Regular readers of Tetrapod Zoology should know that I’m a big fan of cassowaries, and indeed have a serious academic interest in them…
Caption: CASSOWARIES!! (A) Dwarf or Bennett’s cassowary (Casuarius bennetti), (B) Double-wattled or Southern cassowary (C. casuarius), (C) Single-wattled or Northern cassowary (C. unappendiculatus). Images by Darren Naish.
Back in 2014*, Richard Perron and I published ‘Structure and function of the cassowary's casque and its implications for cassowary history, biology and evolution’ (Naish & Perron 2016), a paper that aimed both to get the ball rolling on investigations into cassowary evolution and anatomy and to present a few hypotheses for consideration. I wrote about the paper and our research at TetZoo ver 3 (that article is here). Of course, 2014… 2016… whatever is now a long time ago… sort of… and I’ve been thinking for a while that a major update of that article is required. And thus here we are.
* I generally think of the paper as ‘Naish & Perron 2014’ since I mostly consult the digital version. But the hardcopy version appeared in 2016, meaning that it’s now mostly cited as ‘Naish & Perron 2016’. I very much dislike the perpetual ambiguity we now have about publication dates.
To begin with, it is with some sadness that I must report that Richard Perron – my co-investigator on the issues discussed here – is no longer with us. Richard was probably best known for his association with International Zoo News, an important and very professionally produced publication (he was editor) considered standard reading in the zoo world. However, Richard’s interest in cassowaries was well known and he owned a vast storehouse of information on these birds and research pertaining to them. Some of this was archived at his website, and at least some of his thoughts and conclusions on taxonomy and variation within this perplexing and complex group were published in his 2016 book Taxonomy of the Genus Casuarius (Perron 2016). Richard’s death is bad news for cassowary research and promotion, but I have to say at this point that we might soon enter a new Golden Age of cassowary study thanks to the work of Dr Todd Green. I can’t say any more at the moment, but there’s much to look forward to. Todd is @TheCassowaryKid on Twitter.
Several studies on cassowaries and their casques have been published since Naish & Perron (2016) appeared. These are mostly very much relevant to the ideas and hypotheses discussed here and the text has been modified and updated accordingly.
Caption: I’ve never seen a cassowary in the wild, for shame. But the UK has had captive individuals in several collection for as long as I’ve been around, and I’ve spent appropriate time observing them. This Southern cassowary was photographed at Edinburgh Zoo in 2012. Images: Darren Naish.
Even today, comparatively little is known about cassowaries and a huge number of questions exist wih respect to 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 2013 book Searching for Pekpek: Cassowaries and Conservation in the New Guinea Rainforest does a 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).
Caption: cover of Andrew Mack's Searching for Pekpek, a really interesting, enjoyable read on the trials and tribulations involved in doing fieldwork on New Guinea. Do get hold of a copy!
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 or Northern 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).
Caption: montage depicting the three currently recognised extant cassowary species. (A) Double-wattled or Southern cassowary (Casuarius casuarius). (B) Double-wattled cassowary in profile. (C) Single-wattled or Northern cassowary (C. unappendiculatus). (D) Dwarf or Bennett’s cassowary (C. bennetti). Photos by D. Naish and R. Perron.
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 2016). This has implications for cassowary biogeography and evolutionary history.
Caption: phylogeny generated from molecular sequences: this is a modified version of the cladogram published in Naish & Perron (2016), with the spelling of 'westermanni' corrected.
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. 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.
Caption: mounted cassowary skeleton - so much neat anatomy, and so little work done on it. Image: Open Cage, CC BY-SA 2.5 (original here).
Anyway, the primary aim of our study was to 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.
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.
Caption: Naish & Perron’s (2016) montage depicting casque anatomy. Note the big air space in the casque (visible in D) and the mass of trabeculae filling its anterior part (visible in B).
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 2016). 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.
Caption: here’s a bonus image not relevant to the issue of casque anatomy but interesting nevertheless. We’re seeing the left side of the dorsal surface of the head, showing the bluish, thickly wrinkled skin, peppered across much of its surface by bristles. Avian bristles are sometimes modified feathers but are sometimes evolutionarily novel. Image: Richard Perron.
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 2016). 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.
More recently, Saber & Abdelhafez (2022) provided a brief evaluation of casque anatomy that repeats many of the observations we’d already made, but added extra details as obtained via x-ray. Their specimen was also different from ours in that it didn’t have the big empty space we discovered in the casque’s posterior region, this area instead being filled with trabeculae. The cause for this variation is unknown – it could be due to their specimen being from a Southern cassowary rather than the Northern cassowary we studied – and they noted that further study of this variation is needed.
Caption: dissection scenes from the cassowary episode of Inside Nature's Giants. A reddish mush, combined of numerous bloody fibres, is revealed in the casque's interior. Image: (c) Windfall Films.
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 2016). 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.
A novel hypothesis – or, at least, an implied hypothesis – was published by Eastick et al. (2019) in their intriguing and quite impressive paper ‘Cassowary casques act as thermal windows’. Richard and I had missed that a previous study (Phillips & Sanborn 1994) had actually already suggested the same idea. Eastick et al. (2019) showed how the casque heats up and functions as a heat dump as body and ambient temperature increase (things were especially obvious over 30° C). The implication from the paper is that the casque might thus be a specialised heat dissipation structure, and they even suggested that cranial crests in certain Mesozoic dinosaurs “may have also used such appendages to cope with tropical environments” (Eastick et al. 2019, p. 1).
