These are camels. Apparently, in the desert, their humps are used to store fat for long trips, so they don't die when they encounter nothing but sand and cacti for days on end. However, in captivity, the humps serve a very different function, and that is storing zoo employee drugs. A small incision in the hump is made, excess fat is removed (sort of a camel liposuction) and all manner of illicit drugs can be stored with minimal impact on the camels. Of course the drugs need to be stored in plastic bags or else some is lost to the camel, which can also result in erratic behavior. This is why camels are the most dangerous animal at the zoo.