Who's the authority on salamanders and newts here?

[Peter]

Yawning crocodiles:





I'd guess stretching evolved as an elaboration on yawning, maybe as tetrapods became more active and flexible and put higher demands on the rest of their skeletal muscles beyond their jaw muscles.  In newts and salamanders, the jaw muscles are the only ones that need to deal with prolonged and intense exertion; they often clamp down on a large worm and spend 30-60 minutes slowly eating it, exerting continuous pressure on it with their jaws to hold onto it and periodically shaking their head from side to side to keep the worm from burrowing into the soil while they gradually swallow more and more of it.  If they get exhausted before they're done, the worm escapes.  With the albino axolotls, the jaw muscles become visibly pink from the increased blood supply to them when they're wrestling a worm. 

All the other muscles have quite modest demands placed on them, and don't need to function for long periods or at high intensities.  Locomotion in newts and salamanders is characterised by very frequent resting; they might walk or swim for a few seconds or a few tens of seconds and then rest for at least as long, and they don't keep it up for very long even with the frequent breaks.

They're also not all that flexible; they can't lick their own cloacae like dogs and cats and they don't have much dynamic range to their movements.  Their bodies are kind of intermediate in flexibility between fish and mammals.

[ZEGH8578]

Thanx, that's all sensible input. I didn't realize amphibians were so handicapped, compared to even lizads, whom can be quite active, but still require that lengthy rest afterwards. Like you imply, stretching may be something unique to warm-blooded, fully active creatures. Cows walk non-stop while they graze, chickens walk around non stop, both are warm blooded.
That 2nd crocodile's yawn is awesome btw, it looks like he's really trying to urge it on

[Peter]

The life-strategies of amphibians are a bit different from lizards.  They're mostly adapted for colder temperatures, so they don't have the comparatively high metabolic rate of a lizard that's been basking in the sun, and instead of having claws, teeth, scales and the ability to quickly escape from predators, they rely on hiding, being nocturnal and having toxic glands in their skin, often with warning colouration, either all over the body or just on the belly.  The ones that just have warning colours on their belly have an unken reflex where they show off their underside when threatened.  Some of them will roll over and play dead, too. 

They also have much less sturdy and less well developed skeletons than lizards that don't provide much in the way of attachment points for muscles and wouldn't stand up the forces of large, powerful muscles acting on them.  Here's a skeletal comparison:

[ZEGH8578]

Indeed, most striking is the vertebral collumn! I have seen many drawings and schematics of amphibian skeletons, but I always assumed they were simply flawed or rushed drawings, or I just failed to appreciate the implication of the much more delicate skeleton. Seeing how slowly a salamander walks across flat ground, compared to the quick, and energetic scurrying of a lizard, it does make sense of course.
I love still learning new stuff!