Cutaneous water collection by a moisture-harvesting lizard, the thorny devil (Moloch horridus)

Abstract

Moisture-harvesting lizards, such as the Australian thorny devil, Moloch horridus, have the remarkable ability to inhabit arid regions. Special skin structures, comprising a micro-structured surface with capillary channels in between imbricate overlapping scales, enable the lizard to collect water by capillarity and transport it to the mouth for ingestion. The ecological role of this mechanism is the acquisition of water from various possible sources such as rainfall, puddles, dew, condensation on the skin, or absorption from moist sand, and we evaluate here the potential of these various sources for water uptake by M. horridus. The water volume required to fill the skin capillary system is 3.19% of body mass. Thorny devils standing in water can fill their capillary system and then drink from this water, at approximately 0.7 µl per jaw movement. Thorny devils standing on nearly saturated moist sand could only fill the capillary channels to 59% of their capacity, and did not drink. However, placing moist sand on skin replicas showed that the capillary channels could be filled from moist sand when assisted by gravity, suggesting that their field behaviour of shovelling moist sand onto the dorsal skin might fill the capillary channels and enable drinking. Condensation facilitated by thermal disequilibrium between a cool thorny devil and warm moist air provided skin capillary filling to approximately 0.22% of body weight, which was insufficient for drinking. Our results suggest that rain and moist sand seem to be ecologically likely water sources for M. horridus on a regular basis.

Passive water collection with the integument: mechanisms and their biomimetic potential

“…Rain, fog Louw, 1972;Robinson and Hughes, 1978 Acrochordus granulatus Continued 'superhydrophilic' (Fig. 3) (Comanns et al, 2016a). Such waterfilling has been considered as pre-wetting, which is likely a preparation for faster uptake of water from more-efficient sources, such as damp sand (Comanns et al, 2016a).…”

“…3) (Comanns et al, 2016a). Such waterfilling has been considered as pre-wetting, which is likely a preparation for faster uptake of water from more-efficient sources, such as damp sand (Comanns et al, 2016a). Chemical modifications of the integument are found in secretions of hylid frogs (Toledo and Jared, 1993;Tracy et al, 2011) or 'hydrophilizing' components in waxes of flat bugs .…”

“…Lasiewski and Bartholomew (1969) have surmised that condensation underlies the water ecology of the desert-dwelling spadefoot toads Scaphiopus hammondii (synonym: Spea hammondii) and Scaphiopus couchii. Based on their considerations, it has been speculated that condensation also serves as a potential water source for some moisture-harvesting lizards (Gans et al, 1982;Schwenk and Greene, 1987;Comanns et al, 2011Comanns et al, , 2016a. The spikes of the skin, it is thought, act as condensation foci, in particular for dew, which is regularly found in deserts (Gans et al, 1982;Beysens, 1995).…”

“…The spikes of the skin, it is thought, act as condensation foci, in particular for dew, which is regularly found in deserts (Gans et al, 1982;Beysens, 1995). Although laboratory conditions facilitate greater condensation than natural habitats, it has been shown that condensation on lizard skin is very unlikely to provide water in sufficient quantities for drinking (Comanns et al, 2011(Comanns et al, , 2016a. Thorny devils (M. horridus) can collect about 0.2% body mass by condensation (Withers, 1993;Comanns et al, 2016a), whereas 0.75% body mass gain by condensation has been reported for the western banded gecko Coleonyx variegatus (Lasiewski and Bartholomew, 1969).…”

“…The Namib desert is famous for its fog (Shanyengana et al, 2002); depending on the location, fog events occur 40-200 days per year (Seely, 1979;Shanyengana et al, 2002). Furthermore, there are also infrequent rain falls that must be considered (Comanns et al, 2016a).…”