Eating liquid blood (formally known as hematophagy) is a moderately popular way of life in the animal kingdom. Red blood cells are good sources of protein and iron. But they’re low in vitamins and they are not easy to digest. Critters with a bloody diet often share some particular features: they have sensory systems keenly attuned to locating their food source, a way to anesthetize the point of entry, and use anticoagulants to keep the blood flowing. Once they have ingested the blood, they digest the red blood cells with the help of special enzymes or gut microorganisms (which can provide vitamins) and excrete the excess fluid.
The habit has evolved independently several times in different branches of the evolutionary tree; it’s wide-spread among the invertebrates but also occurs in some vertebrates.
Let’s start with some familiar ones (sometimes all too familiar). Although we can’t compete with the Interior in terms of summertime mosquito populations, we have enough of them here, at least in some places, to elicit bad words, bug dope and head nets. Mosquitoes are basically nectar- and fruit juice-feeders, except when it’s time for females to gear up for producing eggs—that requires a more nutritious diet, and the females go for blood.
A mosquito’s food-getting apparatus is an ingenious thing. The slender proboscis consists of a sheath containing several specialized mouthparts. When a female has located a host, chosen a site, lands there, and is ready to feed, the sheath rolls back, exposing two narrow, toothed maxillae that saw a hole in the host’s skin, two narrow mandibles that hold tissue apart while the hole is sawed open, one skinny tube for sucking up blood, and another skinny tube that drips saliva into the wound. That saliva bears an anti-coagulant, dilates the blood vessel, blocks an immune response, and lubricates the whole works.
This clever and complex arrangement is derived evolutionarily from the appendages of the four segments that form the head — quite a change from the earliest insects, which were chewers. However, similar systems for piercing and sucking have originated many times during the evolution of insects, e.g., bedbugs, kissing bugs, some lice, fleas and a variety of plant-suckers too, such as aphids.
Courtesy Photo / Bob Armstrong A female no-see-um tears a hole in a host’s skin with long, tooth-laden mandibles.
Lots of other invertebrates share the appetite for blood. For instance: An array of flies (blackflies, horseflies, deerflies, no-see-ums, etc.) as well as ticks have a slightly less elaborate anatomical approach. They slice or rasp a hole in the host’s skin, shove in the mouthparts, and slurp up the fluid. Other invertebrates of sanguinary habits include leeches, many of which take blood from vertebrates; you may acquire your own sample by wading or swimming in some of the Dredge Lakes. There are nematodes such as hookworms that live for part of the life cycle as intestinal parasites, feeding on blood.
Among other invertebrates, there’s a so-called vampire moth of Eurasia that pierces the skin of vertebrates. There is a large marine snail (Cooper’s nutmeg) that parasitizes electric rays in the eastern Pacific, cutting a hole and inserting the proboscis; the tongue-like toothy radula may rasp away the flesh; other nutmeg snails might also have this habit. The vampire snail and some of its relatives have tiny radulas and apparently just suck blood from sleeping fish in a similar way. A certain spider is reported to get blood from a secondary source—by eating blood-filled female mosquitoes.
Among vertebrates, I found no reports of blood-eating amphibians or reptiles, but some fishes do so. The Neotropical candiru is a tiny freshwater catfish that swims up the outflow from the gill chamber of other fishes and latches onto the gills, which have a plentiful blood supply. Lampreys are jawless fishes of both marine and freshwaters; over a dozen species of them can attach their round, tooth-laden mouth to another fish and use a toothy tongue to scour a wound on the side, then slurping up the blood and bits of other tissue.
A few birds are known to become blood-thirsty, at least at times. Oxpeckers of Africa graze on ectoparasites infesting large mammals and also eat the blood from the wound made by the parasite. Hood mockingbirds live on an island in the Galapagos Islands and sometimes feed on blood of wounded seabirds. The vampire finch, also found in the Galapagos, pecks wounds in seabirds and drinks the blood, particularly when other foods are scarce. The Tristan thrush lives on the islands of Tristan da Cunha in the south Atlantic; it supplements its diet of seeds and leaves, a variety of invertebrates, bird eggs, nestlings (and even adults) by taking blood from penguins.
I was interested to learn that the word “vampire” probably originated as a term for human “witches” and was only later applied to other animals, such as moths and finches. Surely, the best-known vampires are bats, native to the Americas. Vampire bats belong to the taxonomic family of leaf-nosed bats (although they have lost the nose-leaf), a group with varied feeding habits. The vampires may have originated from some fruit-eaters, or perhaps they came from ancestors that picked parasites off of host animals and acquired the habit of taking blood from the wound as well. Three species of bat are known as vampires, two feeding mostly on birds (and sometimes mammals) and one on mammals and large birds.
The one specializing mostly on mammals is known as Desmodus rotundus, and that one is has been studied much more than the other species. A hungry Desmodus bat crawls up to a mammal on foot and clambers on. Its sensitive temperature receptors allow it to locate a blood vessel close to the skin surface. Then the very sharp upper incisors shave off the fur if necessary and slice a hole in the skin, the saliva keeps the blood flowing and provides some pain-killer, and the bat can use its grooved tongue to lap up its dinner. Specialized gut microorganisms are essential to digesting the blood meal. The meal is processed rapidly, to eliminate the extra fluid, so the bat can take off for its roost without carrying a load of fluid; once in the roost, the solid part of the meal is processed.
Young Desmodus bats feed on their mother’s milk, of course, but they soon start getting regurgitated blood from the mother. These bats roost colonially in caves, hollow trees, and other sheltered places; the groups are composed of females, their offspring and a few males (the rest of the males roost separately). Roost-mates sometimes engage in mutual grooming. During that process, females sometimes share blood meals other females, not only their relatives but unrelated females also. That’s a form of reciprocal altruism, which can help maintain social relationships.
• Mary F. Willson is a retired professor of ecology. “On The Trails” is a weekly column that appears every Wednesday.
