Passage Summary: Birds and reptiles likely inherited hissing from an ancient ancestor who used it to scare predators, even though those predators were deaf to the sound at the time.

Reasoning: The passage notes that hissing likely evolved in a common ancestor as a threat, yet the predators of that ancestor were unable to hear hissing sounds.

Analysis: The paradox here is evolutionary: why would a trait evolve specifically as a 'threat device' if the intended audience couldn't even perceive it? To resolve this, we need a piece of information that explains the utility of hissing in a non-auditory context or for a different audience. Look for an answer that suggests hissing provided a survival advantage that didn't depend on the predator's ability to hear. Perhaps the physical action of hissing mimicked a different, more dangerous animal that the predator could see or sense in another way.