Tetrapod Zoology

Tetrapod Zoology

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Hoatzins are no longer exclusively South American and once crossed an ocean


Like a lot of people interested in birds and weird animals in general, I have a great love of the Hoatzin Opisthocomus hoazin (yes, the binomial name does not have the same spelling as the vernacular one).

Hoatzins photographed in Brazil by Cláudio Dias Timm, from wikipedia.

I’m sure most Tet Zoo readers are already familiar with this species, but – for the few of you that aren’t – this is that famously strange bird of the Amazon and Orinoco basins that typically lives in the trees along waterways and digests the leaves it eats in an enlarged crop (Grajal et al. 1989). This ‘foregut fermentation’ style of digestion is unique. Hoatzins are also unusual in possessing crenulated bill margins that are actually used in chewing (Korzun et al. 2003).

Climbing hoatzin chick. Image by J. Arthur Thomson, from wikipedia.

The one factoid that most people know about the Hoatzin is that its chicks possess obviously clawed fingers and use them in climbing. While it’s often stated or implied that this is a primitive feature that links the Hoatzin with Archaeopteryx and other Mesozoic theropods, the truth is that hand claws are actually widespread in modern birds and not at all unusual. If you’re a regular Tet Zoo reader this should sound somewhat familiar, since I covered the subject of manual claws in birds back in June 2010. Oh, ‘Hoatzin’ is pronounced something like ‘what-seen’ or ‘what-son’.

Anyway, I digress. People who pay attention to my twitter feed (I know that at least two of you do) might have noticed my reference to the ‘Neornithine Holy Grail’ a few weeks ago. What the hell was I referring to? As usual I wasn’t being entirely serious, and was also employing a degree of hyperbole. I was actually referring to a paper – published earlier this week in Naturwissenschaften by Gerald Mayr and colleagues – that provides some fascinating new information on hoatzin history and origins, and also identifies two new fossil hoatzin species.

Foro panarium, from Olson (1992).

We need to note to begin with that fossil hoatzins have been known for a while. In 1953, Miller (1953) named Hoazinoides magdalenae from the Middle Miocene of Colombia. It’s known from a partial skull that’s highly similar to that of the modern Opisthocomus but somewhat larger. I should also mention that an alleged hoatzin relative called Onychopteryx simpsoni (named for a partial tarsometatarsus) was reported from the Eocene of Argentina in 1971 (Cracraft 1971). However, the fossil concerned is so scrappy that few experts have been prepared to accept this as a valid hoatzin record. And then there’s Foro panarium from the Eocene of North America, described by Olson (1992) as superficially hoatzin-like. It’s possible that this bird (represented by fairly substantial remains, as you can see here) is allied with hoatzins, but so far only superficial similarities have been alluded to and detailed evidence for a relationship hasn’t been presented.

Two new fossil hoatzins

Mayr et al. (2011) now give us two additional fossil hoatzins. The first is named Hoazinavis lacustris and is known from a set of wing and chest bones, collected in 2008 from the Upper Oligocene-Lower Miocene Tremembé Formation of São Paulo, Brazil. All of Hoazinavis’s bones are very similar to those of Opisthocomus (though there are enough differences to warrant clear taxonomic distinction), but they come from a rather smaller bird (and the bone surface texture shows that it was an adult at the time of death). Given the problematic status of both Onychopteryx and Foro, Hoazinavis is currently the oldest known hoatzin.

Elements of Namibiavis and Hoazinavis, from Mayr et al. (2011). Images (c) Senckenberg.

Then we come to the second hoatzin. It’s known from three coracoids and six humeri, all of which come from Lower Miocene sediments. It isn’t actually a new discovery: it was named Namibiavis senutae by Cecile Mourer-Chauviré in 2003.

As you might already be guessing from its name, the big deal is that it’s not South American – it’s from Namibia! Mourer-Chauviré (2003) originally identified Namibiavis as a member of the seriema-like Idiornithidae but reanalysis shows that it definitely possesses distinctive hoatzin features (Mayr et al. 2011). The original idiornithid identification was certainly understandable since idiornithids (and seriemas) are hoatzin-like in several respects. Indeed, several authors have previously suggested that all of these birds might be close allies (e.g., Mourer-Chauviré 1983, Olson 1985, Mayr and Clarke 2003). Anyway, what little we know of Namibiavis suggests that it was, again, generally similar to Opisthocomus and presumably similar to it in flight abilities and perhaps in ecology and appearance.

