The mutualism between bats and pitcher plants

The Bornean bat Kerivoula hardwickii approaching the pitcher plant Nepenthes hemsleyana. 

Bat inside a pitcher plant roost. Photos by Merlin Tuttle.

This week's paper is in Scientific Reports, lead authored by Michael Schöner at the University of Greifswald in Germany. The research focuses on an extraordinary relationship between the woolly bat, Kerivoula hardwickii, and a pitcher plant, Nepenthes hemsleyana. Pitcher plants are believed to have evolved in areas with low soil nitrogen content. To achieve their necessary nutrients these plants have turned to carnivory. The pitcher part is a liquid trap, usually filled with digestive juices. The pitchers emit odors that attract insects who become trapped in the pitcher where they are digested, providing nutrients for the plant. Nepenthes hemsleyana, however, is different. Rather than emitting attractive odors or containing digestive juices, instead the pitcher of N. hemsleyana is a perfect roost for bats. Woolly bats sleep in the pitcher plant during the day, and poop there, providing nitrogen for the plant. These pitchers have an unusual shape that makes them not only ideal for a sleeping bat, but also easily detected by echolocation. Additionally, the inner wall of the pitcher has a waxy texture that deters egg-laying insects, providing a pest-free home for the bats. This is considered a mutualism, because the bat provides poop to the plant, and the plant provides a roost for the bat. 

The pitcher plant is highly dependent on the bat, as this pitcher is not particularly good at catching insects. Woolly bats, however, will roost other places than these pitchers. They will roost in the old pitchers of other pitcher plant species, and in furled leaves. So how can the pitcher plant depend on a bat that does not depend on it? In other words, what stabilizes this mutualism?

The authors speculate that the mutualism is stabilized by the quality of the pitcher plants as roosts for the bats. The pitchers of this species make such superior roosts that bats will continue to use them even if they have the option to go elsewhere. To test this they had to examine whether bats preferentially used pitchers of this species (Nepenthes hemsleyana) in the field. The authors first radiotracked bats in the field to examine which of the available roosts the bats were using. At one site pitchers of N. hemsleyana made up only 5% of the available roosts but were all occupied by bats. Interestingly, they did not see any bats switching between different types of roosts in the field (if a bat was roosting in furled leaves they continued to roost in furled leaves and did not necessarily switch to pitcher roosts). To confirm this roost fidelity they brought the bats into flight arenas and gave them a range of options. Schöner and his colleagues found that bats which had been found in pitchers in the wild always selected to roost in pitchers. But 21% bats that had been found in furled leaves in the field switched to pitchers in the lab. This asymmetry in roost selection indicates that bats preferentially roost in pitcher plants over furled leaves. To confirm that bats roosting in pitchers versus in leaves in the field were not genetically different (excluding the possibility that they were different subspecies of bats roosting in different places), Schöner and his colleagues conducted population genetics on the bats collected from different roosts and confirmed that there were not genetic differences by roost. It appears therefore that bats learn, either socially or individually, a certain roost type and then stick with that roost type over their lives.

The mutualism between woolly bats and pitcher plants is likely stabilized by the high quality of the pitcher plants as roosts, and a general preference of bats to use them when they are available. Interestingly, however, it appears that there are individual bats that will use furled leaves as roosts for their entire lifetimes. This willingness to use other roosts could buffer the populations of woolly bats from environmental perturbations that reduce the availability of pitcher plant roosts.