A few personal skeleton headcanons that I use in my writing (featured in 'I Am Their Alpha, my latest Undertale fanfic)!!!
- Skeletons do inhale and exhale, and they do need to inhale and exhale (less so than creatures with physical bodies). This allows them to inhale and exhale surrounding magic (which the Underground is full of). They also can experience dizzy spells in places with less magic, but those places are rare since even human cities are filled with unacknowledged magic.
- Skeletons do not need to pop their joints as humans do. Pockets of magic sit between skeleton joints, which do not make popping noises when stretching as it does for humans. Skeletons do make popping and rattling noises during sex, however. So, a human suddenly popping their joints is similar to someone suddenly moaning in a lewd manner.
- All skeletons can manifest either female or male body parts for sex. Though, most skeletons have a preferred gender that they settled after puberty so it's rare that they'll change which gender they summon. Same with voice, the color of their magic, font style... etc. Skeletons will naturally develop these traits, and won't change them since these traits will become a part of their soul.
- Skeletons have a strong sense of smell. For example, if they came across a human's bed they would know the human's gender and be able to guess if the bed had been slept in recently. Similarly, if a human attempted to lie and say that their dog had slept in the bed last night the skeleton would be able to tell if the animal that slept in the bed was in fact a cat.
- Skeletons do sweat, but it consists of magic. They also get a surface layer of dead bone that needs to be scrubbed or washed off, similar to how humans get dead skin or dandruff.
- Skeletons do not have blood, but they will leak marrow. Similar to how humans' oxygen levels will decrease when bleeding, skeletons lose both HP and magic fast when leaking marrow.
