Dr. Schutt explained that hematophagy is a difficult, dangerous trade, in some ways harder than merely killing and eating your prey outright, which is why blood eaters from different taxonomic orders have evolved a similar set of utensils: the hatpin teeth, the natural clot busters and pain deadeners.

Blood feeders must also be stealthy and wily and good at escaping the swats and fury of their often much larger hosts. The common vampire bat, Desmodus, which feeds on large terrestrial mammals, creeps along the ground like a spider and, in addition to flying, can spring straight upward three feet into the air.

The white-winged vampire bat, Diaemus, approaches a potential host chicken so softly and lovingly that the bird is deceived and sweeps it up to its brood patch as though to warm its own chick. Aquatic leeches aim for hidden pockets and crevices: dip your head into leech-infested waters, and the segmented, toothy worms may slip up your nostrils and make a home of your nose.

Moreover, even though we rightly cherish our own blood as the indispensable elixir of our lives, it turns out that, as a foodstuff for others, it is surprisingly thin gruel. Blood is more than 95 percent water, with the rest consisting mostly of proteins, a sprinkling of sugars, minerals and other small molecules, but almost no fat. Tiny creatures can do fine on such light fare, which is why the great majority of exclusive blood eaters are arthropods  bedbugs, ticks, chiggers, female mosquitoes. For larger sanguivores, though, it is as much of a challenge to survive on blood as it is to acquire it. Lacking dietary fat, vampire bats cannot pack on adipose stores and must consume the equivalent of half their one-ounce body weight in blood every night or risk starving to death. And because the water in that blood meal would make the bats too heavy to fly, they must cast off all modesty and urinate freely as they feed.

Small wonder that wholehearted exclusive blood feeding is rare among vertebrates, and that two of the three species of vampire bats are found in such low numbers they are at risk of extinction. The only reason that so-called common vampire bats are common, said Dr. Schutt, is that they have learned to feed on cattle, pigs and other livestock. “They love it when we clear out the rain forest to make way for ranches,” he said.

The only other vertebrates known to subsist solely on blood are certain types of candiru, a poorly studied but floridly feared group of inchlong catfish found in the Amazon and Orinoco Rivers. A hematophagous candiru’s usual modus is to parasitize a larger catfish, infiltrating the host’s gill slits, grasping onto the flesh inside, rupturing blood vessels, pumping out the blood with its highly mobile jaws and then, after a minute or two, darting out again. Yet for at least a century, the fish have been reputed to target the human urethra as well, supposedly enticed by the scent of urine: fish, after all, urinate through their gills. Despite the antiquity and persistence of the legend, there is only one confirmed case, from 1997, of a candiru making its way into a human urethra, where it probably had no time for a blood meal before suffocating to death.