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There have been a number of questions focusing on mythical creatures where the logical approach to answering has been to scale up an existing animal. For example dragon's wings can be extrapolated from other flying animals or the speed of an insectoid creature from insects in our world.

However evolution has proved that animals do not simply scale up and down, mammals do not grow beyond a certain size unless they live exclusively in the oceans. Insects do not grow to several metres in length.

I believe that other ratios come into play such as power/weight and volume/skin surface area when it comes to whether creatures could continue to survive, move/fly and stay warm/cool at a larger scale.

Am I right? Is it an over simplification to say "A bird with a length of A has a wingspan of B therefore a dragon which is C long will need to have a wingspan of D"? What other factors come into play when scaling up real creates to simulate new ones?

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    $\begingroup$ You know this is almost a meta topic of whether this model of answering works... $\endgroup$ – Liath Sep 19 '14 at 8:04
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    $\begingroup$ I know I've seen a popular-science-magazine article recently that dealt with this exactly. I'll see if I can find it during the weekend. In the meantime, the gist was that no, you cannot simply scale animals up in size, IIRC because the stresses placed on various parts of the body for supporting the weight of certain parts of the body becomes a limiting factor, along with the ability to oxygenate and circulate the blood. $\endgroup$ – a CVn Sep 19 '14 at 8:05
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    $\begingroup$ I suggest reading "On Being the Right Size" by J. B. S. Haldane: irl.cs.ucla.edu/papers/right-size.html $\endgroup$ – bdsl Feb 2 '15 at 15:13
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You can scale them up to a certain extent, but there are a lot of limiting factors.

Weight

The main thing is increasing weight - bone and flesh and sinew and muscles no matter how thick are only able to cope with a limited amount of weight. Eventually the creature just cannot support itself against gravity.

This is one reason very large animals tend to be aquatic, the support from water reduces these problems.

This is compounded by problems of scale though. If you double the size of an animal then the following things happen:

  • The bones get twice as thick (four times the cross-section).
  • The animal gets eight times as heavy (as it has got twice as wide, twice as tall and twice as long).

This is known as the Square-Cube Law.

You can immediately see that this isn't sustainable, you are supporting 8 times as much weight with four times as much bone.

Up to a certain size you can compensate for this by proportionally making the bones thicker, the muscles stronger, the legs shorter. For example compare the proportions of an elephant and a horse. Look at how much thicker the legs are on an elephant. Elephant legs - Source: Wikipedia Horse legs - Source: Wikipedia

After a certain scale you just can't do that any more, so you need to increase the strength of the material used to make bones - which would have other evolutionary costs and is still limited - or fundamentally change the design of the creature. For example multiple legs on a long and thin body would support the weight, but raises the question of what the long and thin body is for. Is there any evolutionary advantage to dragging and supporting all that extra body around? Unless such an advantage was found then the creatures would never grow longer than they need to.

Blood Pressure

As animals grow larger it becomes harder and harder to pump blood around and get it where it needs to be. The heart has to grow larger and work harder or you would need to have and synchronize multiple hearts.

Giraffes for example both have twice our blood pressure and have special muscles in their necks to keep blood flowing to the brain. This allows them to stay conscious even when they put their head to the ground to drink then raise it into a tree to feed.

Reflexes

It takes time for signals to travel along nerves, the very fastest send signals at 250mph but most are much slower. If an animal grows too large there will be a substantial time lag between stimulus and response. Either that or you need localized decision centers and at that point you need to start deciding whether you still have one creature with multiple brains and hearts or a colony of separate but inter-dependent creatures.

Heat

Another consequence of the Squre-Cubed Law is heating. The largest mammals have large ears and need to rest or get into water often to avoid overheating, while the smallest mammals have to almost constantly eat and move to keep their body temperature from dropping too low. This is because the surface area of the body which loses heat is increasing based on the square of the size, while the volume of the body which is generating heat is increasing based on the cube of the size.

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  • $\begingroup$ Great answer! Speaking of giraffes I love the Recurrent Laryngeal Nerve factoid en.wikipedia.org/wiki/… $\endgroup$ – Liath Sep 19 '14 at 9:59
  • $\begingroup$ It's the cross section, not the diameter that's important for structural strength. Doubling linear dimensions gives 8 times the load and 4 times the strength to support it, not 2. The load is still getting bigger faster than the strength, but not by as much as you've said. $\endgroup$ – smithkm Sep 24 '14 at 5:59
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    $\begingroup$ Don't forget heat. The largest mammals have large ears and need to rest or get into water often to avoid overheating, while the smallest mammals have to almost constantly eat and move to keep their body temperature from dropping too low. $\endgroup$ – vsz Sep 30 '14 at 20:38
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    $\begingroup$ And food / sustenance. The larger the creature, the more it needs to sustain itself. If the creatures' food source can't replenish quickly enough, they won't survive. $\endgroup$ – Jay Neely Oct 5 '14 at 18:12
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    $\begingroup$ Going in with the "heat" answer: remember, if it gets too massive, the pressure and heat will become such that it will generate its own gravity and have a molten core. Now, THAT would be an awesome animal. $\endgroup$ – ilinamorato Nov 13 '14 at 16:41
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To add to Tim B's answer, rather than to compete with it:

Eyes:

Eyes do not generally scale with the rest of an animal. If you examine a mouse, it has eyes that are proportionally bigger than those of a cat, which are proportionally bigger than those of a human, which are proportionally bigger than those of a whale. Eyes have an optimum size, above which there is no significant advantage in a further increase in size, however when smaller than optimum, any increase in size is significant.

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    $\begingroup$ Yes, that's a good point. The size of eyes is mostly related to the wavelengths of light they need to focus. $\endgroup$ – Tim B Sep 24 '14 at 16:03
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    $\begingroup$ The advantage of big eyes is the ability to gather more light, ie see well in dark places. Owls, lemurs, and most spectacularly the giant squid. Whales have small eyes because they "see" at depth using sonar. $\endgroup$ – nigel222 Feb 25 '16 at 22:59
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Insects several meters in Size

Dragonflies were able to have a much bigger wingspan 300 million years ago (thanks to a much higher oxygen content in the atmosphere, plus warm as today in the equator), animals can actually be scaled quite a bit up.

In this case the limiting factors were feeding the organism enough oxygen to sustain itself, and provide it with enough heat to keep up its functions.

But the scaling up reached its limits here in the oxygenation of the tissue (though the free oxygen was multiple times higher than now), just as the growth of mobile organisms is limited by gravity and the ability of a pump (heart) to distribute enough oxygen in the biomatter for it to stay alive.

If a giant was to exist, it would die from the fact that its heart, rushing to pump blood to its head, would burst trying to stem against the pull of the planet. Only on a planet with weaker gravity could such gigantisms be observed.

http://en.wikipedia.org/wiki/Meganeura

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I would suggest taking the appearances defining the creature and model it on a creature of the size you are looking for.
For example if you were taking a husky and scaling it up to the size of a horse, take the horses proportions and then change what you need to, (tracing paper works well here) shortening the neck, changing the tail...and so on.
Before everyone rants at me for not being scientific, this works as a basis and a broad overview.

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