On the hunt for their nightly meal, a swarm of Geoffroy's Rousette fruit bats bursts out of a Philippines cave in 2008.
In January 2011 a U.S. cave-mapping expedition stumbled upon an unusually high number of pregnant bats in the Monfort bat colony, in the country's southern Mindanao region (see map). The bat species does not usually give birth in January, making the discovery a "big surprise" and forcing the scientists to halt their mapping project, according to Norma Monfort, founder and president of the Monfort Bat Cave & Conservation Foundation.
The cause of the bat baby boom is unknown, although Monfort suspects one factor may be that the cave is protected from humans as an ecotourism site, which allows their numbers to grow. Monfort's family has owned the property for more than a hundred years.
In most of the bats' Southeast Asian range, people either hunt the mammals for food or disturb them while harvesting guano for fertilizer. If people enter a bat cave, nursing mothers can be easily startled, causing their pups to tumble to their deaths, Monfort said by email. (Interactive: Hear bat calls.)
But at Monfort cave, the 1.8-million-strong colony is not only thriving, in 2010 the Guinness Book of World Records deemed it the world's largest gathering of Old World fruit bats.
One of four small openings to the cave, this hole (pictured in 2010) is often referred to as the "maternity ward" and "nursery," Monfort said, since female bats give birth or nurse their pups near the opening.
After the discovery of the pregnant-bat explosion, Rick Sherwin of Christopher Newport University in Virginia installed infrared and thermal-imaging cameras at the cave openings. Sherwin captured rare footage of females giving birth—as well as males mating with pregnant females or females that had just given birth.
Yet the camera footage also revealed a dark side: males acting aggressively toward pups—and sometimes cannibalizing them, Monfort said. Such behavior may be because the bats are too tightly packed, Monfort told the Agence France-Presse in February.
"My greatest and major concern is the overcrowding of the bats," she told National Geographic News via email.
Bats hang near walls crusted with thick, bluish-black guano in the Philippines cave in 2011. (See readers' pictures of the Philippines.)
Before the days of commercial fertilizers, Southeast Asian farmers would heavily harvest guano for agricultural uses. Though not as common nowadays, guano harvesting continues in remote places, where it poses a threat to cave-dwelling bats such as the Geoffroy's Rousette fruit bat, Monfort noted.
Although not currently considered an at-risk species, Geoffroy's Rousette fruit bats have a low reproductive rate, generally producing only one pup a year.
The Monfort cave has an unusual configuration of four horizontal sinkholes (pictured, the third and largest opening) that allow visitors to peer in and see the mammals, Monfort said.
The bats, which hang from the horizontal rims of the cave holes, do not shy away from the light during the day, she said.
And the nightly emergence of the bats—which begins at 6 p.m. rain or shine—"is something to behold," Monfort added.
"At around 8:30 to 9:00 the last of the bats would have left, and yet on the cave walls cling thousands of golden eyes. … One would wonder who they are and why they do not leave with the entire colony."
Bats pack the walls in an undated picture of the Monfort cave. A conservative estimate of the cave's bat density suggests there are at least 640 animals for every 11 square feet (1 square meter), Bat Conservation International founder Merlin Tuttle reported in 2006.
The Monfort cave is unique not only because of its record-breaking number of inhabitants, but also because all the bats belong to the same species. Most bat caves have more than one type of bat, Monfort noted.
In the 1980s caretakers of the Monfort property would tell stories of a "white lady, alleged to be the spiritual guardian of the bat cave," Monfort said. "It was said that she showed herself only to a very privileged few."
In 2010 Manila-based photographer Nelson Rivera joined the lucky minority when he captured shots of this rare albino bat—later nicknamed "Blanca Bella," or "beautiful white."
"She was photographed pregnant last summer, then seen carrying a regularly colored … pup, which disappeared after five weeks," Monfort said, adding that the young bat must have learned how to fly.
Albinism in bats is rare but has been recorded before, including in five species of Indian bat, she added.
Bats are seen roosting on tree roots near the third cave opening in an undated picture.
The animals crowd the rim of the cave all the way down to the floor. Here, natural predators including rats, pythons, and monitor lizards wait for those unfortunate bats that can't find a spot on the walls and must roost near the ground, Monfort said. (Read "Bat Crash" in National Geographic magazine.)
In a constant flurry of activity, the Monfort cave bats fly from one area to another (as seen in an undated picture), socialize, and meticulously clean themselves—especially the males, Monfort noted.
Most fruit bats navigate using their vision alone. But Geoffroy Rousette fruit bats use both sight and echolocation—the radar-like ability of some animals to emit high-pitched sounds and detect obstacles or prey by listening to the way sounds bounce back.
Bats living in the Philippines cave's fourth opening (pictured in 2011) "seem to be of a more mature or older group," Monfort said.
The animals avoid a blank area of the wall where water routinely rushes down to the floor during heavy rains. According to Monfort, "the bats probably know better than to be caught sleeping during the deluge and be washed down to the bottom of the cave, where very thick guano will guarantee their deaths."
Overall, bats' "service to humanity is invaluable," Monfort said. For example, bats mostly eat insects and agricultural pests, including disease-carrying mosquitoes. They also are pollinators of valuable fruits and crops, she said.
"They are the unsung heroes I call my 'angels of the night.'"