The Flying Cow

Hoatzin adult and chick. Notice the claws on
the wings of the chick. Source.

by Elio

Here’s a bird that thinks it’s a ruminant. Or, speaking science, ruminants may have coevolved their singular way to digest plant material with a bird. I am talking about the hoatzin, a tropical pheasant-sized or, in honor of the day, turkey-sized bird of Central and South America that ferments in its crop the leaves it eats. Opisthocomus hoazin, to give his full name, is unique. No other bird—and there are some 9000 species—is known to carry out a pre-gastric (ruminal) fermentation (although a few do something similar in the cecum). Hoatzins are almost exclusively leaf-eaters, so they benefit greatly from having a microbiome that can handle such food, just as cows do. To accommodate this activity, they have an unusually distended crop and a large esophagus.

The hoatzin digestive tract. (A) Location of the crop and expanded esophagus in the hoatzin body. The anterior sternum is much reduced to make room for the large crop. (B) Extended complete digestive tract of the hoatzin. Source.

Baby hoatzins, like all vertebrates, develop into adulthood in a gradual manner. Over time, the microbiome of their crop undergoes changes. Here is what the authors of a study led by Maria Dominguez-Bello of the University of Puerto Rico have to say: In comparison with the adult, the hoatzin chick crop had a greater abundance of Flavobacteriaceae, Clostridiaceae and Lachnospiraceae but lacked phyla DSS1, Deferribacteres and Termite group 1, which were mostly present in adults. Archaea, including methanogens, are also present in abundance in the crop, and so are protozoa. It is all quite reminiscent of the rumen microbiome of ruminants.

Want to know more about the hoatzin? Here is a sample: It has an unfeathered blue face with maroon eyes and a spiky crest on its head, and the sole extant species within the family Opisthocomidae. It is often considered one of the most primitive of birds as the hoatzin chick has two claws on the first and second digits of their wings to help it grip branches and clamber about awkwardly. Actually, the claws on the wings are lost with maturity, but for a while, the bird looks like the extinct Archaeopteryx, the late Jurassic  candidate for a transitional form between dinosaurs and birds.

Not everything about the hoatzin is attractive to humans. It emits a distinctive manure-like smell, which explains its nickname: stink bird. But its attraction to microbiologists is evident. This species has gone its own way, at least among the birds, to accommodate a richly productive and diverse microbial community.

Comments

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definetely i'm going tocheck your other posts. thank you.

[Geraldine Kaye]

I recently saw something [on the Web, of course] about a current theory that both crocodiles/alligators and birds are descended in a branch parallel to the dinosaurs, rather than through them. It mentioned Archaeopteryx as having feathers and possibly warm blood. Ain't science wonderful--at least the theories proliferate!

[Mark O. Martin]

A lovely post, Elio, as usual.

I read the above, and I thought of this:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2664199/

In the Jeff Gordon paper, there is quite a bit about hindgut and foregut fermentation in mammals and the microbiota associated with each.

From the paper: "...We propose a third feature that may be important in driving the community differences between in gut and non-gut environments: the vertebrate gut may be highly unusual as a microbial habitat by combining high abundance, diversity and flux of polysaccharides in an anoxic environment with a constant controlled temperature..."

So the hoatzin microbiota might be quite relevant to this developing paradigm!

[Abe Eisenstark]

Elio replies:
There is a considerable literature about rumen protists. A lab in Poland of T. Michalowski and colleagues seems to be particularly active in this area, but there are others as well. See PubMed.

But I agree with you that microbiologists tend to neglect this exceedingly important and most exciting aspect of the Microbial World. Their loss!

[Psi Wavefunction]

Any info on what kinds of protists? The papers seem to just lump them all as "protozoa", typical of bacterial env sequencing ppl. Seems like it's often assumed that it's always the bacteria that carry out the bulk of cellulose metabolism, while that's not true for some systems, such as termites, where the parabasalids (and oxymonads?) do all the work. Just found a 1993 paper mentioning rumen ciliates (http://www.jstor.org/stable/30163698?seq=9), but that's all I could get. Would be fun to harvest some 18S from that stuff. Really cool stuff though!