It is well after midnight in a silent backyard in the Tehuacan Valley of central Mexico. The greenish image in the video camera’s viewfinder reveals a few ants crawling across a bugle-shaped flower. Then I hear a faint chirping as a receiver-datalogger announces the arrival of a bat wearing one of our radio transmitters. The chirps grow louder and a Mexican long-tongued bat emerges from the darkness. It approaches the flower, which is growing high on the soaring column of a pitaya cactus. The bat hovers for a split second, dipping its face into the open flower, then flies off into the night. And the cycle continues.
Nectar-eating bats have been pollinating columnar cacti such as pitaya in Mexico and the American Southwest from time immemorial. But the arid landscape, once a vast smorgasbord of resources, is becoming fragmented as towns, farms and ranches eliminate essential bat roosting and foraging habitat. Human alteration of natural landscapes is a core bat-conservation issue here and around the world.
Working with Alejandro Casas and Kathryn Stoner of Universidad Nacional Autonóma de México and photographer José Antonio Soriano, and with a Graduate Student Research Scholarship from BCI, I am exploring how human use and management of cacti affects bats – and how bat activity affects the cacti on which so many species depend.
Along the way, we are educating the indigenous Mixtec and Nahua people of the valley about the value of bats, especially in maintaining the popular and important pitaya cacti that grace so many of their yards. We hope our results can help improve cactus management to benefit both bats and humans.
The pitaya (Stenocereus stellatus) provides an excellent system for studying how human use of a plant affects the biology of the plant and the bats that depend on it. The Tehuacan Valley is rich in bat fauna and, not coincidentally, is peppered with patches of plant species, especially cacti, with large, night-blooming flowers that are specifically adapted for pollination by bats.
Moreover, many of the region's colum¬nar cacti are self-incompatible, meaning they must receive pollen from genetically different plants in order to produce fruit. The wide-ranging foraging patterns and high energy needs of nectar-feeding bats make them ideal pollinators for such plants. Traveling long distances, fruit- and nectar-eating bats play key ecological roles in moving pollen and seeds throughout the ecosystem. They create mobile links which connect habitats that would otherwise be genetically isolated.
The pitaya cactus (or xoconochtli, as it is known locally) produces flowers and fruits on slender, spined columns that can reach 12-15 feet (4-5 meters) high. People of the valley collect the fruits and other parts of the plant in the wild, as well as from pitayas ¬cultivated in home gardens. They also manage wild pitaya by leaving desirable plants standing when native forests are cleared for agriculture and pastures.
As they deal with the cacti in these three types of populations (wild, managed and cultivated), residents change the size and density of the resource available to native bats – the lesser long-nosed bat (Leptonycteris yerbabuenae), the greater long-nosed bat (Leptonycteris nivalis) and the Mexican long-tongued bat (Choero¬nycteris mexicana) – that feed on pitaya nectar and pollen.
Pitaya is grown mainly for food, and fruits on cultivated plants tend to be much larger than those of wild plants and to occur in a variety of colors: orange, white, pink and yellow. Fruits found in the wild are small, spiny and have only red flesh. Our research is planned as a springboard studying the potential for a commercial market for pitaya fruit.
Villagers plant pitaya beside other edible and medicinal plants. They collect cactus branches from the wild for transplanting, exchange cactus material with neighbors and cultivate seedlings that establish naturally in their gardens.
But most of the farmers did not realize that the success of their pitaya crops de¬pends on the bats that transfer pollen among plants. So early in our fieldwork, we organized meeting in various towns. We set up a slide projector to display images on the side of the town hall and placed chairs under the porch. As dusk fell, families began wandering in to see the show.
They enjoyed seeing slides of their towns and gardens and laughed when they recognized familiar faces in slides taken during earlier re¬search. They nodded agreement as we explained the importance of planting diversity and using native species in their gardens. Then we surprised them with the fact that about 30 species of beneficial bats live in their area. We explained that many of those bats eat insect pests, others transfer pollen between plants and still others disperse seeds so new plants can grow.
