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

National Wildlife, June-July, 2001 by Michael Tennesen

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."