Think bats in the Chihuahuan Desert, and Carlsbad Caverns leaps to mind. But there’s a smaller bat flight here that’s equally fascinating.
At sunset tonight, a few thousand Mexican long-nosed bats will fly from a cave high in the Chisos Mountains. They’ll disperse to feast on agave nectar — pollinating the iconic plants in the process. These “agave bats” are deeply imperiled by human impacts. For now, they’re holding their own.
Dr. Loren Ammerman, of Angelo State University, has studied Big Bend bats for decades.
“There are 22 documented species in Big Bend,” she said. “It's an excellent place to study bats.”
Each summer, Ammerman conducts a census of Mexican long-nosed bats — listed as endangered since 1988.
With a thermal camera, she records the bats as they leave the park’s Emory Cave. Big Bend is only one stop on their epic journey, and their population, between 2,000 and 3,000, “trickles in,” with numbers peaking in early July.
The bats are known to winter and mate in a single cave — Cueva del Diablo, near Mexico City. While the males stay put, pregnant females then begin a nearly 2,000-mile roundtrip, following a singular resource.
“The bulk of the bats are moving when the agave are flowering,” Ammerman said. “And so as the flowering or the phenology of the plant is changing, they're following that northward.”
Agave nectar is the bats’ main food source, and their migration follows the agave bloom.
Their first known stop is Infierno Cave, near Monterrey. Here, newborn bats learn to fly. Their skills are immediately put to the test, as they join their mothers in the 400-mile flight to Big Bend.
Ammerman and her colleagues have place “pit tags” — similar to the microchips used for pets — into bats at Big Bend and other sites. The findings have only complicated the picture. None of the bats she’s tagged have been detected at other caves. And she’s found that some Big Bend bats disappear from Emory Cave, to return after several nights. That suggests there’s another, as-yet-unidentified roosting site here.
Strangely, she’s learned that juvenile bats linger in Emory Cave long after their mothers depart.
“And at some point they leave,” Ammerman said. “But how do they know where to go? Maybe there's one important adult that stays, and they all follow that one adult. We don't know.”
Indeed, much about the bats’ migration is mysterious. Where do they go from Big Bend? A few bats have been spotted in the Chinati Mountains. They may have a roost there.
One thing is clear: These bats face numerous threats. For tequila and mezcal, producers must harvest agaves before they bloom. The booming market for the spirits is eliminating the bats’ food supply. And despite the cave’s protected status, homes are being built near Cueva del Diablo, where the bats mate. Part of the cave recently collapsed, after homes were built on top of it.
Even in Big Bend National Park, increased drought and wildfires threaten to decimate the agaves upon which the bats rely.
But there are also encouraging signs. In Mexico, the bats were recently found roosting at an abandoned power plant — suggesting they can adapt. And the Sul Ross State University greenhouse, in partnership with Bat Conservation International, is growing agaves to plant in West Texas, to supplement the bats’ food supply.
Ammerman said she’s hopeful these remarkable creatures will continue to haunt the desert night.
“They have a lot of threats,” she said. “They're not out of the woods, and they need help. I feel like they're definitely at a critical juncture right now. I guess I'm pretty optimistic about what these wild animals are going to be able to do.”
