Stature - Ribcage

The ribcage has only one major function and that's to act as a solid cavity in which the diaphragm can pull and push air into the lungs. Between human bipeds and any quadruped mammal the major difference in the ribcages are from the effects of gravity and the relative positioning of the forelimbs.

Both ribcages at the top are viewed from cranial to caudal, you could say a quadruped's deep and a human's wide. The bottom sketches show how the canine ribcage would compare as a biped.

It is unlikely that a canine ribcage could support the rise and fall, expansion and contraction needed if it were biped, it's adapted in the wrong plain. But I like the idea of adding some of these shapes to a biped ribcage. One starting point is the sternum, it builds that frontal plain of the chest - does it need to be vertical, could it be shorter? I number of differences between species exist but large differences are more likely to be due to the size of the organs the animal has.

I took a comparative view of a carnivore and herbivore. 

Take an equine, it's a prey species, its evolved to run, requires large volumes of oxygen to do so, it's ribcage is extensive to maximize its capacity. However, it's a herbivore, and needs a much larger gastro-intestinal (GI) tract for digestion. To aid this the sternum is comparatively shorter to the ribcage than in a canine this flares the ribcage, helping to fit those larger organs in. Our canine doesn't need a large GI tract or even needs to do a lot of running, it's ribcage accommodates for this.

I wanted to look and see how these species variations may shape our biped anthros.

A scribble of ideas, looking how to incorporate some form of shortened sternum or elongated ribcage as per the species.

 

For the moment I rested my focus on what creates the girth or volume of the chest, the relation of the 1st ribs, clavicles and sternum or what looks more like a 'collar and tie'. Drawing the 1st ribs from the spine to the sternum helps give the chest its volume. The length and angle of the sternum helps give our character a puffy chest or more space for a larger abdomen. 

Take a look at how flared they made the ribs on Tavros in the Narnia films. It's a high volume chest, certainly plenty of space for a herbivores GI tract.

S-spine, C-spine

Comparison of axial skeletons of a human and canine, arrows note the number and directions of each spinal curve. (Not to scale).

We encounter an extreme number of issues when converting quadruped anatomy to that of a biped. Essentially it boils down to the evolution of each systems handling of its centre of mass. Human bipeds balance their weight over 2 legs, with the centre of mass moving between each stance leg, however a quadruped dog shifts its weight between fore and hind limbs during movement.

The spines of each system are therefore adapted to cope with each of these extremes. The S-shape spine of a biped has 4 notable curves, creating it's distinctive S shape. This maintains the centre of gravity when standing from the top of the spine down through the feet. These curves also facilitate weight transference with minimal effort. 

The C-shape spine of quadrupeds have only 3 notable curves that are near opposite to biped curves. The C-shape comes from the thoracic and lumbar regions curving upwards. I've drawn the canine above stood upright, this would bring it's centre of gravity in front of its feet, causing it to be unbalanced and fall forward. The curvature of its spine, particularly at the lumbar region would not support the weight of an upright torso, the curve is in the wrong direction.

This shows that we simply can't start our anthropomorphic characters by just standing a quadruped up on 2 feet. This and other anatomical issues make this a poor starting point, however, should we wish to maintain a degree of our chosen animals features in our anthropomorphic bipeds we can start adapting this anatomy into likely possibles. Lets explore these possibilities...