Why are mammals as large as they are and not much larger?

[parshyaa]

• I just wached walter lewin's first lecture on units and dimensions.
• He tried experimentally to show the galelio's argument
• Why are mammals as large as they are and not much larger.
• He argued that if the mammal becomes too massive that the bones will break and he thought that that was a limiting factor.
• But walter lewins results didn't mached the original experiment
• His difference in the d/l values for different animal was deviated from the expected value
• Therefore what is wrong in galelio's thinking or walter lewins results.

 

[SteamKing]

I don't know the details of Lewin's arguments about the size of mammals being maximal, but I should like to point out that mammals range in size from tiny moles and shrews, which weigh only a few grams each, to the giant Blue Whale, which can weigh more than 150 tons, although the latter does dwell entirely in the ocean.

 

[Vanadium 50]

What is the advantage of size? The adult elephant (largest land mammal) already has no natural predators. What selective advantage would they have to grow even bigger?

 

[micromass]

This might be an interesting read: https://en.wikipedia.org/wiki/Square-cube_law

 

[Nugatory]

• Therefore what is wrong in galelio's thinking or walter lewins results.

Perhaps nothing more than so many uncontrolled additional variables that any model based on simple biomechanical square-cube considerations will come with very wide error bars.

 

[Drakkith]

He argued that if the mammal becomes too massive that the bones will break and he thought that that was a limiting factor.

I can't see this being correct. There have been many, many different species of dinosaurs which grew to a MUCH larger size than any mammal ever has, with the exception of perhaps whales.

 

[parshyaa]

What is the advantage of size? The adult elephant (largest land mammal) already has no natural predators. What selective advantage would they have to grow even bigger?

Exactly , I liked your point

 

[Nik_2213]

Uh, didn't the really big dinosaurs have near-fractal, bird-like bones, with better strength/weight than mammalian ?

Didn't they have a different lung arrangement, that allowed more efficient oxygen transfer ??

As for mammalian size, my reading suggests that, barring 'island effect', 'our' elephants' pleistocene mastodon and straight-tusked cousins were significantly larger. IIRC, human hunters still drive down-sizing...

How things could have turned out if our LCA's quadruped plan had not been 'locked in' so very long ago is always fun to consider...

 

[jim mcnamara]

@Nik_2213 Diplodocus and friends (Neosauropoda) were NOT theropod dinosaurs. They are the REALLY big ones at the museum.
http://www.ucmp.berkeley.edu/diapsids/saurischia/theropoda.html
Theropods are the the group from which modern birds evolved. Your bird like reference.

Lung arrangement? I am not sure how that relates to size, except more mass requires more oxygen.

Do you mean air sacs like modern birds have? Yes, it is thought that they were more efficient breathers. But, did you know that atmospheric oxygen levels were low when dinosaurs first became extant? Circa 12% at the start of the Triassic, compared to the ~20% today. Probably why they did really well when other groups like therapsids (progenitors of mammals) did poorly.

Here is discussion
http://www.washington.edu/news/2005...e-great-dying-worse-greatly-delayed-recovery/

 

[Andy Resnick]

• I just wached walter lewin's first lecture on units and dimensions.
• He tried experimentally to show the galelio's argument

Perhaps nothing more than so many uncontrolled additional variables that any model based on simple biomechanical square-cube considerations will come with very wide error bars.

I can't see this being correct. There have been many, many different species of dinosaurs which grew to a MUCH larger size than any mammal ever has, with the exception of perhaps whales.

@Nik_2213

Lung arrangement? I am not sure how that relates to size, except more mass requires more oxygen.

"On Growth and Form", D'Arcy Wentworth Thompson's magisterial book, has a whole chapter applying scaling laws to animals. He shows how basic proportional relationships constrain a variety of organism parameters: larger animals have proportionally shorter limbs (Galileo's law), swimming speeds increase as the square root of animal length (Froude's law), why there are no small animals in polar regions (Bergmann's law), all animals can jump to the same actual, not relative, height (Borelli's law), and a lot more- 'rules' about flying, walking, respiratory systems (bugs and ants don't need lungs, diffusion is sufficient), eyes and ears, rates of growth...

