Carnivorous Plants Eat Bat Poo

Mammals often benefit plants by acting as seed dispersers or pollinators. In turn, the mammals receive a conveniently packaged and highly nutritious food source.

In an unusual twist on plant-mammal mutualism, researchers have recently discovered a carnivorous pitcher plant in Borneo that has a mutualistic relationship with a bat. The pitcher plant (
Nepenthes rafflesiana) lives in nitrogen poor soils, and like many pitcher plants relies on insects as an additional nitrogen source. N. rafflesiana, however, turns out to be relatively poor at capturing insects. When the researchers began to look more closely at these pitcher plant populations, they discovered woolly bats (Kerivoula hardwickii) roosting head down in roughly 20% of the more than 400 pitcher plants they monitored. Equally interesting was that woolly bats chose to roost in only one particular subspecies of pitcher plant (N. rafflesiana elongata).

In the
elongata variety of pitcher plant, the pitcher is more elongate than other varieties; the distance from the rim of the pitcher opening to the fluid averaged 151 millimeters, or three times the body length of a roosting woolly bat (Figure 1). Indeed, some pitchers were found to house more than one roosting bat.

pitcherplant
Figure 1. (a) Aerial pitcher of N. rafflesiana var. elongata. (b) The same pitcher with the front removed revealling a roosting Hardwick’s woolly bat (K. h. hardwickii). (c) The shorter aerial pitcher of a different variety of N. rafflesiana pitcher plant. (From Grafe et al. 2011)

The researchers (Grafe et al., 2011) hypothesized that the bats gain shelter and protection from the pitcher plant, while the plant dissolves and consumes the bat’s feces and urine for added nitrogen. The scientists used tiny radio transmitters to track the bats and stable isotope analyses to determine the source of the nitrogen sequestered by the plant. The wooly bats they tracked only used the
N. rafflesiana elongata variety of pitch plant as daytime roosts. In addition, the bat’s feces and urine provided roughly 33% of the pitcher plants total nitrogen budget.

This isn’t the first case of Bornean pitcher plants in the genus
Nepenthes evolving novel ways to extract additional nitrogen from their environments. In 2009, Clarke and colleagues reported that another species of pitcher plant (N. lowii) is used as a lavatory by tree shrews (Tupaia montana). In this case, tree shrews benefited by consuming the exudates provided by the pitcher plant and the plant benefited from the extra nitrogen provided by the tree shrews when it defecated into the pitcher after feeding.

The tree shrew - pitcher plant interaction appears to be mutualism based on the exchange of scarce food sources, white the bat - pitcher plant interaction is a novel “resource-service” relationship, where resources (nitrogen for the plant) are exchanged for serves (shelter for the bat).

References

Clarke, C., Bauer, U., Lee, C. C., Tuen, A. A., Rembold, K. and Moran, J. A. (2009) Tree shrew lavatories: a novel nitrogen sequestration strategy in a tropical pitcher plant. Biology Letters, 5:632–635.

Grafe, T.U., Schoner, C.R., Kerth, G., Junaidi, A., and M.G. Schoner. (2011) A novel resource-service mutualism between bats and pitcher plants. Biology Letters, doi:10.1098/rsbl.2010.1141 Published online