Casuariidae – Emu & Cassowaries

The Casuariidae are ratites (flightless birds without a keel on their sternum bone), which are native to the tropical forests of New Guinea, nearby islands, and to Australia. The order and family includes the emu, the only extant member of the genus Dromaius, and three cassowaries in the genus Casuarius: the Northern Cassowary, the Southern Cassowary, and the Dwarf Cassowary.

Emus are the second tallest birds (Ostriches are larger) and can reach heights of up to 1.8 meters (6.2 feet). They have long necks, strong legs, and shaggy, double-layered plumage, which helps insulate them from temperature extremes across various Australian habitats.

Cassowaries, on the other hand, are known for their striking appearance, including bright blue skin on their necks, brightly coloured wattles, and a casque (helmet) on the top their heads. Both genera are powerful runners, capable of reaching high speeds and covering long distances quickly. Their feet have powerful, sharp claws, formidable weapons against predators, real or imagined (they have been known to attack and injure humans and pet dogs that approach too closely). Cassowaries, in particular, are considered dangerous, known to use their powerful kick and having an aggressive nature when they feel threatened, although they are generally shy.

Emus and cassowaries have a common trait during breeding; the male assumes responsibility for egg incubation. In emus, females may court multiple males during the breeding season and can become quite aggressive towards other females. After laying a clutch of large, dark green eggs, the female departs, leaving the male to incubate the eggs for about eight weeks. During this period, male emus and cassowaries fast and rely on stored body fat, rarely leaving the nest and dutifully turning the eggs until they hatch.

Emu Dromaius novaehollaniae – ©Sheba_Also 43,000 photos, CC BY-SA 2.0 via Wikimedia Commons

Both families of birds play significant roles in their ecosystems. Emus are important seed dispersers, consuming a wide variety of plant material, while cassowaries are considered ‘keystone species’ in rainforest ecosystems due to their role in seed dispersal, particularly of fruit too large for other animals to process.

Emus remain common in most of the more lightly settled parts of mainland Australia. The overall population varies from decade to decade according to rainfall; falling as low as 200,000 and or rising as high as a million, but a typical figure is about half a million individuals. Although emus are no longer found in the densely settled southern and south-western agricultural areas, the provision of permanent water for domestic stock in arid regions, has allowed the mainland species to extend its range.

Cassowaries feed mainly on fruit, although all species are truly omnivorous and will take a range of other plant food including shoots, grass seeds, and fungi in addition to invertebrates and small vertebrates. All cassowaries are adept at disappearing long before a human knows they are there. Even the more accessible southern cassowary of the far north Queensland rain forests is not well understood.

All cassowaries have feathers that consist of a shaft and loose barbules. They do not have tail feathers or a preen gland. They have small wings that are reduced to stiff, keratinous quills, like porcupine quills, with no barbs and there is a claw is on each second finger. The furcula and coracoid are degenerate, and their palatal bones and sphenoid bones touch each other. These, along with their wedge-shaped body, are thought to be adaptations to ward off vines, thorns, and saw-edged leaves, allowing them to run quickly through the rainforest.

Northern Cassowary Casuarius unappendiculatus – ©www.viajar24h.com CC BY 2.0 via Wikimedia Commons

A cassowary’s three-toed feet have sharp claws. The second toe, the inner one in the medial position, sports a dagger-like claw that is 125 millimetres (5 in) long. This claw is particularly fearsome. Cassowaries can run at up to 50 kph through the dense forest., and can jump up to 1.5 metres and they are good swimmers, crossing wide rivers and even swimming in the sea.

All three species have horn-like, but soft and spongy crests on their heads, called casques, which are up to 18 cm high. Several purposes for the casques have been proposed. One possibility is that they are secondary sexual characteristics. Other suggestions include that they are used to batter through underbrush, as a weapon for dominance disputes, or as a tool for pushing aside leaf litter during foraging. The latter three are disputed by biologist Andrew Mack, whose personal observation suggests that the casque amplifies deep sounds. However, the earlier article by Crome and Moore says that the birds lower their heads when running …full tilt through the vegetation, brushing saplings aside and occasionally careening into small trees. The casque would help protect the skull from such collisions. Cassowaries live on fallen fruit, and spend a lot of time under trees where seeds the size of golfballs or larger fall from heights of up to 30 metres; so, the wedge-shaped casque may protect the head by deflecting falling fruit. At least the dwarf cassowary and southern cassowary produce very-low frequency sounds, which may aid in communication in dense rainforest. This ‘boom’ is the lowest known bird call, and is at the lower frequency limit of human hearing.