Abstract
Many ideas have been put forward for the adaptive value of the cassowary casque; and yet, its purpose remains speculative. Homeothermic animals elevate body temperature through metabolic heat production. Heat gain must be offset by heat loss to maintain internal temperatures within a range for optimal performance. Living in a tropical climate, cassowaries, being large bodied, dark feathered birds, are under thermal pressure to offload heat. We tested the original hypothesis that the casque acts as a thermal window. With infrared thermographic analyses of living cassowaries over an expansive range of ambient temperatures, we provide evidence that the casque acts as a thermal radiator, offloading heat at high temperatures and restricting heat loss at low temperatures. Interestingly, at intermediate temperatures, the casque appears thermally heterogeneous, with the posterior of the casque heating up before the front half. These findings might have implications for the function of similar structures in avian and non-avian dinosaurs.
Highlights
Many ideas have been put forward for the adaptive value of the cassowary casque; and yet, its purpose remains speculative
Homeothermic animals maintain a largely stable internal body temperature that is often different from Ta through the metabolic production of heat[13,14]
The cassowary casque meets the characteristics of thermal windows: uninsulated and vascularised
Summary
Many ideas have been put forward for the adaptive value of the cassowary casque; and yet, its purpose remains speculative. At intermediate temperatures, the casque appears thermally heterogeneous, with the posterior of the casque heating up before the front half These findings might have implications for the function of similar structures in avian and non-avian dinosaurs. The cassowary faces a thermal challenge owing to its large size (up to 160 cm height; with females (60 kg) heavier than males (30 kg)), dark plumage and tropical distribution in Oceania[23,24,25] For these collective reasons, it is possible that the distinctive helmet-like structure upon the cassowary’s head acts as a thermal radiator to remove excess heat. This finding reinforces the possibility that Mesozoic dinosaurs with similar structures[26,27,28] may have used such appendages to cope with tropical environments
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