistc species, perhaps the best known is the tigre or tiger rat snake, Spilotes pullatus, but Pseudoboa neuwiedii also is darker on Tobago. Recently, John Jones from Huston Texas sent me this photo of an Ameiva atrigularis he took in central Tobago. It is remarkably dark for an Ameiva, and it would be interesting to know if all adults in this population are melanistic or this is just in this individual. Ugento and Harvey (2011) have described the sexual dimorphism in this species and have a photo of a specimen from the Isla de Margarita that looks very similar to this specimen.

Of interest is that all of these populations are at the northern edge of their distribution on Tobago.

Literature
Hardy, JD 1982. Biogeography of Tobago, West indies, with special reference to amphibians and reptiles: a review. Bulletin of the Maryland Herpetological Society 18:37-142.

Ugento, GN & Harvey, MB. 2011. Revision of Ameiva ameiva Linnaeus (Squamata: Teiidae) in Venezuela: recognition of four species and status of introduced populations in southern Florida, USA. Herpetological Monographs 25:113-170.

Tanaka K. 2009. Does the thermal advantage of melanism produce size differences in color-dimorphic snakes? Zoological Science. 26:698-703.

White A, Powell R, Censky EJ. 2002. On the thermal biology of Ameiva (Teiidae) from the Anguilla Bank, West Indies: Does melanism matter? Amphibia Reptilia 22:517-528.