Hematophagy is the practice by certain animals of feeding on blood. Since blood is a fluid tissue rich in nutritious proteins and lipids that can be taken without great effort, hematophagy is a preferred form of feeding for many small animals, such as worms and arthropods. Some intestinal nematodes, such as Ancylostomatids, feed on blood extracted from the capillaries of the gut, and about 75 percent of all species of leeches, a free-living worm, are hematophagous. Some fish, such as lampreys and candirus, and mammals, especially the vampire bats, and birds, such as the vampire finches, hood mockingbirds, the Tristan thrush, and oxpeckers also practise hematophagy.
Mechanism and evolution
These hematophagous animals have mouth parts and chemical agents for penetrating vascular structures in the skin of hosts, mostly of mammals, birds, and fish. This type of feeding is known as phlebotomy. Once phlebotomy is performed, blood is acquired either by sucking action directly from the veins or capillaries, from a pool of escaped blood, or by lapping. To overcome natural hemostasis, vasoconstriction, inflammation, and pain sensation in the host, hematophagous animals have evolved hembiochemical solutions, in their saliva for instance, that they pre-inject—and anesthesia and capillary dilation have evolved in some hematophagous species. Scientists have developed anticoagulant medicines from studying substances in the saliva of several hematophagous species, such as leeches. Hematophagy is classified as either obligatory or facultative. Obligatory hematophagous animals cannot survive on any other food. Examples include Rhodnius prolixus, a South Americanassassin bug, and Cimex lectularius, the human bed bug. Facultative hematophages, meanwhile, acquire at least some portion of their nutrition from non-blood sources in at least one of the sexually mature forms. Examples of this include many mosquito species, such as Aedes aegypti, whose both males and females feed on pollen and fruit juice for survival, but the females require a blood meal to produce its eggs. Fly species such as Leptoconops torrens can also be facultative hematophages. In anautogenous species, the female can survive without blood, but must consume blood in order to produce eggs. As a feeding practice, hematophagy has evolved independently in a number of arthropod, annelid, nematode and mammalian taxa. For example, Diptera have eleven families with hematophagous habits. About 14,000 species of arthropods are hematophagous, even including some genera that were not previously thought to be, such as moths of the genus Calyptra. Hematophagy in insects, including mosquitoes, is thought to have arisen from phtyophagous or entomophagous origins. Several complementary biological adaptations for locating the hosts have also evolved, such as special physical or chemical detectors for sweat components, CO2, heat, light, movement, etc.
Many human societies also drink blood or use it to manufacture foodstuffs and delicacies. Cow blood mixed with milk, for example, is a mainstay food of the African Maasai. Marco Polo reported that Mongols drank blood from their horses if necessary. Many places around the world eat blood sausage. Some societies, such as the Moche, had ritual hematophagy, as well as the Scythians, a nomadic people of Russia, who drank the blood of the first enemy they killed in battle. Psychiatric cases of patients performing hematophagy also exist. Sucking or licking one's own blood from a wound is also a common human behavior, and in small enough quantities is not considered taboo. Finally, human vampirism has been a persistent object of literary and cultural attention.
