What is the scientific principle that governs how big animals get?
February 24, 2017 3:03 PM   Subscribe

After seeing the trailer for the latest King Kong movie, it got me thinking about why animals only get as big as they do. Is it because they evolve in an ecosystem and natural selection doesn't want you getting bigger than the food supply can managably feed? Is there a scientific principle that governs this that people have named? Is it a DNA thing? I guess I'm wondering what prevents gorillas from growing 100 feet tall. Could there ever be a mutation that allows this to happen? Thanks!
posted by Sully to Science & Nature (13 answers total) 7 users marked this as a favorite
The square-cube law is a big part of this. Allometry is the biomechanics corollary.
posted by General Malaise at 3:13 PM on February 24, 2017 [7 favorites]

Gravity is one of them, and I think there's a field of biomechanics that covers the details.

There's also this thing called the square-cube law:

When an object undergoes a proportional increase in size, its new surface area is proportional to the square of the multiplier and its new volume is proportional to the cube of the multiplier.

As an object like a gorilla scales upwards, its surface area goes up in squares (i.e. doubling its height means the area goes up something like 2^2 = 4 times) while its volume, and therefore its mass) goes up as in cubes (doubling the height means the volume goes up 2^3 = 8 times). So by comparison to your standard gorilla, a double-height gorilla (doublrilla!) with the same proportions has 8 times the body and 4 times the skin. So biomechanics asks whether the doublrilla can cool itself off with 8 times the body but only 4 times the surface area, or can gorilla heart-tissue pump 8 times the blood, or can the lungs take in 8 times the oxygen? Biomechanics might test the load-bearing strength of gorilla-bones to help speculate whether gorilla bones get stronger as they scale larger (they do, to a point, but they'll always be made of bone). Eventually, something will get too large for flesh to bear the load, os what it comes down to. Large animals on Earth are sometimes pushing at the bounds, but they never exceed it.
posted by Sunburnt at 3:15 PM on February 24, 2017 [9 favorites]

It's quite a long time since I watched it now, but I remember this Christmas Lecture covering this question:

Mark Miodownik - Why Elephants Can't Dance but Hamsters Can Skydive

I could be wrong, but an enjoyable watch all the same.
posted by jonrob at 3:23 PM on February 24, 2017

The classic essay on this topic is "Being the Right Size". At one point the author mentions that because insects' respiratory systems primarily depend on passive diffusion of oxygen, they can't be much larger than half an inch thick. During the Carboniferous Period, though, the concentration of oxygen in the atmosphere was much higher, which allowed for much larger terrestrial arthropods, like this dragonfly with a 25 inch wingspan.

There are quite a few interesting relationships that have been discovered along these lines, such as Bergmann's rule, which states that related species tend to be larger in cold climates and smaller in warm climates, and Foster's rule, which states that large species tend to get smaller when they colonize islands, and small species tend to get larger.
posted by J.K. Seazer at 3:26 PM on February 24, 2017 [11 favorites]

Came in here to say exactly the same two things General Malaise said in the first comment.

This (square-cube and allometric scaling) was an evergreen source of bickering in our long-running D&D campaign (heavy on the titular dragons). It only got resolved (to the extent that it ever did) through recourse to 1) pay no attention to the man behind the curtain and 2) a wizard did it!

I don't think you can appeal to magic for King Kong, so the solution is probably just "don't spoil the cool thing by thinking about it too hard."
posted by sourcequench at 3:32 PM on February 24, 2017

As people point out above, size makes a big difference, and species tend towards a size that fits their ecological role. The square-cube law is a harsh mistress since it dictrates both transpiration rate and proportional strength (ability to withstand damage or exert force is more-or-less proportional to corss-sectional area or bones and muscles, which grows as the square of linear size). So smaller animals are much stronger and tougher relative to their size. That relativity part is important, because an ant doesn't seem very tough --- you can squish it without really trying too hard --- but if you drop an ant from a height that would turn a human into jelly, they'll just wal away, because the kinetic energy imparted to the ant by the fall (proportional to its weight, which is roughly proportional to the cube of its linear size) would be so tiny that even the body we think of as frail wouldn't be impacted by it.

Of course, tiny isn't perfect: the fact that we can squash an ant shows the problem, in that tiny animals, with their inordinately high strength-for-their-size ratio, are utterly at the mercy of larger, less efficient creatures. But get too large, and that inefficiency becomes a liability in its own right: truly huge creatures need to consume enormous quantities just to survive and handle scarcity badly, and they tend to get injured more easily and recover poorly because of their own bulk working against them. So ecology and anatomy plays a tremendous role in just what size range is really viable for a species at all.
posted by jackbishop at 4:19 PM on February 24, 2017

The Megatheriidae sloth was up to 5 tons and 17 feet tall. There were some pretty big critters hanging out once upon a time. Looking at the massive bone structure will give some idea how many groceries this dude needed to maintain life. Lots of high quality rapid renewing vegetation needed.
posted by BlueHorse at 5:04 PM on February 24, 2017

If you're into some conjecture about this topic from a century back I might suggest HG Wells' Food of the Gods.
posted by Rash at 5:30 PM on February 24, 2017

Wikipedia: The largest perissodactyl, and land mammal, of all time was Paraceratherium. It stood 5.5 m (18 ft) tall at the shoulder, a total height of 8 m (27 ft), totally 12 m (40 ft) long and may have weighed 20 tonnes (22 tons), though mass estimates vary. Some prehistoric horned rhinos also grew to large sizes. The giant woolly rhino Elasmotherium reached 20 ft long and 6.6 ft high.

How about a giant rodent at 2000lbs? And 19 more big 'uns.

Pretty sure the biggest impact on the size of these critters was changing climate affecting food supply.
posted by BlueHorse at 5:33 PM on February 24, 2017

I just want to chime in with something about your reference to mutations. A mutation that, for example, didn't stop growth at the end of adolescence is one mutation. A mutation for the denser bones to support that bigger body would be another mutation. A mutation to stop the growth at some point would be yet another mutation.

Every newborn has dozens, if not hundreds of mutations. Most of them don't do anything noticeable. For the noticeable ones to create anything as complete as King Kong there would have to be a large number of complementary mutations. This means that Monster Island would be full of mutant apes that have some, but not all, of the mutation set. Think about that with regard to superpowers. It takes lots of imperfect offspring to produce one successful one.
posted by irisclara at 6:22 PM on February 24, 2017 [1 favorite]

You could watch Episode 4 of The Wonders of Life if you'd like to see Brian Cox meditate upon this question while exploring the cinematographic possibilities of Australia. (I recommend it)
posted by polecat at 9:11 PM on February 24, 2017

I always found it fascinating that animals such as elephants (and humans!) evolved to be much smaller when existing on smaller land masses over long periods of time. Would not the converse be true as well?
posted by xammerboy at 2:59 PM on February 25, 2017

Partly due to the Eltonian Pyramid. "There must be higher amounts of biomass at the bottom of the pyramid to support the energy and biomass requirements of the higher trophic levels." In other words, being big requires a lot of energy. During the Jurassic and Cretaceous, when most of the really big dinosaurs lived, the Earth was warmer, wetter and there was more O2 and CO2 then to today. Meaning that there was more plant biomass at the bottom of the pyramid.

"Why Big, Fierce Animals Are Rare" by Paul Colinvaux addresses the question in more detail.
posted by chrisulonic at 8:44 PM on February 25, 2017 [1 favorite]

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