A rare meat-eating pitcher plant has re-made its insect traps into night shelters for tiny bats to have their feces as reward.

The nepenthes rafflesiana elongate, a variety of the species also known as Raffles' pitcher plant, grows in Brunei’s muggy peat forests. It is remarkably poor at catching insects, unlike its many cousins, capturing about one seventh as many.

Now Ulmar Grafe of the University Brunei Darussalam and colleagues suggest a key to this oddity. The plant has adapted to get extra nutrition not from hunting, but from working as a sort of night hotel for Hardwicke's woolly bats.

These tiny mammals are less than four centimeters long, and biologists knew they could dorm inside the pitchers. Grafe’s team used 17 miniature radio transmitters to learn more about the bats’ sleeping behavior and found out that the plant’s traps were their night shelters of choice.

The pitcher plants receive pay from their guests. Leaves near the occupied pitchers turned out to have about 13 per cent more nitrogen than the bat-less ones, the researchers report in a paper published in Biology Letters. The nutrient apparently comes from the bat guano, which they drop overnight into the plants’ digestive liquid at the bottom.

Such a mutually-beneficial arrangement is not unprecedented. Earlier, plant ecology researcher Jonathan Moran, a professor at the Royal Roads University in Victoria, Canada, and colleagues reported a similar relationship between some other pitcher plants and tree shrews, which eat their nectar while using the pitchers as sort of bio toilets in return.