If you've ever seen a lizard move, you may have noticed that it runs, then stops for a bit, then runs again.

Well, it turns out that lizards—some of them, anyway—can't run and breathe at the same time.

And this seems to be an evolutionary leftover from the time before the ancestors of four-legged creatures adapted to life on land—

one that different groups of animals have found different ways to get around.

For a lizard, running and breathing require the same muscles.

To run, a lizard has to contract its chest muscles one after the other to help it scamper forward.

And to breathe, it has to contract those same muscles in different ways.

And it can't do both at once.

In a 1997 study published in The Journal of Experimental Biology, scientists put iguanas on treadmills.

And they found that the faster the iguanas ran, the harder it was for them to breathe, which doesn't make a lot of sense.

If you're fleeing or chasing down prey, wouldn't you need to keep breathing so you can keep running?

Well, a 1987 paper in Paleobiology suggested that this flaw in lizards' body plan is an evolutionary holdover from when some vertebrates transitioned from living in the sea to living on land.

The paper said that early tetrapods—a group that today includes four-limbed reptiles, amphibians, birds, and mammals—may have evolved from fish that lived in oxygen-poor waters and were only able to swim in short, rapid bursts.

Sort of like how modern lizards move.

And while those fish could breathe pretty well in water, that wasn't quite the case for their descendants adapting to life on land—

despite having a similar way of moving.

It turns out that walking on land takes up a whole lot more energy than swimming does.

Also, that limitation of not being able to run while breathing may have forced these early ancestors to rely more on intense but tiring bursts of movement.

But lizards have adapted to handle this as best they can.

Even if they have to stop and catch their breath, when they do move they can scurry around really fast—

which may help them maximize what they can get out of a muscle configuration that limits their oxygen.

Other lizards have also evolved better ways of breathing while running.

For example, monitor lizards doesn't seem to have a problem breathing while running, even when they're running at a decent clip.

A 1999 study published in Science found that this ability may be due to gular pumping.

That is, monitor lizards—and some other lizard species as well—can expand and contract their throat to pump more fresh air into their lungs.

The scientists in the study showed that when monitor lizards were experimentally prevented from doing this throat pumping,

they breathed similarly to lizards that don't have that evolutionary advantage.

And these sorts of adaptations aren't just limited to lizards.

Most tetrapods have developed ways of breathing more effectively on land.

The diaphragm muscle in mammals, for example, pushes air in and out of our lungs.

Crocodiles have these as well.

And over time, birds changed their locomotor posture, or the way they position their body while moving around, so that they didn't need to use the same muscles for breathing and moving.

Finally, horses and some other animals that can trot or run, like guinea fowl, breathe in a rhythm that coincides with their movements.

This motion helps them move air in and out of their system more effectively.

So while lizards' start-and-stop running pattern may seem like an evolutionary problem, it's actually a side effect of evolutionary innovation.

Since then, animals have adapted through changes in their anatomy, behavior, and overall just making the best, most efficient use of the bodies that they have.

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