Twenty miles east of the Malay Peninsula sits Tioman Island, a speck of land blanketed in dense rainforest and golden, sandy shores. Hundreds of species inhabit the island, including oddities such as a land-walking catfish, a “flying lemur” called a colugo, and most recently discovered, a bat that hunts during the day.
Yes—you read that correctly. Scientists have observed several Blyth’s horseshoe bats zipping after forest insects in broad daylight, day after day.
The behavior distinguishes them from almost every other species of bat in the world—from Mexico to Vietnam—because those bats hunt at night.
“This is a particularly interesting case on the island,” says Marcus Chua, a graduate student at George Mason University who described the horseshoe bats’ behavior earlier this year in the journal Mammalia. “It’s pretty surreal.”
But this isn’t the first bat species known to venture into daylight—it’s the fourth. And the other three also inhabit islands. Scientists have not missed these similarities. Instead, they have used them to answer an outstanding evolutionary question: Why are bats nocturnal?
Forced into Night
Roughly 1,400 bat species live around the world. And whether they eat fruit, nectar, bugs or even frogs, they almost invariably hunt at night.
For at least the past 40 years, scientists have hypothesized that birds pushed bats to become nocturnal. Competition from birds with a taste for insects, like swifts and swallows, or threats from birds with a taste for bats, such as hawks and falcons, could have pressured the flying mammals to leave daylight as early as 54 million years ago, when they evolved into the animals we know today.
Another theory is that bats might also have a low tolerance for heat. Their dark, thin wings easily absorb the sun’s rays. Consequently, bats could risk overheating if they’re out during the day.
John Speakman, a zoologist at the University of Aberdeen, started investigating bat nocturnality in the early 1990s, mostly because he never had a good answer when the public questioned him about it, he says.
To test those hypotheses Speakman sought a place where bats’ predators, competitors, or both were absent—like an island.
He attempted to find an answer on São Miguel island off the west coast of Portugal. The island is home to an insect-eating bat called the Azores noctule, which Speakman had read was commonly seen flying during the day. Given that the island is home to nearly no insectivorous or predatory birds, it seemed a good starting point.
“I think to try and understand why they fly at night, you’ve got to look at the exceptions…[to] see what they’re doing and what’s going on,” Speakman says.
He and a colleague traveled to São Miguel in 1988. And indeed, within a few hours of arriving, they saw a bat flying in daylight, he says.
Although daytime-flying bats are few, Speakman had a knack for finding them, whether on mainland Europe or near the Arctic circle. Islands, however, provided the natural experiments he and others needed to test each theory.
In 1995, Speakman sent a student to Samoa to test the overheating hypothesis with the Samoan flying fox, an endemic bat that soars on rising heat during the day. She compared when the bats emerged to eat fruits with the island’s annual weather changes. The student observed that the bats regularly avoided the hottest hours but still frequently emerged during daylight, Speakman says. These observations suggest that the hypothesis that bats became nocturnal to avoid overheating isn’t correct.
Elsewhere, predators, competitors, or both may be absent, giving bats the chance to emerge during the day. It’s an especially ideal scenario for insect-eating bats because insects are nearly 100 times more abundant during the day.
“This is exactly what happens on a few islands,” says Danilo Russo, an animal ecologist at the University of Naples Federico II in Italy, who later joined the quest to understand bat nocturnality.
Between 2009 and 2010, Russo and his colleagues traveled to São Tomé Island, just west of Gabon. They aimed to record activity of the island’s bat species, Noack’s roundleaf bat. Because the island lacks predatory birds, the scientists suspected bats would emerge in daylight. Sure enough, they found the bats regularly hunting from 9 in the morning until 3 or 4 in the afternoon.
“Bats that fly in daytime are the exception that proves the rule,” Russo says.
What It Means
Each scientist agrees that predation is likely the reason bats are nocturnal. Even the horseshoe bats on Tioman Island flutter around without an aerial predator lurking.
But they’re still cautious about that conclusion. Some of the islands with daytime-flying are also home to other bats that don’t hunt during the day. And considering the tiny sample of bats that do, Speakman says, it’s hard to say anything with confidence.
Discoveries like the one on Tioman Island, however, continue to supply scientists with new opportunities to address the question of bat nocturnality.
“What I think this behavior suggests is that it’s much more plastic [flexible] than we thought,” Chua says.
Jeremy Rehm is a science writer and reporter based in Washington, D.C. His work has appeared in Nature, Science, and National Geographic, among other outlets. Follow him on Twitter.