Blood is over 95% water. The rest comprises proteins, a sprinkling of sugars, minerals, and other small molecules, but almost no fat.
Tiny creatures do fine feeding on such light fare, which is why the overwhelming majority of blood suckers are arthropods: bedbugs, ticks, chiggers, female mosquitos. For larger sanguivores, hematophagy is as much of a challenge to survive as it is to adaptively acquire.
Lacking dietary fat, vampire bats cannot pack on reserves, and so must consume half their 1-ounce body weight in blood every night or risk starving to death. Because the water in a blood meal would make the bats too heavy to fly, vampires freely urinate as they feed.
Despite the challenges, several animals evolved a taste for blood, and the equipment and behaviors that make it a feasible lifestyle, including hatpin teeth, clot buster chemicals, and pain deadeners.
Bedbug habits and senses are well-adapted to find and reside in the living quarters of their prey. Bedbug mouthparts deliver painkillers and anticoagulants to prime victims.
In between feedings, bedbugs prefer nooks and crannies where they are not easily dislodged. Aquatic leeches aim for pockets and crevices on their victims when attaching themselves for a feast.
A vampire bat does not suck the blood of its victims. The bat instead lets subtle physics do the sucking for it. Relying upon capillary action, a vampire bat’s cleft lower lip, perfectly spaced lower incisors, and doubly grooved tongue jointly form a tube through which a victim’s blood is readily pulled up. The anticoagulants in bat spit are so potent that a host animal often continues to bleed long after the vampire has feasted its fill and flown on.
Hematophagy is a difficult, dangerous trade. Blood feeders must be stealthy and good at escaping the swats and fury of their often much larger hosts.
The common vampire bat, which feeds on large land animals, creeps along the ground like a spider. As well as its flight capabilities, a vampire bat can spring straight up a meter to attach itself.
The white-winged vampire bat approaches a potential host chicken so softly and lovingly that a hen may be deceived and sweep it up to its brood patch as though to warm its own chick.
The candiru is a tiny catfish found in the Amazon and Orinoco Rivers. They are enticed by the scent of urine. Fish urinate through their gills. The modus operandus of a candiru is to infiltrate its host’s gill slits, grasp the flesh inside, rupture blood vessels, pump out as much blood as it can in a minute or 2, and dart out again. Larger catfish are candiru’s favored prey.
Vampire finches on the Galápagos Islands live mostly on seeds, nectar, and eggs. They supplement their diet with occasional iron-rich snacks by persistently pecking at the wings and tail region of one of the blue-footed boobies that live there. Once the finch draws blood, other finches line up like customers at a deli counter. Curiously, victimized boobies do not offer much resistance.
The oxpecker is famed for living aboard large mammals – rhinos, buffalo, and giraffes–and plucking ticks off their hosts’ hides. This symbiosis is sanguinary. Oxpeckers press their beaks in the wounds where ticks are lodged and take nips of blood from the beasts they reside on: trading one blood sucker for another.