Effects of a ferret-like spine/ribcage on bipedal posture, possible leg lengths/proportions?

Question

I was recently redesigning some mostly hominid Aliens I designed a long time back in high school and felt were a little lackluster, and since they were already Tall, Wiry, and vaguely Shambolic-Looking before, I came up with the idea of giving them greater flexibility and strength by giving them a spine similar to that of Ferrets and other long, slinky sorts of mustelids, left over from a time in their evolutionary history when they were predominantly arboreal hunters, scavengers, and occasional carrion feeders.

In addition to increasing their overall length/height compared to most humans, this allows them to bend and contort their bodies naturally in ways humans have to train for years to accomplish, possibly even farther (without significant Injury, of course). Their wiry muscularity also makes them deceptively strong, a bit like a coiled spring; this is enhanced by the fact that they also have two sets of arms, which this post suggests that a longer spine and ribcage could help accommodate (one pair is generally longer than the other, and positioned to be better for lifting, grabbing, climbing, and other activities requiring a wide range of motion without getting in the way of the other, while the second pair is used more for jobs requiring fine motor skills closer to the body- both can still be used fairly deftly, however).

When I looked into the skeletal systems of ferrets, minks, and weasels, I also discovered that they have a far more flexible ribcage, which can expand and contract to some degree when compressed or released, and allow the animal to cram into tighter spaces while still protecting their internal organs- this partly appealed to me because I had been playing around with the idea of giving this species the ability to partly unhinge their jaw and swallow larger food items, and some of the other threads I've been reading on here suggest that you'd need a much more flexible ribcage for that to work?

Thing is, however many generations on, they've developed into plantigrade bipeds like us, and I imagine their spines would give them the look of perpetually slouching in a lazy and faintly delinquent-looking fashion, generally only straightening up to their full height to reach tall objects or intimidate potential foes (which actually works really well with the General Personality of most of the characters of this species I've written so far XD); I also imagine that they'd start to get more bent as they aged, and that a lot of them would start using various forms of spine braces and bindings as they got older. These are just all the things I've thought of off the top of my head, I'm not sure what other effects this might have.

However, my main question concerns their legs- Their feet are fairly long, with long toes like those of raccoons, opossums, and some lemurs to help them stabilize when upright, but I'm not sure what kind of leg is better to support their body structure; Long Thighbones with comparatively shorter shins, or Long Shins with comparatively shorter Thighs? Would they have to stay constantly bent-kneed/bowlegged, or could they support themselves with the same stance as our legs usually take? Would their Pelvic situation need to be different? Should I skip plantigrade and look at another leg structure entirely? Should I scrap the whole idea and start over? I don't know nearly enough about legs, can someone just... generally explain how they work to me XD

Answer 1

Considering evolution is a form of adaptation to the environment, I wonder what has lead your ferrets to evolve into plantigrades.

For example, Eohippus' finger-like foot adapted it for living in the bush, whereas horses' hooves are better suited for prairies. Here, I am not talking teleologically: the environment might have triggered evolution by favouring certain mutations or, esle, some mutations might have appeared and then be co-opted for further use (that is, either "environment meets evolution" or "evolution meets environment").

Drawing on your description, it appears to me your ferret-like people (whether intelligent or not) have matched their evolved bipedal posture with new environmental challenges. I presume their environment of early adaptation must have been some kind of grassland, which requires high posture to navigate and ward off predators while, at the same time, it would still contain sufficient hindrances to running to justify the retention of feet whose surface ratio is higher compared to their mass.

The more I think of it, the more your people seem to me to be like thin kangaroos. Kangaroos are marsupials, all right, but they have a bipedality that nicely shows how a species can retain surface ratio while, at the same time, walk (or leap) on two feet only.

This said, I would conceive the thing as follows:

• Femur becomes shorter over the generations
• Posterior paws become slightly longer
• Higher tibia to femur ratio (so, a longer tibia)
• Jumping gait instead of walking gait (this is a major behavioural change)
• You still retain some sort of shambling look
• If the species is intelligent and has spread to new environments after evolving in a primordial one of sort, you may think of them as able to "walk short distances" while, at the same time, leap and jump whenever its convenient. Their civilisation should reflect this.