If you wear Birkenstock with leather lining your feet will sweat like your backside (in a hot weather – good for Europe not for tropical jungle).
That happened to me when I wore my Keen (the leather ones) in 30C weather in tropical climate. I should have bought Keen Clearwater CNX instead as they are much suitable. My leather Keen became the favourite for the local dogs too as they chewed mine up because of the leather smell, so I had to get the local cobbler to stitch them up.
My friends wear Crocs for two reasons. They wear them loosely and secondly it is much cooling than leather plus the fact that it drys quick or at least it does not stink when they get wet.