Ultimately, my colleagues and I found that not only did significant differences exist between the three vampire bat species, but that most of the variation—including unusual feeding methods and social interactions between roost-mates—relates to the bats’ preference for either mammalian or avian blood.
Aside from the relative ease of studying the common vampire bat, its slew of fascinating behavioral, anatomical, and physiological features helped to sustain the exclusive interest in the species. And some of those “common” features do indeed seem likely to apply to all vampire bats. Take, for instance, one of the most fascinating of all vampire bat adaptations, which I observed only once in the three years I kept a colony of Diaemus at Cornell: blood-meal sharing between bats.
In 1984, zoologist Gerald S. Wilkinson, then of the University of California San Diego in La Jolla, first reported that vampire bats in the wild commonly share food by regurgitating blood. Wilkinson, who made his initial observations on Desmodus, determined that about 70 percent of blood-sharing incidents occurred between a mother and her dependent offspring (until around the age of one). Blood sharing between mothers and newborn pups presumably transfers not only nutrients, but also bacteria necessary to an infant’s digestive tract.
Blood sharing between both related and unrelated vampire bats also occurs on a reciprocal basis; that is, bats that Wilkinson had experimentally starved for one night and that then received blood from another individual were more likely to donate blood to that individual when it, in turn, was starved. That reciprocity almost certainly evolved in response to two basic realities: a bat that cannot find a blood meal will starve to death in less than three days, and yet on any given night, as Wilkinson found, about one in fourteen adult bats and fully a third of young vampires-in-training will fail to feed. And so there will be numerous occasions over a vampire bat’s lifetime both to receive and to share food.
Therefore, it’s remarkable but not surprising that Desmodus can remember past donors as well as recognize cheaters—those individuals who try to beat the system by not sharing blood. There’s another way in which bats discriminate among recipients: adult males will share blood with females and young bats, but rarely with other adult males. That makes perfect sense. Why share food with someone who may be your rival for a mate?
There is anecdotal evidence that the white-winged and hairy-legged vampires also share blood, but in contrast to Wilkinson’s in-depth study of Desmodus, this behavior in Diaemus and Diphylla has yet to be studied in detail.
In other ways, Desmodus exhibits unique traits among the trio of vampire bat species. One of the reasons for the common vampire’s success is its ability to feed from the ground—and thanks to humans, they have developed a partiality to cows’ blood. This they often obtain while on the ground, from the region behind the cows’ hooves, an area with relatively thin skin and an ample blood supply flowing close to the surface. Feeding also takes place with the bat riding its prey’s back, where it’s easy for the vampire to reach sensitive areas like the ears.
To feed on the ground, Desmodus has evolved the ability not only to walk and even run on all fours, but to make spectacular, acrobatic jumps in any direction. A flight-initiating jump off the ground is powered by strong pectoral muscles and fine-tuned by elongated thumbs, which are the last things to leave the ground. The thumbs impart precise direction to jumps that can reach three feet in height. That enables the common vampire to escape predators, avoid being crushed by its relatively enormous prey, and initiate flight after a blood meal. This ability to feed efficiently and safely on large mammals, combined with the increasing supply of domesticated livestock, is the primary reason why Desmodus has been so successful in numbers and range.