• Question from:Kelsie

Hello! While looking at cat skeletons - I believe it was cheetahs - I chanced upon an article on the attachment of the shoulders/forelimbs in felines, and the unique rotational ability and flexibility of the forelimbs derived thereof. My interested was piqued, but I've since been frustrated by the lack of available information.

The article called this a 'floating shoulder,' and said, quote: "The cat’s shoulder is a feat of muscle, in that the forelimb is connected to the rest of the body only by muscle. Unlike our collarbone, which connects the shoulder and breastbone, the cat’s vestigial clavicle floats, anchored in place by muscle. The shoulder’s freedom of movement effectively lengthens the cat’s stride and enhances its range of motion." Here's the link, if anyone's interested: Click

Would this 'floating shoulder' still be possible with a more active clavicle that is more or fully connected to the skeleton? Would this 'floating shoulder' be an aid or hindrance in prolonged sprinting (think cheetah, but much faster and with far greater endurance), or any other specialized type of locomotion (arboreal, aquatic, mountainous, etc.)? Would the lack of an active clavicle affect the size and strength of any relevant muscles in either a negative or positive way? Would the total size and/or height and length of the animal in question - say, for kicks, ten to fifteen feet at the shoulder, with long proportions all around from head/neck (horse-like) to tail (lizard-esque) - have any impact on the effectiveness of this floating shoulder/clavicle setup? And, as a side note, would the complete lack of a clavicle in any form have any effect on the creature with this 'floating shoulder' setup, considering the apparent disuse of the clavicle anyway?

Gracias, and I look forward to any and all replies!

~Kelsie

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  • Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China
  • Posts: 256

Reduction or loss of the clavicle is actually normal in both hoofed and carnivorous mammals, so a "floating shoulder" also exists in horses, dogs and many other species. This improves running efficiency because, once the shoulder blade is no longer restrained by the clavicle, it can act almost like an extra limb segment. This results in a lengthened stride, as mentioned on the web page you linked to. Since running speed is equal to stride length times stride frequency (i.e. number of strides per minute, or other unit of time), long stride length allows an animal to run faster without having to move its limbs so quickly.

Even the human shoulder blade, by the way, has a bit of mobility. When you shrug your shoulders, each shoulder blade actually rises relative to your rib cage, sternum and backbone. However, since one end of the clavicle is attached to the sternum and the other to the shoulder blade, the clavicle maintains a constant distance between the shoulder blade and the sternum and prevents the muscles from pulling the shoulder blade too far out of place. Without the clavicle, your shoulder blades would be a lot more mobile than they are.

I assume, although I don't know for sure, that in animals without a clavicle the shoulder region is at least a bit less stable and more prone to damage. I could imagine that in very large four-legged animals, particularly, it might be difficult for the muscles to hold the shoulder blade in its proper position without the added support provided by a clavicle. For cats and many other mammals, however, the benefits of losing the clavicle (or making it so small that it becomes functionally unimportant) clearly outweigh any disadvantages.