If I put my hand into the colony, they'll nuzzle their noses between my fingers in greeting. Coen is the only researcher studying blood as a source of nutrition in vertebrates. She points out that humans, including the British and the Masai, eat blood in small quantities and that blood meal is used as a nutritional supplement in some agricultural animal feeds. Some insects, such as leeches and mosquitoes, subsist solely on blood, but vampire bats are the only mammals in the world for whom blood is their mainstay. Not surprisingly, vampire bats are metabolic wonders.
Common vampires feed to satiation within half an hour and then move away from their prey.
Afterwards, their expandable stomachs are often so swollen they have a hard time flying. But blood is about 80 percent water, and the bats begin urinating immediately upon feeding. Often they will continue to rest and urinate at temporary roosts near their feeding site, until finally they are free of enough excess water to fly home to their main roost in a cave or hollow tree. On the flip side, the vampire's amazing kidneys can also conserve water in much the same way as desert animals do, even though vampires live in moist climates. Because they previously dumped the water taken in with their blood meal, the vampire bats have to conserve water later on for digestion of the high protein portion of their meal which remains.