 

[Andy Resnick]

I don't know the details of Lewin's arguments about the size of mammals being maximal, but I should like to point out that mammals range in size from tiny moles and shrews, which weigh only a few grams each, to the giant Blue Whale, which can weigh more than 150 tons, although the latter does dwell entirely in the ocean.

That's exactly right- if a whale comes out of the water, it doesn't have the muscle strength to breathe- the buoyancy permits larger size, which is advantageous for swimming speed and distance (migration). Note also that the smaller animals have to constantly eat.

 

[Andy Resnick]

Perhaps nothing more than so many uncontrolled additional variables that any model based on simple biomechanical square-cube considerations will come with very wide error bars.

Actually, that's not true. If you plot a variety of scaled organism parameters the statistical variation is rather small.

 

[Andy Resnick]

What is the advantage of size? The adult elephant (largest land mammal) already has no natural predators. What selective advantage would they have to grow even bigger?

Not just elephants: gorillas, hippos and rhinos also practically have no non-human predators. Notice they are also all herbivores, in contrast to large apex predators like bears, lions, wolfs who are considerably smaller is size.

Two reasons why size matters: larger animals can move faster for longer periods of time, and don't don't have to eat as often. But terrestrial animals are constrained in size by falling/impact injuries as well- even seen an elephant jump?
.

 

[parshyaa]

"On Growth and Form", D'Arcy Wentworth Thompson's magisterial book, has a whole chapter applying scaling laws to animals. He shows how basic proportional relationships constrain a variety of organism parameters: larger animals have proportionally shorter limbs (Galileo's law), swimming speeds increase as the square root of animal length (Froude's law), why there are no small animals in polar regions (Bergmann's law), all animals can jump to the same actual, not relative, height (Borelli's law), and a lot more- 'rules' about flying, walking, respiratory systems (bugs and ants don't need lungs, diffusion is sufficient), eyes and ears, rates of growth...

larger animals have proportionally shorter limbs (Galileo's law)

• But walter lewin's experiment clearlly showed that in reality difference in the thickness of femoure(limbs or bone) is not as expected from galileo's law .
• In later video, walter lewin explained that in reality it does not follows galileo's law because of buckling property of bones(ie. Extra bending ability)

 

[jim mcnamara]

@Andy Resnick - thanks! I just ordered the book.

In grad school I encountered "Hall's Law" - relating stride length to femur length. That was the extent of my reading on this.

 

[Andy Resnick]

larger animals have proportionally shorter limbs (Galileo's law)

• But walter lewin's experiment clearlly showed [...] .

How do you mean 'experiment'?

 

[parshyaa]

Not experiment but his calculations for the length of femoure and thickness of different animal is different from the expected values. He showed it by finding length of femoure and its thickness and then comparing it with other animals but the values didn't matched galelios prediction as shown in his first lecture unit and dimension video.

 

[.Scott]

I don't see much discussion about another potential trade-off between big and small: genetic adaptation. As mammals get bigger, their population goes down and the time period from one generation to the next gets longer.

Some quick Googling yields these stats: There were a few million elephants in 1900 and much less today. They have a gestation period of almost 2 years and they take little interest in their peers until about age 17. So that's at least 19 years per generation. Compared to smaller mammals, they are genetically frozen.

In a stable environment where their abilities and behavior are already close to optimal for survival, this should not be a problem. But with a changing environment, I would expect smaller animals to be able to adapt much more quickly - and then evolve into larger species capable of replacing the "dinosaurs".

Perhaps a good way to address this issue is to ask whether, over a period of thousands of generations, elephants, or a subspecies of elephants would become even bigger than they are now. Would a population of large elephants be able to out-compete their smaller cousins for resources?

 

[brainpushups]

Animals are not scaled up versions of other animals, and there are other factors like motion which affect how the scaling of bones would need to work. I recommend reading the link I've provided to the University of Virginia handout on the subject which has some additional details about actual skeletal scaling in animals.

http://galileo.phys.virginia.edu/classes/609.ral5q.fall04/LecturePDF/L14-GALILEOSCALING.pdf