Both Moloch horridus and [...] Phrynosoma cornutum have the remarkable ability to transport water over their skin’s surface to the mouth where drinking occurs. Sherbrooke et al. (2007) Zoomorphology, vol. 126, p. 89

In the most arid regions of the world, several unrelated lizard genera have independently adapted to the lack of water by evolving effective mechanisms of water collection onto, and transport across, the skin surface, ultimately to the mouth for drinking. The most striking example of convergent drinking adaptations in lizards is between the Australian thorny devil, Moloch horridus and Texan horned lizards of the genus Phrynosoma (P. cornutum being the best studied). Moloch is an ‘agamid’ lizard, and Phrynosoma is an ‘iguanid’, representing two distinct groups within the diverse clade Iguania that have been diverging independently in Australia and North America for approximately 150 million years. In spite of their wide evolutionary and geographical separation they have both evolved a suite of morphological features that equips them to make the most of scarce water, as well as supporting a specialised mode of eating ants (‘myrmecophagy’).

Morphological adaptations to drinking

Water from sporadic rainfall and dew comes into contact with the lizards’ outer scales, and moves to the nearest inter-scale hinge joint, where it passes down a narrow duct into a region at the base of the hinge which is expanded, so forming a network of tubular channels within the outer keratinised layer of the skin. The surfaces of the inter-scale hinge ducts and channels are also convoluted, increasing the surface area and potential surface tension of water within the tubular network. Water is rapidly pulled throughout the network by capillary action (as a combined result of cohesion, adhesion and surface tension), often against the force of gravity, until it reaches the mouth. In both species drinking is promoted by hygroscopic mucous secreted from ducts adjacent to the mouth, and by modified keratinous structures (e.g. spines, plates and flaps) at the jaw angle, close to the lips. Once the tubular network is saturated, continuous flow of water into the mouth is sustained by a ‘buccal pumping’ mechanism, in which movements of the tongue and lower jaw (‘hyobranchial apparatus’) generate cycles of negative pressure, drawing water into the mouth. In addition to gathering rain and dew, Moloch horridus can even absorb water from wet sand or soil by rubbing its belly (ventral surface) on damp substrate or kicking sand onto its back (dorsal surface), allowing subsequent water extraction by means of the strong inter-scalar capillary forces. The critical role of their elaborated skin structures in enabling water movement is highlighted by the fact that other lizards very closely related to Moloch (e.g. Australian agamids Ctenophorus, Physignathus and Pogona) or Phrynosoma (e.g. North American iguanids Callisaurus and Uma) possess scales arranged in superficially similar patterns, but they lack the external microstructure of Moloch and Phrynosoma, and as a consequence water either cannot move at all, or only moves very slowly across their skin.

Behavioural adaptations to drinking

In Phrynosoma, movement of water to the mouth by capillary forces within the inter-scalar channels is enhanced by adopting a stereotyped body posture, such that gravity also forces water in the appropriate direction for drinking. The posture is proposed to have originated from a defence pose, and involves slight inclination of the body axis so that the head is lowest and the legs splayed for maximum water capture onto the dorsal surface. More extreme versions of this behavioural adaptation have evolved independently in a number of lizard lineages, elegantly illustrating convergent adaptation to efficient water capture using the dorsal surface of the body. For example, in the agamids Phrynocephalus helioscopus, Trapelus pallidus, T. flavimaculatus and T. mutabilis, the scaly body surface is not adapted for water transport by capillary action, but water is efficiently collected on the dorsal surface of the body and guided to the mouth for drinking by adopting a pose derived from an ancestral threat posture. The head is depressed low to the ground, the body axis steeply inclined, the legs widely splayed, and the tongue protrudes to lick drops of water that form at a point above the top lip (the rostrum).

Parallel adaptations in amphibians

Surprisingly, the notable ability of Moloch and Phrynosma to transport water throughout the skin in both dorsal and/or ventral directions via a specialised network of sculptured skin structures appears to be a character shared with a number of amphibians. For example, evidence has shown that bufonid toads (Bufo sp.) and ‘terrestrial phase’ newts (e.g. Taricha sp.) are capable of moving water from moist substrates over the body surface by means of capillarity generated within a network of convoluted outer skin structures. Unlike the situation in Moloch and Phrynosoma, however, water movement across the skin in amphibians is not an adaptation for collecting sparse drinking water, but rather for preventing lethal dessication when in a terrestrial environment. The evolutionary distance between the Amphibia and the large lizard grouping to which Moloch and Phrynosoma belong (Iguania) is considerable, and, combined with an appreciation of the disparate physical environments (desert vs. temperate terrestrial) they inhabit, demonstrates that selection for a mechanism of water transport through the skin has nevertheless resulted in convergent evolution of the same general solution, namely capillary-action driven networks based on structural elaborations of the external body surface.