The common ancestor’s inability to hear its own hissing does not provide any explanation for why it would have developed hissing in the first place. We need an answer that explains why hissing was advantageous for the common ancestor, even though its predators couldn’t hear it.

Even if the common ancestor defended itself in other ways, we still need an explanation for why the common ancestor developed hissing as a threat device even though its predators could not hear it. There must be some other reason for using hissing as a threat device.

This gives a reason why the common ancestor’s hissing might have been useful against predators: it made the animal seem larger. This explains why it developed hissing sounds, even though its predators couldn’t hear them.

Whether hissing was energetically costly does not explain why the common ancestor used it as a threat device even though predators couldn’t hear it. We need an answer that gives a different reason for using hissing as a threat device.

Even if the common ancestor had few predators, it would still have needed to defend itself against those predators. So, we still need an alternate reason that explains why the common ancestor developed hissing as a threat device.