Exactly what it says on the tin: what would be some biological features of a humanoid that can fit through a gap roughly the size of their head (give or take ~25% of their head's diameter)?

I want a creepy, heavily biomodded character that can actually fit into, say, air vents and wiggle around in them like snakes, but looks relatively normal if they want to.

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    $\begingroup$ Newborns can fit through a hole roughly the size of their head... pnas.org/content/112/18/5655. Adult humans exist in all shapes and sizes en.wikipedia.org/wiki/Dwarfism. Does your story require the character to be creepy and heavily biomodded or will a small human work just as well? $\endgroup$
    – Poseidaan
    Sep 25, 2021 at 6:52
  • $\begingroup$ @Martin-ReinstateMonica I need an adult-sized human, sorry. $\endgroup$
    Sep 25, 2021 at 7:09
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    $\begingroup$ @Martin-ReinstateMonica the thing about newborns is that their body is designed to go through one hole at the expense of a million other things, and its not under their own power that they can do this. Having witnessed it fairly recently, the power the mother has to bring to push the child through and the time it takes would not work to begin with. And people with dwarfism would be even worse as their skull would have grown together while their bones have much more clacium to elastine wich makes it much tougher (which is good for adults so you can move fast). $\endgroup$
    – Demigan
    Sep 25, 2021 at 7:56
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    $\begingroup$ This is pretty easy if you allow me to scale up the head. Is this allowed? $\endgroup$ Sep 25, 2021 at 14:35
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    $\begingroup$ @JohnDvorak No. I basically need something proportionate to an adult human but with some form of internal biological change or a change in the makeup of their muscle/bone that lets them do this. $\endgroup$
    Sep 25, 2021 at 14:44

2 Answers 2


Cleidocranial dysplasia to bring the shoulders together.

hypermobile shoulders

Cleidocranial Dysplasia - A Case Report

Persons with the syndrome sometimes lack collarbones which gives them hypermobile shoulders. I understand that cats (except lions) also lack collarbones which accounts for some of their axial hypermobility.

Your person can flex and fold the pelvis because he or she has hypermobile pelvic joints.


There are 3: the pubic symphysis in front and the two sacroiliac joints. If these are hypermobile the sides of the pelvis can move relative to each other. There is not a muscle to pull the sides of the pelvis together like the person above had pulled her shoulder together, and so your character would need to fold them in manually then fold them back out.

That does not feel good when it happens to hypermobile people and some have surgery to lock the pelvis in place with metal. That is how your character is biomodded: it does not hurt her to flex in these ways and she can pop back into place.

Mostly she uses her ability to rescue children and animals who have fallen into tight spaces. And she plays bass (sometimes sitar!) and sings backup in her band.


Substitute some bone for cartilage.

Your ears retain their shape, but can also be folded or deformed pretty easily. It's because they're supported by cartilage.


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