Biomedical Ephemera, or: A Frog for Your Boils

The Juvenile Hoatzin (Opisthocomus hoazin)

It should first be noted that all birds are dinosaurs (order Saurischia, clade Theropoda), not just descendents of dinosaurs - modern genetic analysis strongly supports this cladistic organization. But given what we're too often taught in schools, birds and dinosaurs are hard to reconcile in many peoples' minds.

The juvenile hoatzin, however, makes it easy to see the reptilian traits that once dominated the early birds, and displays the unused genetic codes that lurk in the genome of modern avians. When they hatch, they're equipped with lizard-like claws in front of their wings. Their use is described here, but in short, they use them to return to their nest and avoid predators. Their claws disappear by the time they leave the nest, having grown together into the metacarpals that support the wing structure.

Another fascinating trait of the hoatzins is their vegetarianism and their digestive tract. They have gut flora and fermentation similar to ruminants, which no other bird has. This is actually what leads to their being called "stink birds" - they exude a lot of stench with the fermentation process. The gut fermentation is so important to the hoatzin that the flight muscles attached to their keel are significantly reduced, to allow for more space for the stomach. They are weak flyers because of this. After a large meal, an adult hoatzin can spend up to two days doing almost nothing, allowing the leaves and greenery to have their nutrients released by their symbiotic gut flora.

Images: Top: Attitudes of the juvenile hoatzin while climbing Second row, left: Hoatzin nest with two eggs - Note proximity to water Second row, right: Two hoatzin chicks preparing to dive, after appearance of threat from above Third row, left: Hoatzin chick demonstrating strong swimming abilities Third row, right: Hoatzin chick demonstrating poor locomotion on land Bottom: Detail of hoatzin chick climbing, using neck, feet, and claws.

Tropical Wild Life in British Guinea, Vol 1. Curated by William Beebe, 1898.

Opisthocomus hoazin - The Hoatzin

This newly hatched hoatzin chick has something that other birds don't - clawed forefingers, giving it the ability to ascend trees with surprising dexterity, long before it's able to fly.

During breeding season, hoatzin adults are somewhat gregarious, and their nesting areas are prime targets for predators. As the adults don't have any significant defenses other than distraction tactics, when a nest site is attacked, they fly around noisily and try to draw the attention of the predators. Being almost the size of pheasants, they generally aren't attractive targets for eating, but the chicks are.

As the adults distract the predators (or attempt to), the hatchlings dive into the waters of the seasonally-flooded forests below, and scoot beneath the surface to hide. This is when those over-sized feet and clawed forefingers come in handy: after scooting away, when the danger passes, the chicks can climb back up the trees to their nest! The somewhat clumsy flight of the adults is extremely non-conducive to flying down to water-level to rescue offspring, so this trait of the chicks has helped keep the species alive, despite "panic" situations being not uncommon.

It's worth noting that while the bird bears a striking resemblance to Archaeopteryx in its skeletal form, the claws are not a retained trait from its ancestors. In fact, the direct ancestors of all gamebirds had relatively normal wings, and this is a "new" adaptation, probably an atavism from the ancient lizard-like genes still present in its DNA.

[Photograph by J. Arthur Thompson, British Guiana, 1922]