Totally cool vet fact
|December 11, 2011||Posted by Melinda under Uncategorized|
Ready for your totally cool vet fact of the day??!?!?!?!
I think as endurance riders we all get the whole size of the animal and the cost of size when it comes to locomotion.
If you need a bit more background, here’s an example I’m trying to recall from memory – I probably have all sorts of details wrong so read it more for the “ideal” than to quote me on the particulars.
If you want to skip all this and go to the cool stuff at the bottom, that fine too!
I read a study a few years back that said one hypothesis for why large horses have more trouble staying sound is related to the size of the cannon bone and diminishing returns. Let me explain. The body mass of the horse can increase much faster than the cannon bone. Let’s assume you have an average weighted rider and average weight horse. The total combined weight of horse+rider needs a certain size cannon bone in order to successfully handle the load. If you start doing calculations, what you realize is that it’s impossible for the cannon bone size to proportionally increase at the same time body mass is increasing. Thus, in a larger horse you have proportionally less cannon in the big horses that is “left over” for carrying a rider – even a small one. The opposite is true if we DECREASE the weight/size of the horse. It’s easy for a small horse to have proportionally more cannon bone – plenty of cannon bone left over to carry that rider.
OK. With this information floating around in the back of my head, I matter of factly went through my locomotion slides in preparation for the midterm on Friday. Yep – big animals move slowly and more cheaply. Yep – big animals are proportionally weaker (for example, big draft teams may be able to pull more than their smaller counterparts, but it’s a smaller percentage of their body weight). There’s a slide that the text is all jumbled up on, but from what I can gather larger animals have proportionally weaker legs that corresponds with a lesser angular excursion of rear limbs or some sort thing. No big deal. I glazed over some slides where they equivilated the mass to volume of an animal, did something fancy with the muscle cross section as a function of force and maximum force generated is basically equivalent to muscle x-sec, blah blah blah blah. Then I have a statement that as mammals increase in size, the become relatively weaker because the cross section area of the muscle (which they are interpreting as force) is doesn’t increase as fast as body mass and thus maximum force as a function of body weight is decreased.
Interesting, but so far nothing I haven’t heard in some variation over the years.
Then I get to this statement: “AV Hill predicted mammals of different sizes should be able to raise their centers of mass the same height when jumping”. What??????
Apparently, jumping is a whole ‘nother ball game from forward locomotion.
Let me put this in a different perspective – SIZE HAS NO BEARING ON HOW HIGH AN ANIMAL CAN JUMP.
Remember we are talking center of gravity measurements in terms of displacement – so assuming that the center of gravity is just behind the animals shoulder, the horse starts off about 4 feet higher than the rat, BUT total vertical displacement distance of that point on the animal is identical.
Anyone have a guess on what the magic number is for a vertical jump?
Turns out it’s about 2 meters. (reminder – we are talking vertical displacement of center of gravity, not the height of the jump from the ground)
500kg horse – 2.47 m
400kg mule – 1.98m (from a STAND – this was a coon jumping record I think)
80kg human – 2.43m
0.25kg Galago – 2.25m (also from a stand)
So I got curious and tried looking up this Hill guy my professor had referenced. 🙂
A good bio is here
Here’s a reference to a relevant paper –
Hill, A. V. 1950. The dimensions of animals and their muscular dynamics. Sci. Prog. 38: 209-230.
I can’t actually find the paper itself, but it referenced all over the place. If someone finds it and can share (copywrite laws and all that….) can you let me know? I’d love to read the original paper.
Then, (I remind you, I should be STUDYING right now), I started looking up videos of the horses and other animals referenced in the presentation, and let me tell you – watching the horses jump gave me goose bumps.
Cindy the greyhound (be patient and watch the second jump, where they follow up with a fabulous slow motion shot. The first jump on the video is just a warm up)