A bat and a carnivorous plant in Borneo enjoy an unusual mutually beneficial relationship, according to a new paper.
The discovery, outlined in the latest Royal Society Biology Letters, represents only the second known case of a mutualistic association between a carnivorous plant and a mammal. The other case was reported in 2009, when scientists saw three tree shrews pooping into the pitchers of another carnivorous plant.
Although both the bat and the carnivorous plant prey on insects, they are not in competition with each other.
"The bats do not eat the often putrefied insects from the pitcher fluid," says lead author Ulmar Grafe. "Even if they wanted to, the pitcher tapers too much to allow the bat access. The bats literally get stuck, that is they wedge themselves in the pitcher below the girdle and cannot wiggle further down towards the pitcher fluid."
Grafe, a University of Brunei Darussalam biologist, and his team made the determinations after placing transmitters on the backs of Hardwicke's woolly bats that were caught in a Brunei Darussalam peat swamp forest.
Tracking the bats, the researchers discovered that many chose to rest and sleep in the aerial pitchers of the carnivorous plant Nepenthes rafflesiana, variety elongate.
Chemical analysis of the plants found that some 33.8 per cent of their nutrients came from the bat poop and urine.
Mothers use plants to suckle pups
Further investigation of the plants revealed that they put little energy into trapping insects. They released low amounts of insect-attracting volatile compounds and produced little digestive fluid.
Instead, the plants devoted energy towards enticing bats to roost by growing elongated, narrow and cylindrical pitchers that create snug, cozy hideaways for roosting bats.
"In mother-juvenile pairs, the mother will suck the pup in the roost," says Grafe, who explains that most of the plant's pitchers provide enough space for two bats stacked on top of each other.
"The average number of bats per plant at any given time was only approximately 2.5 percent," he adds. "Realise, however, that it may be sufficient for a plant to attract a bat once in its lifetime as the reward is very high."
Coincidental use leads to regular
He and his colleagues suspect the bat-plant relationship evolved after some bats happened to roost in the plant.
"Coincidental use may have evolved into regular and exclusive use if the pitcher plant responds by making the roost more attractive," he says.
Matthew Struebig, a researcher at the University of Kent's Durrell Institute of Conservation and Ecology, was based in Brunei for a while and witnessed a mother bat and pup together in one of the carnivorous plant's pitchers.
"This is a great study that, for the first time, exposes the mutualistic link between a small woolly bat and its pitcher plant host," says Struebig. "There have been anecdotal reports of this bat species being found in pitchers, but most people considered this to be only a temporary arrangement."
Struebig explains that bat experts consider such temporary spots to be a "day roost" or a "satellite roost," "a convenient stop-over on the way to the bat's main home after a night of busy foraging." In this case, though, the pitchers appear to serve as the bat's primary home.
Both he and Grafe now wonder if other carnivorous plants in the region enjoy beneficial relationships with bats and different animals.
"Many pitcher plants grow in remote mountainous areas," says Grafe. "Borneo, the world's third largest island, has the highest species richness of any region and most species have not been investigated in much detail."