In my fantasy world I have a species of reptilian humanoids.

  • Their body mass, lifespan and overall intelligence is about the same as humans. This probably means similarly sized brains.
  • They are NOT cold blooded. They have a human-like metabolism.
  • The female gets internally fertilised and then lays an egg (rarely multiple eggs, like twins in humans).
  • Their body plan is like bipedal dinosaurs. They walk bipedally on two legs, but with a horizontal spine like dinosaurs, NOT upright with a vertical spine like humans. (This probably influences the shape of the hips, which influences the maximal egg size.)
  • They are highly social and live in tribes, towns or cities like humans.
  • They are generally monogamous and mate for life.
  • The atmosphere and available nutrients are similar to those on Earth.

I want to determine how long time their eggs take to hatch. There's two periods involved:

  1. The time from the moment of impregnation to the moment the egg is laid. I'm going to call this gestation, but there might be a more proper term for it.
  2. The time from the moment the egg is laid until it hatches. I believe this is called incubation.

Of course I can just pull numbers out of my rear end and say that gestation takes 3 months and incubation takes 6 months. But I'd like it to be more qualified than that.

Important factors include the sizes of the hatchling's brain, the egg itself and the female's birth canal. But I don't know about egg-laying animals enough to set them.

What are good arguments for why the gestation and incubation periods should be one thing and not another?

Preferably I would like the gestation period to be short. My reasoning is that the shorter the gestation period, the smaller the burden that automatically falls on the female, which can influence their gender roles.

Thanks in advance!

Here is pictured the Avisapiens saurotheos by C. M. Kosemen and Simon Roy. My lizard men don't look exactly like this, but similar.

  • $\begingroup$ A horizontal spine can be a big problem depending on things like how big of a brain you want for your creatures, especially if you're talking dinosaur-like skulls. Remember T-Rex had a brain as big as a dog's. $\endgroup$ Jun 16, 2021 at 17:12
  • $\begingroup$ @ProjectApex : Why is a horizontal spine a problem? Do you mean because of the weight of a large head which the spine and neck need to support? $\endgroup$ Jun 16, 2021 at 17:25
  • $\begingroup$ Mostly yes. Although corvids and parrots show it doesn't take a colossal brain to have decent intelligence, the process of relying more on muscles to support the skull means you're going to worry about how the weight limits are being shared between the brain, skull and muscles in the head see how particularly large land animals tend to have proportionally smaller heads). Another thing to keep in mind is how heavy the counterbalance (be it a tail, an elongated pelvis or something similar) needs to be to make up for the weight on the front. $\endgroup$ Jun 16, 2021 at 17:33
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    $\begingroup$ I've heard it argued that warm-blooded organisms aren't good candidates for laying eggs because the metabolic demands are so high that the mother would need to lay an egg bigger than themselves to allow for gestation to be complete. You might want this process to look almost like marsupial gestation, where the parents can continue to supply nutrients to the egg over time, and perhaps the egg undergoes a metamorphosis at the end to transform into a hatchling. $\endgroup$
    – DWKraus
    Jun 16, 2021 at 18:28

3 Answers 3


Slight frame-challenge (but only slight).

Lizards living in moist areas lay eggs with leathery shells rather than hard brittle ones as the leatheryness allows for the absorption of moisture to keep the developing young hydrated.

As an evolutionary aside, lizards are thought to have developed from the first ampibians in the late Carboniferous (about 250 M years ago). Amphibians returned to water to spawn, the spawn volume, once hydrated being sometimes sixty times the volume of the adult female that ejected it.

Do you see where I'm going with this?

If this ability, to lay partially dehydrated eggs were retained, the volume increases vastly when layed in a safe watery place, allowing the birth of a self-sustaining egg surrounded in nutritious jelly to develop. Something the size of an apple would be produced turning into a beachball. Gestation might just be a matter of a week or so. The parents would need to take it in turns to continually circulate the water over the egg to maximize oxygen absorption. Perhaps a community acted together to protect their collective young and keep their gaseous exchange going.

The next stage of development would involve hatching after perhaps 2-3 months, hunting small-ish fish and aquatic mammals, being fed strips of meat by their parents all to build their bodies. A diet comprising more plants would come later.

Now like toad's tadpoles they'd develop into their adult shape, gaining feet, hands, lungs and a tougher skin. Emerging from the water they're metabolism would start to change, perhaps being able to internally generate their own heat, also growing the rudiments of a feathery covering. And so on until full adulthood.

If this seems improbable, take a look at this, the top row showing the starting point for different vertebrates, the bottom row showing the end point. Birds, mammals, fish - all start looking roughly similar.

enter image description here


You could do it 2 ways

1: Long gestation, long incubation, long period between eggs. Juvenile is fairly mature on hatching. This would be analogous to humans with lots of investment in infrequent juveniles.

2: Short gestation, short incubation, short period between eggs. Juvenile is not mature on hatching. This could be the pattern for a species with many juveniles, low initial investment in each, and historically high infant mortality.

The second could be interesting because it would mean your people are not just green humans. Lots of nonsentient juveniles are everywhere like little animals. They don't get intelligent until the third molt.

This would make for an interesting fiction because maybe certain of these juveniles start following a specific adult around when they feel it is that time. Playing the odds the chosen one is probably not a parent; even if it is her or she does not know that. But if that individual is chosen that is the one who then provides the parental care once the third molt takes place. Some individuals might get chosen by juveniles a lot and so have a lot around them, growing up. The juveniles have their reasons.

Your individual is quite old and no juvenile has ever chosen him, until now.


Humans aren't a good model for what's possible. Due to how the human skull grows, the size of the brain is largely limited by what can be pushed through the mother's hips, but if skulls grew differently, the brain could start small and grow to a large size during infancy. It even does this during pregnancy (obviously), but because the lungs aren't ready yet, it can't do it outside the womb.

Thus, the two important factors for this species (assuming they come from an alternate Earth, and have biology similar to ours) is that the lungs are developed enough by the time their size exceeds the ability to absorb oxygen through the shell, and that their able to feed at hatching (either latching onto nipples if the species lactates, or to be fed by the parents).

The ostrich makes a decent example here. They are large like humans (though not quite the body plan you want). Their eggs are sizable, and have about 42 days of incubation time. Emus on the other hand go for a full 55-60 days I believe, but have smaller eggs. Those are only examples though, and if your story required a longer duration, that wouldn't be implausible.

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    $\begingroup$ Could you please elaborate on the part about the skull and about the limits on how the brain needs to grow? What is special about the human skull that affects how much the brain can grow after birth? $\endgroup$ Jun 16, 2021 at 16:36

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