A casque of thousands

Cassowary and palm seeds
Photographer: Liz Gallie

A recent study examined the internal anatomy of a cassowary casque in an attempt to better understand its evolution and ecological function.

The Southern Cassowary (Casuarius casuarius) is one of Australia’s most distinctive birds yet surprisingly, there is still much to learn about this majestic creature. This is of particular concern given the important role that cassowaries play in maintaining tropical rainforest diversity and structure. Some of the many things we don’t know about cassowaries are: how they communicate with each other, how they move within the landscape, what their mating rituals are, and details of their dietary habits.

 

Cassowary casques

Cassowaries are one of only a handful of animals to have a casque, the ‘helmet-like’ structure that sits atop their heads. Made of keratin, the key structural material found in hair and nails, it is a defining feature of the species. However, despite its prominence, we don’t really know why the casque exists.

A research team headed by Dr Darren Naish from the University of Southampton has provided new anatomical information about the cassowary’s casque that is helping to address this important knowledge gap. Dr Naish’s team has, for the first time, described in detail the anatomy of the casque, offering ideas as to its evolution and function.

Six hypotheses, some anecdotal, have previously been proposed to explain the casque’s presence. Dr Naish’s team evaluated each hypothesis using the anatomical information they obtained dissecting a cassowary skull (see Table 1). Based on their observations, the researchers concluded that the most likely functions of the casque are its use as both a visual and acoustic socio-sexual aid. Aurally, it is likely that the casque acts as a resonance box, with male cassowaries making low frequency, guttural noises to attract a mate. The researchers suggest that the casque helps amplify and direct vocalisation as the male lowers its head towards its potential partner.

Dr Naish’s team confirms that there is very little known about cassowaries generally and that much more work needs to done on their anatomy, genetics, physiology, ecology and behaviour. Stating that their paper is 'unashamedly speculative in places', they hope their work will promote discussion and further investigation, suggesting areas for future research foci.

 

Table 1: Discussion on casque function based on anatomical description by Dr Naish’s team

The cassowary has its casque because it functions as a ....

Likelihood

Reasons

Resonance box used in low frequency communication

Most likely

See discussion in the text

Sexual display 

Plausible, but more research needed

Unconfirmed indications that casque height differs between males and females. 

Weapon used during combat with other cassowaries

Extremely unlikely

The casque is too light and fragile. No observational data to support this theory.

Ratites (large, flightless birds of Gondwanan origin, many now extinct) usually fight by kicking

Tool for moving foliage on the rainforest floor

Unlikely

Only one published observation of this behaviour

Unfavourable position and structural form of the casque

Tool to knock hanging branches to dislodge fruit

Unlikely

Lack of observational data

The casque is too light and fragile

Unfavourable position and structural form of the casque

Head protection when moving through the forest

Unlikely

Lack of observational data

The casque is not robust enough

 

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