While working with white-winged vampire bats, I discovered that they move differently from the common vampire, and not only because they have shorter thumbs. Perhaps Diaemus bats once initiated flight similarly to their aggressive, spring-loaded cousins, but now their movements are more deliberately paced and show little sense of urgency. When placed on a force platform—which measures the forces generated by an animal as it moves across the surface—white-winged vampires would give a little hop or two, then scuttle off to find a dark corner to hide in.
Watching Diaemus feed in trees, rather than terrestrially like Desmodus, I learned why the former doesn’t need to catapult into the air. Approaching a roosting bird from below its perch on a branch, a white-winged vampire will move slowly and stealthily, always keeping the branch between itself and the underside of its intended avian prey.
Once situated beneath the feathered lunch wagon, Diaemus picks a potential bite site, usually on the bird’s backward-pointing big toe, the hallux. Feeding from that particular digit keeps the bat better hidden from above than if it were to feed on one of the forward-facing toes. After licking the chosen site for several minutes, the bat inflicts a painless bite with razor-sharp teeth, which characterize all three vampire bat species. White-winged vampire bat (Diaemus) climbs onto the back of a hen, which crouches as if mounted by a rooster. The bat will then feed, typically from the rear portion of the bird’s fleshy comb. The bite is never violent and very often occurs as the bird shifts position slightly. Anticoagulants in the bat’s saliva will keep blood flowing from the tiny wound well after the bat has drunk its fill.
Still hanging below its completely oblivious prey, Diaemus begins feeding, and within five minutes it begins peeing. To meet its energy needs, the vampire must drink close to half its body weight in blood at each meal, and blood is about 80 percent water. So the bat’s digestive and excretory systems have evolved to unload the excess quickly: the stomach lining is rich in blood vessels that absorb water and shunt it straight to the kidneys. Diaemus deftly avoids soiling itself while it eats by extending one hind limb sideways and downward. After feeding for fifteen to twenty minutes, the bat releases its thumbs from the branch, hangs briefly by its hind limbs, then drops into flight. Initiating flight in this manner means that there is no need for Diaemus to jump in the manner of its terrestrially feeding cousin, Desmodus.
On numerous occasions, my colleagues and I have observed Diaemus feeding on birds from the ground. Supporting its body in a low crouch, as compared with the extreme upright stance of Desmodus feeding on a cow, the white-winged vampire is adept at hopping around, rather comically, in pursuit of a feathered blood meal. Although ground locomotion has not been reported in the wild, we proposed on the basis of this behavior (and the possession of robust hindlimb bones) that white-winged vampires have made a relatively recent return to the trees, thus avoiding competition with their ground-feeding cousins.
During the terrestrial feeding bouts of our white-winged vampires, we also recorded a parasite–host interaction that rivaled chick mimicry on the “weird-o-meter.” When a bat leaped or climbed onto a chicken’s back to get a meal, a male chicken would quickly grow agitated and dislodge the bat with a shake and a peck. A hen mounted in this fashion, however, would immediately assume a crouching posture, giving the bat the opportunity to scuttle forward and bite the back of the bird’s head or its fleshy comb. The hen would maintain this crouch until after the vampire bat had finished feeding and hopped off. With a little research into poultry behavior, we learned that this was the exact posture taken by a hen while being mounted by a male bird—for a completely different purpose.
Another way that Diaemus differs from Desmodus and Diphylla is the presence of a pair of cup-shaped oral glands located at the rear of its mouth. When Diaemus gets upset or Diaemus possesses a unique feature among the three vampire bats, a pair of cup-shaped oral glands that project forward when the bat is agitated or during displays of dominance. The glands emit a fine spray of musky-smelling liquid. engages in battles for dominance, these glands are projected forward and can be seen quite easily when the bat opens its mouth. Diaemus simultaneously emits a strange hissing vocalization and a fine spray of musky-smelling liquid from the oral glands. Although a detailed study remains to be performed, the oral glands of Diaemus appear to be employed in self-defense, as well as in communicating information such as status, mood, and territorial boundaries to others of its kind.