Caption: Eastick et al.’s (2019) figure on how the different body parts of cassowaries change temperature according to ambient temperature. Notably, casque temperature obviously increases (relative to other body parts) at high temperatures.
I am currently of the opinion, however, that any thermal function that the casque has is incidental: the fact that it might have this role does not require that this should be assumed to be the primary driver of its evolution. By analogy, bovid horns have a known thermoregulatory function and vary in form, size and keratin thickness according to climate (Taylor 1966, Picard et al. 1988, 1996), while an important thermoregulatory role has also been proposed for antlers (Stonehouse 1968, Ohtaishi & Too 1974). Indeed, Eastick et al.’s (2019) results show that other areas of the cassowary’s body could be considered equally specialised for heat exchange, namely the end part of the bill and the legs. Given that bills and legs are specialised for their primary functions and were then secondarily co-opted in these birds for heat dissipation, I think that the same can be argued for the casque. If it has a role in thermal control, this is – I posit – an exaptation.
Caption: opinions are divided over whether this image of a sitting Single-wattled cassowary is cute or highly disturbing. Image: 22Kartika, CC BY-SA 3.0 (original here).
In contrast, we (Naish & Perron 2016) hypothesised 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). As support for this, we noted that the casque is angled by the birds such that they direct their low-frequency, guttural vocalisations at prospective partners (Naish & Perron 2016). The possibility that cavernous subdermal blood sinuses might play a role in amplifying the booming calls that cassowaries make had earlier been suggested by Starck (1995), though note that this proposal doesn’t discount the possibility that the casque also has an acoustic role.
I have since, however, absolutely revised this opinion (that is, the part positing a vocal function), and I thank cassowary expert Todd Green for sharing his thoughts on this issue too. In what feels to me like a very useful follow-up study to our paper, Brassey & O’Mahoney (2018) published observations of a Southern cassowary based on CT-scanning, and one thing they drew attention to was lack of connection between the spaces within the casque and the respiratory system. The casque thus has no proper connection with the airways, leading Brassey & O’Mahoney (2018) to “posit that low-frequency calls of cassowary are … generated and resonated in the trachea rather than the casque” (p. 4). I fully agree: videos I’ve seen since 2014 make me think that sounds made in the syrinx are not broadcast to the outside via the casque, but via inflation of structures at the base of the neck. You can see the neck base (and maybe adjacent parts of the thorax too) inflating and deflating when cassowaries vocalise, but exactly what’s going on here is unstudied so far as I know.
Caption: these stills from a Mission Beach video show a cassowary preparing to vocalise (at left), and then contracting and inflating its neck as it vocalises (at right). The back of the neck and parts of the rest of the body can be seen to vibrate as the vocalising occurs. Images: Mission Beach Cassowaries youtube channel (original video here).
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 plausible that they’re the sort of birds where we might predict mutual sexual selection to be at play (Naish & Perron 2016).
Caption: stalwarts of the mutual sexual selection literature: Crested auklets (Aethia cristatellus) at left; ruffled wing feathers of a Black swan (Cygnus atratus) at right. Images from Hone et al. (2011).
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).
The actual anatomical homology of the bones forming the casque wasn’t important to our study, but we felt it appropriate to mention it in passing: based on images provided online by Larry Witmer’s group (showing casque development in cassowary chicks), we hypothesised that the casque might be formed from outgrowths of the frontal bones, though we did note that this was provisional and that “A study of casque ontogeny is sorely needed” (Naish & Perron 2016, p. 509). This proposal is wrong, or at least simplified: Mayr (2018) – who took us to task for saying that work on cassowary casque development had never been published (we were wrong: William Parker had published relevant observations in 1866) – argued that the casque involved the mesethmoid at its base, with novel contributions from the nasals forming the bulk of the casque’s height (Mayr 2018). Green & Gignac (2020) then showed that both the mesethmoid and a novel ‘median casque element’ form the midline of the casque while parts of the nasals, lacrimals and frontals form its edges. It would seem, then, to be a multi-part structure.
Caption: Green & Gignac’s (2020) digital render of a juvenile Southern cassowary in (a) left lateral, (b) dorsal, (c) anterior and (d) posterior view, showing bony configuration of the casque.
And that about wraps things up for now. As I emphasised when I first wrote about this research in 2015, much of what’s been written about here is very much provisional, and we’re still 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! Having said that, if we look at the cassowary-themed papers that have appeared since Richard and I published our 2014/2016 paper (Brassey & O’Mahoney 2018, Mayr 2018, Eastick et al. 2019, McInerney et al. 2019, Green & Gignac 2020, Saber & Abdelhafez 2022), it feels to me that interest in these birds and their remarkable anatomy is building — so much so that we might talk of being in a cassowary research revolution.
On that note, we will revisit cassowaries again soon, I promise!
Caption: aww, cassowaries… aren't they great? Bennett's cassowary image by J. G. Keulemans. Image in public domain.
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, February 2011
Did dinosaurs and pterosaurs practise mutual sexual selection?, January 2012
Getting a major chapter on birds – ALL birds – into a major book on dinosaurs, August 2012
Thor Hanson’s Feathers: The Evolution of a Natural Miracle, May 2012
The ‘ghosts’ of extinct birds in modern ecosystems, December 2013
Controversies from the world of ratite and tinamou evolution (part I), March 2014
The Age of Maximum Cassowary, April 2014
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