Journey of the Nectar Bats
The annual migration of winged mammals from Mexico to Arizona, vital to plants north of the border, is at risk
- Michael Tennesen
- Jun 01, 2001
THE DESERT SKY changes from red to black over the Southwestern Research Station in the Chiricahua Mountains of Arizona. There, Katy Hinman, a graduate student at State University of New York-Stony Brook, positions a night-vision video camera underneath an agave plant and aims the lens at the plant's flowers. Her goal is to find out who is taking the agave's nectar under the cover of darkness.
The next morning, Hinman plays the videotape. She watches creatures the size of sparrows hover into view and flit from blossom to blossom, burying their long snouts into each flower. The agave, also known as the century plant, blooms only once in its life, sending a branched stalk as high as 25 feet into the air, tipped with clusters of whitish flowers that first open after dark. "I've watched nectar bats visit a cluster of flowers 150 to 300 times in a night," she says.
Hinman's tape is the most recent evidence of a vital but largely unheralded ritual in the deserts of the Southwest. On warm nights in the late spring and early summer, desert plants such as agave, and cacti such as organ pipe, cardon and saguaro burst forth with flowers that may last only a night but are filled with energy-rich nectar and pollen. As if on cue, more than 100,000 nectar bats flocking north from their homes in central Mexico arrive at just this moment to share in the desert's ephemeral feast.
But this carefully choreographed annual rite is at risk. Destruction of habitat along the bat's migratory path, deep-rooted fears of these harmless creatures, cattle grazing, mine closings--even the thirst for margaritas and other tequila-based drinks--are putting these animals in jeopardy all along their migratory route. That route stretches from tropical forests in the Pacific lowlands of central Mexico, north through the Sonoran Desert and into southern Arizona. And since the bats play key ecological roles in these habitats, "if you destroy the bats in caves in Mexico or abandoned mines in the U.S., you affect ecosystems along that entire migratory route," says Hinman.
Nectar bats eat nectar and fruit from a variety of desert plants. Unlike insect-eating bats, these animals rely on pollen as a protein source. Nectar bats have shorter ears, bigger eyes and longer muzzles and tongues than their insect-eating relatives. They also make flapping sounds and are much more direct in flight, whereas insect-eating bats are silent in the air, zigzagging back and forth in their pursuit of prey. The tongues of nectar-eating bats are also long and narrow, with brushy surfaces at the end. "It allows them to extract a lot of nectar in a very brief visit," says Ted Fleming, professor of biology at the University of Miami, "since most of their visits last less than a second."
Although no nectar bats live exclusively in the United States, 3 of the 34 species in Latin America journey north of the border to spend their summers here. Two are found in the Sonoran Desert in Arizona: the lesser long-nosed bat and the Mexican long-tongued bat. The other, the greater long-nosed bat, ranges through southern New Mexico and into Texas. Both of the long-nosed bats are endangered due to habitat destruction and their habit of roosting in large numbers in a few caves.
Each spring pregnant female nectar bats leave their mates on Mexican wintering grounds and fly north, following the blooming schedules of various cacti and agaves. They arrive in the United States when these plants are bursting with nectar and pollen, which provide a bounty of energy and protein for both mother and pup. In turn, the bats--along with birds, moths and bees--play a vital role in the pollination of the Sonoran Desert.
Many of the desert plants have evolved in specific ways to attract their winged pollinators. The flowers of saguaro and organ-pipe cacti, for example, first open after dark when the nocturnal bats are in flight. Cacti have light-colored, large-mouthed, bowl-shaped flowers that are easy to see in the dark. The clustered, funnel-shaped agave blooms have a strong odor like rotting cantaloupe, says Hinman, which makes them easy to locate. Other scientists say the plant's odor is like a bat's, but Hinman counters, "It's hard to tell if nectar bats smell like that normally or because they're sticking their heads in agave plants all night."
Tiny stems called anthers surround the reservoirs of these flowers with little pollen sacs at their tips. When the nectar bats stick their long snouts into the blossoms to lap up the nectar, the pollen sticks to the animals' heads, chests and chins. When the bats move on, they carry the pollen to other plants, where the pollen fertilizes the receptive plants and initiates fruit production.
After a cactus flower is pollinated, its base swells into a fruit filled with seeds. The advent of these fruits comes at a critical time for summering bats and other creatures in the southwestern desert. "It's right before the late summer monsoon, when everything is very hot and very dry," says Janet Tyburec, a biologist with Bat Conservation International (BCI), a nonprofit group based in Texas. "Birds and other animals don't get a lot of moisture from water sources, so they must get most of their moisture from food."
