On an Earth-like planet, I'm imagining a bipedal sapient species whose juveniles have a largely hollow head so large that they can retract the whole body into the head when frightened, the way a turtle or snail can retract into its shell. They lose this ability when the rest of the body grows rapidly during puberty. Plausible?
Subpoints to consider:
- How big would the head have to be to allow this? My first guess is a round head roughly twice as big across as the height of the body below the collar bone. (Compare)
- How would the body parts fit together to leave room in the head for the body? Consider how much attachment would be needed to hold up a head that big.
- How would such a child reach around himself for hygiene and dressing? Or would they be dependent throughout childhood on parents and parent-created tools?
- How does the plausibility change depending on the adult height of the creature? And would it be more plausible underwater with a dolphin tail than with land bipedalism? (Compare)
- Finally, what in evolution would select for this ability? On the one hand, I know intelligent juveniles have to stay small while still learning to conserve energy, so that might lead to the small body. On the other hand, the difficulty of giving birth would appear to put the brakes on evolving a large enough head.