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My world has creatures called "Puff Runners", which are theropod-like reptiles that have beaks rather than teeth, are armless or have tiny useless arms, and they mate for life.

One of the special things about "Puff Runners" is their distinctive gular pouch (similar to that of a frigatebird) which is covered in spikes (like a porcupinefish). Both male and female Puff Runners have them.

They don't use their gular pouch to attract mates but rather use them to especially protect their eggs and chicks (and sometimes in personal defense). First they intimidate the attacker by inflating the pouch, then they charge and ram them with their spiked gular pouch.

So my question: is my "Puff Runner" biologically plausible? Would they able to harm others without harming themselves?

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The gular pouch of a frigate bird is rather soft. It would be unlikely to be able to support spikes firmly enough for its inflation to be able to drive the spikes into another creature, and its lung power would be unlikely to be sufficient to perform the task if the pouch did have the required properties.

To be able to be inflated, a pouch would need to be sufficiently flexible and stretchable so as to be able to be inflated by lung power. To support spikes would require a rigid surface to which to attach them. These would appear to be contradictory requirements.

While puffer fish have a similar mechanism, this occurs in water. Water is effectively uncompressible when compared with air, which allows the water to form an effective part of the mechanism. Since air is highly compressible, and maximum expiratory pressure is likely on the order of 100 cmH2O, which corresponds to 1.4 psi or 9.8 kPa, this is a very low pressure for such an application.

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It Works.

Scales on the soft surface, that lie flat when uninflated but when inflated lift up and present a spike.

Single spike on a large-ish base surface makes for a reasonable support for the spike. Something very much like the Pufferfish.

The primary difference would be that the spikes would need to be restricted to a smallish area, so that they do not present a direct danger to the Puff Runner itself. And as the pouch would be inflated with air not water, it would not present quite as formidable an obstacle. A spine-covered pillow, rather than a spine-covered hard ball as the Pufferfish manages.

P.s. Remember that a Pufferfish's spines are not mere mechanical menace, they also contain some of the most lethal poison known to the biological world.

Relaxed:
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Not so relaxed:
enter image description here

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  • $\begingroup$ it works only if the thing they are attacking has incredibly thin skin, you would need incredible pressure for the ouch to support the spines against significant loaded needed to pierce hide. $\endgroup$ – John Jan 1 at 15:13

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