SKIN RESISTANCE TO EVAPORATIVE WATER LOSS IN REPTILES:A PHYSIOLOGICAL ADAPTIVE MECHANISM TO ENVIRONMENTAL STRESS OR A PHYLETICALLY DICTATED TRAIT?

Abstract

Rates of cutaneous water loss, Ec, in terrestrial reptiles may be affected by the temperature and humidity prevailing in the habitat, by the activity pattern of the reptile, and by the taxonomic position of the species. To find out whether variations in Ec are due mainly to the physical conditions characterizing a particular habitat, or whether they are better attributed to taxonomic position, Ec and skin resistance to water loss (Rs) were compared among eight snake species of the families Colubridae and Viperidae, and among five lizard species of the genus Agama (family Agamidae). These species inhabit different climatic regions (from mesic-Mediterranean to dry and hot desert) and habitats (semi-aquatic and terrestrial) in Israel, and have different daily activity (diurnal and crepuscular-nocturnal). In addition, intraspecific Ec and Rswere measured in nine insular populations of the arboreal lizard Anoliscristatellus (family Polychridae) in the British Virgin Islands. This species is found in habitats ranging from subtropical moist forest to relatively dry scrubby ridges. Significant differences were found between the two snake families: average Ec value of the colubrid snakes (0.38 mg cm² h¹ ) was 35 times higher and their Rs (410 s cm¹) was 3 times lower than those found in the vipers. Within each family, however, Ec and Rs were strongly correlated with the aridity of habitat. In each family, Ec rates of the non-desert species were higher, and their Rs lower than those from desert species. Average Ec and Rsof the agamid lizards were very similar to those for the colubrid snakes. Here, again, lizard species that occupy more humid habitats had higher Ec and lower Rs than the desert-dwelling species. Average Ec of the polychrid Anoliscristatellus (0.4 mg cm² h¹) was similar to that found in both agamid lizards and colubrid snakes, whereas the average Rs (107 s cm¹) was the lowest among all the groups studied. Detailed comparison between different populations of A. cristatellus revealed the same trend: populations from more humid islands had higher Ec and lower Rs than those living in drier islands. Inter specific differences of Ec and Rs in terrestrial reptiles may thus be better attributed to the physical conditions prevailing in the habitat, rather than to the taxonomic status of the species.