I’m a big fan of palaeognaths – the terrestrial bird group that includes the mostly big, flightless ratites and the chicken-sized, flight-capable tinamous. Among the most interesting, most aberrant of palaeognaths are also among the most poorly known. I’m talking about the black-plumaged, elaborately adorned cassowaries of eastern Australia, New Guinea, and various of the islands around and between these two regions. I’ve long had a serious research interest in cassowaries and have been collected data on them (predominantly on museum specimens) since the late 1990s. Alas, other things have long been getting in the way and I’ve never managed to complete any research papers on them. Until a few weeks ago, when cassowary specialist Richard Perron and I published our paper ‘Structure and function of the cassowary’s casque and its implications for cassowary history, biology and evolution’ (Naish & Perron 2014). Today I want to talk about this paper and some of the ideas associated with the research that we did.
Comparatively little is known about cassowaries and yet a huge number of questions exist as goes their evolutionary history, morphology, ecology, behaviour and biology. There are questions about their biogeography and distribution, their systematics and phylogeny, their anatomy and functional morphology, their social and sexual behaviour, their ecology, and so on. Part of the reason for this situation is that most populations are hard to study in the wild, due in part to the terrain, climate, relatively remote location, and politics of New Guinea. Andrew Mack’s recent book Searching for Pekpek: Cassowaries and Conservation in the New Guinea Rainforest does a very good job of explaining how difficult things are (Mack 2013). Mack went to New Guinea to study cassowary ecology (in particular their interaction with fruiting plants) but ended up becoming so embroiled in politics and conservation work that most of the book is about these subjects, not so much about cassowaries (Mack 2013).
Three cassowary species are currently recognised: the Double-wattled or Southern cassowary Casuarius casuarius (the only one that occurs on mainland Australia), the Single-wattled cassowary C. unappendiculatus, and the Dwarf or Bennett’s cassowary C. bennetti. A case has also been made that a fourth species (previously included within the synonymy of C. bennetti) should be recognised. This is the Papuan or Westermann’s cassowary, the correct name for which is C. westermanni (Perron 2011), not C. papuanus as has been thought by some authors (Davies 2002).
There’s no serious doubt that cassowaries are close relatives of emus, but there is doubt as to how the different extant cassowaries are related to one another. We generated a phylogeny from molecular sequences and – to my surprise (but not necessarily to Richard’s) – C. casuarius seems to be the sister-taxon to a C. unappendiculatus + C. bennetti clade (Naish & Perron 2014). This has implications for cassowary biogeography and evolutionary history – two subjects that I can’t cover here today (but will do at some point in the near future).
It’s worth saying in the context of these comments that cassowary phylogeny and taxonomy requires substantial revision. More than 20 species and a much greater number of ‘subspecies’ have been named, at least some of which appear to be worthy of recognition. We’ve made a start with this and, in our paper, provide molecular support for the distinction of C. westermanni (dammit: I’ve only just noticed that we incorrectly have it spelt ‘westermani’ in our cladogram). An expanded molecular database that includes genetic information from more cassowary populations needs to be produced in future. The whole situation is made complicated by the fact that people have evidently moved cassowaries around quite a bit, making it difficult to determine which populations represent the products of natural dispersal and which have been artificially transported.
Anyway, the primary aim of our study was investigate three connected areas: casque anatomy, casque function, and cassowary evolution. Richard obtained the head of a C. unappendiculatus specimen (representing an animal that died in captivity) and sectioned it. We then set about doing some basic descriptive work. You may or may not be surprised to find that scarcely any work of any sort has ever been done on cassowary casque anatomy. You might be surprised to find that scarcely any work of any sort has ever been done because, hey, we’re talking here about a neat and peculiar structure that people are always asking about. Surely, you’d think, it would have formed the focus of some kind of study? But, on the other hand, you might not be surprised to find that scarcely any work of any sort has been done on it because, alas, there are an enormous number of anatomy-themed issues and questions that have never been investigated. [Adjacent cassowary skeleton image by Open Cage.]
Casque form, internal anatomy, and inferred function
The casque’s external keratinous sheath is not as hard as might be assumed. It’s pliable and somewhat leathery in life (except along its anterior and posterior margins). Casque form is extremely variable intraspecifically, there being some indication that casque size and shape reflects health and diet and perhaps individual quality, as well as sex (females seem to have bigger casques). This hypothesis requires proper evaluation and is obviously relevant to the idea that the casque has a sexual display function.
Beneath the keratinous sheath of the casque, its bony core is formed of a dense-boned, twin-layered ‘shell’ that surrounds both an internal mass of densely packed trabeculae as well as a gigantic air-filled space (Naish & Perron 2014). The external ‘shell’ is 2-3 mm thick and formed of thousands of bony cells arranged in a semi-regular, honeycomb-like arrangement. This sort of incredible micro-architecture has previously been described in the skull bones of birds and is also seen elsewhere in the skeleton (Bühler 1988). One day someone will do the required sorts of analysis that will test whether this arrangement confers a mechanical advantage of some sort... or maybe they’ll find that it’s related to something else.
