I don't get why blood pressure is relevant for heart energy expenditure. Animal bodies are full of blood, it's not like hearts pump blood up there, they just pump it into movement.
I also can't imagine how that long neck evolved (and stayed there) without a huge competing advantage. The and usual advantage of large necks is feeding high, so I distrust the claim that an animal with a long neck couldn't keep his head up.
> I don't get why blood pressure is relevant for heart energy expenditure. Animal bodies are full of blood, it's not like hearts pump blood up there, they just pump it into movement.
No, blood does not get pumped in your head with just movement. You don't swing your head below your heart to replace blood in your brain. Somehow the blood needs to be replaced with fresh oxygenated blood.
> I also can't imagine how that long neck evolved (and stayed there) without a huge competing advantage. The and usual advantage of large necks is feeding high, so I distrust the claim that an animal with a long neck couldn't keep his head up.
It is common consensus that Giraffe necks did not evolve just under competitive advantage, and the best proof for that is the fact that their necks are much much longer than any other species. If it was only competitive advantage, other species would have displayed similar trends. Sexual selection is a more likely explanation.
Does high G actually cause a higher or lower blood pressure at heart level? (search engines fail to give an answer) I think the problem there is blood packing to the lower body, and a sauropod would have enough blood and strong enough veins to be able to hold its head upright.
It's not really obvious to me why the sauropod heart->head->heart loop shouldn't be considered as a "siphon". For all the blood that's going up there should be an equal amount of blood going down.
What would be the force actually shutting the capillaries, if the veins themselves aren't collapsing? The upper part of a siphon still has internal pressure, just less than at its base.
If the condition is brought on slowly, i.e. because the animal grew to a large size, I'd think that it would have enough blood to fill their vessels at the increased pressure required to have positive blood pressure everywhere.
I think you may be assuming that veins are rigid, so can create suction at the top, like a siphon, as blood flows back down the neck. If that were true, it would not take much work to move blood through the neck and head. Maybe you are thinking the veins could be braced against surrounding bone structures, to be held open against such suction.
Unfortunately, that cannot work. All the capillaries in the head and brain would also need to be rigid, and strong enough to hold back that much suction. There's no way around actually doing the work to pump the blood up there, and letting it drain back down.
But that isn't a slam-dunk for extreme blood pressure. We have plenty of examples of valves in veins to help get blood back from lower extremities. A little musculature around a pair of such valves on an artery could easily develop into a secondary heart, and a series of them could eliminate much of the need for the actual heart to supply all the pressure, or for the aorta to resist it.
It is not unreasonable to expect to find novel adaptations in an extreme body plan. Supporting the idea in the absence of fossil evidence is hard, even when such a novelty would eliminate the need for such a novelty as absurdly high blood pressure. Biologists resist suggestion of soft -tissue novelties much more than suggestion of physico- mechanical absurdities because they are, after all, biologists, not engineers.
Not so much rigid, and not so much suction, in a closed circulatory system, the column of fluid that is going down pressurizes the bottom just as much as is required to send the fluid back up.
Yes, that would require all capillary vases to support 1 extra atm of pressure against the air, what is a large issue. But it does not require rigidity.
Your point is taken. Looks like I wasn't much clear, but 1 atm is a sure huge amount of pressure, it's not likely that an animal would have this blood pressure, it could at most get some small partial savings from high pressure.
Blood vessels can expand and contract. That's why fighter pilots without legs can actually withstand higher-g maneuvers. That's also why compression socks can improve circulation and reduce swelling in your ankles if you have to stand all day.
I've often wondered if maybe sitting upright might not be the best position for a modern combat pilot. I can see pilot who is laying down pulling far more Gs than even a legless one sitting upright.
I suppose that makes sense, although that would make visibility harder. Maybe if you went full virtual HUD helmets, and didn't need to see out at all... although then you might just want to have drones with AI dogfighting capability?
The other thought about legless pilots is the feet I think are used for the rudder control (pedals), so there is some loss of ability to control without using your feet. While I think computers could probably take this over, in the end it does mean two less things to interact with the plane.
Does this imply that in order to pump blood at the top of a very long neck you don't necessarily need a stronger heart, but just stronger muscles or thicker blood vessels in your legs? In that case the solution to the dinosaurs' problem might be very far from the the heart or the neck.
A few weeks ago I spent an evening reading posts on svpow.com, and I have become a sauropod neck truther. If you just look at how necks actually work in living animals, you can immediately see how arguments based on the "osteological neutral pose" are complete bunk. Here's a place to start: https://svpow.com/2014/11/03/necks-lie-the-complete-story/
It is unfortunate that the page is so mealy. There is a lot of good science going on around the topic.
For a while there had been a sort of consensus around the notion of "neutral posture", the joints held at about the middle of their range. By examination of extant animals that was shown to be absurd; hardly any animal maintains anything like such a posture, not even those you might expect to, like alligators. It is much more common for one or more joints to be "at the stop". From owls to rabbits, a deeply S-curved neck is the norm.
Another folly was sticking bones together to see how far they could bend between stops. That both under-and overestimated flex range. Under, because thick cartilage in joints often separated bones, providing more room; and overestimated, ignoring limits on extension of connective tissue.