In Chinango, the center of our research, Ana Pérez Ortiz spoke for her neighbors, who murmured assent, saying she’d had no idea that bats eat insects or nectar. Almost all the groups we spoke to expressed surprise at just how valuable bats were – not only to the environment but directly to their communities.
Most had heard of vampire bats, which can be harmful to livestock. We noted repeatedly that vampires, uncommon in the area, are greatly outnumbered by beneficial bats. Misplaced efforts to eradicate vampires can be devastating to valuable species and produce serious economic damage in their valley.
We brought a fruit-eating bat to our presentation in Chinango. It was a real hit, with everyone leaning forward for a closer look at the undersized creature. Teenage boys laughed nervously, but girls cooed “que linda” (“how cute”) and reached out to touch its downy fur.
We spent most of our days mapping the location of cacti and took samples of each one for genetic analysis. As we walked through the pitaya gardens, we identified flower buds that would open during the night – buds that were swollen to double their size and had a diamond shape with light pink to white edges on the bracts (the structures that protect the unopened flower).
At night, we would return to those plants with cameras. Between 9 and 10 p.m., bracts would begin to fold back, exposing the internal cone of white petals and hundreds of pollen-filled stamens surrounding the long slender pistil in the center of the flower. At its base, the flower would produce nectar. When bats dipped into the flowers to sip the nectar, their faces would be coated with pollen. They would carry that pollen to the next flower along their way, transferring it to that pistil. By sating their hunger for nectar, the bats ensure pitaya flowers are cross-pollinated and can produce fruit.
To document the bats’ foraging activity, we focused Sony low-light digital video cameras on opening flowers and set them to record all night. We placed mist nets near our filming site to capture, measure and tag nectar-feeding bats. We also attached a tiny radio-transmitter to each bat, then released it.
Watching the videos the next day, I would record details of the bat visits: how each approached the flower, the number of wing beats as they hovered and how far each put its face into the open flower. When one of our radio-tagged bats appeared in the videos, I could add data about the specific bat and check the datalogs in our receiver to determine how often that particular bat returned to the garden for nectar.
Townspeople often joined us in the field. Homeowners such as Apuleyo Hernández Ocampo, Maura Hernández Ocampo and Rodríguez Pimentel Delphino pointed out cacti in their gardens and described the type of fruit each produced. A few people joined us in the late evening as we set up cameras or extracted bats from mist nets. A local property owner, Lorenzo Hernández Ocampo, joined us as we worked outside of town in the managed and wild populations.
During two summers of fieldwork, we collected 316 plant samples, tagged 23 bats and collected 393 hours of video that recorded 7,103 bat visits to pitaya flowers. The primary visitors were lesser long-nosed bats and Mexican long-tongued bats, although other flower visitors included large hawk moths. We found, however, that bats are the only effective pollinators of pitaya cactus: If there are no bats, there will be no fruit.
My results clearly show that the home gardens are a very important resource for Tehuacan Valley bats, es¬pe¬cially in June and July. On some nights, these flowers received bat visits every five minutes. Managed and wild pitaya populations tended to receive fewer pollination visits, which might account for their lower fruit production.
My data are still being analyzed. The results will give us a much better idea of bat-visitation rates in cactus populations under different forms of management, while genetic information from the cacti will let us determine just how bats are moving pollen between populations.
Documenting the critical role of nectar-eating bats in the production of pitaya fruit should prove a valuable tool in encouraging conservation efforts. Documenting details of that interaction will help develop effective strategies for conserving nectar-eating bats in the patchy environments that they must share with people.
Jennifer Cruse-Sanders is a biologist who studies the impact of human activities on native plant populations. She is completing post-doctoral research in evolutionary biology at Rancho Santa Ana Botanic Garden in Claremont, California, and will join the faculty at Salem College in Winston-Salem, North Carolina, this fall.