Cassowaries, Casuarius spp., are large, flightless, Palaeognath Birds related to Emu’s and Ostriches, which inhabit the dense forests of northern Australia and Papua New Guinea. These Birds have a distinctive helmet-like casque, made up of bone with a keratinous covering, the purpose of which has eluded biologists for over 200 years, with suggestions having been made that the structure might be used in combat, as a sound-enhancing device to help the Cassowaries’ booming call carry further, to protect the head from damage when running at speed through vegetation, or possibly as a secondary sexual characteristic.
A Southern Cassowary, Casuarius casuarius. Lone Pine Koala Sanctuary.
In a paper published in the journal Scientific Reports on 13 February 2019, Danielle Eastick of the Department of Ecology, Environment and Evolution at La Trobe University, Glenn Tattersall of the Department of Biological Sciences at Brock University, and Simon Watson, John Lesku, and Kylie Robert, also of the Department of Ecology, Environment and Evolution at La Trobe University, publish the results of an investigation into the possibility that the casque of the Cassowary is a thermal window used in the regulation of body temperature.
The idea that Cassowaries might use their casques to aid temperature control was first made in 1994, in a paper by Polly Philipps of the University of Illinois, Urbana-Champaign, and Allen Sanborn of Barry University, as part of a wider study into temperature regulation in Palaeognath Birds. However, Philipps and Sanborn were only able to observe a single Cassowary, at an ambient temperature of lower than 30°C (i.e. lower than the average temperature in the areas where Cassowaries live), so while the idea that a Bird as large as a Cassowary (females can reach 160 cm in height and weigh as much as 60 kg), that lives in tropical forests and has a thick coat of black feathers, might use a well vascularised bony casque to help regulate its temperature seems good, there has been, to date, no proof of this.
Eastick et al. were able to examine 20 Cassowaries at a range of ambient temperatures, using a Testo 875i hand-held thermal imager. This revealed that at high temperatures Cassowaries shed the most heat through their casques, the tips of their bills and their legs (beaks and legs are known to be used as heat exchanges in a wide range of Birds), while at low temperatures (i.e. when the Birds were losing heat involuntarily) the Cassowaries lost the most heat through their necks, their eyeballs and the hindpart of their bills. This strongly suggests that when they are hot Cassowaries pump blood to the casque, neck and beak in order to facilitate heat loss, while when they are cold the most heat is lost from areas they cannot afford to cut the blood supply to. They also noted that when they were hot the Cassowaries frequently dipped their casques into water, something which would improve heat loss.
Thermal image of a Cassowary at an ambient temperature of 5°C. Eastick et al. (2019).
Thermal image of a Cassowary at an ambient temperature of 25°C. Eastick et al. (2019).
Thermal image of a Cassowary at an ambient temperature of 35°C. Eastick et al. (2019).
Eastick et al. also observe that as the temperature rises the rear of the casque begins to shed heat before the front, suggesting that the Cassowaries are able to fine-tune their temperature control by increasing the blood supply to different parts of the casque sequentially, something previously observed in the beak of the Toco Toucan, Rhamphastos toco, which is also used in temperature control.
They also note that casques and casque-like structures are seen in a wide variety of Birds living in tropical climates, including Hornbills, Bucerotidae, Helmeted Guineafowl, Numida meleagris, and the Maleo, Macrocephalon maleo. They also note that such structures are known to have been present in a wide range of Pterosaurs and Dinosaurs, making it possible that these animals also regulated heat in a similar way.
Eastick et al. also observe that as the temperature rises the rear of the casque begins to shed heat before the front, suggesting that the Cassowaries are able to fine-tune their temperature control by increasing the blood supply to different parts of the casque sequentially, something previously observed in the beak of the Toco Toucan, Rhamphastos toco, which is also used in temperature control.
A Toco Toucan, Rhamphastos toco. Bernard Dupont/Wikipedia.
They also note that casques and casque-like structures are seen in a wide variety of Birds living in tropical climates, including Hornbills, Bucerotidae, Helmeted Guineafowl, Numida meleagris, and the Maleo, Macrocephalon maleo. They also note that such structures are known to have been present in a wide range of Pterosaurs and Dinosaurs, making it possible that these animals also regulated heat in a similar way.
Parasaurolophus cyrtocristatus, a Dinosaur with a prominant crest that may have been used in tempertature control. Wikimedia Commons.
