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I've seen them yawn quite a few times. They are stationary on the rock and suddenly they opened their mouth and yawn.

Oh mine. They have big mouth.

Quite a magnificant sight. Too bad I do not have camera on hand.

Yeah I've seen mine yawn a few times. But it happens so quick and as soon as you're scream "look!" it's all over. haha

If it happens frequently or regularly this could be a sign of illness, RI. Infrequently or seldom is OK. Do they seem healthy otherwise? How often do they yawn?

I've caught mine do it on occasion. They seem to do it when they have been basking for long periods of time and are half asleep on the dock.

They can yawn underwater too, and yes they sure do have big mouth
Always wish I could shoot a picture of my turtle while he's yawning.

When they yawn does it mean that they are sleepy?
how long a day do they usually sleep?

I saw mine yawn once. As soon as he got on the basking dock, he turned, looked at me, and yawned. As if to say I bore him or something.

I don't know how long my turtle sleeps, been trying to figure that one out.