The first known naturally iridescent eggs have just been identified and are laid by a bird known as the great tinamou, aka "mountain hen," according to a paper published in the latest issue of the Journal of the Royal Society Interface.
Great tinamou eggs are so luminous that viewers actually perceive them as changing color when they are seen from different angles.
"Tinamous have some of the most colorful and glossy eggs of all birds, and here we show that an extremely smooth eggshell cuticle produces their mirror-like sheen," Branislav Igic, lead author of the paper, and colleagues wrote. "Furthermore, we reveal the presence of iridescence on the blue eggs of the great tinamou, an optical effect that has not been previously reported for avian eggs."
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Great tinamous, native to Central and South America, are about the size and shape of a small turkey. Their body coloration is a bit drab (with colors ranging from olive green to white), but they help the bird to stay well camouflaged in their rainforest habitat.
Charles Darwin himself treasured a tinamou egg from a different species. The egg was found after Darwin's death in his collections. At some point during his lifetime, Darwin accidentally cracked the egg, but the still-shiny specimen went on display earlier this year.
For the new study, Igic, a researcher from the University of Akron's Department of Biology and Integrated Bioscience Program, and the other researchers used chemical analysis and a barrage of high-powered magnifying devices to study tinamou eggs.
They determined that an extremely smooth cuticle produces the glossy appearance of tinamou eggshells. The cuticle is composed of calcium carbonate, calcium phosphate and, potentially, organic compounds such as proteins and pigments.
How does it change color? Not all color is pigment-based. A crystal, for example, may look rainbow colored from certain angles even though it is clear, simply because of the way it reflects light. Some insect bodies and bird feathers also exhibit radiant colors due to how their structures impact light.
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In this case, it appears that the eggs have the best of all color types. The researchers carefully removed the eggshell cuticles to see what would happen. The gloss and iridescence were lost, but the background coloration remained. This suggests that color comes from both iridescence and pigments.
But the real mystery is: Why would a prey species like these birds lay such eye-catching eggs? The scientists don't propose this idea, but I have to wonder that the optical illusion produced the eggshells might somehow make them invisible to certain predators.
The researchers instead suggest that females might want other females and males to find the eggs easily. Females sometimes lay their eggs within clutches of others, necessitating the ability to quickly find the eggs of different mothers.
"Bright eggs may also 'blackmail' males into comparatively high incubation attendance to conceal conspicuous eggs, thereby shortening their incubation time and reducing the risk of predation," the scientists wrote.
They also mention that smooth eggshell surfaces may prevent water from clogging pores in the shell that could impede gas exchange. They added, "A highly reflective eggshell surface may also help prevent damage to the embryo from solar radiation."
The developing tinamou chicks might therefore thrive in a perfectly comfortable environment with something akin to an auto heat shield (only better looking) wrapped completely around their colorful enclosures.
Photo: Tinamou eggs. Credit: Marcos Massarioli, Wikimedia Commons