Killer Plant Turns Cute

The children's magazine I edit has a science news feature in every issue, with six or seven quirky stories. It's hard to say exactly how I choose these stories out of all the science news I read each month. But if I had to quantify the process, I'd say I give one point to any item about robots or dinosaurs, two points for cute animals, an extra half-point for anything miniature, and about eleven points for poop. So this story emerged as a clear winner.

In the appealing-sounding "peat swamp forest" of Borneo, a team led by ecologist Ulmar Grafe studied the carnivorous pitcher plant Nepenthes rafflesiana elongata. Compared to the typical variety of its species, the elongata variety is lousy at catching insects for food--seven times worse, in fact. So how does it survive? A clue came when the team discovered tiny bats roosting inside some of the pitchers.

N. rafflesiana elongata has extra-long pitchers that the tiny bats (a subspecies called Hardwicke's woolly bats, with a body just an inch and a half long) can comfortably fit inside. Grafe suspected that the plant and bat had evolved a mutualistic relationship: the bats dozed safely inside the pitchers, and the plants digested the poop their guests left behind. To test this, he glued tiny transmitters onto the bats' backs and tracked which plants they visited. Then he analyzed the nitrogen composition of the pitcher plant leaves.

As Grafe had guessed, the pitchers that housed bats (about a quarter of the elongata pitchers observed during the experiment) had significantly higher nitrogen levels than those that were bat free. The plant hasn't gone totally vegetarian, but supplementing its insect diet with bat droppings seems to have been a successful evolutionary strategy.

The pitchers grow with a tapered shape that bats can wedge their heads down into, so they don't have to cling to the slippery pitcher walls with their feet. The pitchers also have a reduced amount of fluid stored inside; after all, it's hard to convince guests to stay the night when they'll be sleeping with their heads next to a pool of digestive juices. The plant has apparently renovated itself to become a convenient bat hotel. The researchers even found a couple of mother bats roosting with their babies.

No one's ever found a relationship like this before, where a carnivorous plant and a mammal work together. It's almost a heartwarming story--if you can forget about the poop.