There are organisms on Earth that don’t need sunlight to survive, some don’t even need oxygen, but all living things on this green and blue planet need water.

This need for water isn’t a problem when your environment has an abundance of it, but for those in drier climates they have had to adapt more inventive ways to survive, with some lizards being able to drink water simply by touching it.

Living with a scarcity of resources encourages animals and plant alike to develop wonderful adaptations, leading to amazing diversity. One such adaptation has been developed completely independently by 3 different lizards on 3 different continents, and this trick it to collect water using their skin allowing them to drink water by simply standing in puddles or in the rain.

This incredible ability was highlighted in Attenborough’s documentary Seven Worlds: One Planet. During an episode dedicated to the weird and wonderful wildlife found in Australia, the Thorny Devil’s party trick of was revealed.

The Thorny Devil, Agamidae Moloch horridus, is a species of lizard endemic to Australia and lives in the harsh arid deserts and sandy plains of central and western Australia. The Devils’ highly specialised jaws and tongues make it easy to hunt down and eat ants, but impossible to drink water the conventional way or even to lick water droplets.

The rain-harvesting ability was first reported by PA Buxton in 1841. Buxton described the Australian Thorny Devil as a “repulsive animal” while noting that it “has the ability to absorb water through its skin after raining”

In the 1970’s Wade Sherbrooke observed the very same phenomenon in Texas Horned lizards (Iguandae Phygnosoma cornutum), and once again in 1987 in Horvath’s Toad-headed agama (Agamidae Phrynocephalus horvathi) by Schwenk and Greene.

These three lizards have all independently evolved the ability of “water-harvesting” despite being completely isolated from each other.

So where do these lizards collect water from, and how do they transport it to their mouths? Let’s start with how they transport the water.

The secret lies in the lizards skin. Between their roughly hexagonal shaped scales, the lizards possess small channels which accumulate water which is then transported to the mouth using capillary action. There is some debate whether the lizards need to open and close their mouths to “pull” the water through the tubes like a straw, or if capillary action alone is enough. There is a hypothesis that there are glands by their mouths that secrete hygroscopic mucus which helps pull the water in the right direction, although this has not yet been proven.

Capillary action draws liquid through a small space/channel without the assistance from and often in opposition to gravity. The narrower a tube is the stronger the capillary action, and the further and faster the water is transported.

A study found that there is a hierarchical structure to the channels, with the large channels dividing into smaller channels and sub-capillaries. The sub-capillaries extends the distance that the water travels by 39% and could reduce the volume of water required to fill the channels and allow drinking to start [2], fully taking advantage of the physics of capillary action.

DEEPER DIVE – If the diameter of the space is small enough, the surface tension of the liquid (the property of wanting minimal surface area, which is why water forms droplets) and adhesive forces (how much two different particles cling to each other) between the liquid and the wall of the container causes the liquid to move.

This is an incredible adaptation sure, but what about when there are no puddles? A study in 2016 [4] evaluated the Thorny Devil’s potential sources and narrowed them down to falling rain, puddles, water droplets from dew & condensation, moist soil, and thermally facilitated condensation.

It is not yet clear if all the capillary channels need to be full to allow drinking to occur, but the study carried out testing on the assumption that they would need to be. They found that an equivalent of 3.19% of the lizards body weight was needed to fill the capillary channels to allow drinking to occur, so evaluated each water source by exposing the lizard to each source and calculating the water uptake via mass gain and counted jaw movements. Although the study could not definitively rule out condensation and moist soil providing sufficient water to drink, they could “pre-wet” the skin. Pre-wetting prepares the skin for faster uptake of water, so when they are exposed to rain or puddles they can pull the water into their capillaries faster. Thorny devils have also been seen shovelling moist sand onto their back, using gravity to absorb water from the sand.

The hexagonal arrangement of their scales covers a larger surface area, a larger number of connections compared to other shaped scales increases the speed of water movement, and even increases rate of condensation [5].

It is incredible to think that each of these lizards evolved the same niche adaptation despite being separated by millions of years of evolution, each of them using micro-architectural adaptations to use physics to their advantage. Nature continues to absolutely amaze me…

Some of the many articles I read

[1] Uptake of water by the lizard, Moloch horridus, Bentley and Blumer, 1962

[2] Adsorption and movement of water by skin of the Australian thorny devil (Agamidae: Moloch horridus), Comanns, Esser, Kappel, Baumgartner, Shaw & Withers, 2017

[3] Laser-Based biomimetic functionalization of surfaces: From moisture harvesting lizards to specific fluid transport systems, Comanns, Winands, Arntz, Flocke & Baumgartner, 2014

[4] Cutaneous water collection by a moisture-harvesting lizard, the thorny devil (Moloch horridus), Comanns, Withers, Esser & Baumgartner, 2016

[5] Moisture harvesting and water transport through specialized micro-structures on the integument of lizards, Comanns, Effertz, Hischen, Staudt, Bohme & Baumgartner, 201