Mikey skeletal anatomy! Click for better quality
Box turtles are the kinds of turtles who can fully fit into and close their shells along a single hinge in their plastron, which once closed is extremely hard to break into for any kind of predator especially if they cannot get a proper grip on the dome of the shell. Box turtles are also usually found in plains rather than ponds, and while they need water to survive and keep hydrated and can swim well, they’re not generally considered aquatic. Box turtles do have an interesting adaptation due to their more northern range of being able to survive freezing temperatures for a short period of time by slowing their metabolism outside of seasonal brumation (similar to hibernation, but more of a dormant state) if absolutely needed.
After watching TMNT ‘03 where the turtles meditate to save oxygen and prevent suffocation in a broken airlock, I thought, why not mix their ninja ability to conceal their body heat and breath and mix it with a turtles natural ability to brumate? Perhaps the brothers could enter into a completely dormant state on command with enough training in order to survive extreme situations or astral project in some form. Just a fun thought to tie their strong mystic powers with their physical abilities!
Although I use the term heterothermy to refer to them being able to vary between generating their own body heat while active and relying on environmental temperature while inactive, the intricacies of understanding thermal regulation in animals eludes me. If there’s a term that fits better, go ahead! I’m not a biologist so you probably know better lol
These are all my personal head cannons! You can use em’ as reference or not, whichever you prefer :)