A predator that can't hunt won't last very long. So when biologists found a carnivorous plant in Borneo that was bad at catching insects, they were puzzled. Just what does it eat to stay alive? The answer, a new study reveals, appears to be bat guano. The enigmatic plant makes a snug roost for tiny bats, which drop nutritious excrement into their host's digestive fluid.
Ecologist Jonathan Moran first noticed something odd about the Raffles' pitcher plant (Nepenthes rafflesiana) when he was a Ph.D. student in the 1980s. Most plants of this species lure flying insects into their pitchers—leaflike cups that are several centimeters deep—with a cloying floral aroma, ultraviolet decorations, and tempting nectar. When the bugs drown in a pool of digestive fluid at the bottom, the plant soaks up nutrients that it can't get from the poor, swampy soil where it grows. But Moran found that a rare variety of the species, N. rafflesiana elongata, wasn't good at catching bugs. It had long, narrow pitchers that lacked scent and flashy patterns, and which captured seven times fewer insects than did those of the common variety.
A couple of decades later, ecologist Ulmar Grafe of the University Brunei Darussalam was searching Borneo's muggy peat forests for tadpoles in pitcher plants when he and his students happened upon a bat roosting in an N. r. elongata pitcher. The researchers dismissed it as happenstance until months later, when Grafe read Moran's earlier study of the odd pitchers. "It just clicked," Grafe recalls. "I said, 'That's it! The bats are going to make a contribution.' "
During 7 weeks of daily pitcher patrols, Grafe and his team found bats in more than one-quarter of the 223 N. r. elongata plants they checked. They were all Hardwicke's woolly bats (Kerivoula hardwickii), tiny mammals less than 4 centimeters long. And they were picky about their housing. To track where they slept, the team glued mini radio transmitters, each lighter than a paper clip, onto the backs of 17 bats. For the several days that their transmitters lasted, those bats slept only in N. r. elongata pitchers, the researchers report online today in Biology Letters. Twice, mother bats even snuggled into the same pitcher with a son or daughter.
As pitcher plants go, N. r. elongata is especially well suited to the bats. There's room for a bat or two above the plant's digestive fluid. And the tubes are so slender that the mammals can wedge themselves in rather than struggle to grip the slippery walls. Although the bats sleep head down, they probably share the common bat habit of turning around to go to the bathroom.
The sleeping arrangement works out well for the plants, too. Leaves adjacent to pitchers that housed bats had about 13% more of the essential nutrient nitrogen than did the leaves attached to batless pitchers. Indeed, the well-nourished leaves absorbed about one-third of their total nitrogen from bat guano, the researchers concluded after testing leaf blades for a rare nitrogen isotope that is most abundant in carnivores (and carnivore poop).
"It's an awesomely cool natural history story," says Aaron Ellison, an ecologist who studies carnivorous plants at Harvard University's Harvard Forest in Petersham, Massachusetts. But he wonders how interdependent the plant and bat really are. By tracking only bats collected from N. r. elongata pitchers, he says, Grafe's team may have missed individuals that used other roosting spots and overestimated the importance of pitchers to the species as a whole.
Moran, however, feels that his mystery is solved. Now a professor at the Royal Roads University in Victoria, Canada, he says he, too, happened upon a couple of bats nestled in N. r. elongata pitchers, but he "never put two and two together." Just last year he and his colleagues described how some other pitcher plants rely on tree shrew feces for nutrients, and only then did he wonder whether bats might be fertilizing the plants. Indeed, pitcher plants are turning out to be less carnivorous than biologists once believed—another species digests leaf litter. With most of the 120-odd Nepenthes species hardly studied, Moran predicts that the plants will yield more surprises "that we can't even dream of yet."