Things would have been better if biologists used the notions of "least upper bound" and "greatest lower bound" on numbers, and sought to successively narrow them, as physicists do. But they're biologists.
Notions about practicality of extremes of blood pressure have been hard to investigate, because no living animal has any need for anything close to what they would have needed. It would not be unreasonable to hypothesize two-chambered helper hearts in the neck, but no evidence of any such thing has been interpreted from fossils, and it would be quite a novelty, so risky for a working paleontologist to suggest.
But do they have a two-chambered heart? Birds have a 4 chambered one and crocodiles have a 3 chambered one, which IIRC, is in some ways more complex that's very well suited to their current style of ambush hunting.
Apparently the current view is that crocs abandoned one of their chambers, which seems to me an astonishing thing to achieve.
We can probably assume that the croc relatives that lived in trees or thundered across the tundra had four-chambered hearts. I gather most of them were taken by the mass extinction early in Jurassic era that opened the way for the dinosaurs.
The crocs we have now must have all evolved from little caiman-like things, all their bigger relatives done in by the K-T event.
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[ 2.9 ms ] story [ 68.4 ms ] threadI also can't imagine how that long neck evolved (and stayed there) without a huge competing advantage. The and usual advantage of large necks is feeding high, so I distrust the claim that an animal with a long neck couldn't keep his head up.
No, blood does not get pumped in your head with just movement. You don't swing your head below your heart to replace blood in your brain. Somehow the blood needs to be replaced with fresh oxygenated blood.
> I also can't imagine how that long neck evolved (and stayed there) without a huge competing advantage. The and usual advantage of large necks is feeding high, so I distrust the claim that an animal with a long neck couldn't keep his head up.
It is common consensus that Giraffe necks did not evolve just under competitive advantage, and the best proof for that is the fact that their necks are much much longer than any other species. If it was only competitive advantage, other species would have displayed similar trends. Sexual selection is a more likely explanation.
https://www.scienceabc.com/nature/animals/why-giraffes-have-...
I am no biologist (or physicist) but if a bulk(¿) liquid moves against gravity, then surely it needs assistance?
Getting the blood in a vessel to "just move" if that vessel leads many meters up, has got to be more difficult than if it weren't.
¿ I know capillary effects exist, and plants don't exactly get water up themselves by magic but ...
If that were the case, G forces wouldn't make people black out. https://en.wikipedia.org/wiki/G-LOC
It's not really obvious to me why the sauropod heart->head->heart loop shouldn't be considered as a "siphon". For all the blood that's going up there should be an equal amount of blood going down.
Because blood vessels are flexible. Siphoning would be disastrous for the body - capillaries would snap shut under the suction and we'd die.
Unfortunately, that cannot work. All the capillaries in the head and brain would also need to be rigid, and strong enough to hold back that much suction. There's no way around actually doing the work to pump the blood up there, and letting it drain back down.
But that isn't a slam-dunk for extreme blood pressure. We have plenty of examples of valves in veins to help get blood back from lower extremities. A little musculature around a pair of such valves on an artery could easily develop into a secondary heart, and a series of them could eliminate much of the need for the actual heart to supply all the pressure, or for the aorta to resist it.
It is not unreasonable to expect to find novel adaptations in an extreme body plan. Supporting the idea in the absence of fossil evidence is hard, even when such a novelty would eliminate the need for such a novelty as absurdly high blood pressure. Biologists resist suggestion of soft -tissue novelties much more than suggestion of physico- mechanical absurdities because they are, after all, biologists, not engineers.
Yes, that would require all capillary vases to support 1 extra atm of pressure against the air, what is a large issue. But it does not require rigidity.
You just wrinkled my brain. Is there an actual group of fighter pilots without legs who are taking advantage of this?
The other thought about legless pilots is the feet I think are used for the rudder control (pedals), so there is some loss of ability to control without using your feet. While I think computers could probably take this over, in the end it does mean two less things to interact with the plane.
For a while there had been a sort of consensus around the notion of "neutral posture", the joints held at about the middle of their range. By examination of extant animals that was shown to be absurd; hardly any animal maintains anything like such a posture, not even those you might expect to, like alligators. It is much more common for one or more joints to be "at the stop". From owls to rabbits, a deeply S-curved neck is the norm.
Another folly was sticking bones together to see how far they could bend between stops. That both under-and overestimated flex range. Under, because thick cartilage in joints often separated bones, providing more room; and overestimated, ignoring limits on extension of connective tissue.
Things would have been better if biologists used the notions of "least upper bound" and "greatest lower bound" on numbers, and sought to successively narrow them, as physicists do. But they're biologists.
Notions about practicality of extremes of blood pressure have been hard to investigate, because no living animal has any need for anything close to what they would have needed. It would not be unreasonable to hypothesize two-chambered helper hearts in the neck, but no evidence of any such thing has been interpreted from fossils, and it would be quite a novelty, so risky for a working paleontologist to suggest.
We can probably assume that the croc relatives that lived in trees or thundered across the tundra had four-chambered hearts. I gather most of them were taken by the mass extinction early in Jurassic era that opened the way for the dinosaurs.
The crocs we have now must have all evolved from little caiman-like things, all their bigger relatives done in by the K-T event.