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I seem to have heard before that some dogs have some sort of anatomy that lets their jaws "lock," so that even when they're dead they can still be hanging on.
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Pit bulls do NOT have locking jaws. No dog of any breed has ever been found to possess a mechanism in their jaw which would allow them to "lock" their top and bottom jaw together. There is no such thing as a locking jaw!
Taken from
here. Not a very credible site, but I highly doubt that pitbulls can lock their jaws. I think they just have extremely strong jaws.