The benefit to the plants for allowing animals to take their fruit is that the animals disperse the plants' seeds. Some birds take mouthfuls of cactus fruit to trees to eat, and the seeds they drop or defecate there may sprout and grow. Nectar bats, which may take cactus fruit in desert basins and feed on it while flying to mountain roosts as far as 30 miles away, spread the seeds much farther.
The bats' future is at risk, however. Fleming took a drive last year along the bats' migratory corridor. "On over half the route, the desert wildlands or tropical dry forests have been converted to agricultural fields, which produce much of the winter vegetables exported into the United States," says the scientist. "Resorts and RV parks are springing up all along the coast. Cactus populations are few and far between. Bats are relegated to feeding at little mountain stops along the way, which, like islands, have survived development."
Bats in Mexico suffer not only from habitat loss but from superstitions. When Merlin Tuttle, founder and executive director of BCI, went to Mexico in the early 1990s to study bats, he looked for mines where the long-nosed bats congregated. "When I asked about the location of the mines, people were surprised I would go near them. They told me that if a bat urinated on you, you would go blind."
During the past decade, the legend of Chupacabra, half-man/half-bat, has spread through Mexico and parts of Latin America. The mythical creature is said to appear after dark to suck the blood out of livestock and people. Inflamed by the myth, some people have attacked bat roosts. BCI sponsored a survey by Mexican bat biologist Arnulfo Moreno that documented alarming losses at eight caves in Mexico where locals had attempted to burn the bats out.
Tequila drinkers may also indirectly take a toll on the bat corridor. The same agave plants that provide nectar for the creatures also are used to make tequila and mescal in Mexico. Just before the agave flowers, the plant's heart is bursting with sugars and carbohydrates. Agave growers harvest the plants, cut out the hearts, roast them and use them for fermentation. Unfortunately, the harvest occurs before stalks and flowers are produced, thus eliminating an important food source for the bats.
Conservationists are encouraging growers to plant hedgerows of agave along fields and roads, and to allow these plants to flower. But a bigger problem than legitimate growers may be moonshiners--people with illegal stills and no legitimate sources of agave--who harvest wild agave to make bootleg tequila and mescal.
Cattle grazing has detrimental effects as well, both to the plant communities and to the bats they support. Cattle eat all accessible cactus parts-- flowers, buds and developing fruit--from ground level to about five feet. "But their major impact is through the consumption or trampling of seedlings," says Fleming. "Cattle like to eat the new cactus shoots, and they like to hang out in shady places, under mesquite and ironwood trees, where seedlings struggle to get established."
Having a place to stay is as important along the migratory route as having something to eat. Female nectar bats that travel north in spring assemble in maternity roosts or abandoned mines, and each gives birth to a single pup in May. Such roosts may contain anywhere from a few thousand to 100,000 female bats. But abandoned mines on private property in the United States are liabilities to owners, and many have been closed. To counter this trend, biologists have installed mine gates that will let bats in and keep humans out. "But sometimes, you put up the gate and the bats abandon the cave," says Karen Krebbs of the Arizona-Sonora Desert Museum.
Conservationists are also trying to educate citizens on both sides of the border about the importance and vulnerability of nectar bats in the environment. BCI has launched a joint effort with the Arizona-Sonora Desert Museum and ecologists in Mexico City to locate key roosting caves and start education programs in nearby towns. The goal of the program is to change citizens of these communities from threats to bats, to stewards for them.
The program includes a series of storybooks in Spanish and English for children, along with games and exhibits. The results have been fantastic, reports Steve Walker, director of Latin American programs for BCI. "We've started a number of Friends of Lucia Clubs," he says. "Instead of spray-painting graffiti at the caves, the kids are having workdays to go up and scrape the graffiti off. Plus they're building trails and viewing areas. And the kids are bringing parents in to look at the exhibits."
Tim Snow, a biologist with the Arizona Game and Fish Department, says that education is helping to increase support for bats in the United States. "We get a lot of requests from schools for education programs. It's the parents that harbor the old myths about bats, but the kids who really get excited about them." Hinman agrees. "Bats don't have a good public image. People think bats, they think vampires and rabies. But a lot of that is changing. I'm finding more and more people who actually realize that those are misconceptions. The education programs are working."
Hinman sees a need not only for public interest, but for scientific attention, too. "There is still a whole lot more that we don't know about these creatures," says Hinman, such as "how important they are to the plants, how they are moving seeds and pollen throughout the environment, and how they are being affected by development and population growth along their migratory route."
But she smiles, raises her finger, and notes, "We're making progress."
Michael Tennesen spent a week with bat biologists last spring for this story. Photographer Merlin D. Tuttle is executive director of Bat Conservation International.