Global range of the extant hoatzin, from wikipedia.

So, it’s official – hoatzins are no longer exclusively South American; they're also known from south-western Africa. This is an exciting discovery given that hoatzins have always been regarded as quintessentially South American. What might this discovery mean for hoatzin history and evolution? And how on earth could ancient hoatzins be present in both Africa and South America?

“No” to vicariance

For starters, you can forget the idea that this distribution is best explained by vicariance: that is, by the idea that both South American hoatzins and Namibiavis descend from a population that was once continuous across both landmasses, but then evolved into two lineages once the Atlantic formed.

You can forget this because Africa and South America rifted apart in the Cretaceous, and were substantially separated by a young Atlantic by the Late Cretaceous. There are no indications that hoatzins are anywhere near old enough to have pre-dated the opening of the Atlantic. Indeed, the opening of the Atlantic even pre-dates the appearance of Neoaves (the major clade that includes all crown-birds except palaeognaths, waterfowl and gamebirds) and probably Neognathae (waterfowl, gamebirds and neoavians) and Neornithes (palaeognaths and neognaths).

“No” to northern or southern over-land routes

Could the earliest hoatzins have dispersed northwards (using North America, Greenland and Europe as stepping stones between Africa and South America), or southwards (using Antarctica)? Neither proposal is parsimonious, for three reasons. Firstly, there aren’t any fossils that might support either model of dispersal (unless Foro really is a stem-hoatzin). Secondly, living and fossil hoatzins are all tropical birds, likely constrained to waterside habitats and a consistent supply of a certain kind of edible foliage. It would be a real stretch to imagine extinct hoatzins surviving in, and travelling through, the cool temperate and sub-Arctic regions of the extreme north and south.

Thirdly, stretches of water separated South America from North America, South America from Antarctica, Africa from Antarctica and Africa from Eurasia during parts of the Eocene, Oligocene and Miocene. Indeed, both South America and Africa were island continents during much of the Oligocene and Miocene. In short, positing over-land dispersals of this kind would potentially involve several over-water dispersal events, and would also require that early hoatzins travel into cool parts of the world that they almost certainly weren’t suited for. Incidentally, these objections also apply to other tetrapod groups (like, apparently, phorusrhacids) found in Africa and South America but not elsewhere.

Palaeomap of the Early Oligocene, from Mayr et al. (2011), showing distribution of the four fossil and modern hoatzins. Image (c) Ron Blakey, Colorado Plateau Geosystems, Inc.

If both vicariance and over-land dispersal are out the window, how else might we explain hoatzin distribution? The only reasonable answer is trans-Atlantic dispersal. Hoatzins must have crossed the Atlantic Ocean some time during the Cenozoic. In view of their limited flight abilities, it’s unlikely that they flew, even if we do allow for the presence of a narrower Atlantic and various island archipelagos. Rafting is the only alternative.

Because we regard hoatzins as South American birds, it might seem natural to assume that the group is ancestrally South American and that its presence in Africa represents a west (South America) to east (Africa) dispersal. The fact that the oldest hoatzin – Hoazinavis from the Late Oligocene or Early Miocene – is South American could be interpreted as support for South American ancestry.

However, while there are a few groups of animals that have moved across the Atlantic from west to east - like manatees (Domning 2005) and thrushes (Voelker et al. 2009) - animals that have rafted across the ocean have otherwise all moved from east to west. Caviomorph rodents, platyrrhine primates, various amphisbaenian and gekkotan lizards, and others, are thought to have rafted from Africa to South America. Remember that these ‘rafts’ shouldn’t be imagined as small, simple structures, but as sometimes enormous, complex messes of live vegetation, tree trunks and sediment. They more resemble floating islands than ‘rafts’, and can be tens of metres long. The only half-decent image I can find of such a structure is this painting of an Amazonian raft [it comes from an encyclopedia series called The Joy of Knowledge, published by Mitchell Beazley in 1987, but I can’t find an artist to credit]. It does a nice job of showing the ‘raft’ to be complicated and with structure and areas of refuge, but ideally I want a picture showing a much larger raft.

There’s some suggestion that, prior to the formation of the Central American landbridge in the Pliocene, winds and currents made it easier for volant animals to move from South America towards Africa (Voelker et al. 2009). However, most data indicates that moving westwards across the Atlantic was easier in the past, as it is now. For these reasons, Mayr et al. (2011) favour the idea of an east to west dispersal and thus support the idea that hoatzins began their history in Africa. As Mayr et al. (2011) note, because hoatzins typically live in or on the vegetation that flanks very large, powerful rivers they might be at special risk of being swept out to sea during times of excessive flooding. Furthermore, their folivorous habits might have proved advantageous as goes transportation on a massive pile of floating vegetation. This is the first proposed instance of trans-oceanic rafting by a group of birds.

