The cassowary (Casuarius casuarius) is a large, flightless bird that bears a prominent helmet (or casque). The function of the cranial structure has attracted considerable curiosity and speculation for nearly two centuries and animal experts have proposed various theories, including that it’s a protective weapon used for fighting other animals or a means of attracting the opposite sex. According to a new study published in the journal Scientific Reports, the casque acts as a thermal radiator or ‘thermal window,’ offloading heat at high temperatures and restricting heat loss at low temperatures.
Cassowaries use their casque much in the same way as some other birds use their beaks for heat exchange. Image credit: Lolame.
“Our results are quite compelling and it’s highly probable this is what the casque is actually used for,” said Danielle Eastick, a researcher at La Trobe University in Melbourne, Australia.
“It’s really exciting to think we may have solved a mystery that has baffled scientists for so long.”
Using a handheld thermal imaging device, Eastick and colleagues obtained readings from 20 captive cassowaries, from Victoria through to northern Queensland and in different weather conditions.
The images showed that the birds released minimal heat from their casque when the weather was just 5 degrees Celsius and the greatest levels when the mercury reached 36 degrees Celsius.
“Cassowaries face a thermal challenge owing to their large size (up to 5.2 feet, or 1.6 m, height; with females heavier than males, dark plumage and tropical distribution in Oceania,” Eastick said.
“Just as humans sweat and dogs pant in hot weather or following exercise, they offload heat from their casque in order to survive. The hotter the ambient temperature, the more heat they release.”
The ‘thermal window’ explanation may provide a rare glimpse into the physiology of dinosaurs.
“Our finding reinforces the possibility that Mesozoic dinosaurs with similar structures may have also used such appendages to cope with tropical environments,” the scientists said.
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Danielle L. Eastick et al. 2019. Cassowary casques act as thermal windows. Scientific Reports 9, article number: 1966; doi: 10.1038/s41598-019-38780-8
