Shells in all shades
THANK THE DINOSAURS FOR THIS BIRD TRAIT
PICTURE A bird egg: Perhaps it’s the cocoa brown of a free-range chicken. Or a robin’s creamy blue-green. If it’s a quail egg, it has inky speckles. Those colours and variations, according to a new study published last week in the journal Nature, share a common origin. Eggshell colour, in all its glory, is a trait that comes from long-ago animals: the dinosaurs.
Birds, strange as it seems, are living dinosaurs, the last of a lineage that otherwise went extinct 66 million years ago. Before this work, though, many biologists predicted that modern birds, not their ancestors, developed colourful eggshells. Eggshell theories were “kind of all over the place,” said study author Mark Norell, a paleontologist at the American Museum of Natural History in New
York. Some people hypothesised colours evolved several times independently in the history of birds; others suggested it happened once as birds became the animals we know. Colourful eggs, this study concludes, are much older.
“The discovery of a single origin of eggshell colour in dinosaurs is a wonderful reminder that modern birds inherited many traits from their dinosaurian ancestors,” said Mary Caswell Stoddard, a Princeton University evolutionary biologist who was not involved with this research. “We eat eggs for breakfast, but they hold so many clues to the evolutionary past.”
Smash an eggshell into its molecules, and you’ll find only two types of pigment among the wreckage. A molecule called biliverdin is the source of green (a decades-old experiment linked biliverdin to green splotches that sometimes appear in bruises). Another molecule, protoporphyrin, provides the rusts and browns. These pigments mix like watercolour paints to produce the entire colour palette of bird eggs.
A few years ago, Mark Hauber, an ornithologist at University of Illinois at Urbana-Champaign, and his colleagues designed a protocol to examine biliverdin and protoporphyrin in eggshells, including shells from a giant extinct bird called the moa. The authors of the current study ran with this idea and “did a much better job than we did,” by including a range of dinosaurs, Hauber said.
Study author Jasmina Wiemann, now a doctoral student at Yale University, began her search for colourful eggs with a dinosaur named Heyuannia huangi, an oviraptor with a beak like a parrot’s. Other scientists told her she was “wasting her time,” she said, because they assumed dinosaur eggs lacked pigment.
Using Raman spectroscopy, a nondestructive analytic tool, Wiemann looked for the molecular signatures of protoporphyrin and biliverdin in the oviraptor eggshells. Heyuannia huangi, she discovered, laid bluegreen eggs.
This new research expands her previous study. Wiemann, Norell and Tzu-Ruei Yang, at the University of Bonn in Germany, examined eggs from 18 species. The non-extinct animals included an alligator and the domesticated chicken. The dinosaur crowd was a diverse bunch: oviraptors and other bird ancestors, but also sauropods (long-necked dinos) and hadrosaurs (duck-billed dinos), which aren’t closely related to birds.
The study authors selected their shells carefully so they could know what sort of dinosaur laid the eggs. “We only accepted specimens for the research where there is either an embryo inside the egg,” Norell said, “or that there was an adult that was closely associated with the clutch of eggs.” Though shell colours faded over millions of years, as the eggs became fossils, traces of the pigment molecules remained.
Sauropods and hadrosaurs did not have colourful eggs. Theirs were like alligator eggs – white. Dinosaur relatives of birds, though, had colourful eggs, including spots and both pigment types. (If you’re wondering why supermarket eggs are white, that’s a human invention, the result of farmers who specifically raised chickens with genes for white eggs. Jungle fowl, the wild cousins of chickens, lay brownish eggs.)
The dinosaurs in this study came from the Cretaceous period, late in the dino line. Norell said it is possible that “more primitive” animals, like a Tyrannosaurus, may have laid colourful eggs, but the scientists lacked older shells to test.
These results confirm that Wiemann’s discovery of blue oviraptor eggs was not a fluke, Hauber said. “Extinct dinosaurs laid eggs just like the rest of the birds do,” he said.
Egg patterns are a window into dinosaur nesting habits. “A pretty easy prediction - but we’re the first ones to have real evidence for it - is that the origin of coloured eggs is associated with the origin of an open nest,” Norell said. Crocodiles and turtles bury their white eggs, which means they don’t need to be camouflaged. But many birds have coloured eggshells that camouflage their eggs in nests that are exposed to the elements and predators.
Hauber offered three other reasons, in addition to camouflage, why birds have colourful eggs: One, pigments can act like a parasol or sunscreen, protecting the embryos within from too much heat. Two, some parent birds use spot patterns to recognise their own eggs if they live in large colonies (or to prevent freeloading birds, like cuckoos, from sneaking their eggs into a brood). And three, pigment deposits are like molecular mortars, strengthening the shell’s structure. These are all possibilities for dinosaurs, too. “We put on our thinking cap now!” Hauber said.
So listen well, crayon manufacturers and paint mixers: Robin egg blue is fine, but oviraptor blue is the original.
An assortment of fresh chicken eggs