Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Luckily, I don’t look as bad as I smell (Image: David Tipling/Getty)

Species: Opisthocomus hoazin

Habitat: in trees along rivers and swamps in the Amazon and Orinoco basins of South America

Loud and colourful, it is about the size of a pheasant and sports wiry head feathers that resemble a punk’s hair spikes. With oddly primitive-looking features and no close relatives, the hoatzin is an evolutionary enigma.

Everything about it is weird. Unlike the majority of birds, it gets most of its food from leaves, which it ferments in its foregut, much like a cow. Also unlike other modern birds, young hoatzins have claws on their wings that they use to climb trees – relics of their dinosaur ancestry.


Now fossil evidence suggests they did not evolve in South America as once thought, or even in its former neighbour Africa, but in western Europe. These punk pheasants, it seems, are French.

Feathered cows in trees

The hoatzin – pronounced “wot-seen” – lives in trees, where leaves account for more than 80 per cent of its diet, supplemented by flowers and fruit.

The bird ferments this mixture in its foregut, which is adapted from the common avian crop and oesophagus. There, bacteria break down the cellulose and toxins that deter other animals from eating the leaves, making a rich mix absorbed in the bird’s intestine. The process is similar to digestion in cows, and hoatzins have a distinctive cow-like stink. The few other birds that eat foliage digest it differently.

The foregut helps hoatzins thrive in their environment, but it comes at a cost. “It takes such a large volume that their breast muscles are reduced and shifted backwards,” says Gerald Mayr of the Senckenberg Research Institute in Frankfurt, Germany. This limits the birds to flying only a few hundred metres: just enough to reach nearby trees.

That’s not the only oddity: baby hoatzins have a pair of claws on each wing. The birds nest during the rainy season, and chicks jump out of their nest into the water beneath to escape predators, then use their claws to climb back up the tree. Other bird species have tiny residual spurs, but those of young hoatzins are quite well developed.

The combination of wing claws and poor flight made hoatzins look primitive and like extinct bird-relative Archaeopteryx, which lived 150 million years ago, but Mayr says that is misleading. “Many Archaeopteryx reconstructions were inspired by hoatzins,” he says. Actually, hoatzins belong to a group containing almost all modern birds called “neoaves“.

Quest for relatives

The relationships of hoatzins to other species are “the most enigmatic among birds”, says Marcel van Tuinen of the University of North Carolina at Wilmington. They don’t seem to fit anywhere.

In the early 1900s, ornithologists grouped hoatzins with the similar-looking galliformes: heavy ground birds like chickens and pheasants – virtually the only birds which aren’t neoaves. Early genetic studies in the 1980s and 1990s suggested cuckoos were their closest relatives, but later studies failed to confirm this.

Whole genome studies have also failed to find a match. “There is nothing else like it,” says van Tuinen. The most recent genomic analysis says the birds are part of an ancient lineage with no close relatives (PLoS One, doi.org/mq2). Hoatzins appear to have split from other birds during the rapid diversification after the mass extinction that wiped out the dinosaurs.

Native American?

For many years palaeontologists thought that hoatzins evolved in South America. Many odd South American animals evolved there because it was isolated for tens of millions of years, until the Panama land bridge formed, connecting it to North America.

However, Mayr has now found hoatzins from the Old World. In 2011, he studied fossils of the 17-million-year-old Namibian bird Namibiavis, and realised it was a hoatzin (Naturwissenschaften, doi.org/fnjzkc). It was the first example from outside South America.

Mayr also identified the oldest unquestioned hoatzin on record. Hoazinavis lacustris lived in Brazil 22 to 24 million years ago.

Globe-trotters

Now Mayr has found an even older example, which lived 34 million years ago in what is now France. Protoazin parisiensis is more closely related to the modern hoatzin than the ancient Namibiavis.

Together with another African fossil discovered in 2012, the find suggests hoatzins evolved in the Old World. Later, one line spread to America and the rest became extinct in Europe and Africa.

That means South American hoatzins are a relict of a widespread group, not a native group that evolved locally. Mayr says there are other birds that now live only in tropical America, but which once also lived in the Old World, including hummingbirds, seriemas and the frankly ridiculous potoos – which one bird guide described as “little more than a flying mouth and eyes”. Old World hoatzins may have been wiped out when tree-climbing mammalian predators spread to Africa and Europe.

How did these birds cross the Atlantic when they are such poor flyers? The similarity between the European hoatzin and the modern South American form suggests a northern route. But, so far, no hoatzin fossils have been found in North America. A 52-million-year-old bird called Foro panarium from Wyoming has been called a hoatzin, but Mayr is not convinced because it lacks many of their characteristic features.

Alternatively, Mayr says the birds may have hitched a ride from Africa to South America on a floating raft of vegetation tens of metres across. Primates and rodents are thought to have followed the same route.

Journal reference: Naturwissenschaften, DOI: 10.1007/s00114-014-1144-8