Structure Casques are found in a number of species, including most
hornbills, all
cassowaries, the
maleo, the
horned guan, the
helmeted guineafowl and several species of
curassow. In most of these species, the casque is a
bony extension of the or
skull that is covered with a
cornified layer of skin. However, in cassowaries, a foamy, elastic layer of
collagen sits between the bone and the skin. Hornbill casques grow from an area of
vascularized tissue at the front of the skull. In most species, the casque is primarily hollow, with a network of bony filaments at the posterior end. The structure starts small in youngsters and develops over time, and at maturity is typically larger in males than in females. For larger species with larger casques, the growth process may take as long as six years. In general, if male and female casques of a species are similarly sized, then they tend to be differently colored, and if they are similarly colored, they tend to be differently sized. Hornbill species that live in dry, open areas tend to have smaller casques than those that live in forested areas.
Functions Casques may serve different functions in different species, and may serve multiple functions in a single species. In the hornbills, the casques of males and females of each species differ in size, shape, structure, and color, and the casques of young birds are different than those of adults. These various differences may aid in the recognition of potential mates or competitors. Casques on the
bill, particularly those that run the length, or nearly the length, of the , may help to strengthen a long, curved beak, which can allow a stronger bite force at the bill's tip. Some species use their casques for fighting with other members of the same species. Male
helmeted hornbills, for example, clash their casques together in mid-air combats that can last up to two hours. Male
great hornbills also bash their casques together, sometimes in aerial combat, sometimes while one of the two birds is perched.
Indian grey hornbills casque-butt both in aerial battles, and in clashes between perched and flying birds. While most instances involve two males, clashes can also occur between members of a mated pair. Heat exchange is a primary function of cassowary casques. Studies have shown that the casques efficiently shed heat at high temperatures and help to restrict heat loss at lower temperatures. Cassowaries have been seen dunking their casques into water when temperatures were high. Some theories that have been advanced in the past for cassowary casques – that they provide a "helmet" to protect the birds' heads as they move through the forest, that they serve as a "shovel" during foraging, or that they are used during fights with conspecifics – have now been largely discounted due to a lack of field observations confirming those uses.
Problems and threats Casques are regularly subject to injury and disease. Injury can be either self-induced, or caused by
conspecifics or environmental factors. Invasive
squamous cell carcinoma is a common issue, particularly in the
great hornbill.
Rhinoceros hornbills and
helmeted hornbills have long been hunted for their casques, which are used for carvings. Items made from
hornbill ivory date back more than 2000 years in
Borneo and more than 1000 years in
China. Helmeted hornbills are particularly sought, as their casques are densely solid. Although they are protected by law throughout their range, they are killed at unsustainable rates; between 2011 and 2014, for example, more than 1100 skulls were seized from poachers in
Indonesia's Kalimantan region alone. ==Reptiles==