Most epizoochorous burs attach to hair on the body or legs of the host animal, but a special class of epizoochorous bur is known as the trample-bur (or trample-burr). Several species of "Tribulus", "Harpagophytum", and "Grielum" produce fruit in the form of trample-burs. As the name suggests, they attach themselves to the animal when trampled. They may hook onto the legs of animals as the large hooks of "Harpagophytum" do, sometimes causing serious injury, but sometimes hooking onto the leg of say, an ostrich, apparently without causing discomfort. It also might penetrate a hoof or foot pad or the tires of a vehicle, only to be shed after being carried for a considerable time and distance; most "Tribulus" and "Grielum" species are specialised for such attachment, variously being flat, but with upward-directed spikes as in say, "Grielum humifusum", or shaped like a caltrop as in some species of "Tribulus" that have achieved the status of cosmopolitan weeds by sticking to the tires of aircraft.