The project started with an accident. Four years ago, Steven Portugal was working at the University of Birmingham and studying how the colours and structures of bird eggs evolved. As he leant across his desk, he knocked over a large jug of distilled water into a box full of eggs.
“My first thought was: Oh crap, I’ve spilled lots of water,” he says. “My second thought was: Why does that one look so strange?”
On most of the eggs, the falling droplets flattened out and ran off in streaks, just as they would do on most surfaces. But on just one egg, they sat as a nigh-perfect sphere. As Portugal poked the egg, the drops ran off but kept their shape—spheres till the very end. You can see the same effect on many other natural surfaces, like the leaf of the lotus plant, the rim of a pitcher plant, or the body of a springtail. These are so good at repelling water—the technical term is superhydrophobic—that drops refuse to flatten upon them.
The special egg belonged to the common guillemot (Uria aalge), a seabird found across Britain, northern Europe and the northern Pacific. It’s vaguely penguin-like in appearance, and nests in vast, dense cliff-side colonies.
First, Portugal wanted to check that the guillemot’s egg is actually unusual. To do that, he measured the angle that a water droplet makes when it sits upon the egg. The bigger this ‘contact angle’, the more spherical the droplet stays and the more water-repellent the surface is (see image below). That angle would be 180 degrees for a perfectly water-repellent surface, and zero for a perfectly absorbent one. The guillemot egg achieved a respectable 120—less than a lotus leaf, but more than all the other eggs, which came in between 60 and 100 degrees.
Lotus leaves repel water because they are lined with rows upon rows of tiny studs, which trap pockets of air between them. The guillemot’s egg works along similar lines. When Portugal studied their shells under a powerful microscope, he found a textured landscape of microscopic mountains and valleys. “It looks like the Himalayas,” he says. There are hundreds of little cones and, on top of each of these, even smaller cones. “It’s very similar to what you see on a lotus leaf. The water sits on top of the cones rather than going down into the surfaces between them.”
Are these structures unique to the guillemot? Portugal found out by studying the shells of 450 different bird species, including every single one that breeds in Britain! “The guillemot structures weren’t present on any of these,” he says. I couldn’t find anything similar in the others.” The only exception was… the only other species of guillemot (Uria lomvia).
“It’s been like a 4-year Sherlock Holmes mission trying to figure out the function of these structures,” says Portugal, who has been systematically ruling out possible explanations. For example, he knew that guillemot parents rest their eggs on their feet while at the nest. Maybe the cones stop the eggs from rolling off, by increasing the friction between them and their parents’ feet?
It seems unlikely. Emperor penguins also incubate their eggs in this way and their eggshells don’t look anything like those of a guillemot. Portugal even managed to get some feet from dead birds and tested them against their respective eggshells, using a machine that measures the friction between two surfaces. The guillemots didn’t stand out.
Guillemots also nest in colonies of thousands, and each bird can apparently recognise its own egg among the surrounding hordes. Other British seabirds don’t share this ability. Portugal wondered if the guillemot’s egg surface might reflect light in such a way that makes each one unique, but after lots and lots of reflectance measurements, he couldn’t find any evidence for that.
Finally, he has narrowed his possible explanations down to two. First, and perhaps most obviously, the water-repellent cones have evolved to repel water. The birds nest on cliffs, and the eggs are regularly splashed by sea spray. Their cones help to keep them dry. Second, the eggs are self-cleaning. When water droplets runs off them, they carry dirt and microbes in their wake.
Both traits are important because guillemots don’t make proper nests. They lay directly onto bare rock, and the eggs “get covered in sea salt, spray, and their parents’ and friends’ faeces,” says Portugal. “They’re not the tidiest of birds.”
All eggs need to be able to exchange gases through their shells, so the embryos can get oxygen and get rid of carbon dioxide. They also need to lose enough water to create an air pocket, so the developing chick can breathe before hatching. If the eggs are encrusted by salt and faeces and dirt, the chicks would die.
Other seabirds have similar concerns, but they mitigate them with a less laissez-faire approach to egg-laying. Puffins, which are closely related to guillemots, nest in burrows. Shearwaters? Burrows. Gannets? They nest on cliffs but they actually make nests. Only the guillemots, which plop their eggs onto exposed cliff faces, need a self-cleaning surface.
Portugal, now at the Royal Veterinary College, is writing these discoveries up for publication, and is presenting his work today at the Society for Experimental Biology 2013 Annual Meeting in Valencia. He’s interested in what his fellow biologists will make of his hypotheses, but so far the reception has been positive. “People have been mesmerised by these surfaces,” he says. “You look at an egg and think it’s an egg. You don’t expect that there would be these weird structures.”
More on water-repellent structures:
- Killer plant super-slippery material that repels everything
- Incredible skin helps springtails to keep dry underwater and always stay clean
- Caterpillars vomit detergents to wreck ant waterproofing
- Death from disorder: scientists uncover secret of the velvet worm’s quick-setting slime
- Bacteria cities cannot form on a super-slippery surface, inspired by meat-eating plants
Note: I’m not at the conference. This post is based on interviews with Portugal, and a copy of his slides and data.