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The smallest known vertebrates are frogs that range down to 0.01 grams. I would take that as evidence that at smaller sizes an endoskeleton is more of a liability than an asset.

1-2 grams is a lower limit of what has evolved across all phyla of warm-blooded animals. Are there physical reasons that this would be the case?

One I can imagine is that any smaller and the surface-area/volume ratio makes it too expensive to regulate body temperature, but that's just a broad guess.

feetwet
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    This is a guess, not a proper, sourced answer, but my bet is that below ~2 grams is where a warm-blooded organism has so much body surface area relative to internal volume that it can't eat quickly enough to make up for body heat loss. It's why shrews eat several times their body mass daily. – KEY_ABRADE Apr 16 '23 at 19:24
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    @KEY_ABRADE so that would be a sort of surface/volume constraint, but on digestive-system processing capacity rather than heat regulation. Sounds plausible, given that (AFAIK) hummingbirds consume almost pure sugar, which I assume is the most efficient/concentrated means of getting bio-available energy. – feetwet Apr 16 '23 at 19:29
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    Is there any reason to believe that the current smallest known vertebrates actually represent the lower bounds? We know that the current upper bounds are not the largest ever (for terrestrial mammals at keast). – kmm Apr 16 '23 at 21:01

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