It is somewhat uncommon for lions to actually climb trees. There are no more than 2 populations in whole world of such lions that do actually climb trees as one of their day after day behavior. One of these populations is found within the Ishasha sector which is found in the south part of the well known Queen Elizabeth National Park within Uganda. The other population is found in Lake Manyara National Park in the Southern part of Tanzania.
It is supposed that this type of lions climb trees as a way of protecting themselves against the numerous biting tsetse flies on the ground level, whereas other people claim the they actually climb into the branches to escape from the heat on the ground and enjoy the cool breeze; nonetheless the reason why they climb up into the tree branches remains unknown.