Creatures that make their homes in the desert have to be tough and adaptable. To overcome the searing heat and lack of water, many have developed strategies that help them to avoid the sun at critical times. Sandfish lizards (Scincus scincus) from the North African desert resolve the challenge by plunging beneath the surface of the dunes and burrowing down into sand at a comfortable temperature. Yet how these reptiles breathe in an environment where most would suffocate was a mystery. Not only must they breathe in sufficient oxygen while entombed in sand, but they must also avoid inhaling grains into their delicate lungs. According to Anna Stadler, from Johannes Kepler University Linz, Austria, another burrowing species – the Colorado Desert fringe-toed lizard (Uma notata) – is equipped with a U-shaped kink in its respiratory tract, which traps sand particles that are then exhaled. Might subterranean sandfish use a similar twist to keep their lungs sand-free?
First, Stadler, Werner Baumgartner and Wolfgang Böhme needed to find out how well the lithe creatures protect their delicate respiratory tissue from the abrasive grains, and when they dissected animals that had died from natural causes, they were impressed to find that the sandfish's airways and lungs were entirely sand-free. However, when they reconstructed the 11-mm-long airways from 3D images of the respiratory tract, there was no evidence of a sand trap kink. And although they found tissue that could swell and trap particles, it was not large enough to close the airways and account for the lizard's sand-free lungs. ‘In general, the only thing that was interesting is that there is mucus and cilia and there is a narrow part that gets wider and then it gets narrow again’, says Stadler.
With no evident filtration system, Stadler and her PI, Werner Baumgartner, decided to build a model of the minute respiratory system to try to discover how the tiny animals were excluding sand from their lungs. Realising that she would have to scale up the model if they were to have any hope of making meaningful measurements, Stadler teamed up with Michaela Huemer, Martin Riedl, Stephanie Shamiyeh and Bernhard Mayrhofer to print a 3D model that was 7.8 times larger; ‘It was not that easy’, says Stadler. The team also had to switch to using a helium atmosphere – to ensure that the gas flows in and out of the model respiratory system were realistic – in addition to replacing the fine sand with coarser grains. Meanwhile, Boštjan Vihar and Mathias Günther attached a tiny sensor to the lizards’ chest to measure their breathing while submerged, and the team was surprised to find that although the lizards inhaled slowly when buried, they seemed to ‘cough’ the air out of their lungs in just 40 ms. The team then buried the respiratory system model in sand, simulated the lizard's breathing pattern with a pump and saw that the model remained sand-free. Finally, Stadler calculated the flow pattern through the airways and realised that the air speed decreases dramatically at the point at which the airway widens above the section coated in mucus and cilia when the animals inhale.
Stadler suspects that this drop in air speed allows the sand particles to fall and become trapped in the mucus before the lizard exhales hard to effectively cough the sand out of its airways. So instead of using a physical obstruction to protect themselves from suffocation, sandfish appear to use a dynamic filter to prevent their lungs from becoming clogged with sand.
