Finding a snug place to sleep isn't always easy in Borneo's peat swamp forests, but Hardwicke's woolly bats have a pretty good setup. They like to sleep inside a plant called Nepenthes hemsleyana — a carnivorous plant that feeds on bat poop. It's one of nature's amazing reciprocal relationships that work out really well, even if it sounds downright weird. But there is one issue with this arrangement: these plants can be hard to find. Fortunately, the shape of their "pitchers" — the part that can be used to catch insects and, err, house a sleeping bat — are ideal for reflecting sound, according to a study published today in Current Biology. This means that bats can "spot them" using echolocation after a tiring night of flying around.

The plants are giving a "'shout back' to the bats to come on over."

"We mostly think about plant signaling as targeting sight and smell, but here these plants are not so much giving a 'shout out' but a 'shout back' to the bats to come on over," explains Rohan Clarke, an ecologist at Monash University in Australia who didn't take part in the work.

To attract insects, Nepenthes hemsleyana use pitchers filled with a digestive fluid — which might be bad for those slumbering bats, right? Not quite. As it turns out, these plants have an uncommon shape that allows the bats to "stick inside the pitchers like a cork on a bottle," says Michael Schoner, a bat ecologist at the University of Greifswald and a co-author of the study. And only the bottom part of the pitchers are filled with fluid, so the bats never come in contact with it.

Bat poop, however, reaches the digestive fluid just fine — which is good because these plants are kind of bad at catching insects. "It's a very poor insect trap," Michael Schoner says. "It captures seven times less insects than its closest relative." This means that allowing bats to find pitchers to sleep in is pretty important for the plant's survival. And that's where the plant's shape comes in.

Credit: Ch'ien C. Lee

To find out if the pitcher's shape could serve as an effective "acoustic reflector," the researchers used a sonar device that they placed at different angles away from individual N. hemsleyana plants and its closest relative, Nepenthes rafflesiana — a plant that doesn't host bats. They measured both species' ability to return sound to their source. The researchers found that N. hemsleyana was able to produce a very loud reflector echo, and that the directional information contained in that echo was specific to that plant species. Those two features should help the plants stand out, acoustically, in cluttered surroundings, the researchers suggest.

The plants are bad at catching insects, so they "eat" a lot of bat poop

Still, the scientists needed to know if the bats' echolocation calls were suited to the pitchers. Hardwicke's woolly bats are known for producing short, high-pitched calls, so the researchers recorded the last five calls produced by a handful of bats as they approached the pitchers. By analyzing these recordings, the scientists found that calls were ideal for localizing and classifying the vegetation that surrounds the bats. And incidentally, the researchers think that these bats produced the "highest frequencies ever recorded in bats" — with starting frequencies of up to 292 kHZ.

Armed with this information, the scientists then tried to test how good these carnivorous plants are at attracting bats. In this experiment, the researchers enlarged the pitchers of certain plants, removed them, or left them intact. Then, they recorded the time it took bats to find hidden plants.

The bats needed a lot less time to find enlarged and intact pitchers

The scientists found that the bats needed a lot less time to find enlarged and intact pitchers compared with plants devoid of pitchers. And when it came time to choose a plant to actually sleep in, the bats chose to enter unmodified pitchers more often than plants with removed or enlarged pitchers. So, although the bats seem attracted to enlarged reflectors, they don't seem to identify them as N. hemslayana when it's time to nap — a decision that may be related to the acoustic cues they're getting from the carnivorous plants.

The study provides "strong evidence" that the bats use the ultrasound reflection "signature" of their plants to find the roosts and to distinguish them from pitchers of other species, says Ulrike Bauer, an ecologist who specializes in plant-insect interactions at the University of Bristol and who didn't take part in the study. Humans tend to underestimate the powers of plants because they are so different from us — animals are much easier to grasp, she says. "But plants have just as many amazing tricks up their sleeves as animals."

Clarke agrees, adding that he thinks the findings are especially noteworthy because plant-bat relationships that include sound reflectance are "relatively rare." And so far, these relationships have only been found in the neotropics, in situations where bats pollinate flowers — instead of situations where they defecate in them.

Humans tend to underestimate the powers of plants

The researchers don't know if the plant's shape is an adaptation developed specifically to help bats locate them. "We assume that it's the case," says Caroline Schoner, a bat ecologist also at the University of Greifswald and a co-author of the study. "But at the moment we cannot prove exactly that it's really developed for the bats." Still, the fact that none of the 300 to 500 other pitcher plants have this shape, or this relationship with bats "gives us some clue," Michael Schoner says. The Schoners, who are married, plan to tackle this question next.

The study highlights the importance of peat swamp forest environments in Southeast Asia, Caroline Schoner says. The bats rely pretty heavily on these plants, so if humans damage these forests, they will affect the bats, too. "The study shows how connected the different organisms are," she says. "If you take one out, then the whole ecosystem is in danger, because there are so many other species that rely on this one organism."

But beyond the conservation factors lies the "this is totally awesome" line of thinking. When researchers discovered that some plants consume animals for nutritional purposes, that was considered pretty novel. Then, a few years ago, scientists realized that bats gain a safe roost site in return for their droppings, which enhance plant nutrition in these same carnivorous plants, Clarke says. "Now, we've learned that these plants are acoustically attractive to the bats," he says — a finding that shows, once again, "just how complex the natural world can be."