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Very straightforward question, how big can a gecko get before the van der Waal force becomes insufficient to carry their body? In life, each of a gecko's four feet has a clinging strength of up to 20 times the animal's body weight. A possible response to the lizards growing mass might be thicker sturdier limbs and wider feet but for the sake of the question let’s assume they keep the same proportions.

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Here’s a cute gecko for your troubles.

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  • $\begingroup$ Apparently it might be "a lot" given that the wikipedia entry for "van der_Waals_force" (en.wikipedia.org/wiki/Van_der_Waals_force) includes : In May 2014, DARPA demonstrated the latest iteration of its "Geckskin" technology by having a 100 kg researcher (saddled with 20 kg of recording gear) scale an 8-metre-tall (26 ft) glass wall using only two climbing paddles. Tests are ongoing, but DARPA hopes one day to make the technology available for military use, giving soldiers Spider-Man-like abilities in urban combat. $\endgroup$
    – JonSG
    Jul 29, 2021 at 22:23

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This is another case where the square-cube law comes into play: the mass of the hanging gecko scales up with the cube of the length, while the contact area which ensures the gecko is sticking to the surface only grows with the square of that length.

$M \approx l^3$

$S \approx l^2$

If we take for good your figure

each of a gecko's four feet has a clinging strength of up to 20 times the animal's body weight.

and considering that the ratio between mass and feet surface goes linearly with the length, a gecko 20 times the length of an actual gecko would be the maximum size that the gecko feet clinging mechanism could hold.

That would mean that the gecko could have a mass of about 8000 times a current gecko, with feet surface 400 times larger.

For reference with a real world animal, considering that a gecko can be about 20 cm long, this fictional gecko would be about 400 cm, or 4 m long. Good luck seeing something the size of an alligator hanging on the ceiling above your head.

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    $\begingroup$ That's really impressive for a wall-clinging lizard. The average male gecko weighs about 75 grams, multiplied by 8000 that gives us a gecko weighing about 600 kg. That would be terrifying! $\endgroup$ Jul 30, 2021 at 11:29
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    $\begingroup$ Which means that in many situations the actual limit will be the strength of the wall rather than the strength of the gecko. The hull of an oceangoing vessel or exterior of a building would be strong enough, but interior walls and ceilings... not so much. $\endgroup$ Jul 30, 2021 at 11:38

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