@MaddieM4 In the grander scheme of things we'd probably have to classify birds as a kind of reptile. If crocodiles and lizards are both reptiles then so are birds: Crocodiles are more closely related to birds than they are to lizards.

Anyway, the problem with breathing while moving is specific to lizards and tuataras. The phenomenon is called Carrier's Constraint (after a paleontologist who documented it in prehistoric animals): Animals that flex their bodies side-to-side while moving are going to constantly compress one lung while expanding the other, pushing stale air back and forth between lungs rather than expelling it. It doesn't help that most lizards don't have dedicated respiratory muscles - given the aforementioned constraint, they wouldn't actually help much.

Other reptiles don't have this problem: Turtles don't flex their bodies at all (and breathe using a sling-like muscle wrapped around their lung - obviously, they can't expand and contract their chests!), snakes only have one lung, and archosaurs (crocodilians and birds) have unidirectional lungs - air travels in a single continuous loop through their lungs and out again, rather than sloshing back and forth into a bunch of "dead-ends" like in mammal (or lizard) lungs. Birds use air sacs both to pump and to keep stale and fresh air separated; crocodiles use valves and muscles*. It's a pretty sophisticated system, arguably more advanced than what we mammals have.

Monitors are lizards, and move like lizards - but they've solved the problem by evolving a large bellows-like pump in their throats, with which they can force fresh air into their lungs (which then expels stale air). They've also separately evolved a variant of the "looping" structure of archosaur respiration.

Some agamids and iguanids have a pragmatic quick-fix: They'll get up on their hindlegs and run bipedally, which avoids the side-to-side flexing.

@MaddieM4 *) Crocodilians have an *extreme* degree of control over their respiration. All the muscles they use for respiration are under the direct control of their central nervous system, rather than the autonomic nervous system - in other words, breathing is always conscious act for them.

This is useful if you live in water and sometimes get into fights where you might be knocked unconscious for a short while every now and then: It's better to stop breathing than to immediately inhale a lungful of water and drown.

According to Igor's reptile vet, this also makes them very difficult to work with if they're not cooperative: If you sedate them, they stop breathing. If they need surgery, they must *always* be fully intubated.

@MaddieM4 Yep.

Fun little detail about him: He once mentioned in an off-hand remark that the only animal bite that had sent him to the hospital was a housecat ... and he'd been bitten by agamas, pythons, iguanas, turtles, monitors and one crocodile.

"Wait.... you've been bitten by a *what*?!" :-P

All Danish reptile vets are on-call for the zoos, and he once had to come look at a young crocodile with an upset stomach. It bit his arm and didn't let go for almost 10 minutes - and as he said, there's not really anything you can do but wait - it's impossible for a human to force even a small crocodile's mouth open.

(Though I am somehow more impressed by the fact that this guy has performed eye surgery on snakes. It is amazing to me that there are people who can and do surgically treat eye disease in snakes.)