Recent comments in /f/askscience

atomfullerene t1_jdwspcc wrote

Reply to Why are nonhuman erect bipedal animals so rare? by violetmammal4694

It's less about adaptive value or not and more about body plan.

Your basic tetrapod body plan is pretty lizard-like (in turn, it's very basically a fish with four legs stuck on). The key thing about this body plan is that the main axis is horizontal. Makes sense, after all the ground is horizontal and that's what you are moving across.

Most tetrapods, heck, most vertebrates keep this basic orientation (for exceptions, see seahorses). Swimming, flying, walking, the body tends to be held horizontally. And this goes for bipedal movement too, which is actually pretty common. If the hind legs are longer than the front legs, and the animals has a big tail (like most tetrapods), it's pretty easy to get up on two legs. Tends to happen in lizards when they run, they are essentially just popping wheelies. Loads of dinosaurs also went around on two legs, with the body horizontal and the tail out behind for balance.

But this doesn't really work with most mammals, which tend to have piddly little tails and long front limbs. Which is why you rarely see any sort of bipedal mammal.

Now like I said, most vertebrates go around horizontally. What's an exception? Stuff in trees. Climbing, hanging from branches, that often puts the body vertical. And requires good balance too. Various primates will go vertical and even bipedal in trees, running along branches and doing that sort of thing, or swinging and leaping around upright.

When we get to apes, you have no tail at all, so there's no hope of going around horizontal-bipedal like a normal tetrapod. But Gibbons like to spring around in trees, run along branches, and walk upright quite a bit when forced onto the ground. Apes that spend less time in trees need to get around more efficiently. Chimps and gorillas seem to have adopted knuckle walking, hominids seem to have improved the original upright stance for efficient use on the ground.

But it's only the odd confluence of lack of a significant tail and preexisting history of upright orientation that makes upright movement plausible in hominids.

As for penguins, you'll note they also lack significant tails and (unlike most birds) have the feet at the very rear of the body to improve swimming efficiency. Which also leaves them stuck with no other options.

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banestyrelsen t1_jdwph4s wrote

Reply to Why are nonhuman erect bipedal animals so rare? by violetmammal4694

Bipedalism is actually not that rare among animals like us, ie primates. Lemurs and gibbons walk on two legs on the ground (though lemurs tend to skip more than walk). 20 species of gibbons and they all do this. For some tree climbers it just seems to be the most comfortable way to move around on the ground.

It seems to have been the same with our ancestors because full bipedalism was already present right at the start of human evolution with Australopithecus, which still had a brain not much larger than a chimp. So maybe bipedalism is not something we evolved for any particular reason, maybe it was just how we started out as a byproduct to how we moved in the trees, and when we started living on the ground more we had to work with it.

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OneShotHelpful t1_jdwo8iz wrote

Reply to comment by ebinWaitee in Why are nonhuman erect bipedal animals so rare? by violetmammal4694

Archosaurs are a common ancestor of everything we consider a dinosaur and a lot of things we don't, like crocodiles. If you only want to go back as far as the ancestor of all dinosaurs, then Ornithodera is what you're looking for and it's also a biped.

Birds are theropods, which are one of the three big groups of dinosaurs. They are the ones that survived to today and kept the bipedal bodyplan all the way through.

Ornithicians are things like triceratops. Many became quadripetal, but things like the duck billed dinosaur didn't.

Sauropods are things like brontosaurus. They probably became quadripetal early after the split because I don't know of any bipedal examples.

If you look at quadripetal dinosaurs you might notice a tendency for the front limbs to be short and underdeveloped or have odd rounded shoulders. That could be called a remnant of the bipedal ancestry.

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