why are things the size that they are?
December 2, 2018 6:58 AM Subscribe
Why is it that every species isn't 33% smaller or 25% bigger? Why are people on average the height that they are, and cats the size that they are? Why isn't every form of life 10% smaller?
Is it a question of needing more resources if you're bigger, but if you're smaller, being weaker, and thus finding an equilibrium? Is it about brain size? What dictates size, and why aren't all the sizes different from what they are?
You'd think I'm asking for a child, but no. My husband is curious!
Is it a question of needing more resources if you're bigger, but if you're smaller, being weaker, and thus finding an equilibrium? Is it about brain size? What dictates size, and why aren't all the sizes different from what they are?
You'd think I'm asking for a child, but no. My husband is curious!
There are, for example, about 40 species of cats in all different sizes, so there’s no single perfect size. There are lots of subtly different ways to be a cat. Obviously there’s a very different ecological niche for an animal that can kill a zebra compared to one that eats mice, although it’s much less easy to see what distinguishes more similar species.
posted by Bloxworth Snout at 7:21 AM on December 2, 2018
posted by Bloxworth Snout at 7:21 AM on December 2, 2018
If what you are asking is why can't a specific creature just be scaled up (a cat, a hummingbird) , then thats a question about physics. Our universe has scale. And these scaling laws apply to metabolic processes within the body. This leads to elephants having to have large feet and giraffe's having to have complex mechanisms for dealing with blood pressure changes because of their long necks.
See also: Animals tend to evolve over larger sizes over time.
Kleiber's law: for the vast majority of animals, an animal's metabolic rate scales to the ¾ power of the animal's mass
posted by vacapinta at 7:21 AM on December 2, 2018 [5 favorites]
See also: Animals tend to evolve over larger sizes over time.
Kleiber's law: for the vast majority of animals, an animal's metabolic rate scales to the ¾ power of the animal's mass
posted by vacapinta at 7:21 AM on December 2, 2018 [5 favorites]
From a physics standpoint, part of the reason is that things scale with size at different rates. So you can't just double the size of something and expect everything to behave physically the same.
Most notably would be the strength of support structures. If you double the dimensions of something, the weight is multiplied by 8 - it's a cubic scaling because you've got three different direction that get multiplied by 2: 2 x 2 x2. But the strength of support structures like exoskeletons or bones would only get multiplies by 4 - since it's based on the cross sectional area. So as things get bigger it becomes harder to support them. That's why an elephant has to be built a lot more solidly than a gazelle. And why large mammals have to have bones to support them while small insects can get away with just having an exoskeleton. An ant the size of a human would probably collapse under its own weight.
Another similar scaling difference is heat generation vs. dissipation. How much heat a creature generates is going to scale with its weight - cubically again, so a doubling of length multiplies it by 8. However, how much heat ends up getting conducted off depends on surface area. That is also only going to get multiplied by 4 if you double the length. So a larger creature, all things being equal, might have more of a problem cooling off while a smaller one might have more trouble keeping warm enough.
And there's a number of other similar things, like diffusion rates, that provide reasons for things to be one size over another. The specifics of course, I'd have to leave to biologists.
posted by Zalzidrax at 7:22 AM on December 2, 2018 [8 favorites]
Most notably would be the strength of support structures. If you double the dimensions of something, the weight is multiplied by 8 - it's a cubic scaling because you've got three different direction that get multiplied by 2: 2 x 2 x2. But the strength of support structures like exoskeletons or bones would only get multiplies by 4 - since it's based on the cross sectional area. So as things get bigger it becomes harder to support them. That's why an elephant has to be built a lot more solidly than a gazelle. And why large mammals have to have bones to support them while small insects can get away with just having an exoskeleton. An ant the size of a human would probably collapse under its own weight.
Another similar scaling difference is heat generation vs. dissipation. How much heat a creature generates is going to scale with its weight - cubically again, so a doubling of length multiplies it by 8. However, how much heat ends up getting conducted off depends on surface area. That is also only going to get multiplied by 4 if you double the length. So a larger creature, all things being equal, might have more of a problem cooling off while a smaller one might have more trouble keeping warm enough.
