Convergent Rain Drinking Behavior in Moloch horridus, Phrynocephalus mystaceus and Chamaeleo namaquensis

Moloch horridus, commonly known as the thorny devil or thorny lizard, exhibits unique adaptations for survival in its arid Australian desert habitat. One of its remarkable behaviors is the capillary rain drinking behaviour, which allows it to efficiently gain moisture from its environment, particularly during rare rainfall events.

The thorny devil is well-suited to the harsh, dry conditions of its habitat. When rain falls, the lizard leverages its specialized physical features to capture and utilize water. The surface of its skin is covered in small, spiny protrusions and grooves that function to channel rainwater directly to its mouth. This adaptation is crucial, as the thorny devil primarily relies on capturing moisture rather than drinking from standing water sources, which are scarce in its dry environment.

During a rainfall episode, the thorny devil positions itself to maximise water collection. As raindrops hit its body, the grooves on its skin channel water toward its mouth. The lizard can effectively gain moisture through capillary action, which is the movement of water through small openings or spaces. This allows the lizard to take advantage of even a light drizzle, which might not provide sufficient water for other species.

Additionally, Moloch horridus can also collect dew that forms overnight. By strategically positioning its body, the lizard can gather condensation on its spiny surface, utilizing this moisture to supplement its hydration needs.This capillary rain drinking behavior illustrates the incredible adaptations of Moloch horridus to survive in an arid environment. By efficiently capturing and gaining available moisture, the thorny devil can thrive in conditions that would challenge many other species. This behavior is a striking example of how animals can evolve specialised traits to cope with their ecological challenges, ensuring their survival in harsh conditions.

Same adaptation has evolved in the desert agamids of the genus Phrynocephalus (Central Asia) and in Chamaeleo namaquensis (Angola, Namibia & RSA). It is to be noted that the water is not penetrating the skin and "absorbed" in strict sense of the term, but directed using:

• capillary action of the interpaces between scales and 

• gravity through special head-down postures 

towards the mouth opening and nostrils, where it is sucked inside of the mouth/nasal cavities, transported to larynx and swallowed. 

The advanced capillary action in Moloch is well demonstrated in the linked videos.