Following the Nectar Trail

Learning more about the diets of lesser long–nosed bats is helping to unravel the secrets of their long seasonal migrations. . .

by Theodore H. Fleming

Biologists have long suspected that the annual migrations of lesser long-nosed bats (Leptonycteris curasoae) northward from Mexico into southern Arizona and back again are timed with the flowering and fruiting of various food plants. But not until recent studies were we able to provide concrete evidence.

In the spring of 1989, Merlin Tuttle, several colleagues, and I began studies of these endangered bats. Our initial field work was designed to determine their importance to three species of columnar cacti in the Sonoran Desert ecosystem of northwestern Mexico (BATS, Fall 1989, and National Geographic, June 1991). What these studies revealed is that lesser long-nosed bats play a key role in the reproductive success of organ pipe and cardon cacti and are also important, though to a lesser degree, to saguaro cacti. Lesser long-nosed bats, in pollinating the flowers and dispersing the seeds of the fruit of these cacti, ensure that some of the most familiar sights of the southwestern desert–the giant cacti–will continue to survive, and along with them, the health of an entire ecosystem.

As is true of many scientific investigations, our work raised new questions. One of the things we wanted to learn was whether Leptonycteris, which we believe migrates from the tropics and subtropics of Mexico to the Sonoran Desert of northwestern Mexico and the southwestern United States each year, is a dietary specialist. Does it feed exclusively on cactus and agave flowers year-round? Or does it change its diet seasonally with its migrations?

One of the techniques we are using to study the diet of Leptonycteris is to determine the stable (non-radioactive) carbon isotope composition found in bat tissue. In the early 1970s, geochemists, and more recently, archaeologists and ecologists, began using this technique to trace the movement of carbon and nitrogen through ecosystems and populations of plants and animals.

There are two stable isotopes of carbon: the common carbon 12, which comprises about 99% of the earth’s atmospheric carbon, and the much less common carbon 13. During photosynthesis, these two isotopes are incorporated in different proportions into plant tissues. The ratios of carbon 12 and carbon 13 found in plant tissues tells scientists what kind of photosynthesis, or photosynthetic pathway, a plant uses. In turn, the ratio of carbon isotopes found in animal tissues reflects the composition of the kinds of plants they eat–directly if they are herbivores, or indirectly if they are insectivores or carnivores.

Most temperate-zone plants, including most desert plants (except succulents), as well as many tropical trees and shrubs, use a photosynthetic pathway known as C3 (denoting a 3-carbon compound). Their carbon isotope ratio differs from that used by plants using a pathway known as C4. Many tropical grasses, which include such plants as corn and sugarcane, use this type of photosynthesis. A third group, referred to as CAM* plants, are those adapted to dry climates, mostly growing in deserts. All CAM plants are succulents and include cacti and agaves. These plants are unique in that they open their stomata, the pores in their surface, at night and close them during day–the opposite of the way other plants behave. In harsh desert climates, this adaptation conserves water and prevents dehydration by slowing water loss.

If lesser long-nosed bats are dietary specialists, feeding on cactus and agave flowers and the fruit of cacti year-round, their tissues should contain CAM carbon year-round. But if they switch from feeding on C3 plants when they are in the southern part of their range, to feeding on CAM plants, such as cacti and agaves, when they are in the northern part of their range in spring, summer, and early fall, the carbon composition of their tissues should show a cyclic variation over the course of a year.

To see which scenario was correct, we evaluated the carbon composition of minute amounts of tissue, mostly taken from dried museum skins collected throughout Mexico and southern Arizona at different times of year. Since carbon isotopes in preserved tissue do not change through time, the age of the tissue was of no consequence. Working in the laboratory of Dr. Leo Sternberg at the University of Miami, we obtained pure carbon from each sample and determined the specific isotope ratios.

The ratios supported the “seasonal switching” scenario. Bats captured during the fall and winter months in southern Mexico were feeding mostly on flowers of C3 trees and shrubs. In February, however, isotope ratios began to change, and by June, tissues from both northern and southern sites indicated that Leptonycteris was feeding exclusively on cactus or agave plants. In fall, the pattern reversed as the bats migrated south and once again began feeding on C3 plants.

These results, coupled with published data on the blooming times of bat-visited cacti and agaves, indicate that lesser long-nosed bats migrate along a broad “nectar corridor” consisting of night-blooming columnar cacti in the spring and agaves in the fall. When the various species of giant cacti begin to bloom in the southernmost part of the bats’ range in the early spring, the bats follow the flowers north. As one cactus species finishes blooming, another begins, providing the bats with a continuous supply of nutritious nectar during the journey. For their return south, agaves provide the trail of blooms.

It was interesting to discover that bats collected throughout the year in Baja, California contained only CAM carbon, indicating that lesser long-nosed bats in Baja are feeding year-round on either cactus or agave flowers. This suggests that, unlike their mainland-Mexico cousins, the Baja Leptonycteris are non-migratory. Published data on the blooming times of agaves in southern Baja indicate that some species flower in the late fall and winter, providing bats with flowers when agave blooms are absent in the Sonoran Desert of mainland Mexico and the southwestern United States.

