I always worry when that happens that its going to be permanent, or damage the nerves...

How important is it not to sleep on your arm like that? Like is it a minor annoyance or major health hazard?

Any tips for how to make your arm not fall asleep when sleeping on your side? Like a certain shoulder position or place to avoid pressure? Pillow position?

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All the bones in a bats wing and in a hand but you don't say a bat flies with its hands

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That's a great start. Thanks! Looks like we're still early on in our understanding of this.

The great apes seem to have a pretty dominating spot too. Not sure if they count as a proper biped though since they will often drop to all fours, for example when they’re charging at a person with death in their eyes and let out a furious roar that would soil the pants of even the bravest of men.

Bipedal animals that primarily move with their legs are efficient long-distance runners but less nimble than comparable four-legged animals. They do well on open plains, typically. Humans, ostriches, and kangaroos all fall into this category.

These types of open environments can't support as much biomass or biodiversity as, say, forests. So even if there was an equal distribution of forests and plains across the world, the plains could support far fewer species overall and thus there would be fewer species optimized for this type of long-distance running.

And honestly, it's not a terribly effective body plan. A lot of flightless birds go extinct when they come in contact with quadrapedal mammals, from the terror birds of old to modern species that are currently being wiped out by invasive rats and cats in isolated island habitats. Kangaroos survive because they live on the only continent without mammalian megafauna. The only things big enough to regularly threaten them are slow-moving reptiles like monitors and crocs.

Humans are kind of a fluke. We developed tool use before we were fully bipedal, and even with that advantage, our ancestors were preyed upon by big cats and other quadrapedal mammals. The loss of two functional limbs for locomotion is a huge risk, even with tools.

Kangaroos and wallabies stand erect if they are watching out for threats or just trying to see what is going on. They can see further when they stand erect.

When they are grazing their spine is horizontal but their front limbs are not used for mobility or stability. Their tail does the stability function.

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Ah. I didn't have sound on. That makes perfect sense. Personally I wish we could have more respect for elephants freedom. They're too intelligent for us to just have as pets.

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The thing that makes it appear that elephants can run is that they can walk at 25mph/40kph. I have seen people running a 4-minute mile on an elevated indoor track that was made so you could clearly see how fast those men were going. That’s 15mph. Elephants are fast.

There are two major kinds of airplane fuel: jet fuel, and avgas.

Avgas has lead in it. It is used by piston engine airplanes. Little trainer planes. Cessnas and so on. There aren’t so many of them and they’re small so overall they don’t use enough fuel to make a huge difference to your health.

Jet fuel is basically like kerosene or diesel fuel. It is not leaded. It’s not exactly nontoxic but no, that big ass jet is not spewing tons of lead behind it.

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You mentioned penguins but forgot all the other birds. We all (those of us with spines at least) evolved from 4 legged things. The only reason to get up on two legs is if you have a really great use for those two front legs that makes it worth the mobility and stability sacrifice.

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