In the late 1970s, Wade Sherbrooke was watching lizards out of his window. He had collected some spiny Texas horned lizards and placed them in a wire mesh enclosure next to his home. Then it started raining.
As he watched, the lizards arched their backs upwards, lowered their tails, and rhythmically opened and closed their mouths. "Those guys are drinking!" he said to himself.
Living in the Chihuahuan Desert on the border between the US and Mexico, these spiny lizards have evolved ways to make the most of rare rainfall. In between their scales are microscopic channels that catch and carry water. By opening and closing their mouth, they drink through their scales like sipping through a series of straws.
Sherbrooke wasn't the first to observe this sort of behaviour. About 50 years earlier something similar had been seen, half the world away.
By opening and closing their mouth, they drink through their scales like sipping through a series of straws
The Australian thorny devil was described as a "repulsive animal" by PA Buxton in 1923. But Buxton also noticed that the devil "bears tubercles and circles has the ability to absorb water through its skin after raining". Another researcher likened the lizard's skin to "blotting paper" in its ability to spread water.
In 1993, after years of observations, Sherbrooke coined the term "rain harvesting" for the behaviour. About 15 years later he worked out how the harvesting happened: studying the lizard's scales under a microscope he realised their undersides – where two scales overlap – formed a network of tiny channels that collected water. The two species have both evolved a similar strategy – and the same channels haven't been documented in any other species.
Until now, that is. Earlier this year a study revealed a third lizard species that harvests rain through microchannels. Meet Horvath's toad-headed agama, a small insectivorous lizard that lives in the Araks River Valley of Armenia, Iran, and Turkey.
Covered in a mottled mosaic of spines and scales, this species blends into its desert habitat of bare rock, sand, and sere vegetation. But despite their camouflage, they are still easy to catch – if you know where and when to look.
High-resolution microscopy revealed semi-circular tubes where one scale overlapped with its neighbour
One population calls the foothills of Mount Ararat – the tallest peak in Turkey – home. This is where Melodi Yenmis, a PhD student from Ege University in Izmir, Turkey, collected the lizards while they were sunbathing to warm their cold blood. "There are several ways to capture a lizard actually, but bare hands is always the best," she says. "It's the least dangerous way for the animals."
After an 18-hour drive back to Izmir, Yenmis placed the round-headed lizards in specially made terrariums, each replicating their natural habitats. Her experiments could then begin.
First, she added water. Spraying the lizards revealed similar behaviour to that observed in Texas horned lizards and Thorny devils – an arching of the back and opening and closing of the mouth. They were drinking.
But the similarities go deeper than that – much deeper. Using high-resolution electron microscopy on thin slices of the lizard's skin, Yenmis could look at the scales' intricate topography. Her images revealed the presence of semi-circular tubes where one scale overlapped with its neighbour. "[They] cover all the body like a net," she says.
Those are the only three lizards that have ever been examined at that depth
Yenmis sent her images to the doyen of rain harvesting research – the recently retired, Sherbrooke. To him, the anatomy looked very familiar indeed. "It just jumped out at me that this was the same internal structure, those semi-tubular capillary things, that I was seeing both in Texas horned lizards and in thorny devils."
Separated by thousands of miles and millions of years of evolution, these three species have solved the same problem of a waterless environment. "It's convergent evolution on a micro-architectural level," says Sherbrooke, who was a co-author on the new paper – and other lizards could have converged on the solution too.
"Those are the only three lizards that have ever been examined at that depth," Sherbrooke says. "But people don't usually run out in the middle of a rainstorm and try to take observations on lizards. We miss lots of things in life."
Using droplets of water dyed with red food colouring, Yenmis was able to track the movement of water on the lizard's skin. Did it just spread in both directions within the channels? Or was it directional? Like a river flowing to the ocean, it preferentially flowed one-way – to the mouth.
We assume some kind of active mechanism at the mouth
This all happens with no mouth movement, or input of any kind from the lizard. "It all happens passively," Yenmis says.
How? A study published earlier this year may provide an answer.
This same one-way water flow is also seen in Texas horned lizards, but not thorny devils. By measuring the size of the tiny channels – or capillaries – of individual scales, Philipp Comanns from RWTH Aachen University in Germany found that their width wasn't constant along their length. They were thinner at the end that pointed towards the lizard's mouth – like a funnel.
This shape directs the flow of water forwards because of the way liquids behave in tiny channels. Surface tension and adhesive forces let liquids flow through the channels – and this "capillary action" becomes stronger as the channel narrows, drawing liquid towards the narrower end of the funnel.
But this capillary action isn't sufficient alone to help the lizards drink. Whenever the water came to a junction between one scale and another, it would have to jump from the narrow end of one funnel into the wide end of the next funnel, which the water is reluctant to do.
"As soon as the liquid approaches one of the abrupt widening points, which we call singularities, there the liquid would stop," says Comanns.
Steel versions of the water harvesters could be a big benefit to industry
To prevent such blockage, each capillary is paired to another through small interconnections. These are located in a staggered fashion so that, at each interconnection, even if one water flow is halted by a singularity, its neighbour is still moving. This way, it can pull the halted water up to speed, across its wide obstacle. This water flow will then return the favour, and so it continues back and forth.
"I had no idea what [they] discovered was going on," says Sherbrooke. "I didn't even realise that it was directional."
And yet both Sherbrooke and Comanns aren't convinced that this unidirectional flow is vital to the rain harvesting mechanism for lizards. The opening and closing of the mouth, they believe, is more important. This pulls water from the channels, like a sucking liquid through a web of straws. "We assume some kind of active mechanism at the mouth," says Comanns.
That begs the question: why do the tiny channels have directionality at all? "There must be some benefit," says Sherbrooke. "It must give them water under certain circumstances, maybe when they wouldn't get it otherwise. That's a hypothesis and a question to be addressed still."
Whatever its use for the lizards, Comanns is hoping to use it for our own benefit. He has already built plastic versions of the lizard water harvester, and thinks similar surfaces made from steel could be a big benefit to industry.
For instance, they could find a role in engine lubrication systems. "If you go for a vacation for several weeks, the lubricants are moving to the bottom of everything due to gravity," he says. This is a problem when the car is restarted. "You have a very high abrasion at the beginning because there's low lubrication."
To prevent this, Comanns hopes to design surfaces that can passively funnel lubricants from one place to another through tiny capillaries, like water on the backs, bellies, and legs of Texas horned lizards and Horvath's toad-headed agamas.