Animal sizes in low gravity environments

[GregM]

"why size matters in animal behaviour? How can a hamster survive falling from the top of a skyscraper, ants carry over 100 times their own body weight and geckos climb across the ceiling? In the first of this year's Christmas lectures, Dr Mark Miodownik investigates . He reveals how the science of materials - the stuff from which everything is made - can explain some of the most extraordinary and surprising feats in the animal kingdom. By the end, you will understand why you will never see an elephant dance" - British Royal Institute of Science

Hello everyone. This is a revised form of a previous attempt to get some information on scale of animals, structures in relation to gravity.
If gravity was lower, would animals be able to grow larger? Say if we terraformed Mars. Would animals on Mars evolve to be bigger than those on Earth? That there are 150 ton blue whales in the ocean where specific gravity ( density of object vs density of its medium ) is close to zero is evidence i think to support this possibility. ( Do I need to cite a peer reviewed journal paper for the existence of blue whales? )

If yes, how much larger could animals grow in say, half g gravity?
Thanks,

 

[berkeman]

That there are 150 ton blue whales in the ocean where specific gravity ( density of object vs density of its medium ) is close to zero is evidence i think to support this possibility. ( Do I need to cite a peer reviewed journal paper for the existence of blue whales? )

Your previous thread start was deleted with a warning because you were asking if Earth's gravity was substantially lower in the time of the dinosaurs (which is nonsense) without supplying any valid references. This thread start is a reasonable question after I move it to the General Discussion forum.

 

[DaveC426913]

if Earth's gravity was substantially lower in the time of the dinosaurs ... without supplying any valid references

Does Canadian science fiction count as a valid reference?

https://www.goodreads.com/book/show/264943.End_of_an_Era

 

[BillTre]

There is a lot of research on animal size and how it relates their physiology and biophysics.

Something often suggested to "explain" larger insects in the Carboniferous and Permian periods is a higher oxygen level, which would allow animals support greater metabolic needs. Does not have universal consensus.

These time periods had the first amniotes, vertebrates that are not required to lay their eggs in water (reptiles vs. the earlier amphibians).
However, the legs of these early amniotes did not extend directly under the body where they could most easily support great weigths, but came out more laterally. This is a non-optimum geometry for supporting a lot of mass. Legs extending directly down from the body (like an elephant) can more easily support the weight.
Changing the way the legs came off the body would have required changes in how the legs develop in the embryo. It would seem that this is not the easiest thing to change in evolution, since it took a long time for that to evolve.
Dinosaurs, having their legs in this more supportive position, had an design advantage, among vertebrates, in getting large.

Elephant legs are a good example of physical support limitations.
The elephants weight goes up due to its mass, which is approximately a third power function of its linear length. The strength of the leg to support the weight is based on its cross-sectional area, a second power function. This is why elephant legs are proportionately thicker when compared with lighter animals.
There are lots of other possible adaptations for large size.

Birds are lighter, in part because some of their bones are air filled and part of their respiratory system. This makes the lighter, thereby mitigating some of their weight problems. Some dinosaurs may have also used the strategy to deal with large heavy bodies. (Birds evolved from dinosaurs.)
There are a lot of natural engineering ways to deal with size. Some species will not have to evolve certain features if their anatomy and physiology can not be easily modified that direction.

Effects on body size evolution is a very complex issue, affected by many different things.
Why were there mammalian megafuana up to a few thousand years ago, that are now largely gone?
It is usually explained as due to either humans or changes in climate. Way different from gravity.

These might have been an slight increase in the Earth's gravity since the times of the dinosaurs, due to meteorites adding mass. The Chicxulub impact could have both added and removed mass.
Other than sources like these, the Earth's gravity (which is based on its mass) would not have changed much.
Compared to the overall mass of the earth, this would probably be so small as to be insignificant.
Other than a change in the mass of the earth, there is no reason to expect the strength of the Earth's gravity to change.

 

[A.T.]

If yes, how much larger could animals grow in say, half g gravity?

From a purely biomechanical perspective apply this:
https://en.wikipedia.org/wiki/Square–cube_law#Biomechanics

But this ignores complex matters like sufficient oxygen supply etc.

 

[DaveC426913]

From a purely biomechanical perspective apply this:
https://en.wikipedia.org/wiki/Square–cube_law#Biomechanics

But this ignores complex matters like sufficient oxygen supply etc.

Although I think lungs are similarly subject to the square-cube law.

 

[GregM]

>if Earth's gravity was substantially lower in the time of the dinosaurs
> (which is nonsense)
thanks for answering my first question. i think there's room for improvement wrt the answering process though. Honestly i wouldn't be surprised if many intelligent kids on seeing a dinosaur documentary and then the royal lecture on the square-cube law would put 2 and 2 together and conjecture lower Earth gravity for dinos : you should form a friendly policy of welcoming them instead of insta-locking their thread as punishment for practising the first step of science.

