at the Wild Asia exhibit in the bronx zoo, one of the leopards (a black one) is behind glass, not bars... but he often posititions himself near one of the vents in the glass, were he (she) can push his nose up against it, and smell the other animals..
the flicks of the tail indicate intrest.
because of this, the leopard is often lying still, and you can clearly see the different shades of color black in the spots.