Mass, gravity and proportionsThe most important stress that influences biomechanical structures is the pull of gravity. For example, the maximum height of an organism is given by the work of its structure to oppose its weight: E=mgh → h=E/mg. Since E (the energy output) and m (the mass) are both proportional to the volume, they cancel each other out, and therefore the height is only inversely proportional to gravity: h ~ 1/g. That means that, if the gravity doubles, the maximum height of an organism in divided in half. The same is true for the maximum height reached by jumping: since it's independent from mass, a jerboa and a kangaroo jump roughly to the same height, but if the gravity increases n times, this height will become 1/n times greater (that is, smaller).
Another consequence of changing gravity is the different burden upon the bones (or equivalent structures). The pressure the bones have to withstand is independent from the cross-sectional area, but it's directly proportional to weight, which is itself the product of mass and gravity: therefore, everything else being equal, the cross-sectional area of bones is directly proportional to gravity, and their radius to its square root (A ~ g, r ~ √g). If either the mass or the gravity double, the bones have to become √2 = 1.4 times wider.
Similar changes in the body shape can be computed in a similar way (remember that increasing the gravity n times is functionally equivalent to increasing volume by n times, and thus to increasing length by n3. For example, the torso width, neck length and leg length are all proportional to the square root of body length (~ √l).
What are the lower and upper limits of size? The smallest known organism capable of metabolic activity (therefore excluding viruses) is the parasitic bacterium Mycoplasma genitalium, with a diameter of 200-300 nm and a mass of about 10-13 kg; the smallest organism able to survive on its own (therefore excluding parasites too) in Pelagibacter ubique, about 400 nm long. The smallest known eukaryote, that needs a cell much more complex that any bacterium, is the alga Ostreococcus, 800 nm wide. Finally, the smallest known animal is the crustacean Stygotantulus stocki, 0.094 mm long. Being eukaryotes, animals need cells much larger than the minimum size; it has been calculated that a human being built with Mycoplasma-sized cells would have a mass of 50 mg and a height of 5 mm, though it's unlikely that cells that small would be able to support complex life.
As for the upper limit, largest sequoias, already built with a pillar-like shape, can weigh over 1000 tons, but they get to this size only because they don't have to move, something that puts much more stress on the structure; the largest known animals (blue whales and largest sauropods) have a mass of roughly 100 metric tons, or 105 kg. Perhaps on planets with weaker gravity the maximum mass would be higher: since mass is proportional to the cube of linear dimensions (such as height), and therefore to the cube of 1/g, we can deduce that the maximum mass is inversely proportional to the cube of gravity (m ~ 1/g3).
The limits of relative gravity on inhabitable planets are believed to be 0.2 and 2.2: that would lead leading to maximum masses of 12500 tons and 9.4 tons, respectively. Anyway, since the support that whales get from buoyancy doesn't seem to affect the result much, it's likely that the limit to size is given by other factors, such as the retention of metabolic heat and the increasingly difficult blood circulation.
GRAVITY PROVEN
GG NO RE CARDhttp://speculativeevolution.wikia.com/wiki/Speculative_biomechanicsThis post was edited by caswallen on Jan 21 2018 06:41pm