Previous comments on the casque’s interior have noted that some sort of undetermined liquid or sludge is present inside (Crome & Moore 1988, Jones et al. 2003). There is, in fact, no liquid or sludge: the internal mass of trabeculae is quite fragile, so much so that if you push hard with your finger you can break right through it, and what we think has happened is that these descriptions refer to blood haemorrhaged from the various vessels present throughout the structure (Naish & Perron 2014). Some of you might recall the Inside Nature’s Giants episode in which Graham Lauridsen, Joy Reidenberg and Mark Evans dissected a cassowary casque to reveal a messy, bloody, wiry pulp that left them somewhat confused. What they were actually looking at was the bleeding, broken mass of internal trabeculae. [Image below by 22Kartika.]
Several possible functions have been proposed for the casque, though note that most of the ‘functions’ have been put forward in anecdotal fashion, not as hypotheses backed by data (Naish & Perron 2014). It might, some have suggested, serve as a protective helmet when the bird runs through the forest, as a weapon, as a digging or foraging or ‘fruit-knocking’ tool, or as a resonating device or sound-collector. Or perhaps it’s a sexual ornament, a possibility that looks likely given indications that its size is sexually dimorphic.
We hypothesise that a visual, sexual display role is most likely and that it co-evolved with the use of the casque as a resonating device (cf Jones et al. 2003, Mack & Jones 2003). We note that the casque is angled by the birds such that they direct their low-frequency, guttural vocalisations at prospective partners (Naish & Perron 2014). The possibility that cavernous subdermal blood sinuses might play a role in amplifying the booming calls that cassowaries make has also been suggested (Starck 1995), though this doesn’t discount the possibility that the casque also has an acoustic role.
Of mutual sexual selection and ‘species recognition’
One final thing (for now). Cassowaries are what we call elaborately monomorphic. That is, those extravagant display structures (brightly coloured, carunculated skin, dangling wattles, cheek flanges, and casques) are not limited to one sex, but present (and developed to similar degree) in both. Ergo, whatever the function of these structures, they are likely being used by members of both sexes in similar ways. We suggest that mutual sexual selection might be at play – the phenomenon in which members of both sexes are evaluating potential partners on the basis of quality and fitness (I should note the longer-term interest I and my colleagues – Hone et al. (2011) – have with respect to mutual sexual selection and its distribution in archosaurs fossil and modern). Given that male cassowaries play an extensive role in parental care, it’s certainly seems plausible that they’re the sort of birds where we might predict mutual sexual selection to be at play (Naish & Perron 2014).
Furthermore, the fact that the different cassowary species (all of which differ with respect to the form and configuration of the casque, wattles and so on) mostly occur naturally in non-overlapping environments and locations is in keeping with arguments that ‘species recognition’ is not a significant mechanism driving the evolution of these sorts of structures (Hone & Naish 2013).
I want to finish this article by emphasising the fact that the paper I’ve been discussing here is very much provisional as goes various of its conclusions: we’re at an essentially preliminary stage in terms of examining many of the issues here, and more work, and more data, is needed. Some of this work is underway. Indeed, we’ll be coming back to the fascinating world of cassowaries at some point later this year.
For previous articles on cassowaries and other palaeognaths, and on some of the issues mentioned in this article, see...
- Walter Rothschild and the rise and fall of Sclater’s cassowary
- Did dinosaurs and pterosaurs practise mutual sexual selection?
- Getting a major chapter on birds – ALL birds – into a major book on dinosaurs
- Thor Hanson’s Feathers: The Evolution of a Natural Miracle
- The ‘ghosts’ of extinct birds in modern ecosystems
- Controversies from the world of ratite and tinamou evolution (part I)
- The Age of Maximum Cassowary
- Ratites in trees: the evolution of ostriches and kin, and the repeated evolution of flightlessness (ratite evolution part II)
Refs - -
Bühler, P. 1988. Light bones in birds. Los Angeles County Museum of Natural History, Science Series 36, 385-393.
Davies, S. J. J. F. 2002. Ratites and Tinamous. Oxford: Oxford University Press.
Hone, D. W. E. & Naish, D. 2013. The ‘species recognition hypothesis’ does not explain the presence and evolution of exaggerated structures in non-avialan dinosaurs. Journal of Zoology 290, 172-180.
Hone, D. W. E., Naish, D., & Cuthill, I. C. 2011. Does mutual sexual selection explain the evolution of head crests in pterosaurs and dinosaurs? Lethaia 45, 139-156.
Jones, J., Mack, A. L. & Nelson, D. A. 2003. Low-frequency vocalisations by cassowaries (Casuarius spp.). Auk 120, 1062-1068.
Mack, A. L. 2013. Searching for Pekpek: Cassowaries and Conservation in the New Guinea Rainforest. Cassowary Conservation & Publishing, New Florence (PA).
Mack, A. L. & Jones, J. 2003. Low-frequency vocalizations by cassowaries Casuarius spp. The Auk 120, 1062-1068.
Naish, D. & Perron, R. 2014. Structure and function of the cassowary’s casque and its implications for cassowary history, biology and evolution. Historical Biology doi: 10.1080/08912963.2014.985669
Perron. R. M. 2011. The taxonomic status of Casuarius bennetti papuanus and C. b. westermanni. Bulletin of the British Ornithologists’ Club 131, 54-58.
Starck, J. M. 1995. Comparative anatomy of the external and middle ear of palaeognathous birds. Advances in Anatomy, Embryology and Cell Biology 131, 1-137.