>From a purely biomechanical perspective apply this: (square cube law)
yes that law and allometric scaling and a little thinking is what got my first thread locked. Elephants have legs like tree trunks or support columns. What other animals look structurally like elephants in their legs?

>Elephant legs are a good example of physical support limitations.
Are they? They are outclasssed by dino legs by a factor of 12. Elephant legs seem to be at only 8% capacity, therefore they are a bad example of limitations

 

[collinsmark]

Here's a link to the full 1 hour lecture video with the correct aspect ratio and better audio.

https://www.rigb.org/christmas-lect...elephants-cant-dance-but-hamsters-can-skydive

 

[GregM]

 

[DaveC426913]

If gravity was lower
if we terraformed Mars
half g gravity

 

[Mark44]

Your previous thread start was deleted with a warning because you were asking if Earth's gravity was substantially lower in the time of the dinosaurs (which is nonsense) without supplying any valid references.

@GregM, why would you think that Earth's gravity might have been substantially lower in the time of dinosaurs (i.e, earlier than about 63 million years ago)? Do you suppose that the mass of the Earth was, say, 50% or 60% of what it is now? Geologically speaking in terms of the age of the earth, that wasn't that long ago.

 

[DaveC426913]

@GregM, why would you think that Earth's gravity might have been substantially lower in the time of dinosaurs

I'm not sure this is a productive line of enquiry, considering the history of the thread. Speculating about lower gravity on Earth is what got the original post locked in the first place.

 

[Mark44]

I'm not sure this is a productive line of enquiry, considering the history of the thread. Speculating about lower gravity on Earth is what got the original post locked in the first place.

My purpose here was to show why the reply of "nonsense" was given.

 

[GregM]

>why would you think that Earth's gravity might have been substantially lower in the time of dinosaurs?
It appears to me to be an immediately result of the square cube law and the size of many prehistoric bones.

SCL : ' cross sectional support structures cannot scale proportionally with mass in scale transforms, hence a specific object design scaled to be bigger reaches a maximum practical size '

Elephants seem near the limit of what is practical today. Yet with the same thick support column style legs, sauropods got to about 12 times the mass of Elephants.

If we'd like to qualify this with a calculation, wiki says bones have a max compressive strength of about 170MPa. What is the csa of a Elephant's shin bones?

 

[GregM]

>I'm not sure this is a productive line of enquiry, considering the history of the thread. Speculating about lower gravity on Earth is what got the original post locked in the first place.

On the contrary Dave, its when apparent contradictions in understanding are found and analysed that enquiry is at its most productive.

 

[BillTre]

>I'm not sure this is a productive line of enquiry, considering the history of the thread. Speculating about lower gravity on Earth is what got the original post locked in the first place.

On the contrary Dave, its when apparent contradictions in understanding are found and analyzed that enquiry is at its most productive.

In that case, your weighting of the vague and not fully understood biomechanical knowledge of dinosaurs vs. mammals over the immensely more fully understood physics of gravity is way off.
Much of what you are saying about the biology can be questioned (even by yourself):

Elephants seem near the limit of what is practical today. Yet with the same thick support column style legs, sauropods got to about 12 times the mass of Elephants.

If we'd like to qualify this with a calculation, wiki says bones have a max compressive strength of about 170MPa. What is the csa of a Elephant's shin bones?

This is your job to do if you believe in this stuff.
Where's the wiki reference?
Its not for someone else to take their time proving your conjecture. That's lazy.
What is the relevance to the overall discussion?

 

[GregM]

@Biltree Whatever. Its not my job to force spoonfeed you ideas you don't want to consider.

Gravity + Square-Cube-Law + ( comparison of Elephant vs Saropod Bones ) -> Earth Gravity was weaker in the past.
rephrased for the faint hearted as:
With the square-Cube-Law in mind would animals evolved in lower gravity be able to grow bigger?

If you don't enjoy or see purpose or insight in considering this question, please don't reply to the thread.

 

[berkeman]

Thread is closed for Moderation (there's a surprise)...

 

[Mark44]

It appears to me to be an immediately result of the square cube law and the size of many prehistoric bones.

This "immediate result" completely ignores what is known about the size of the Earth (its mass) throughout history, and therefore the force due to gravity.

In that case, your weighting of the vague and not fully understood biomechanical knowledge of dinosaurs vs. mammals over the immensely more fully understood physics of gravity is way off.

Amen to that.

Gravity + Square-Cube-Law + ( comparison of Elephant vs Saropod Bones ) -> Earth Gravity was weaker in the past.

And what could have been the possible cause of Earth's gravity being weaker?

@Biltree Whatever. Its not my job to force spoonfeed you ideas you don't want to consider.

On the contrary, this was your thread, so it is your responsibility to come up with references that bolster your assertions, not ours.

This thread will remain closed.