The cassowary achieves its luxuriously glossy plumage through modifications to feather shape similar to the hair structure of some mammals with scale-like body coverings, according to a new US study.
This mechanism likely evolved gradually over time, the researchers suggest, with an ancestral bird losing its barbules (feather branches) and the cassowary developing a distinctly thickened central feather shaft.
And that may help scientists better understand vertebrate colouration more generally.
While other birds achieve a silky sheen through a keratin film in their tiniest barbules or through flattened feather barbs, cassowaries’ characteristic black gloss appears to arise from the structure of the central feather shaft.
Structural colours that arise from the branches within feathers, manifesting in a range of iridescent hues, have been observed in many modern birds but have not been reported in members of the palaeognath clade, which includes cassowaries, ostriches, and emus.
In their study, Chad Eliason from the Field Museum of Natural History in Chicago and Julia Clarke from University of Texas at Austin compared the reflectance ratios of 32 feathers from 17 palaeognath species.
They used atomic force microscopy and transmission electron microscopy to understand the structural basis of cassowary sheen, finding that their feathers are not noticeably smoother than those of related species and that they possess no keratin film.
This led them to measure aspects of feather microstructure with light microscopy, noting differences between shiny and matte species. They suspect different genes and regulatory components underlie the varying feather parts that can contribute to structural colour.
“These kinds of observations are key to understanding how colour evolves and also inform how we think about extinct species,” Clarke says.
The pair also explored the feathers of Calxavis grandei, a cousin of the cassowary that lived 52 million years ago, thanks to a well-preserved fossil that includes imprints of its feathers.
“You can look at a fossil slab and see an outline of where their feathers were, because you kind of see the black stain of melanin that’s left over, even after you 50 million years or so,” says Eliason.
“We peeled off little flakes of the fossil from the dark spots of melanin, and then we used scanning electron microscopes to look for remnants of preserved melanosomes.”
Examining these feather imprints on a microscopic level, they saw the shape of the pigment-producing melanosomes in the leaf-like barbules of the feathers. The melanosomes were long, skinny, and green bean-shaped, which in modern birds is associated with iridescence.
The study is reported in a paperin the journal Science Advances.