Every day, I and my undergraduate assistant Kim Brockmann fed a Snapple bottle full of cow’s blood to our captive vampire bats. Our colony consisted of twenty-two animals—eleven common vampire bats (Desmodus rotundus) and eleven white-winged vampires (Diaemus youngi)—and we maintained them for two years while I was doing my graduate work at Cornell University. One of the keys to our success was giving them the opportunity to feed on a live hen once a week.
It was on one of the first of those special feeding days that I noticed two of the white-winged vampires doing something incredible. They crawled across the floor of their feeding enclosure like a pair of spiders, and then one of the bats made a bold approach to a rather large hen. The bird cocked her head to one side, eyeing the bats. Her beak could have severely injured or even killed them, so I got ready to intervene. Sharing my concern, perhaps, one of the vampires stopped a couple of inches beyond pecking distance. The other bat, however, crept even closer, and then, amazingly, it nuzzled against the hen’s feathery breast. Instead of becoming alarmed or aggressive, the bird seemed to relax. The vampire responded by pushing itself even deeper into what I would later learn was a sensitive section of skin called the brood patch: a feather-free region, densely packed with surface blood vessels, where body heat is efficiently transferred to the hen’s eggs or to her chicks. As I watched, the hen reacted to the bat by fluffing her feathers, hunkering down—and closing her yes.
My God, I thought, these bats have learned to mimic chicks!
What was most remarkable to me was that in all likelihood chick mimicry wasn’t innate behavior written into the D. youngi DNA over millions of years. It had probably developed in less than a thousand years—since humans brought domesticated fowl to South America. Were vampire-bat mothers teaching this cuddle-up trick to their young?
So enthralled was I at this wonderfully diabolical maneuver and its implications that I didn’t notice that the second vampire had disappeared under the hoodwinked hen’s tail feathers—not until several minutes later, that is, when a thin trickle of blood appeared on the floor behind the bird. Through the gloom of the darkened enclosure I could see a small puddle forming, glistening like red tinsel.
Vampire bats feed solely on blood, and their adaptations to the peculiar challenges of that diet make them among the most highly specialized of all living mammals. Only three bat species out of the 1,100 in the order Chiroptera qualify as vampires. As I began to take an interest in these creatures, I noticed that vampire-bat researchers (with a few notable Mexican and South American exceptions) hadn’t done much with the two rarer vampire bat species—Diaemus youngi, described above, and the hairy-legged vampire, Diphylla ecaudata. Instead, most of their research and nearly everything that had been written about vampire bats dealt solely with the common vampire bat, Desmodus rotundus. I wondered why. The bat experts I consulted told me confidently that all vampire bats would act similarly, but how could that could be so? With overlapping ranges and a coveted resource (blood), wouldn’t the species be competing with one another, and wouldn’t it be likely that differences in behavior and anatomy had evolved to reduce that competition?
Perhaps the reason for the near-exclusive focus on D. rotundus can be found simply in the name “common.” This species is numerous across a widespread range that includes Mexico and Central and South America; furthermore, it has been maintained successfully in captivity for more than seventy years, with some individuals surviving for as long as twenty years. The hairy-legged and white-winged vampire bats, by contrast, are far more difficult to locate and capture within their more limited ranges, and they long had a reputation for being difficult to maintain in captivity. As a result, even though local scientists in places like Trinidad and Brazil, where the less common vampires live, had been aware of differences among the vampire species for years, it wasn’t until the very end of the twentieth century that the mainstream scientific community began looking at each of the three vampire bats as separate and distinct. Hence, the door was wide open for the comparative work I’d proposed to undertake, and for new discoveries like the one described above concerning chick mimicry.