Intrigued by the physiology of an all-blood diet, a researcher gets acquainted with acquainted with a colony of rare white-winged vampire bats . . .
Intrigued by the physiology of an all-blood diet, a researcher gets acquainted with a colony of rare white-winged vampire bats . . .
By Metta Winter and Claudia Coen
Claudia Coen takes exquisite care as she pours blood from the Snapple bottle into four sections of an ice cube tray. Each day at noon she carries this high protein diet to the vampire bats in a secured room at her South American research station in Cali, Colombia.
She mustnt lose track of the schedule, because vampire bats need to eat every day in order to survive. Nor can she be cavalier about whats on the menu. These shy, intelligent creatures are fussy eaters indeed, dining on virtually the same thing day in and day out. Even the slightest variation will cause them to sicken and die.
It is this dependency on a singular food source that intrigues Coen, a graduate student from South Africa specializing in nutritional physiology and ecology at Cornell University. She originally returned to graduate school to study cheetahs, but became interested in vampires upon hearing another graduate students casual remark: We feed cow blood to the Diaemus six days a week, then put them with live chickens once a week--otherwise they get sick and die. (The chickens, incidentally, are not adversely affected.)
The Diaemus are Diaemus youngi, commonly known as white-winged vampire bats. They are shy, rarely seen animals that live in trees and feed primarily on bird blood--as compared to common vampire bats, Desmodus rotundus, which are abundant, widely distributed, and feed primarily on mammal blood [BATS, Spring 1991].
Coen became intrigued with the white-winged vampires and couldnt stop asking herself, What is there in chicken blood that these bats must have every week to live? While she was studying these remarkable creatures in the laboratory at Cornell, she maintained the only known captive colony of white-winged vampires outside of Latin America. When she moved her research to Latin America, she passed the colony on to the Burnet Park Zoo in Syracuse, New York, along with a detailed feeding protocol.
These animals have been given such an awful name, Coen comments, yet it comes from legends that predate their identification by thousands of years. And even more curious is the fact that vampire stories are prominent in the cultures of India, Africa, and Europe--countries where the animal has never even lived. So all over the world theyre thought to be such villains, but really theyre not. People just need to be exposed to them.
Despite their dicey reputation, vampire bats have much to distinguish themselves. The strikingly appealing white-winged vampires--a non-scientist might even think them cuddly--have a social organization thought to be on the level of some primates. No longer than a business card and weighing a little more than a first class letter, these brown-furred creatures with white-tipped wings live in groups numbering from a pair to more than 20. (Common vampires, however, often forms colonies numbering in the hundreds.) They hang together, play with exuberance (by chasing, huddling, and wrestling), groom each other, and will even regurgitate their own food and give it to a member of the colony who hasnt found a meal, making this sacrifice today knowing the same would be done for them tomorrow if needed.
When one of their number is even momentarily removed, then returned, to the cage, they greet each other both vocally and through a ritual of pressing their noses into each others armpits and wrapping their wing membranes tight around in a hug of welcome. Although shy of strangers, Coens vampire bats now greet her in the same way.
When I enter the room, theyll come forward in the cages. They answer back when I talk to them with high-pitched sounds made by sucking through my teeth. If I put my hand into the colony, theyll nuzzle their noses between my fingers in greeting. Theyre enchanting!
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. They might be expected to have a high metabolic rate like other bat species and birds; but to the contrary, their metabolic rates are low. Once set to lap up their meal (they lap like a dog, not suck as folklore would have it), vampires consume 50 to 100 percent of their body weight in blood. 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 vampires 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.
In contrast to common vampire bats, Coen has observed that captive white-winged vampires maintain a close association with their prey. Settling on the chicken, some feed and sleep intermittently for a period of up to eight hours. Coen describes how chickens become used to the presence of the bat and appear quite unperturbed as they bed down or move around with a bat nestled in their feathers. After feeding, the bats retreat to a roost site which is often in the same tree as the one in which they feed, eliminating the need for long distance travel.
Regular monitoring of blood samples by a veterinarian indicate that Coens laboratory chickens have neither contracted diseases nor become anemic, and they continue to lay eggs. Their role in the bat studies appears to cause them no distress. Ive had hens for up to four years, and I become as fond of them as I am of the bats, Coen says. Human accounts and observations of Coens chickens attest to the fact that the bite of a vampire bat is worse in the imagination than in experience. Although most people need never worry about being bitten by a vampire bat, reports from some who have been bitten say they were not even roused from their sleep. Vampire bats are silent and their bites are painless. Coen describes what she sees in her lab once a week:
First the bat chooses a wound site. On chickens, its often the nape of the neck, the comb, the cloaca, or a foot. Then it licks the wound site for a long time. This licking seems to soften and numb the site. It then bites out about an eighth-of-an inch chunk of skin with teeth so sharp its like being slit with a razor blade. After the wound has bled for a while, the bat puts its tongue in and rolls it around the wound, and in doing so, injects an anticoagulant so the blood wont clot. Then it laps up the blood like a dog, one drop at a time, its grooved lower lip and specialized tongue acting as a funnel. Soon after the bat has finished, the anticoagulant wears off so the chicken doesnt bleed to death.
