In Tel Aviv, Israel, you might be privy to a strange sight: bats darting from tree branch to tree branch in broad daylight. These Egyptian fruit bats (Rousettus aegyptiacus), unlike most other species of bats, are diurnal and have great vision. But they aren’t just using their eyesight as they forage for ripe fruit, researchers reported April 11 in Current Biology; these fruit bats are echolocating during the day.
Until recently, researchers thought that Egyptian fruit bats could only echolocate in dark caves. But a few years ago, researchers discovered that the bats could echolocate outside at night and in dimly-lit lab settings.
For two years, photographer Yuval Barkai took thousands of photos of the bats on their daily excursions around Tel Aviv. Wanting to learn more about what the animals were doing as they foraged, Barkai approached Yossi Yovel, a coauthor on the earlier paper and a biologist at Tel Aviv University. In the photographs, Yovel and biologist Ofri Eitan, another collaborator on the previous echolocation project and a graduate student at Tel Aviv University, noticed something unusual.
“I said, ‘Look, the bats are smiling,’” Eitan tells The Scientist, referring to the upturned corners of the bats’ mouths—a sign that the bats are producing tongue clicks. “‘It looks like they’re echolocating in the daytime.’” Many of the photos pictured smiling bats, even at hours when it looked like they should be able to see perfectly well.
The researchers went out into the field to test whether the bats could indeed echolocate in the daytime. Eitan followed Barkai to a location in Tel Aviv just outside a theater, where Barkai knew the bats were active during the day. There, Eitan set up his laptop and audio recording equipment and, to his surprise, recorded the bats clicking.
To test whether the bats were echolocating for a reason or merely clicking, Eitan performed similar experiments in other locations around Tel Aviv. In one, he set up his equipment at an artificial pool of water that’s a popular drinking spot for bats, which fly downward toward the water, collect it in their wings, and fly upward again. He found that the bats increased the frequency of their clicks as they approached the water, but the rate of the clicks decreased as they flew upward, showing that the echolocation likely has a function: to help the bats judge distance and keep them from crashing into objects.
The pool was also helpful in another way: it had a large, decorative wall in its center. Bats far away from the wall barely echolocated as they flew away, while bats close to the wall echolocated at higher rates, likely to prevent themselves from running into it, Eitan says.
The bats also echolocated as they approached fruit trees, when landing on the branches, and when they were taking off, the team found. They increased the rate of their echolocation clicks as they approached trees. In previous experiments, the researchers had also found that the bats rarely echolocate when commuting from one foraging site to another, as they’re flying over the tops of trees and buildings.
Eitan and his colleagues say in their paper that the bats are likely using both echolocation and vision to navigate during the daytime, especially when performing complex tasks. While in theory the bats don’t need echolocation and can solely rely on vision to get around, Eitan explains that integrating both sensory modalities, akin to how we use both our eyes and ears for navigation, is likely helpful. “We hypothesize that echolocation helps them measure distances much more accurately” than vision alone, he says.
The researchers don’t know if the phenomenon is restricted to just this group of fruit bats, or indicative of the species as a whole. Of the 1,400 species of bats in the world, fewer than a dozen are diurnal, and similar behavior occurs in other species of diurnal bats. Eitan explains that the diurnal, insectivorous species lack the sharp vision the fruit bats have. In the future, he says that he and his colleagues would like to investigate how common the phenomenon of daytime echolocation is among Egyptian fruit bats and whether it is related to urbanization. The lack of eavesdropping predators in urban areas—animals that could detect the bats’ clicks, making them easier to hunt—may make echolocation more common there, he speculates. Doing similar experiments in the countryside, he says, could be a next step.
Previous studies have indicated that echolocation evolved when bats became nocturnal, a shift in schedule that allowed bats to avoid daytime predators such as birds of prey. But “observations such as this one tell us that echolocation must be so hard-wired in bats that it keeps being used even when their sensitive eyes would suffice,” Danilo Russo, a bat researcher at the University of Naples Federico II in Italy who was not involved in the study, tells Science News. “This is especially interesting in the species covered in the study, which still largely relies on vision besides using echolocation.”