And there's a number of other similar things, like diffusion rates, that provide reasons for things to be one size over another. The specifics of course, I'd have to leave to biologists.
posted by Zalzidrax at 7:22 AM on December 2, 2018 [8 favorites]
Just some thoughts off the top of my head about plants:
Competition for light and nutrients is important. It can be helpful to be taller than neighboring plants so they don't shade you out. Or to have a bigger root system. But growing bigger also means you need more water and nutrients and those may be in short supply in your environment, limiting how big you can get. (That's why in the dry shortgrass prairie, for instance, trees grow only along streams.) The length of the growing season is also a factor. (So trees are shorter at high elevation.) The stability of the environment is important, too. Plants that grow in stable environments (like forests that experience infrequent fires or other disturbances) have time to grow taller than plants that grow along eroding, flood-prone riverbanks or places where fires are frequent.
posted by Redstart at 7:35 AM on December 2, 2018
Competition for light and nutrients is important. It can be helpful to be taller than neighboring plants so they don't shade you out. Or to have a bigger root system. But growing bigger also means you need more water and nutrients and those may be in short supply in your environment, limiting how big you can get. (That's why in the dry shortgrass prairie, for instance, trees grow only along streams.) The length of the growing season is also a factor. (So trees are shorter at high elevation.) The stability of the environment is important, too. Plants that grow in stable environments (like forests that experience infrequent fires or other disturbances) have time to grow taller than plants that grow along eroding, flood-prone riverbanks or places where fires are frequent.
posted by Redstart at 7:35 AM on December 2, 2018
On Being the Right Size.
It's old, so the science may not be current. I will never forget this line, though,
"You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes."
posted by ZeroDivides at 8:44 AM on December 2, 2018 [6 favorites]
It's old, so the science may not be current. I will never forget this line, though,
"You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes."
posted by ZeroDivides at 8:44 AM on December 2, 2018 [6 favorites]
Consider that island gigantism and island dwarfism are both commen phenomena.
For example there are island sloths that are tiny, compared to ther continental kin, while the same island hosts butterflies that are enormous, relative to their kin.
So in effect, on small islands many species seem to have experienced normative forces on body size.
Which is to say, it’s subtle, there are no easy answers, and yes this is an open area of research. The Haldane article linked above is probably the single best thing for a modern layperson to read about this question.
posted by SaltySalticid at 9:09 AM on December 2, 2018 [2 favorites]
For example there are island sloths that are tiny, compared to ther continental kin, while the same island hosts butterflies that are enormous, relative to their kin.
So in effect, on small islands many species seem to have experienced normative forces on body size.
Which is to say, it’s subtle, there are no easy answers, and yes this is an open area of research. The Haldane article linked above is probably the single best thing for a modern layperson to read about this question.
posted by SaltySalticid at 9:09 AM on December 2, 2018 [2 favorites]
I realized that my comments above about plants mix together factors that can keep an individual plant from reaching its full potential height and factors that help determine what potential height a species evolves to have. The same factors can have both effects. In a place where there's a lack of water or a short growing season or a lot of fires, some of the plants may be species that could grow bigger in a different place. But many of them will be species that evolved to be optimized for that environment. They aren't designed to grow to be big because that just wouldn't ever be feasible in their environment.
posted by Redstart at 9:58 AM on December 2, 2018
posted by Redstart at 9:58 AM on December 2, 2018
I'm with vacapinta and Zalzidrax at least a bit. It's the math. You start with a scale of 0..∞, but various factors of existence scale at different rates along polynomial lines.... some scale at x, some at x², some at x³, etc.
Sufficiently near the 0 mark the difference between these scales is small and minor adjustments in x don't change much. As x grows from 0, the differences become significant.
Think about it as a graph. The x is just that straight line, the x² is that parabola, the x³ starts climbing up even faster up than that x² parabola.
At some point in size, those curves are going to surpass what is possible in the environment with the properties of material.
Gravity overtakes the strength of bone. The average temperature of the air and the thermal transfer from your surface to environment is overtaken by the heat produced by your volume. The amount of energy you need to function overtakes what you can acquire. (Think about it, even if you can take in nutrients from your entire surface x², the volume of your self that needs sustaining grows at x³). If you just scale up, your mouth doesn't grow fast enough to feed your body. :)
posted by zengargoyle at 10:48 AM on December 2, 2018
Sufficiently near the 0 mark the difference between these scales is small and minor adjustments in x don't change much. As x grows from 0, the differences become significant.
Think about it as a graph. The x is just that straight line, the x² is that parabola, the x³ starts climbing up even faster up than that x² parabola.
At some point in size, those curves are going to surpass what is possible in the environment with the properties of material.
