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.
  • $\begingroup$ What's the size of this bipedal sapient species in adulthood? Are you just looking for ratios? Plausibility will change greatly if they are humans sized compared to mouse sized. $\endgroup$
    – Samuel
    Nov 30, 2015 at 22:28
  • $\begingroup$ You have a point about square cube issues. I'll add that as a fifth point. $\endgroup$ Nov 30, 2015 at 22:45
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    $\begingroup$ So basically, they need to carry a suit of armor around their head such that, if needed, they can pull everything up inside that suit of armor? What about approaching the problem backwards? Allow the children to huddle into some ball-ish shape, and then figure out what you'd have to do to armor that shape while still leaving a hole for them to stretch out from. Seems to me like they'll be unwieldly little mongrels. You may have invented the first species which becomes less clumsy at puberty! $\endgroup$
    – Cort Ammon
    Nov 30, 2015 at 22:57
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    $\begingroup$ @tepples, I'm thinking it should be underwater if you want this system of them being able to retract into their heads to work. They are just too top-heavy otherwise. A leathery-shelled torso like armadillos have might be more feasible if you want them to be land-dwellers. Or the head and torso could be the same. I'd also expect them to have more than two legs and not to stand upright. $\endgroup$ Dec 1, 2015 at 7:52
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    $\begingroup$ I'm just imagine regular human kids with retractable heads and I'm legitimately terrified. $\endgroup$
    – JD Solomon
    Dec 4, 2015 at 14:44

2 Answers 2


Have the children be a sort of snail or octopus, that can retreat by oozing its way into the shell. During puberty, the bones form/develop, blocking off this ability, or possibly the whole form hardens and is thus unretractable. A good question is whether or not this shell ever grow. Does it weaken and become more expandable as the brain gets bigger?

Another thought: perhaps these creatures lay really massive eggs that grow quickly and harden, but not before the attached baby breaks off the bottom part and grows out. If so, this structure could be rather large, maybe torso-size.

  • $\begingroup$ Note that I'm skipping bones entirely and making them something that could actually pull it off. $\endgroup$ Dec 1, 2015 at 18:02

With bones, the creatures would be very like us, but capable of holding a larger weight, thicker bones, etc. They would be skinny with double jointed limbs. To fit into the shell, they would simply curl into the fetal position. They would have to have eyeholes in the shell, or else some sort of tremorsense, to detect things. They would also have something on bottom, like my first example, something hard like shelled feet. This would be used as a sea snail uses its plug, and a hermit crab its large claw. Something protective from the bottom, and possibly equipped with a deterrent like claws.

On the other hand, more turtle-like, they would have to include stunted-looking pre-folded limbs, and rather stubby.


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