Male sandgrouse are flying sponges, soaking up large amounts of water in their feathers in faraway desert watering holes before flying for many kilometers to give their thirsty children a drink
A small group of birds, the sandgrouse, that dwell in Africa’s forbidding deserts have long mystified and delighted observers because they absorb and carry water in their breast feathers whilst flying long distances to water their thirsty chicks. How do they do this? Thanks to the use of high resolution microscopes and cutting-edge 3D technologies, a team of researchers at Johns Hopkins University and Massachusetts Institute of Technology (MIT) have solved the mystery of how sandgrouse feathers can hold so much water (Figure 1).
“It’s super fascinating to see how nature managed to create structures so perfectly efficient to take in and hold water”, lead author, systems engineer Jochen Mueller, an assistant professor in Johns Hopkins’ Department of Civil and Systems Engineering, said in a statement. Professor Mueller’s research expertise exists at the intersection of science, application and design, and his primary interests are using 3D printing and manufacturing along with computational design to create smart materials. He currently holds eight patents.
“From an engineering perspective, we think the findings could lead to new bio-inspired creations”, Professor Mueller pointed out.
Meet the feathered muses and learn about their Superpower
Sandgrouse are a small family of birds that many people are unfamiliar with. To the casual observer, they closely resemble pigeons and doves in both shape and size (they are distant relatives), although a closer look will reveal some differences, particularly in their toes and legs.
They mainly live on the ground, feeding on seeds and, when they can find them, insects. They are cryptically colored with sandy grey, buff, or brown plumage that is often mottled or barred to blend into the arid treeless landscapes of their home. They have long, sharply pointed wings that enable them to fly swiftly, and they also run very fast on their deceptively short legs. Although sandgrouse commonly are seen in large flocks, they form pairs in breeding season. They nest in a shallow scrape on the ground, located many kilometers away from water to avoid predators. The chicks are precocial, leaving the nest shortly after their downy feathers have dried, and can feed themselves, although they cannot fly for a month. As a result, obtaining water is a problem.
To satisfy the chicks’ thirst, male sandgrouse fly as far as 32 kilometers (20 miles) to a watering hole, dunk their bodies in the water, and carry it back to the nest – in their belly feathers. This collected water can comprise as much as 15% of a male sandgrouse’s total body weight. Yet, he can carry this load of water at speeds that can reach 65 kilometers per hour (40mph). As much as half of that water can evaporate during the male bird’s half-hour-long flight back to the nest, where his thirsty chicks use their bills to squeeze water from his belly feathers.
There are 16 species of sandgrouse and the males of the species are the only birds with specially adapted belly feathers that can carry water. This water-carrying ‘superpower’ of sandgrouse was first reported in 1896 by Edmund Gustavus Bloomfield Meade-Waldo, better known as EGB Meade-Waldo, a British ornithologist who was breeding the birds in captivity. His painstaking observations of their behaviors were dismissed as ‘fantasy’, but finally, 60 years later, he was proven correct.
“He saw them behaving like this, and nobody believed him!” Professor Gibson said in a statement. “I mean, it just sounded so outlandish.”
EGB Meade-Waldo’s original report was confirmed in 1967 by two ornithologists who published their own detailed observations of wild sandgrouse visiting African watering holes (ref). They reported that male sandgrouse feathers could hold about 25 milliliters (5 teaspoons) of water, which the birds collected during the course of five minutes or so of dipping and fluffing their belly feathers in water. Unfortunately, the technology didn’t exist in 1967 to allow those researchers to further investigate the special feather structures in any detail.
Professor Mueller and collaborator and co-author, materials engineer Lorna Gibson, who is the Matoula S. Salapatas Professor of Materials Science and Engineering and a professor of mechanical engineering at MIT, set out to better understand how these unusual feathers work. Professor Gibson has been studying feather microstructures for years, and was curious to learn more about sandgrouse feathers.
“We just wanted to see how it works”, Professor Gibson explained simply. “The whole story just seemed so interesting.”
To conduct this investigation, Professor Mueller and Professor Gibson obtained several belly feathers from the specimen of a male adult Namaqua sandgrouse, Pterocles namaqua, held in the collections of the Harvard University Museum of Comparative Zoology, which maintains a collection of specimens comprising about 80% of the world’s bird species. They then investigated and greatly magnified the feathers, both dry and wet, using the full array of modern technologies available — scanning electron microscopy, microcomputed tomography, light microscopy and 3D videography. Professor Mueller and Professor Gibson closely examined the feather shafts, each just a fraction of the width of a human hair, and the even tinier individual barbs and barbules (Figure 3).
Professor Mueller and Professor Gibson discovered that sandgrouse feathers are structured differently from other birds’ feathers. In the inner zone of the feather, the barbules have a helically coiled structure close to their base and then a straight extension (Figure 3e). In the outer zone of the feather, the barbules lack the helical coil and are straight. Both zones lack the grooves and hooks that ‘zip’ together to hold the vane of contour feathers together in most other birds.
Professor Mueller and Professor Gibson then observed that, when wetted, the coiled portions of the sandgrouse feather barbules unwind and rotate so they end up perpendicular to the vane. This creates a dense forest of fibers that can hold water through capillary action. At the same time, the barbules near the tip curl inward, helping to retain water.
Inspired, Professor Mueller described the feather structures as “magnificent.”
“That’s what excited us, to see that level of detail”, Professor Mueller elaborated. “This is what we need to understand in order to use those principles to create new materials.”
What sort of new materials could be created using the sandgrouse feathers as a model and inspiration? Professor Mueller and Professor Gibson expect their findings will underpin future engineering designs requiring controlled absorption, secure retention, and easy release of liquids. For example, in desert regions where water is scarce but fog and dew occur regularly, incorporating this feather structure into huge nets to be used to efficiently collect water.
“You could imagine this could be a way to improve those systems”, Professor Gibson explained. “A material with this kind of structure might be more effective at fog harvesting and holding the water.”
Professor Mueller proposed that a water bottle with an inner feather-like structure to keeps the contained liquid from sloshing around while someone moves with it — a hydration pack or water bladder that could do this would be particularly appreciated by runners.
Professor Mueller also imagines a new sort of medical swab that is easier to use, “where you can efficiently soak up liquid, but it’s much easier to release it.” Adding that just such a release feature was an issue for collecting COVID-19 nasal test samples during the pandemic.
Professor Mueller and Professor Gibson plan to 3D print similar structures for use in investigating and developing commercial applications.
Jochen Mueller and Lorna J. Gibson (2023). Structure and mechanics of water-holding feathers of Namaqua sandgrouse (Pterocles namaqua), Journal of The Royal Society Interface 20(201):20220878 | doi:10.1098/rsif.2022.0878