The Hoatzin: Misfit, Belcher, Genetic Mystery

Deep in the wilderness lurks a fat, foul beast that stinks of manure, barks and caws in gutteral tones, and produces offspring with sharp claws in unusual places. Nobody quite knows where it came from, but we do know where to find it: in the trees of the Amazon jungle, crouched on low branches that hang over rivers. Sound sinister? It’s called a Hoatzin—and once you get to know it, it’s about as goofy and awkward a bird as you can find.

The Hoatzin never quite got hang of the whole “being a bird” thing. Not that it doesn’t try to fit in—on the outside, Opisthocomus hoazin looks like a mish-mash of half a dozen other birds, with the scruffy crest of a Guira Cuckoo, a Cassowary’s bright-blue face, the body of a chicken, and a long, stiff hawk’s tail. And the Hoatzin can fly, though it's a clumsy, reluctant flier at best. (If you have a hard time imagining how the word “lumbering” could be applied to a bird in flight, just take a look at the Hoatzin trying to get from one tree branch to another.)

But then there are the little idiosyncrasies that give the species away as something of an oddball in the avian world. Case in point: Hoatzins apparently never got the genetic message that they aren't dinosaurs anymore, and shouldn't be growing claws on their forelimbs. To be fair, Hoatzins do have claws in the normal places—but baby Hoatzins also have claws sprouting from their stumpy little wings. There is, of course, an evolutionary explanation for this odd appendage: Hoatzins build their nests on tree branches that extend out over water, which doesn't leave their chicks many exit strategies for when a hungry snake or monkey is heading their way. So when the featherless chicks feel threatened, they hop out of the nest and bellyflop into the water below. Once the danger has passed, they paddle to shore, and use their wing claws to clamber up the tree and back into the nest. 

The wing claws disappear by the time the Hoatzin matures, at which point the birds can pretty much pass as normal, at least from the outside. From the inside it’s another story, because the Hoatzin’s other decidedly non-avian feature is its stomach, which can best be described as bovine. Hoatzins are the only birds in the world that eat nothing but leaves, which, compared to seeds and fruit, aren’t very nutritious, and are hard to digest. So to accomodate this diet, the Hoatzin has evolved a multi-chambered digestive tract with lots of little “stomachs,” where the leaves can sit for a while and be digested by friendly bacteria. During the digestion process, the bacteria release methane that the bird then belches out, producing an olfactory aura that's landed the Hoatzin a less-than-flattering nicknamed: the stinkbird. So much for fitting in.

The Hoatzin’s many anatomical anomalies have long made it a seductive research subject for scientists, who've been intent on solving one big mystery: Where the hell did this thing come from?  For decades, anatomists were baffled. No other birds in South America have claws like the Hoatzin, or eat leaves like the Hoatzin, or even look like the Hoatzin, so tracing its lineage seemed like a dead end. Then, thanks to the discovery of a little thing called DNA and the advent of genetic mapping, modern scientists began to pick apart the Hoatzin's genes to figure out what they’re most closely related to. But for a while, even that yielded no definitive answer. One group of researchers claimed, based on tiny snippets of DNA, that the Hoatzins were related to a group of African birds called turacos. This seemed to make a lot of sense: Some turaco chicks have claws on their wings, and some turacos eat leaves to supplement their diets. And then there's the fact that fossils suggest that Hoatzins actually evolved in Africa or Europe, like the turacos, and then drifted over to South America on rafts of vegetation. But other scientists doubted the turaco connection, using longer sequences of DNA to posit that Hoatzins were most closely related to doves. Finally, in 2014, scientists started sequencing the Hoatzin’s genome as part of a massive project to map out bird evolution. This study found Hoatzins were most closely related to, of all things, cranes and plovers.

The latest twist of events, though, explains why scientists had such trouble: The Hoatzin is truly in a class (or, to be specific, a "sister group") of its own. Research published just this week estimates that the species branched off from the rest of the avian tree about 65 million years ago, and is the only species in its group today.  Perhaps this shouldn't come as too much of a surprise—the Hoatzin always was the black sheep among birds. Every family has one; they're the ones who keep things interesting.

Correction: Due to an editing error, the original version of this story stated that the Hoatzin branched off from the avian tree about 65 years ago. That would have been neat, but it was, of course, 65 million years ago.