A northern cassowary shown in visible light at left and ultraviolet light at right reveals the bright green-blue biofluorescence of its casque.
Credit: Todd L. Green
On a warm night in Florida, Todd Green walked up to a six-foot-tall cassowary named Ginger with a flashlight and a strange expectation.
The light was ultraviolet, invisible to human eyes. Green, an anatomist and paleontologist at the New York Institute of Technology College of Osteopathic Medicine at Arkansas State University, wanted to know whether the bird’s casque — the helmet-like structure rising from its head — would glow.
The owner with him was skeptical. “I went out with one of the owners that night, and he was actually kind of making fun of me because he didn’t think anything was going to happen,” Green told National Geographic. Then the light hit Ginger’s head. “It was one of the brightest fluorescences I’ve ever seen,” he said. “I audibly gasped.”
Green’s new study now shows that cassowary casques, which look dull compared with the birds’ electric blue necks and red wattles, can blaze in green-blue patterns under ultraviolet light.
The finding does not prove that cassowaries use those patterns to communicate. But it suggests that one of the strangest structures in the bird world may look far more dramatic to cassowaries than it does to us.
A Dull Helmet That Was Never Really a Helmet
The casque of Ginger, a southern cassowary at the Cassowary Conservation Project in Florida, glows blue-green under ultraviolet light. She was the first living cassowary in the new study found to show this hidden fluorescence. Credit: Todd L. Green
Cassowaries already have a fearsome reputation. Native to the rainforests of Australia and New Guinea, they can weigh more than 140 pounds and carry dagger-like claws. They are often described among the world’s most dangerous birds. In tropical forests, cassowaries swallow fruits whole and spread seeds across the landscape, helping maintain the forest itself.
But it’s their casques that have puzzled scientists.
“It’s a bizarre structure,” Green told National Geographic. “When you see it, it kind of looks more like a horn, but really is nothing like any horns that animals have.”
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The casque has been proposed as a tool for defense, heat regulation, sound production or display. But it is not a sturdy battle helmet. It is a bony structure covered in keratin, the same family of proteins found in human hair and fingernails. Both male and female cassowaries have them, which also makes simple explanations based on male competition less convincing.
The new study tested whether the casque might be more visually important than it first appears.
A southern cassowary shown in visible light at left and ultraviolet light at right reveals the bright green-blue biofluorescence of its casque. Credit: Todd L. Green
Many birds can see ultraviolet light, a slice of the spectrum beyond human vision. Feathers, bills and other body parts that look plain to us can carry signals that birds may detect. Cassowaries, like other ancient lineages of birds, have visual pigments that suggest sensitivity in the UV range.
Green and his colleagues examined 95 adult cassowaries, including preserved museum specimens and living birds. They exposed casques from the three living cassowary species — the dwarf, southern and northern cassowary — to ultraviolet light at 365 nanometers and 385 to 395 nanometers.
Southern and northern cassowaries showed strong fluorescence across large areas of the casque. Dwarf cassowaries, whose casques are typically black or dark gray, showed little to none.
Moreover, southern cassowaries tended to glow toward the rear of the casque. Northern cassowaries showed different patterns, sometimes patchy and sometimes covering most of the structure.
Casque biofluorescence patterns differences between Casuarius species. (a) Casuarius bennetti, (b) Casuarius casuarius, (c) Casuarius unappendiculatus rothchildi, and (d) Casuarius unappendiculatus rufotinctus preserved dry specimens in left lateral view under 385–395 nm ultraviolet light. Credit: Scientific Reports.
A Clue for Living Birds and Extinct Dinosaurs
Credit: Nelson Illustrates, Instagram.
Like all birds, cassowaries are technically avian dinosaurs. Their prehistoric-looking headgear may offer some clues about how some dinosaurs may have used fluorescence.
“Cassowaries are among the best living analogs of many extinct kinds of dinosaurs,” said Darren Naish, a paleozoologist at the University of Southampton who was not involved in the research.
Paleontologists often look to cassowaries when thinking about the crests and ornaments of extinct dinosaurs, including some hadrosaurs and oviraptorosaurs. Those structures may have helped animals recognize one another, attract mates, intimidate rivals or display status. But fossils rarely preserve the soft tissues and colors that would have shaped how those ornaments looked in life.
“If we can come to understand what modern animals are using this strange headgear for,” Green told Science, “we can take it back into the fossil record to better understand extinct dinosaurs.”
If cassowaries can detect these signals in the wild, the casque may stand out during displays. Cassowaries stretch their necks and raise their bodies in ritualized encounters, making the casque the highest point of the animal’s profile. In a dim forest, where green leaves absorb much UV light, a UV-reflective casque could create contrast at dawn, dusk or under filtered canopy light.
That could help cassowaries judge size, age, identity or species. It could also help them avoid dangerous fights. A clear signal can be useful when two large, territorial birds meet and neither benefits from injury.
The authors note that their work does not directly prove the casques function as species signals. Cassowaries may see broad UV contrast from the casque, not the detailed glowing patches captured in the lab. Or the patterns may arise from the structure or chemistry of keratin without serving a social purpose.
Still, the findings are a good reminder that animals often live in visual worlds humans cannot see or even comprehend. Flowers, feathers, fish and even some mammals can carry ultraviolet or fluorescent cues that only become obvious when researchers look beyond human color vision.
The glow may also help humans. Because the fluorescence remains visible in old museum specimens, curators could use UV light to identify incomplete or faded cassowary specimens. Field researchers might one day use UV-sensitive cameras to distinguish individuals, much as whale researchers identify humpbacks by tail flukes.
“This is one of those huge, high profile, career-defining discoveries,” Naish told National Geographic. “It’s a very, very good set of results.”
The next step is harder than shining a flashlight in the dark. Scientists need to test whether cassowaries actually respond to UV cues under natural rainforest light. Until then, Ginger’s glowing casque remains a tantalizing glimpse of a hidden signal — one that was always there, waiting for the right eyes to see it.
The findings appeared in the journal Scientific Reports.