The only photo I have of a hoatzin skeleton. Yeah, it's terrible.

As noted earlier in this article, uncertainties over the affinities of hoatzins make it difficult to work out where hoatzin ancestors and close relatives ‘should’ be distributed. Hoatzins have at times been considered close to gamebirds, cuckoos, or pigeons (Sibley & Ahlquist 1973, Hedges et al. 1995, Hughes & Baker 1999, Sorenson et al. 2003), all of which are widely distributed and not clearly associated with any specific ‘ancestral’ area. Hoatzins have also been considered close to the exclusively African turacos (supposed fossil turacos from Europe are likely misidentified: Mayr 2009). They’ve also been suggested to be close to the South American seriemas (e.g., Mourer-Chauviré 1983, Olson 1985, Mayr and Clarke 2003). However, because close fossil relatives of seriemas were present across Europe and North America during the early Cenozoic, it isn’t necessarily clear that a possible link between hoatzins and seriemas would constrain the hoatzin ‘centre of origin’ to South America.

An approximate tree for neognaths (though with some groups in controversial positions). Where do hoatzins go? Take your pick.

And of course I could say a lot more about the possible affinities of hoatzins, since it’s one of the most contentious and debated issues in neornithine systematics (see also Fain & Houde 2004, Ericson et al. 2006, Hackett et al. 2008). Mayr et al. (2011) don’t come down in favour of any one particular hypothesis on hoatzin affinities, but they do note that a close relationship with turacos is most consistent with an African origin.

Given both the enigma and controversy that’s surrounded hoatzin affinities and the various morphological and physiological peculiarities evolved within the hoatzin lineage, any news on the group’s history and evolution is of great interest. I’m very excited to know that hoatzins once inhabited Africa, and the fact that these birds seemingly once rafted across an ocean makes them all the more interesting. As usual, we must hope that – in time – additional discoveries will tell us more about the history of this most remarkable group of birds.

For previous Tet Zoo articles that mention or discuss hoatzins, see...

Refs - -

Cracraft, J. 1971. A new family of hoatzin-like birds (Order Opisthocomiformes) from the Eocene of South America. Ibis 113, 229-233.

Domning, D. P. 2005. Fossil Sirenia of the West Atlantic and Caribbean region. VII. Pleistocene Trichechus manatus Linnaeus, 1758. Journal of Vertebrate Paleontology 25, 685-701.

Fain, M., & Houde, P. (2004). PARALLEL RADIATIONS IN THE PRIMARY CLADES OF BIRDS Evolution, 58 (11), 2558-2573 DOI: 10.1111/j.0014-3820.2004.tb00884.x

Grajal, A., Strahl, S. D., Parra, R., Dominguez, M. G. & Neher, A. 1989. Foregut fermentation in the hoatzin, a Neotropical leaf-eating bird. Science 245, 1236-1238.

Hackett, S. J., Kimball, R. T., Reddy, S., Bowie, R. C. K., Braun, E. L., Braun, M. J., Cjojnowski, J. L., Cox, W. A., Han, K.-L., Harshman, J., Huddleston, C. J., Marks, B., Miglia, K. J., Moore, W. S., Sheldon, F. H., Steadman, D. W., Witt, C. C. & Yuri, T. 2008. A phylogenomic study of birds reveals their evolutionary history. Science 320, 1763-1768.

Hedges, S. B., Simmons, M. D., van Dijk, M. A. M., Caspers, G.-J., de Jong, W. W. & Sibley, C. G. 1995. Phylogenetic relationships of the hoatzin, an enigmatic South American bird. Proceedings of the National Academy of Sciences 92, 11662-11665.

Hughes, J. M. & Baker, A. J. 1999. Phylogenetic relationships of the enigmatic hoatzin (Opisthocomus hoazin) resolved using mitochondrial and nuclear gene sequences. Molecules and Biological Evolution 16, 1300-1307.

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Sorenson, M. D., Oneal, E., García-Moreno, J. & Mindell, D. P. 2003. More taxa, more characters: the hoatzin problem is still unresolved. Molecular Biology and Evolution 20, 1484-1499.

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The views expressed are those of the author and are not necessarily those of Scientific American.

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