Biomedical Ephemera, or: A Frog for Your Boils

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]