Cassowaries’ strange headgear helps them stay cool in the heat

https://www.newscientist.com/article/2193508-cassowaries-strange-headgear-helps-them-stay-cool-in-the-heat/

Cassowaries

Cassowaries have unusual fin-like structures on their heads

Erica Olsen/FLPA

Cassowaries, the second largest birds in the world after ostriches, have a fin-like structure on their heads that has long been a mystery. Now biologists have determined that it acts like a radiator, helping them to shed heat in Australia’s sweltering summers.

Many functions have been proposed for the cassowary head fin, called a casque. Some thought it was a weapon, but since it is flexible, that seemed unlikely.

Others suggested it is a sexual ornament, although it is present in both males and females. Another possibility is that it acts as a resonance chamber, helping the birds make low, booming noises. However, emus and ostriches make similar noises without such an instrument.

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Danielle Eastick of La Trobe University, Australia, and colleagues had another idea: it could be a “thermal window” – an organ that helps to regulate body temperature. Such organs have a large surface area and a rich blood supply that can be turned up when the animal needs to lose heat, or restricted when it needs to retain heat. Rodents’ tails, elephants’ ears and toucans’ bills all work in this way.

To investigate, Eastick took readings with a thermal imaging device on 20 cassowaries in zoos and wildlife parks from Victoria to Queensland, in temperatures from 5 to 36°C.

Cassowary

Keith Allen / Alamy Stock Photo

In cold weather, the birds restricted blood flow to the casque, allowing it to drop almost to ambient temperature. In hot weather, blood flow in the casque increased. At the highest temperatures, 8 per cent of heat exchange over the cassowary’s body was through the casque – a lot for a small organ.

With large, round bodies covered in feathers, losing heat is a struggle for cassowaries. Emus and ostriches lose some heat by flapping their wings, but cassowaries’ wings are so small that they are little help.

Several dinosaurs had similar-looking features on their heads, including Anzu wyliei, the recent discovery corythoraptor and some members of the “duck-billed” hadrosaurid family.

We don’t know whether dinosaurs were warm-blooded like modern birds and mammals, or cold-blooded like modern reptiles, or something in between. But it’s plausible that those with crests used them for heat exchange too.

“These structures are so similar to the ones that you see on dinosaurs,” says Eastick. “It makes you think if cassowaries are using them for thermoregulation, it’s really possible that dinosaurs did as well.”

Journal reference: Scientific Reports, DOI: 10.1038/s41598-019-38780-8

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