People commonly ask about how large an animal can get, and the answer is that they can be pretty enormous, but there is a limit. There could never be an animal as large as Godzilla, for example.

This is because of the cube-square law, which shows that volume increases at a faster rate than area. Once an animal gets too big, their volume is just too much to sustain. An animal the size of Godzilla would need to consume far too many calories to support its body than it could actually get in through its mouth. That is even assuming its legs could somehow support the massive weight.

Even if an animal evolved to somehow overcome those issues, a mammal with that much volume would quickly overheat and die because its surface area could not get rid of heat fast enough. A reptile or other cold-blooded animal would likely freeze since the outside surface area could not get enough heat into the body.

So, that explains why animals (especially mammals) can’t become overly massive, but what about the other direction? How small can mammals be?

Well, there is a limit here as well and it is also due to the cube-square law. Scientists don’t know the exact minimum size that a mammal must be, but the Etruscan shrew (Suncus etruscus) is pretty close to that limit weighing just 1.8 grams (.063 ounces). It is as heavy as one playing card.

An extinct animal, the Batodonoides vanhouteni was slightly smaller at 1.3 grams (.046 ounces) and it is the smallest animal ever known. It is unlikely that any smaller ones will ever be discovered, at least not on Earth. Paul David Polly is a vertebrate paleontologist at Indiana University-Bloomington and also a Research Associate at the Field Museum in Chicago. He explained:

“The tiny Batodonoides was operating near the minimum size limit imposed by mammalian physiology. At this size, animals are hard-pressed to gather food fast enough to maintain a constant body temperature.”

These animals are very small and that means that they have a very small volume of mass compared to their surface area thanks to that cube-square law of nature. This means that they need to run a very fast metabolism to create enough heat to sustain their life.

The Etruscan shrew has been studied and it was found that their heart beats at a rate of 25 beats per second in order to keep up with its needs. It has to eat about 25 times per day and take in more than its body weight of food just to survive.

Fortunately, they typically live in an area where food is abundant and they reproduce extremely rapidly for mammals, which has allowed them to survive so far. Any significant change to their environment, however, could be trouble.

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