After looking at concept art of various humanoid creatures portrayed in science fiction I noticed many artists tend to make the abdominal muscles of their creatures/aliens look similar to those muscles found in humans and other primates.

I would like to know if a non-primate mammal/reptile/bird does become humanoid (perhaps through the process of convergent evolution) how different would their abdominal muscles realistically look like compared to those of primates?

enter image description here

  • $\begingroup$ The function of muscles is to pull. He abdomen is shaped that way in order to bend the spine by pulling smaller sections instead of smashing the chest of the animal into the pelvils. So just ask yourself, what function do the muscles in your creature have? $\endgroup$
    – user75689
    Commented May 23, 2020 at 20:36

2 Answers 2


Not a lot different, really.

Well, as it turns out, with evolution being what it is, animals being descended from earlier kinds of animal; it should come as no surprise that the anatomy is going to be relatively familiar across species.

The images shown here demonstrate that even between amphibians and mammals, there's quite a lot of similarity --- the musculature comprises homologous structures. You can clearly see the pectorals, rectus abdomini (six/eight pack), the external obliques.

As for the motivations of fantasy artists, I've noticed that, anatomically speaking, when it comes to werefolk of one kind or another, artists tend to plop a cat or dog head onto a human body, cover it with fur, give it an anatomically incorrect tail and call it a day. They don't make tiger people's bodies look very tigerlike! I think the reasons for this are twofold: first, trying to figure out what a feline body will look like after millions of years of evolution into an upright posture is no easy task. Bones will lengthen and change shape; joints will move differently; muscles will alter their place and function. The resulting body might look something like a human body -- maybe thinner and more flexible -- but its shape will be quite different even though the underlying muscles will all have human homologues.

The second reason is boobs. Yep. Whenever we see a Werecat or Werewolf or Werewhatever female character in fantasy, what do we see? Wide human hips and big human boobs high up on the chest. If you've ever owned a girl cat or dog, you know that wide hips and high boobs are not a thing among most animals. I suspect that a real Werecat would retain a relatively narrow pelvic girdle and six or eight tits along the abdomen. Not very sexy for movies, but, that's reality check!

Here's a cat:

enter image description here

Here's a dog:

enter image description here

Here's a frog:

enter image description here

  • 3
    $\begingroup$ Ahem. The technical term for your second reason is "fan service" 😉. That said (and also note this and this), if your critters have human-like intelligence, they very well may have human-like hips for the same reason humans do; we need them to give birth. $\endgroup$
    – Matthew
    Commented May 1, 2020 at 2:46
  • 2
    $\begingroup$ except it will not be the same for birds and reptiles who have ribs all the way down, many even have gastralia which is like an extra set of ribs for the belly. it is not even true for salamanders, frogs are just weird and have lost a lot of bones. $\endgroup$
    – John
    Commented May 1, 2020 at 3:22
  • $\begingroup$ I wanna see a six-pack of a chicken, please. $\endgroup$ Commented May 1, 2020 at 4:57
  • $\begingroup$ @AdrianColomitchi -- LMGTFY. $\endgroup$
    – elemtilas
    Commented May 1, 2020 at 5:15

Only if it is a mammal.

if it is mammal not much will be different, mammals are pretty conservative in their anatomy. frogs have a similar muscle pattern, frogs are really specialized and derived and have lost a lot of bones(this is also why they should not be used for high school dissections, they are misleading).

but Basically all other vertebrates have ribs all the way down, the major exceptions being crocodiles (which have gastralia so still no six pack) and frogs which can have six packs and a few squmates which have a lumbar region too but it is pretty rare.

If they are crocodiles, birds, dinosaurs or any of the archosaurs, it is impossible. These groups have gastralia which is an extra set of ribs in the belly. A therapod for instance will not have a six pack the closest it could get would be a 40 pack thanks to all the gastralia.

dinosaurs enter image description here

Crocodiles enter image description here

birds enter image description here


there are plenty of other complications as well, in birds for instance the wing muscles cover the entire stomach reaching the pelvis even, and you have things like turtles with shells.


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