I saw several colonies of these impressive ants at widely separated locations in the valley. The linked photos show different specimens in one colony, variously on a tree trunk and on leaf litter near Yosemite Falls, elev. ~4000 ft. (1220 m), afternoon. They are not aggressive but are expert at hanging on, so that removing them from hands and clothing takes some work. These two individuals, on a tree trunk, seem to be greeting or identifying each other.