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A yukkuri wiggling its bottom. Tends to be associated with it trying to get into a tight spot or it being held up (as in attempting to get loose). A panicking yukkuri might also wiggle it's bottom as they're trying to run away, though it's unclear if it actually makes them move faster or not
Yukkuris also wiggle their bottom when they are happy (e.g. they eat a very delicious food), as part of a dance, and sometimes as a form of exercise.
There are some instances when shithead yukkuris wiggle their bottoms to taunt or insult another yukkuri or a human which almost never ends up well for them. These bottom wiggling taunts are usually accompanied with them declaring how easy and beautiful their bottom is, or sometimes even demanding the insulted yukkuri/human to lick their anyaru
Also seen when a yukkuri's bottom is separated from their head in which case the bottom will autonomously wiggle similar to when lizards lose their tails.