At a ranch outside the capital city of Brasília, Uieda and I set up my infrared video camera at sunset. We aimed the camera upward, into the branches of a fig tree, for it was there that the resident guinea fowl went to roost at dusk.

Several hours after nightfall, as I stared bleary-eyed through the camera’s viewfinder, a pair of dark shapes flew past the sleeping birds.

“Wilson, check this out,” I whispered.

My friend, who had been dozing on the chair next to mine, was instantly alert. Less than a minute later, we performed the aerial reconnaissance a second time.

Uieda whispered a single word: “Diphylla.”

After that we saw nothing for several minutes—until a tiny pair of glowing spots appeared beneath one of the roosting birds. I hit the zoom on the camera, focusing in on the twin points of reflected light. They were eyes! Uieda traced a dark silhouette on the screen, and I could just make out Diphylla’s upside-down head peeking out from the guinea fowl’s feathery breast. “Dinnertime,” he said.

“This is different from Diaemus,” I responded.

Rather than feeding from below the branch, Diphylla was actually hanging from the bird! Even more interesting, photographs taken by Wilson Uieda and his colleagues at another site clearly showed that Diphylla was using its opposable calcar to get a grip on the body of its avian prey. Unlike the white-winged vampire, which generally hangs from a branch and feeds from the toes of perching birds, Diphylla made many of its bites around the cloaca—the common opening for the digestive, urinary, and genital tracts found in many non-mammalian vertebrates, such as birds.

Several days later, we visited a cave that was home to a small colony of Diphylla. Using the infrared camera again, we recorded three hairy-legged vampires as they moved across the stony ceiling. Not only were the bats walking upside down, they were moving backward (not really strange, since bat knees face backward). What was unique was the way they led with their hind limbs, carefully seeking a secure purchase before taking a step—and using their “sixth digits” much as a rock climber would use his thumbs.

After scrambling around the cave ceiling for a few minutes, the vampire bats disappeared into a narrow crevice. I left the cave elated that we’d been able to support my hypothesis with observations in the field. What had begun as a surprising observation back in New York City ended with the discovery that, just like the panda’s radial sesamoid bone, the hairy-legged vampire bat’s calcar had been co-opted for a new role as an opposable digit.

Vampire bats have long been prime candidates for superstition and folklore-based fear. Only relatively recently have they gone from barely glimpsed creatures of the night to subjects of thoroughgoing scientific research and increasing open-mindedness. Rather than presuming that the three vampire bat genera are similar, researchers are currently studying these mammals with an eye toward variation. As a result, we are discovering intriguing adaptations and behaviors related to blood feeding. But we’ve also come to understand that two of the three vampire species (Diaemus and Diphylla) urgently need our help if they are to avoid extinction over the next few decades. The welcome shift in the vampire bat’s public image may be coming just in the nick of time.

This article was adapted from Dark Banquet: Blood and the Curious Lives of Blood-Feeding Creatures, by Bill Schutt, © 2008. Reprinted with permission from Harmony Books, a division of Random House, Inc. All rights reserved. Click here for ordering information.

Web links related to this article:

• North American Symposium on Bat Research
• Science at the American Museum of Natural History (Science and research at AMNH)