Blood is an important biological material for vertebrates: transporting oxygen, carbon dioxide, hormones, minerals, food and wastes around their bodies, blood is crucial to their lives. For some animals—known to biologists as hematophagous species—blood is even more important, as it represents their primary food source. Hematophagy is widespread in the animal kingdom, and is practiced by a number of invertebrates, fish, birds and mammals.
Common Traits among Hematophagous Species
Sometimes widely separated, distantly related species evolve similar lifestyles, morphologies and behaviors; scientists call this phenomenon convergence. Examples of convergence abound among hematophagous species: most species that feed on blood produce saliva with anticoagulant and pain-killing properties, which increase the flow of blood and reduce the likelihood that their host will notice them, respectively. Additionally, structures like sharp teeth and piercing mouthparts are common among bloodsucking species.
A variety of invertebrates, including insects, arachnids and worms, consume blood. Insects such as assassin bugs (Reduviidae), mosquitoes (Culicidae) and bedbugs (Cimex lectularius) live by feeding on the blood of various host species. Male mosquitoes do not feed on blood, but females require it to deposit eggs, otherwise they will die without reproducing. Though they superficially resemble insects, ticks are arachnids—relatives of spiders and scorpions—that subsist on the blood of other animals. New World leeches primarily live an aquatic lifestyle, while those of the Old World often live on land. Hematophagous invertebrates are vectors for the spread of a variety of diseases; mosquitoes spread West Nile virus and malaria, ticks spread Rocky Mountain spotted fever and Lyme disease, and assassin bugs transmit deadly Chagas disease.
Lampreys form the order Petromyzontiformes, an ancient lineage of fish that some scientists think is the sister group to all vertebrates with jaws. Lamprey mouths have circular rows of teeth and a rasping tongue; most species attach themselves to living fish, open a wound and then suck out blood and other bodily fluids. In some cases the lampreys weaken or kill the fish, and invasive, introduced lamprey species have become a commercial problem as they deplete native fish populations.
Given that the diversity of finches found in the Galapagos Islands was rich enough to help inspire Darwin’s theory of natural selection, it shouldn’t be surprising that one of these enterprising species has evolved to drink blood. Sharp-beaked ground finches (Geospiza difficilis)—sometimes called vampire finches—eat a variety of prey; they tend to feed on whatever food is available, including sea bird eggs, nectar and seeds. Some of those that live in low-lying areas with a low food supply engage in hematophagy, typically by alighting on the backs of boobies (Sula spp.) and pecking the birds until they bleed.
Perhaps among the most famous hematophagous animals, the various species of vampire bats get their name from their habit of lapping up blood from other animals. Using their sharp teeth to make a small incision in their host’s skin, the bats feed greedily on the high-protein food source. Bats will feed for about 30 minutes; sometimes their anticoagulant-laden saliva is so effective that blood continues to drip from the host animal for some time after the bat has left.
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