Gravity overtakes the strength of bone. The average temperature of the air and the thermal transfer from your surface to environment is overtaken by the heat produced by your volume. The amount of energy you need to function overtakes what you can acquire. (Think about it, even if you can take in nutrients from your entire surface x², the volume of your self that needs sustaining grows at x³). If you just scale up, your mouth doesn't grow fast enough to feed your body. :)
posted by zengargoyle at 10:48 AM on December 2, 2018
The ideal size for any species depends a lot on the other species around it. Plants that are smaller than all the plants around them may get outcompeted for light and nutrients. Animals that are bigger may be less vulnerable to predation or less able to hide from predators. The best size will depend a lot on what the predators are. Being bigger may allow a predator to kill more easily but also means it needs more food and could be more likely to starve. The best size depends on what prey is available, and on what other predators live in the same area and what they're specialized to prey on. That's probably why animals on islands often end up bigger or smaller than similar species on the mainland - the collection of other species they live with is different.
That doesn't exactly answer the question, "Why isn't every form of life 10% smaller or 25% bigger?" But I think it does sort of help to answer it. There are upper and lower limits that have to do with physics and biology. For instance, this article talks about the upper height limit for trees. There are going to be environments where pressure to outcompete other species pushes one or more species to be as big as it can get. Those species aren't going to stop evolving to be bigger until they have to. They wouldn't have evolved to be 10% smaller than they are now because that wouldn't be the most advantageous height. But they couldn't be 25% bigger either, because that isn't physically possible. If some species in an ecosystem are at their upper or lower size limits, that exerts some control over the possible sizes that will work for all the other species, because they're all affecting each other. Making everything 10% bigger or smaller isn't an option if something is already as big or as small as is physically possible for its structure and energy input/output.
posted by Redstart at 10:57 AM on December 2, 2018 [2 favorites]
That doesn't exactly answer the question, "Why isn't every form of life 10% smaller or 25% bigger?" But I think it does sort of help to answer it. There are upper and lower limits that have to do with physics and biology. For instance, this article talks about the upper height limit for trees. There are going to be environments where pressure to outcompete other species pushes one or more species to be as big as it can get. Those species aren't going to stop evolving to be bigger until they have to. They wouldn't have evolved to be 10% smaller than they are now because that wouldn't be the most advantageous height. But they couldn't be 25% bigger either, because that isn't physically possible. If some species in an ecosystem are at their upper or lower size limits, that exerts some control over the possible sizes that will work for all the other species, because they're all affecting each other. Making everything 10% bigger or smaller isn't an option if something is already as big or as small as is physically possible for its structure and energy input/output.
posted by Redstart at 10:57 AM on December 2, 2018 [2 favorites]
The complexities of this question are nicely illustrated by the case of the giant insects:
The largest wings of any living insect belong to the Queen Alexandra birdwing butterfly and the atlas moth. They can span 10 to 12 inches across. But even these giants are puny compared to the insects of prehistory. Meganeura, for example, was a dragonfly that lived 300 million years ago and each of its wings was the length of my arm. Why do such behemoths no longer exist?posted by jamjam at 11:37 AM on December 2, 2018 [10 favorites]
The prevailing theory, proposed around a century ago, is that the Earth’s atmosphere used to have much more oxygen—more than 30 per cent in the Permian, compared to just 20 today. This vital gas sets an upper limit on how big animals can be. The seething quantities of past eras allowed flying insects to fuel faster metabolisms and larger bodies.
Matthew Clapham and Jered Karr from the University of California, Santa Cruz have now found some strong evidence to support this idea, after analysing more than 10,500 fossilised insect wings. It took almost 18 months to collect the entire data set, but it clearly showed that the maximum wingspans of flying insects neatly tracked the oxygen in the atmosphere for their first 150 million years of evolution. As the gas reached its peak during the Permian, the insects were at their largest. As levels later fell, the insects shrank.
But this neat correlation stopped between 130 and 140 million years ago, during the early Cretaceous period. Even though oxygen concentrations started climbing from a Jurassic low of 15 per cent, for the first time in their history, the insects didn’t follow suit. If anything, they got smaller. They had finally encountered something that limited their growth more than the oxygen in the air: birds.
On top of the physical constraints like scaling laws (calculus) and competition with neighboring individuals-of-many -species (partial differential equations are hard enough, Diophantine versions even worse), the physical environment has a lot of variability (statistics), so species survival may depend on *not* optimizing for the current conditions. Or being suitably plastic to changes.
After the Haldane paper, try The Beak of the Finch, even though it's only loosely about the size of the whole finch. It's just a great adventure book and immersed in all these factors at once, demonstrated in the beautiful severe Galapagos.
posted by clew at 12:09 PM on December 2, 2018 [3 favorites]
After the Haldane paper, try The Beak of the Finch, even though it's only loosely about the size of the whole finch. It's just a great adventure book and immersed in all these factors at once, demonstrated in the beautiful severe Galapagos.
posted by clew at 12:09 PM on December 2, 2018 [3 favorites]
The original question sounds a little like Poincaré's doubling: if everything in the universe suddenly became twice (or half) the size, would we be able to notice?
