By Andrew Stuart
Agave is one of the great resources of arid North America. It was a staple food for Native Americans. It’s notable for its beauty – its luminous floret and towering stalk. Agave spirits – tequila, mezcal – are prized exports.
But as close as the relationship between agave and humans has been, one mammal in our region has an even more intimate relationship with the plant.
Each summer, Big Bend National Park is home to a population of endangered Mexican long-nosed bats. As the glow fades on the Chisos’ ramparts, the bats set out from a mountain cave. They’re hungry, for only one thing: agave flower nectar.
The relationship benefits both bat and plant. Human activity imperils that bond.
The bat roost at Emory Cave – a deep fissure, near the Chisos summit – was first recorded in the 1930s. But its occupants’ story was mysterious. Scientists have begun to unlock those secrets.
Dr. Loren Ammerman, of Angelo State University, has researched bats in Big Bend for 22 years. Until recently, she was the only U.S. mammalogist studying the Mexican long-nosed bat, also known as the greater long-nosed bat.
“Bats are mysterious,” Ammerman said. “It’s very hard to study them, since they’re mostly active at night, and out of sight. Luckily we’ve gotten new technologies that are helping us to study them a little bit better. We have acoustic monitoring system, so we can hear their echolocation. I use a thermal-imaging camera, in order to detect bats at they fly out of the cave.”
Long-nosed bat mating is thought to occur in a single cave – Cueva del Diablo, near Mexico City. Leaving the males behind, the expecting females set forth from their tropical home, to follow the agave bloom. In winter, they feed on a variety of plants. Moving north, agave is it.
Mothers give birth during migration. Emory Cave is home from June through August. Along with El Infierno Cave, near Monterrey, Mexico, it’s the bats’ only known “maternity cave.”
Weighing 30 grams, sandy brown or yellow, the bats are finely adapted for nectar feeding.
They have muscular forearms. Insect-feeding bats are built to swoop and dive. But strong forearms allow this bat to maneuver around flowers.
Then, there’s the tongue.
“You have to see this on real high-speed camera footage – you can’t just watch it,” Ammerman said. “They extend their tongue, and there’s these little projections that fill with blood, and it makes it have more of a bristle tip. And the bristle goes into the nectar and it traps all the nectar, and they pull it back into their mouth. So they don’t have to sit there for very long. All they do is, slap my tongue into the nectar, pull it back, and I’ve got a mouthful of sweet stuff. So that’s pretty cool.”
How do they find agave at night? Scientists don’t know. Echolocation may play a role.
Hummingbirds, beetles and moths visit agave during the day. But agaves here can only be pollinated at night. The bats likely play an important role in plant reproduction and genetic diversity.
Biologists noted population declines in the 80s. The bat was listed as an endangered species in Mexico, and, in 1988, in the U.S.
Now, with a grant from Bat Conservation International, Ammerman and others are re-assessing the bat’s condition. Through 10 years of monitoring, the Emory Cave population has fluctuated between 1,500 and 3,000. Ammerman is installing digital chips in bats, to track their movements.
The bats face multiple pressures. Targeting vampire bats, ranchers in Mexico sometimes kill entire bat-cave populations. Development has destroyed agave habitat. And tequila and mezcal production are a major factor.
To make spirits, agave hearts must be removed before the plant flowers. Surging demand for tequila and mezcal means fewer and fewer flowering plants.
Mexican scientist Rodrigo Medellin had led a unique effort – working with producers to let some agaves bloom. Producers are now marketing spirits as “bat-friendly.”
Such efforts are critical, Ammerman said, if the bat is to survive.
“We know a lot about what is required for them,” she said. “It’s just going to be up to humans to not destroy a food source that we know is very important for this species.”
You probably won’t see it, but in the Big Bend night, bats and agaves are enacting an ancient exchange