My female never hisses or puffs up, but my male does it the second I walk by his cage. He is getting better about being held, but still fights it most of the time.
The reason is probably a defense mechanism that has been maintained even though they are captive animals. Instincts never really go away... Mimicking an aggressive animal often times will scare away would-be predators. The hissing seems to mimic a rattlesnake sound, and the puffing up makes them look bigger than they are.
It's all for show, and that's why I love this species!
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Robert Charvat
1.1 het albino Western Hognose