The act of Puff Puff is a yukkuri defensive measure. Like with the puffer fish and certain lizards, the yukkuri sucks in air to make itself larger in order to look more intimidating to would-be attackers/threats.
Puffing up is generally effective against most animals. Animals generally judge strength by size, thereby a large animal is a strong animal. So, by increasing its size, a yukkuri can fool a animal in theory. There are many cases, however, of yukkuri who are still slain or eaten despite puffing up.
Puffing up (or doing "puff-puff") tends to anger humans more than anything else in Yukkuri behavior. This is mainly because the yukkuri adds provocative behavior to its puffing up, with "I can beat you with my both hands sawed off"-style speech. By combining the two, they tend to provoke humans to feel challenged to a fair duel by a thing they generally consider to be just a manjuu. Therefore, puffing up generally causes the human to get annoyed enough to actively kill the yukkuri.
With Pet Yukkuri, the yukkuri may puff up in order to protect its owner, believing its strong enough to fight off a would-be attacker. Pet yukkuri tend to puff up at other pet yukkuri, trying to establish a form of dominance. Much like how a regular yukkuri would puff up at a passing human, some pet yukkuri's have been known to puff at their owners if they are displeased. While rare in properly trained pets, new pet yukkuris, pets that were once wild, or shitheads are well known for puffing up at their owners and saying rather cruel things. While the proper thing to do is punish the yukkuri, some owners will outright kill their pet if it puffs at them, either because of annoyance or because they believe puffing up is a sign of a "shithead".
There has been reports of people choking to death after eating a living yukkuri whole, which puffs up in the throat and blocks the airway. Also, due to their makeup, melting only makes things worse. Therefore, it's advised, if one is going to eat a yukkuri, that said yukkuri be dead, properly chewed up, or, to save the trouble, just don't eat a yukkuri.
Some yukkuri are known to puff-puff against other yukkuri, mainly to intimidate their opponent or to show displeasure. While smaller yukkuris and koyukkuri are intimidated right away, most yukkuri don't seem to fear puffing up.
It has been seen in koyukkri that they will puff up at their parents if they are offended or feel upset. Most of the time the parent yukkuri are taken back by this act, believing that their child's despleasure is a sure sign they have failed as a parent. However, on some occasions, the parent yukkuri will puff back at its own koyukkuri or even punish them.
Against Inanimate objectsEdit
Being that they're young, most koyukkuri grow out of this habit as they age. Though it has been reported that yukkuri new to cities are seen puffing up at cars, which usually leads to the yukkuri being run-over by said car.