I think he reversed that list- roo is very stretchy (very nice and buttery too )
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I personally haven't had any experience with them, but the guys at Sartorialoft, Archive, Atelier, Darklands and Wikipedia say that Kangaroo is thin which makes it very malleable and buttery soft and the layout of the fibers makes the tensile strength the strongest of the leathers.
This is also in relation to the way Guidi cuts the various leathers.
The list definitely isn't reversed. Horse is not stretchier than calf in my experience.
Tensile strength is not stretch-ability, but ability to resist rips, I think. Donkey, I've ways heard, is very very non-stretchy (which is why people recommend sizing up 2 on donkey sometimes). But regardless, other than donkey everything is sized the same. Avoid donkey and you're good.
Ah, I'm actually very unfamiliar with Donkey Leather so I removed it from the 'ranking'. Wasn't aware that it wasn't stretchy. Included bison which is just about the same as horse, but with grain.
The cobbler just lops off the first layer of your heel and shaves a bit of your sole to put on the vibram anyway. So as long as you don't wear the heel down past the first layer it should be fine.
I am not a fan of the weird texturing though. The same style is coming in shell cordovan for AW13, which will be a beauty!
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I've worn mine without vibram and they have been fine, even in rain. If you notice, the heel is not full leather, but a ring of rubber on the outside.
Once the heel wore down a bit, I put heel taps and nothing more.