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The Hoatzin is a jungle bird whose only remarkable trait, other than looking fabulous, is that their wings have fingers.

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Is it possible that these wing fingers could evolve into arms? What about legs? What pressures would lead to such an evolution?

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  • $\begingroup$ Yes, the pressure of needing arms.. $\endgroup$ – dot_Sp0T Nov 4 '16 at 16:56
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    $\begingroup$ Since wings evolved from arms, there is no reason they could not evolve back.... $\endgroup$ – Thucydides Nov 4 '16 at 21:38
  • $\begingroup$ I'm thinking maybe due to irreversible climate changes all the tropical fruits and nuts developed hard shell to retain more water etc, these Hoatzin may grab any sharp objects to pry open the thick shell similar to monkey. They can also play barrel of Hoatzins with their spouse and chicks during typhoon season... $\endgroup$ – user6760 Nov 5 '16 at 6:13
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I don't know exactly why you want the hoatzin to evolve hands, and the birds already have legs, but the best answer is to make the birds need to climb. At present, only the chicks need to climb, so only the chicks have claws, but the adults are pretty bad fliers, so it wouldn't take too much to push them to stop flying all together. This simplest way for this to happen would be for the climate to change a little.

Let's say we have a rain-forest just perfect for the modern hoatzin. It's got lots of trees with lots of branches perfect for hoatzin nests, and lots of leaves for the birds to eat. Now, hoatzins really only glide from branch to branch, they don't really flap much because their muscular structure really isn't set up for flapping. If the hoatzin didn't have a lot of branches fairly close together, it would have a right difficult time getting about. Of course, in a rain-forest there are lots of trees with lots of branches all very close together because there is plenty of sunlight and water for all of them. What would change this could be a climactic shift, I would propose to a wetter but cooler climate.

All this would take is some cloud cover.

With less sunlight, the trees devote all their energies to being taller and they put all their branches up top, where they can gather sun. The lower levels of the jungle are plunged into almost total darkness, very bad for plants, so the trouble of branches all over the place is cleared up. When the birds fall out of the nests, or out of the trees, or end up in the water for whatever reason, they have to climb to get back up into the trees. There is no longer the option to fly up, especially not out of the water.

Then, with clouds comes rain. Lots of rain. The hoatzin already lives where there is a lot of standing water, so it wouldn't take too much to flood the rain-forest floor. This would take care of all that pesky underbrush that the birds might use to hop-fly back up into the trees. Think swamp: tall trees sitting in water, not too much cluttering up the way-up from the ground-level.

The hoatzin sits on its eggs more to keep them cool than to keep them warm, so the birds can't be away from the nest for too long. Climbing up and down the trees isn't the fastest thing to do, so a cooler climate would help select for climbers, or at least, not select against climbers.

The original needs for climbing (get back to the nest, climb out of the water) are still present and are exaggerated. What's more, this shift in climate can occur with some rapidity. It wouldn't take eons and eons for a jungle to turn into something of a swamp, which means that the birds will be forced to select for a right-now trait (like claws that remain into adult-hood) instead of a slower trait (like a progressively better flight muscular structure), but it will take long enough that the birds would have time to evolve, not just be wiped out by the sudden change in conditions.

As the rain-forest becomes more and more swamp-like and the foliage changes, the hoatzin will have even less need or opportunity to fly. In the beginning, the birds will still fly between the tops of the trees, but with the expanding breadth of the tree-tops, they won't have to. As the trees strive to collect as much light as possible, their canopies will become less vertical and more horizontal, so the hoatzin will be able to hop more than fly, and its progressively longer-lasting claws will help it to do so. When these original trees die, the dark ground level will not permit as many saplings to take hold, so in perhaps a hundred years or more, when the first trees have died and the second generation has begun to rise, the forest will be less dense, meaning the trees don't have to compete as much for the light and their canopies will thin out again. At this point, the hoatzin has experienced some 30-40 generations since the environment began to favor adult claws. That's not a lot, but it's enough to give the hoatzin 2.0 a good basis of flightlessness because along side the selection for claws, the hoatzin is experiencing a lack of selection for flight structures. The reason the modern hoatzin does not fly well is because the structures needed for it to eat leaves conflict with the structures it would need to fly, so when the balance is tipped, relieving the need for flight while holding the need for digestion steady, we will see the worsening of flight structures. With the trees becoming farther apart, the hoatzin will have to get from one to the other, but it's flight ability has been reduced to clumsy gliding at best. The birds will have to climb back into the trees when they attempt to glide and miss, but they will also have to start developing better ways to get about in the trees. There will be a die-off as the second generation of trees replaces the first generation, and birds with longer wings will be selected for because they can reach farther to get from branch to branch. The half of the wing that follows the claws will get shorter as the part that precedes the claws gets longer, bringing the wing into a more arm-like appendage. The same pressures will select for longer legs as well, leading to a long-armed, long-legged, clawed bird that spends almost all of its time in the tree-tops. Eventually, it might be necessary for the swamp to begin to return to the rain-forest conditions (as we know, monkeys do not live in swamps {except the Allen swamp monkey, but it dives, so I'm not counting it} for the same reason: nothing to swing from/to) which will provide the hoatzin 2.5 the structures needed for flightless, grabby locomotion. The end result will be the hoatzin 3.0, a feathery, egg-laying creature with long scaly legs,and long arms with claws at the ends that swings through the jungle almost like an ape. If you then wanted the hoatzin to evolve into a human-oid, you need simply follow the ape-to-human evolutionary track.

