Nectar Bat Rescue Tied to Finding Elusive Lairs

Bijal P. Trivedi
for National Geographic Today

August 28, 2001

Scientists are searching for the roosting places of an endangered
species of bat that survives almost entirely on the nectar of agave
plants.

The population decline of greater long-nosed bats
(Leptonycteris nivalis), which migrate between central Mexico,
the Big Bend National Park in Texas, and the southwestern tip of New
Mexico, has been attributed to disturbances of the bats' roosting sites
and the loss of their food sources.

"There used to be tens of thousands of these nectar bats in the Chisos caves in Big Bend National Park, but these numbers have fallen to somewhere between five and three thousand," said ecologist Brian Keeley of Bat Conservation International in Austin, Texas. He is leading the efforts to find roosting sites along their migratory path.

Many agave crops in Mexico, where the plant is grown primarily for tequila production, have been destroyed by bacterial and fungal diseases. The huge losses have led plantation owners to harvest wild agaves, wiping out the bats' primary food source.

Another threat comes from a public backlash against destruction by vampire bats in Mexico. The response has been general destruction of the roosting sites of many species of bats, including greater long-nosed bats.

"Loss of forests and native wildlife have caused vampire bats to turn to cattle for their meals," said Keeley. "Frustrated ranchers roll smoking tires or set off dynamite in the caves to kill the bats."

Search for Agave Groves

Keeley believes that once the lairs of greater long-nosed bats are found, scientists will gain a better understanding of the size and health of the population. The discovery also should lead to better protection of the caves where the bats roost.

The strategy to seek out roosting sites entails trapping nectar bats in nets suspended in and around the groves of agave, where the bats are expected to feed. The researchers then attach a radio transmitter to each captured bat as a means of tracking it to its roost.

Until now, the only known agave patch and roosting site of these nectar bats in the Big Bend National Park have been in the Chisos Mountains. In June, Keeley and his colleagues surveyed west Texas mountain ranges for more agave groves large enough to attract bats. "We fly low, at about 500 feet (152 meters), and the agaves are big, striking plants that are very easy to spot from a plane," said Keeley.

The agave plant is a blue-green rosette of thick rubbery leaves about three to four feet (one meter) in diameter. When an agave blooms, it sends out a thick woody stalk almost 15 feet (4.5 meters) high, which produces bunches of pale yellow flowers.

"There is a lot of evidence that the bat and agave co-evolved," Keeley explained. "This plant is perfectly tuned to the needs of the bat, releasing its nectar and pollen at 10 p.m., when the bats are out foraging."

Each plant can produce about eight liters of nectar, he added, which explains why the nectar bats rely almost solely on agaves as a source of food.

Nectar bats' dependency on agaves isn't all one-sided. Keeley said the large genetic diversity of agave species is thought to result from pollination by bats across a wide swath extending from central Mexico to the southern United States.

Ongoing Study

In three days of searching, Keeley's team identified four agave clusters in the Chisos, Davis, Glass, and Chinati mountain ranges of Texas.

"We set up nets around the agaves, and then sleep next to them," Keeley said. "Every few hours we get up to remove the bats and tag them."

However, the team failed to find any roosting sites of the greater long-nosed bats this year. One possible explanation is that the bats don't always return every year to roost in the Chisos Mountains. Keeley hopes to have better luck next year in the three-year study.

"We also didn't have access to the best agave clusters in the Glass and Chinati ranges because these were on private land," he said, adding that next year the team will try to get the landowners' permission to survey and study those areas.

Meanwhile, the team is encouraging that similar surveys and research efforts be conducted in Mexico in the coming winter.

The study is part of Programa Para la Conservacíon de Murciélagos Migratorios (Program for the Conservation of Migratory Bats). The program is a collaborative project by American and Mexican scientists to protect bats that migrate annually between the two countries.

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