I think most, all?, of the elk burgers that I have eaten are a hundred percent elk. I dont't know, they hold together fine, but it's probably not the best idea to over cook them. A good tip I've picked up is to hand form the burgers donut style without the hole in the middle. I think it's the connective tissue that makes it contract when it cooks, so a regular flat raw burger looks like it balls up in the middle after it's cooked.
To me, elk's quite a bit different than venison, it knows how to turn plain old grass into some good eating, where deer taste better when they're eating farmed crops. The last big old elk I helped a buddy winch out of a drainage hung almost fourteen feet when the helper kid hooked its tied front legs to an overhead rail and pushed it in the shop. A good game processor can not be underestimated.