What we can learn from determining the ratio of carbon isotopes in bat tissue has tremendous implications for bat conservation, especially for the management of endangered and migratory species. It has shown us that migratory Leptonycteris are highly dependent on plants of only two families (Cactaceae and Agavaceae) for food during a large part of the year. Factors that adversely affect columnar cactus and agave plants will also affect lesser long-nosed bats. And the status of these endangered bats will also, in time, affect the cacti and agaves.

Bat biologist Donna Howell showed that, without bats, the pollination success of Agave palmeri dropped to 1/3000th of normal. And while agaves suffer from diminished populations of bats, wild agaves in Mexico are being intensively harvested for production of “bootleg” tequila. As agave plants disappear, will the bats eventually have insufficient food for the return journey south? We can only speculate.

In the truest sense, they may not be able to live without each other. Their well-being, in turn, affects many other species of animals who also rely on the plants of these two families for food and shelter. Highly co-evolved ecosystems like the Sonoran Desert are fragile. To be concerned about the bats, we must also be concerned about the plants that provide them with food along their migratory routes. The nectar corridor must remain intact if lesser long-nosed bats are to complete their annual migratory cycle.

[bio]
Theodore H. Fleming is a Professor of Biology at the University of Miami at Coral Gables, Florida. He recently was named as head of the endangered species recovery team for lesser long-nosed bats.

[footnote]
* Crassulacean Acid Metabolism was named after the botanical family Crassulaceae, where this mode of carbon fixation–the process by which plants convert CO2 into organic material (sugar)–was first discovered. Plants in the family Crassulaceae are succulent herbs and small shrubs, including typical rock garden ornamentals such as Sedum.
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[side article]

A New Riddle

Our studies of Leptonycteris also led to some fascinating discoveries about the diet of another desert bat, the pallid bat (Antrozous pallidus). In the Sonoran Desert, pallid bats often share night roosts with lesser long-nosed bats. Night roosts are places bats use to rest and digest their meals. Upon entering these roosts during an evening’s feeding, they often were caught in the mist nets we set for Leptonycteris.

We immediately noticed something odd about the pallid bats we were capturing: their faces were often covered with pollen! Since pallid bats are insectivorous and also well-known for their propensity to capture large arthropods, including scorpions, we wondered why they were covered with pollen. Were they visiting cactus plants to catch insects, and only inadvertently picking up pollen, or were they really after the nectar and pollen?

Using the same carbon isotope technique as on Leptonycteris, we discovered that Antrozous was also obtaining some of its dietary carbon from cactus plants. What our results did not tell us was if the carbon was coming directly from cactus nectar or from insects that specialize in feeding on cactus plants. Since the only insects found regularly in cactus flowers are small beetles, which would hardly make a nutritious meal for a bat that normally feeds on large insects, such as crickets and grasshoppers, it seemed possible that Antrozous was visiting the flowers to drink nectar and not to eat insects.

We wanted to know more. My Mexican graduate student, Gerardo Herrera, James Findley of the University of New Mexico, and I conducted a geographic survey using the carbon composition found in museum specimens of Antrozous collected from locations where bat-visited cacti and agaves are present (the Sonoran Desert) or absent (parts of western United States away from the Sonoran and Chihuahuan Deserts).

The results generally supported our hypothesis that pallid bats in areas with spring- or summer-blooming bat flowers contained more CAM carbon than those taken in the same months in areas lacking bat flowers. Like lesser long-nosed bats, pallid bats from Baja were especially rich in CAM carbon, indicating that they too were feasting on the nectar from cactus and agave flowers. In contrast, tissues of control species of bats such as the California leaf-nosed bat (Macrotus californicus), big brown bat (Eptesicus fuscus), and Mexican free-tailed bat (Tadarida brasiliensis)–all known to be entirely insectivorous and collected in the same places and at the same time as the pallid bats–were completely different in their carbon composition.

These results suggest that pallid bats, in addition to being insectivore-carnivores, are nectar feeders in some parts of their geographic range. The presence of bat flowers that have evolved to attract nectar-feeders such as Leptonycteris appears to have provided an evolutionary opportunity for an insectivorous bat to become a flower visitor. This may offer insight into how early bats first began to co-adapt with plants.
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[sidebar]
For further reading:
Stern, K. R. 1991. Introductory Plant Biology. Fifth Edition. Wm. C. Brown, Publishers. Dubuque, Iowa.

Howell, D. R. 1980. Adaptive variation in diets of desert bats has implications for the evolution of feeding strategies. Journal of Mammalogy, vol. 61, pp. 730-733.

Tuttle, M. D. 1991. Bats, the cactus connection. National Geographic, vol. 179, no. 6, pp. 130-140.