Granted, this all involves a lot of time, a lot of things happening just right, and a lot of assumptions, like that the trees the hoatzin eats would survive this kind of change long enough for the hoatzin to switch foods, but it is, in theory, given ideal circumstances, possible.

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Quick note: Only hoatzin chicks have claws on their wings; these claws are lost after maturity.

What pressure could lead to further development of claws? Well, it appears that the claws have emerged because of the pressure of local predators. Hoatzin chicks will run away from the nest when the alarm call is given, using their claws to climb more effectively. They'll also drop into the water, then use their claws to climb back up to the nest. Thus once the chicks are able to fly, they no longer exhibit the behavior that encouraged them to emerge in the first place. You'd need a use for those claws in adults to encourage them to be retained and developed further.

Perhaps a flightless species of hoatzin might develop a need for hands for the same reason primates have them - climbing and gathering food. Finding a food source that benefits from more dexterous tools than just a beak might be a good stimulant for such a change as well.

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As pointed out the real question isn't how to further develop the claws, there is plenty of pressure for that already, climbing helps them in many ways. What you need to do is remove the pressure to keep flight capable wings. to really push it you could have them increases in size, the bigger they get the less flight is going to be achievable. Lack of predators could easily do both, a mass extinction freeing up lots of niches and killing off all the large predators would be enough.

One interesting feature, the creature will have 3 fingers, birds lost the 4th and 5th finger long before they were birds.

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"Whose only remarkable trait, other than looking fabulous, is that their wings have fingers"? I'd have to heavily disagree with you there- Hoatzins have a couple of other remarkable traits as well. The Haotzin is the last surviving member of an order of birds which recent genetic studies have now shown branched off in its own direction more than 64 million years ago, literally as the ashes were settling shortly after the extinction event which wiped out the non-avian dinosaurs. And their most remarkable feature, IMHO, isn't the fact that juvenile Hoatzins possess finger claws.

Even more notably, the Hoatzins are the only birds to have ever evolved a digestive system which utilizes bacterial fermentation to break down the vegetable material they consume, in the same manner as cattle and other mammalian ruminants do. In the place of the mammalian ruminants' rumen (a specialized stomach for bacterial fermentation), in a case of parallel evolution, the hoatzin has evolved an unusually large crop instead, folded in two chambers, along with a large, multi-chambered lower esophagus. Its stomach chamber and gizzard are much smaller than in other birds. Their crop is so large as to displace the flight muscles and keel of the sternum, much to the detriment of their flight capacity.

And you might also be interested to know that the fossilized remains of an extinct species of Hoatzin has been found in Namibia, some 18 million years ago, proving that indicating that, in the African-South American interchange which faciliated the colonization of South America by new world monkeys, rodents, bats and other species, at least a few Hoatzins went the other way and successfully colonized Africa too (along with at least one species of terror bird). As such, full flightlessness seems like an extremely likely evolutionary path for the Hoatzins to take.

So, how's about this for a scenario; over in Africa, as the climate gets increasingly arid, and the mangroves and rainforests begin to be replaced by savannah, instead of simply dying out as they did, some of these African Namibiavis Hoatzins manage to cling on by abandoning their arboreal lifestyle and adopting a more terrestrial lifestyle instead; rapidly losing their flight ability in the process, and becoming fully terrestrial. Their unique pseudo-ruminant digestive system bestows them a tremendous evolutionary advantage as avian browsers and grazers, potentially enabling them to outcompete contemporary ostriches.

And without the limitation to their maximum size imposed upon them by retaining their vestigial flight ability, with their far more efficient digestive systems granting them access to more nutrition, and with the considerable evolutionary pressures placed upon them by African predators, these African hoatzins rapidly grow to become far larger than even ostriches; large enough to rival the mass and height of the largest ruminants, the giraffes (which they could also conceivably compete on a level footing with), and feasibly, even some of the dinosaurs of old (with mummified hadrosaur remains indicating that they also possessed enlarged crops most closely resembling those of Hoatzins). As a result, these end up looking something like this;

Megahoatzin?

The original purpose of these Hoatzins' finger claws, for climbing, has long been rendered redundant by this stage. But these African Megahoatzins still re-evolve arms, and refine their finger claws, in an evolutionary development paralleling those of the gigantoraptors, ground sloths and calicotheres when they all took the same evolutionary path. Equipped with increasingly large and developed claws, they use them to strip leaves, tear off branches, and as potent and formidable defensive weapons against any predators who attempt to attack them or their nesting sites.

Perhaps even also as legs, with a sub-order evolving to become semi-quadrupedal as an adaptation to ground-feeding grazing, just like hadrosaurs did; with this semi-quadrupedal stance also enabling these Megahoatzins to become larger and heavier still, conceivably rivaling the size of the largest ornithopods. What do you reckon; does it sound interesting?

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