If every species suddenly became 10% smaller, presumably not much would change. But, well, there is no mechanism for all species to suddenly change. There is a mechanism for a species to change, very slowly. It will if there's some benefit to it, and otherwise probably won't.
For a given species, changing its size probably means changing its niche. A predator getting smaller might not be able to catch its accustomed prey. Getting larger might just increase its energy requirements without allowing it to catch larger prey.
posted by zompist at 12:47 PM on December 2, 2018 [1 favorite]
If every species suddenly became 10% smaller, presumably not much would change. But, well, there is no mechanism for all species to suddenly change. There is a mechanism for a species to change, very slowly. It will if there's some benefit to it, and otherwise probably won't.
For a given species, changing its size probably means changing its niche. A predator getting smaller might not be able to catch its accustomed prey. Getting larger might just increase its energy requirements without allowing it to catch larger prey.
posted by zompist at 12:47 PM on December 2, 2018 [1 favorite]
This is a good and interesting question, as evidenced by the responses above.
I was just dropping back in to also point you to the wikipedia entry on Allometry which also covers many of the ideas expressed above.
posted by vacapinta at 2:22 PM on December 2, 2018 [2 favorites]
I was just dropping back in to also point you to the wikipedia entry on Allometry which also covers many of the ideas expressed above.
posted by vacapinta at 2:22 PM on December 2, 2018 [2 favorites]
I believe that there are some hard-ish limits on the size of flying animals given the strength, stiffness, and power generation abilities of biological materials. For large fliers, wings are most efficient when they are long and thin. Long+thin+holding up lots of weight = high mechanical stress. Large birds do some clever things with the feathers at their wingtips to mitigate some of the efficiency losses of short wings, but even with those tweaks the strength-to-weight and power-to-weight ratios of bones, tendons, feathers and muscles mean that there's a limit on what can get off the ground.
That doesn't mean it'd be impossible for new biological materials to involve.
posted by clawsoon at 3:27 PM on December 2, 2018
That doesn't mean it'd be impossible for new biological materials to involve.
posted by clawsoon at 3:27 PM on December 2, 2018
As well as all of the other excellent points made above, one other factor might be temperature: Bergmann’s rule suggests that endothermic (i.e. warm-blooded) animals will tend to be bigger in colder weather and smaller in hotter weather, because of the ratio of surface area to volume: the volume of an animal is what’s inside it, a collection of cells metabolising and producing heat. And that heat can only be lost from the animal at its surface.
Smaller animals have more surface area per unit volume, so it’s easier for them to lose heat (an advantage in hot places) and larger animals have less surface area per unit volume, so it’s easier for them to retain heat (an advantage in cold places).
You can prove this to yourself by doing some easy maths with a cube: a 1*1*1 cube has a volume of 1, and a surface area of 6 (ratio of 6) whereas a 10*10*10 cube has a volume of 1000 and a surface area of 600 (ratio of 0.6).
Of course, this is just one factor among many, and probably most useful when comparing different members of the same species, or the average size of animals during an ice age versus the average size today. You can still have elephants in hot countries! (They just need to increase their surface area by evolving big ears...)
posted by chappell, ambrose at 9:19 PM on December 2, 2018
Smaller animals have more surface area per unit volume, so it’s easier for them to lose heat (an advantage in hot places) and larger animals have less surface area per unit volume, so it’s easier for them to retain heat (an advantage in cold places).
You can prove this to yourself by doing some easy maths with a cube: a 1*1*1 cube has a volume of 1, and a surface area of 6 (ratio of 6) whereas a 10*10*10 cube has a volume of 1000 and a surface area of 600 (ratio of 0.6).
Of course, this is just one factor among many, and probably most useful when comparing different members of the same species, or the average size of animals during an ice age versus the average size today. You can still have elephants in hot countries! (They just need to increase their surface area by evolving big ears...)
posted by chappell, ambrose at 9:19 PM on December 2, 2018
(As you can see from both my comment and the comments about the size of bones in larger animals, a lot of biology hinges on the difference between square and cube functions!)
posted by chappell, ambrose at 9:25 PM on December 2, 2018 [1 favorite]
posted by chappell, ambrose at 9:25 PM on December 2, 2018 [1 favorite]
This thread is closed to new comments.
posted by Anticipation Of A New Lover's Arrival, The at 7:11 AM on December 2, 2018 [15 favorites]