In addition to the behavioral differences in the way vampire species feed, there are differences in the type of food they feed on. When blood is analyzed in a laboratory as a food, it becomes apparent that blood is not blood is not blood, Coen says. The composition of carbohydrates, proteins, fats, and minerals is not the same between mammalian and avian blood. Thus she asks, Are the differences in feeding behavior between Desmodus and Diaemus due to the differences in the behavior of chickens and cows alone, or are they driven by the distinct nutritional requirements of each bat species?
Coen explains that the quality of a diet is reflected in the way an animal responds to it. Common vampires are well adapted to the high-protein, low-glucose, low-fat diet of cow blood while white-winged vampires are not. Some white-winged vampires eat cow blood in captivity, indicating that they are able to switch from their optimal diet when it is not available; however, they appear to suffer from some toxic effect or limiting nutrient in the cow blood diet. When forced to subsist on cow blood for long periods of time they become lethargic, dehydrated, and often die. In some cases, white-winged vampires refuse to eat cow blood at all.
Because the common vampire bat feeds mainly on domestic livestock, it has become a pest of economic significance to the livestock industry. The immediate practical value of my studies, explains Coen, is that with a better understanding of the vampire bats feeding behavior and nutritional physiology, we can develop more effective control techniques specific to vampires as agricultural pests. This is of particular importance since they transmit diseases, such as rabies, as a consequence of their dietary needs and feeding strategies. I regularly receive inquiries from local farmers looking for information to help them control vampire bats feeding on their herds.
Coen is concerned about some of the local methods used to control vampire bats, in which whole caves are often dynamited or burned, killing not only the vampire bats that live there but non-target species of beneficial bats as well. She is working with Danilo Valencia at ICA (Instituto Colombiano Agropecuario) in Colombia to develop an education program for local farmers about bats and common vampire bat control. A cornerstone of their program is BCIs Spanish-language video on bat conservation and vampire bat control. In addition to their efforts in education and conservation of bats, Coen and Valencia will conduct a long-term regional study of the distribution and abundance of the common vampire bat in the Cauca Valley of Colombia.
Over the course of three years, Coen plans to determine how the vampire bats assimilate and digest blood. Soon she will be off in search of the third, most elusive species of vampire, the hairy-legged vampire bat (Diphylla ecaudata), which feeds solely on birds, but about which little else is known. Shell use a metabolic chamber she designed to conduct experiments comparing the nutrients the bats assimilate from diets in the wild with nutrients from diets provided in captivity. Shell also explore differences in the ways the three genera of vampire bats use different types of blood as a source of food. As she continues to apply science to the subject, we can only hope it will remove some of the mystery from this most notoriously mysterious of animals.
Metta Winter is a staff writer at Media Services, Cornell University. This article is based on a story that first appeared in ALS News, the alumni publication of the College of Agriculture and Life Sciences, Cornell University.
We thank Winter, Claudia Coen, and ALS News Editor Elizabeth Bauman for their permission and assistance in reprinting and updating the article. Many thanks as well to BCI member W.D. Burrows of Frederick, Maryland, for bringing the article to our attention.
Pharmaceutical Companies Learning from the Vampires
After many years of study, researchers have discovered the secrets of the anti-coagulants in vampire bat saliva, allowing them to create a drug that prevents blood clotting. The drug, facetiously named Draculin, is based on a protein isolated from the bat saliva, which can now be synthesized by recombinant DNA technology. The French company Rhône-Poulenc applied for a patent on this process in at least 80 countries last year.
Combatting Vampire Problems in Latin America
There is no denying that the common vampire bat has caused serious problems for ranchers in Latin America who are concerned about disease transmission to livestock. Common vampires have been quick to take advantage of the abundance of livestock on new ranchland created by deforestation.
But there are some simple solutions to the problem [BATS, Spring 1991], and since 1993 BCI has been working with a leading vampire bat authority, Dr. Rexford Lord, to teach them to ranchers. Through local seminars and wide distribution of the Spanish-language video he helped BCI produce, Lord has introduced the principles of bat conservation and vampire control to more than 20 Latin American countries. Word of mouth about his popular lectures has helped spread the message and gain important new sponsors and partners.
There is ample evidence that Lords program is proving far more successful than we initially envisioned. For example, newspaper articles about rabies outbreaks in Mexico now frequently mention cattlemen meeting to watch the BCI video. With a growing number of converts and the support of this program by BCI members Tom, Marilyn, and George Fifield, it should only be a matter of time before Lords message brings about a large-scale change of attitude in Latin America.
Coen enjoys the company of a group of white-winged vampire bats in her lab at Cornell last year. Although she holds her captive bats bare-handed, she warns that no wild bats should ever be handled without gloves.
The refrigerator at Claudia Coens former lab at Cornell was stocked with cow and chicken blood. The amount shown here would typically feed the bats for 120 days.
While feeding on the tail of a cow, this common vampire bat is simultaneously ridding itself of excess water from the blood it is taking in. This process of urinating as it drinks allows the bat to carry home the nutritious part of the meal without the excess weight of the water.
Left: White-winged vampire bats at the Cornell lab. These bats now live at the Burnet Park Zoo in Syracuse, New York.