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Cheetahs are fast. You know this. But which is faster: a cheetah, or a Tyrannosaurus rex?

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Pretty much everyone over the age of four has heard that cheetahs are the fastest land animal. These long, lean cats can accelerate from zero to 72 kilometers per hour in 2.5 seconds, which is ridiculously fast.

Faster than a lot of cars. But if you stop to think about it, it kind of seems like a /bigger/ animal ought to hold the land speed title. Pit a T. rex against a cheetah, and wouldn't the T. rex leave the cheetah in the dust?

The T. rex had longer legs and more muscles than a cheetah, so it seems like the cheetah would lose the race /and/ get devoured at the finish line. But alas, T. rex was probably as useless as a sprinter as it was at playing piano with those tiny little arms. And that has to do with the way muscles work.

Skeletal muscles are made of bundles of fibers, and those fibers come in two kinds: slow twitch and fast twitch. Slow twitch muscle fibers contract slowly, and are used by the body for activities that require endurance. Fast twitch muscle fibers contract more quickly, and are good for rapid actions like jumping or sprinting.

Slow twitch muscle fibers can keep doing their thing for a long period of time, while fast twitch muscle fibers tend to get fatigued pretty quickly and use up a lot of energy. Big animals have more muscle mass, and therefore more fast twitch fibers. So logically, big animals /should/ be faster than smaller animals, but there's a catch.

The fast twitch muscles of larger animals run out of fuel a lot more quickly than muscles of smaller animals. Because their mass makes them heavier, it takes longer for a large animal to accelerate. So some biologists think large animals are simply incapable of reaching their potential top speed -- because they use up all their fuel before they can get there.

So in terms of speed, it's not the big animals that rule, and it's not the small ones, it's the medium-sized ones. In fact, in 2017 researchers hoping to answer this question published a model for predicting maximum speed in the journal Nature Ecology & Evolution. The model uses body size alone to make predictions about animal speed, and it's accurate not just for land animals, but for birds, insects, and fish too.

They developed this maximum speed model to answer questions about why there seemed to be an upper limit to animal movement, and what the consequences of those limits were for individual species. In other words, the model can help us understand more about why some animals traded speed for size, and vice versa. The relationship between size and speed was so robust, they can even use it to predict the speed of /extinct/ species, even though we'll never see them in motion.

Like Velociraptor, which it predicts would have been roughly twice as fast as T. rex. Cheetahs are the clear winners of the modern land speed game, even though they are roughly the same size as some other slower animals, like the leopard. The difference between cheetahs and their slower cousins can be found in certain physical characteristics that give them an edge.

For example, the cheetah's spine is longer and more flexible than the leopard's — or any cat species — which lets it maximize the length of its stride. Other physical characteristics, like paw pads that provide traction, a large heart, liver, and adrenal gland, an aerodynamic body, and swiveling hips give it the edge it needs to claim the “fastest land animal” title. So a cheetah could definitely outrun a T. rex, but take heart.

If you put the two species together in an ultimate fighting ring, the T. rex would win every time. Just as long as it doesn't have to outrun anyone. Or play the piano.

Thanks for asking, and thanks to our patrons for bringing you this answer. We are so grateful to have the coolest community of supporters out there. If you'd like to join them, check out [ ♪OUTRO ].