Thermal Adaptation - Cutaneous Water Transport

Internal body temperature is affected by the availability of water to the cells. Osmoregulation is the process of water transfer between cells, and as a direct impactor of body temperature, is closely linked to the thermoregulation of animals. Desert inhabitants are threatened by dehydration, and will perish without adaptations allowing them to access water found in plants, ground surface or underground. The thorny devil is a lizard inhabiting large areas of arid Australian land, where rainfall is irregular and surface water largely unavailable (Sherbrooke 1993). The adaptations that allow the thorny devil to access water, thus regulate internal temperature will be discussed in this post.

(Fig.1 - Thorny Devil (Moloch horridus) in walking stance)
(Clemente et al 2004)

The thorny devil has hygroscopic skin, allowing for accumulation of water to ‘stick’ to the skin surface. Spines and thorns on the skin increase surface area, aiding condensation of water. However, the thick scales it possesses for predatory protection are non-permeable, so an adaptation must have developed to allow transportation of water to the mouth. The thorny devil achieves transportation of water through interscalar channels on its back, in some cases ‘drawing’ water from its legs and stomach to the jaw. The channel is formed by the scale hinge joint that is interconnected with all scale hinges on the body, and composed of a very thin layer of keratin. Capillary forces fill the channels with water and physical pumping mechanisms of the jaw and upper body depress local water pressure, pulling water up through the channels toward the mouth (Sherbrooke 2005). The thorny devil can also access water by walking through wet spinifex and lying on wet substrates such as undersides of rocks. However wet vegetation and rocks are rare occurrences in the desert, so it depends predominantly on accumulation of water on the skin, and subsequent transport to the mouth through capillary channels.

(Fig.2 - Thorny Devil (Moloch horridus) in drinkning stance (A) and in process of jaw movement (B))
(Sherbrooke 1993)

The thorny devil is specifically adapted to living in extremely arid and dry conditions. Its ability to transport water along its capillary inter-scalar channels is gravity defying and fascinating. Extreme environmental conditions are correlated with complex adaptations, and the thorny devil is a perfect example of this. The evolution of species can be achieved in seemingly uninhabitable conditions, as long as genetic variation and enough time is available. 

References:

Sherbrooke, W.C 1993, Rain-drinking Behaviors of the Australian Thorny Devil (Sauria: Agamidae), Journal of Herpetology, Vol. 27, No. 3, pp. 270-275

Sherbrooke, W.C 2005, Functional morphology of scale hinges used to transport water: convergent drinking adaptations in desert lizards (Moloch horridus and Phrynosoma cornutum), Zoomorphology (2007) Vol. 126, pp. 89–102

Figures:

Fig.1: Clemente, C.J, Thompson, G.G, Withers, P.C, Lloyd, D. 2004, 'Kinematics, maximal metabolic rate, sprint and endurance for a slow-moving lizard, the thorny devil (Moloch horridus), Australian Journal of Zoology, Vol. 52, page 488

Fig. 2: Sherbrooke, W.C 1993, Rain-drinking Behaviors of the Australian Thorny Devil (Sauria: Agamidae), Journal of Herpetology, Vol. 27, No. 3, page 272