The cassowary is a large, flightless bird that bears a prominent helmet (or casque). The function of the casque has attracted considerable curiosity and speculation for nearly 200 years1, yet its purpose remains unclear2. Early descriptions of the casque referred to it as a horny structure3,4, a horn-covered bony growth5 or hoof-like material6, giving rise to the most widely recognised idea that the casque is used as a protective structure for moving at high speed in dense vegetation5,7,8 and during fights with other animals5. Others have suggested that the casque is a secondary sexual characteristic9, although this hypothesis might not reflect the reason for its origin since both males and females are casque-bearing. Alternatively, the casque has been proposed to act as a resonance chamber to amplify the cassowary’s low frequency boom10; however, anatomical data argues against this idea11. One report12, of a single cassowary offered a possible thermal function, but the casque was outside the focus of the study, and the animal was studied only at ambient temperatures (T a ) less than 30 °C.

Homeothermic animals maintain a largely stable internal body temperature that is often different from T a through the metabolic production of heat13,14. Many animals have evolved morphological structures, or adapted existing body regions, known as ‘thermal windows’, for heat exchange13,15,16,17,18,19,20. A pre-requisite that enables a structure to exchange heat with the environment is that it must be highly vascularised and uninsulated13,21,22 to enable some of the warmth in blood to be dissipated. Vessels in these body parts are superficial, facilitating heat exchange before the blood is recirculated towards the core. It is crucial that blood flow is adjustable to these regions13,17. At low T a , vessel walls constrict, limiting blood flow to the area and enabling the warm blood to flow within the body. At warmer T a , vessels dilate, which facilitates the cooling of blood before returning to the deep interior of the body.

The cassowary casque meets the characteristics of thermal windows: uninsulated and vascularised. Casques are keratinized, overlying a body crown and network of trabeculae surrounded by dorsoventrally aligned canals containing blood vessels making up an extensive vascular network2,6,9. Moreover, 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 Oceania23,24,25. 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. We provide evidence that the casque serves (at least in part) as a thermal window. This finding reinforces the possibility that Mesozoic dinosaurs with similar structures26,27,28 may have also used such appendages to cope with tropical environments.