Only three mammalian species are sanguivorous—that’s blood feeding—and they are all bats. Blood, apparently, is not that nutritious. It has almost no carbs, fats, or vitamins; its high iron levels can disrupt heart, liver, and pancreas function; its obscenely high protein and salt levels can cause renal disease if nitrogenous waste products build up. It contains pathogens. It clots.

Vampire bats have some obvious adaptations to allow them to survive on their limited and macabre diet. They have sharp incisors and canines, clawed thumbs on their wings, and use infrared sensing to find blood vessels in their prey. Their kidneys are hyperactive so that they can effectively excrete the urea that comes from all the protein in blood, and they have enhanced immunity to deal with blood-borne pathogens.

How they manage all that, however, remains largely unknown.