Something Is Fishy About This Rare Bat's Diet

Spain's long-fingered bat uses echolocation to catch aquatic prey.

(Photo: Antton Alberdi / UPV/EHU)

Apr 11, 2014· 2 MIN READ
Richard Conniff is the author of House of Lost Worlds: Dinosaurs, Dynasties, and the Story of Life on Earth and other books.

For researchers on the Mediterranean coast of Spain, near Valencia, the mystery began when bat excrement on the floor of a cave turned out to be loaded with scales, suggesting that an insect-eating bat species had instead turned to fishing.

It wasn’t a complete surprise. Bats are amazingly diverse, with 1,240 species (that's 20 percent of all mammals), and they’ve had 50 million years to develop a multitude of quirky behaviors. Different bat species are known to eat almost anything—insects, of course, but also fruit, leaves, flowers, nectar, pollen, and, yes, blood. It might be genuinely surprising if somebody said bats catch and eat songbirds, except that researchers caught bats doing just that in 2007.

Frogs too. Early this year, researchers in Central and South America reported on how the male túngara frog’s love song produces a widening pattern of ripples on the surface of the water. Bats have learned to use that pattern as a flight path to cruise in and pluck up the unfortunate Lothario for dinner.

Still, Ostaizka Aizpurua-Arrieta and her coauthors on a new study in the journal PLOS One wanted to find out how Spain’s long-fingered bat learned to fish. "It was a special challenge for me because we didn't think fishing was among the habits of the long-fingered bat,” says Aizpurua-Arrieta. These bats use echolocation to hunt down their main food source, immature midges, at the surface of the water. But the sonic pulses the bats emit for echolocation can’t penetrate below the surface, to where fish live. The long-fingered bat also weighs no more than a third of an ounce, which is “why it is difficult to imagine it fishing.”

And yet, as the study progressed, researchers discovered one bat that had, in a single hunting bout, gobbled down 15 fish. Even more odd, the bats were preying on a species that is a newcomer to their neighborhood. Public health officials introduced Gambusia holbrooki around the Mediterranean in the 1920s for mosquito control. The mosquitofish, as they are known, went on to displace native fish species in eastern Spain, including two that are now critically endangered—a pattern that’s been repeated often enough for mosquitofish to rank among the 100 most invasive species in the world.

Deciphering how the long-fingered bats learned to fish was a bit of a detective caper. The researchers first trapped 15 bats as they were returning from their first feeding of the night to their roost at a limestone cave in Montgó Nature Reserve. They held these bats long enough to examine their droppings with a field microscope and identify the four bats with the highest concentration of fish scales in their diet. Then they attached tiny tracking transmitters to those four bats, and set them free. When the target bats went out to forage again, the researchers tagged along by car and on foot, ending up at two artificial ponds on a nearby golf course. There, they were able to set up high-speed cameras and record the behavior.

Both ponds turned out to be loaded with mosquitofish, which constantly broke the surface to catch their prey. Much as in the study of bats catching túngara frogs, the bats were able to use their echolocation to lock onto the resulting ripples. They instantly swooped down and snatched up the fish with their elongated legs. Even the largest fish was only a little over an inch in length and only 4 percent of the bat’s own weight—well within its carrying capacity, even after pigging out on multiple fish.

The study raises two interesting possibilities. Because intensive agricultural development has damaged many of the water bodies on which it depends, the long-fingered bat is now categorized as a vulnerable species. The new findings could provide a means for rebuilding populations. But the study suggests that the bats themselves may already be adapting. “We cannot rule out a learning process being involved in the intensity of fishing,” the researchers write.

They don’t say if this fishing behavior “is definitely new for the species from an evolutionary point of view” or merely an adaptation of occasional past fishing on native species, to take advantage of the opportunity to feed on more densely populated mosquitofish.

Either way, it’s a reminder: Never underestimate a bat.