Abstract
Moisture-harvesting lizards, such as the Australian thorny devil Moloch horridus, have remarkable adaptations for inhabiting arid regions. Their microstructured skin surface, with channels in between overlapping scales, enables them to collect water by capillarity and passively transport it to the mouth for ingestion. We characterized this capillary water transport for live thorny devils using high-speed video analyses. Comparison with preserved specimens showed that live lizards are required for detailed studies of skin water transport. For thorny devils, there was no directionality in cutaneous water transport (unlike Phrynosoma) as 7 µl water droplets applied to the skin were transported radially over more than 9.2 mm. We calculated the total capillary volume as 5.76 µl cm−2 (dorsal) and 4.45 µl cm−2 (ventral), which is reduced to 50% filling by the time transportation ceases. Using micro-computed tomography and scanning electron microscopy of shed skin to investigate capillary morphology, we found that the channels are hierarchically structured as a large channel between the scales that is sub-divided by protrusions into smaller sub-capillaries. The large channel quickly absorbs water whereas the sub-capillary structure extends the transport distance by about 39% and potentially reduces the water volume required for drinking. An adapted dynamics function, which closely reflects the channel morphology, includes that ecological role.
Highlights
Adaptations in nature to limited resources such as water scarcity are well studied
Coloured water droplets applied to the skin of live thorny devils rapidly entered the skin channels and spread over the skin surface in all directions from the point of application
We found for the thorny devil, as has been previously reported [19,21,23,25], that water is transported in channels between the scales rather than spread over the skin surface, as most of the scale surfaces are not covered by water during transport
Summary
Adaptations in nature to limited resources such as water scarcity are well studied. Some snakes, toads, arthropods and even mammals have been found to survive water scarcity by using their body surface to collect water from various sources [1,2,3,4,5,6,7]. Like the Australian thorny devil (Agamidae: Moloch horridus) and the Texas horned lizard (Iguanidae: Phrynosoma cornutum), have a spectacular ability to harvest environmental moisture using their skin [17,18] These lizards can inhabit arid environments presumably in part because of their ability to collect water on their skin when drinking water is inaccessible [19,20,21,22,23,24,25]. The recently proposed term ‘moisture harvesting’ is perhaps the most appropriate term, as it describes the different kinds of water acquisition more comprehensively [19]. Such moistureharvesting systems have interesting biomimetic implications. Biomimetic studies of these lizards are invaluable, with respect to adapting biological mechanisms for anthropological roles of water acquisition e.g. for surface wettability and directional liquid transport [19,30], sampling/collection of target material for detection systems [31], or lubrication [32]
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