Volume 30, Issue 3, Fall 2012

Living Inside a Deadly Trap

Woolly bats use carnivorous pitcher plants as roosts

By Caroline R. Schöner and Michael G. Schöner


Searching for students ... for research on the interactions between woolly bats and pitcher plants in Brunei.

That notice, posted by Professor Ulmar Grafe at the University of Würzburg in Germany, certainly got our attention. These are carnivorous plants that consume insects – how would mammals interact with them? And where is Brunei? Professor Grafe later explained that he had discovered Hardwicke's woolly bats (Kerivoula hardwickii) regularly roosting in pitcher plants in Brunei, on the tropical island of Borneo, and wanted to investigate that relationship. He invited us (two students) to join his research team. We jumped at the chance.

This peculiar pattern had been mentioned a few years ago by pitcher-plant researcher Charles Clarke, who wrote: "It is worth noting that the bats were not trapped by the pitchers."

So in June 2009, we traveled to Brunei, a small sultanate with a huge share of primary forests that are home to at least six species of the Kerivoula genus. These "woolly bats" are generally small, mostly weighing less than 10 grams (about a third of an ounce), and occur in Africa, Asia and Australia. Their echolocation calls are often adapted to foraging in very dense vegetation, and some Kerivoula bats were known to use unusual roosts, such as bird nests and furled ginger or banana leaves.

The pitcher plants, of the lethal genus Nepenthes, typically grow in nutrient-poor soils in waterlogged, peat-swamp forests. They are vines that reach heights of up to 65 feet (20 meters). To compensate for the lack of nutrients, such as nitrogen and phosphate, these plants have developed trapping structures shaped like pitchers and partly filled with liquid. When insects or other small arthropods fall into the trap, they drown in the liquid and are digested by enzymes produced by the plant.

We did not have to search long before we found our first Hardwicke's woolly bats roosting in pitchers. To examine their roosting behavior, we attached miniature radiotracking transmitters to the backs of all captured bats.

Then the adventure began. Using an unwieldy antenna, we followed each bat for up to 12 days through the dense, swampy jungle. We were astonished to find that all the woolly bats in our study area roosted only in Nepenthes hemsleyana pitchers. We never found them in tree holes, caves or other common bat roosts. Instead, each bat was settled in, head first, above the digestive fluid in a well-defined region – a girdle-like structure below which the pitcher tapers significantly. The bats fit so perfectly that they don't even use their feet to hold onto the pitchers' walls.

Unlike other Nepenthes species, the digestive fluid inside the Nepenthes hemsleyana pitchers is limited to the lowest part of the cone, so the bats never come into contact with it. Normally these bats roost alone, but some pitchers provide enough space for a mother with its pup.

While roosting in the pitchers, bats can hardly be seen from the outside. So pitchers provide a secure roost that helps bats avoid detection by predators. All things considered, pitcher plants seem well suited for the bats. But what's in it for the plant?

Previous studies found that Nepenthes hemsleyana captures seven times less prey than other, closely related species of pitcher plants. Perhaps bat feces serves as a kind of fertilizer that compensates for the lack of nutrients. To test this hypothesis, we collected tissue samples of plants that had been occupied by bats and compared their nitrogen content to pitcher plants that were not hosts to bats. Laboratory results found that plants used by bats gained more than 33 percent of their nitrogen from bat droppings. We now have strong evidence that the relationship between pitcher plants and woolly bats demonstrates a mutualism that benefits both partner species.

During the final week of our first stay in Brunei, we made an intriguing new discovery: While studying a new area, we found woolly bats roosting in another pitcher-plant species: Nepenthes bicalcarata. And bats' use of this species showed big differences from the way they use N. hemsleyana.

Woolly bats roost in hemsleyana pitchers only when they are completely intact. But their use of bicalcarata pitchers was limited to those that were wilted or had their bases punctured by little holes. The explanation is obvious. Undamaged N. bicalcarata pitchers contain so much digestive liquid that there is very little space for the bats to roost. It is not yet clear whether these holes are part of the pitchers' normal wilting process or if the bats themselves modify the pitchers by nibbling the holes.

So, awash with data and new impressions, we went home to Germany knowing one thing for sure: We had to come back to Brunei to learn more about this fascinating connection between woolly bats and pitcher plants.

Supervised by Gerald Kerth and Ulmar Grafe, we returned to Brunei in 2011 to continue our research, this time in different peat-swamp forests. Using harp traps and radiotracking, we discovered that in some areas, bats roost exclusively in only one of the two pitcher plant species, while in other locations, the individuals switch freely between the two species.

Now we are trying to determine whether bats typically prefer one pitcher plant species over another and why they should be so choosy. We also hope to learn whether Hardwicke's woolly bats and N. hemsleyana plants have co-evolutionary adaptations for one another, and exactly what each partner gains from this relationship – and what price each pays for it.

Bats and pitcher plants thrive together in unique ways within Borneo's peat-swamp forests, but they are only a small part of the wondrous biodiversity that depends on this invaluable habitat. A better understanding about how species interact in these ecosystems is a key to their protection. We can only protect what we know.

Caroline R. Schöner and Michael G. Schöner are Ph.D. candidates at the University of Greifswald, Germany. Both are analyzing the interactions between pitcher plants and woolly bats in Brunei.

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