Tree shrews and nocturnal rats in the forests of Borneo have a unique relationship with carnivorous pitcher plants. The mammals defecate, and the pitchers are happy to receive.
A study published on May 31 in the Journal of Tropical Ecology shows a species of giant mountain pitcher plants (Nepenthes rajah) supplements its diet with nitrogen from the feces of tree shrews (Tupaia montana) that forage in daylight and summit rats (Rattus baluensis) active at night. When the small mammals lick nectar from the underside of the pitcher’s lid, they stand directly over the jug-shaped pitcher organ.
The pitchers grow in nutrient-poor, acidic soils and require a supplemental source of nitrogen. Carnivorous plants usually digest bugs, grabbing nitrogen from protein. The Bornean pitchers consume nitrogen-rich feces.
 A summit rat, Rattus baluensis, licking the lid of a Nepenthes rajah pitcher. A fecal pellet is visible, dangling from the rat above the pitcher’s mouth. Photo by: Konstans Wells |
Researchers have already documented tree shrews using two species of Nepenthes pitcher plants as toilets. A third species provides daytime roosts for Hardwicke’s woolly bats in exchange for guano.
However, this is the first instance of a carnivorous plant interacting with two different mammal species, and the first attempt to measure how the plants attract the dining and defecating animals.
“The fact that one of the mammals visits the pitchers during the day and the other one visits during the night indicates that this plant is involved in one of these mutualistic interactions around the clock, which is quite interesting,” said Charles Clarke, an ecologist at Monash University, Malaysia campus, who published a similar study at the same time, in an interview with mongabay.com.
The research team, led by ecologist Konstans Wells of the Biodiversity and Climate Research Centre BiK-F, Frankfurt am Main, Germany, set up camera traps to record activities at eight pitcher plants in the Kinabalu National Park in Sabah, Borneo. The scientists also collected droppings for 61 days.
 A tree shrew, Tupaia montana, on a Nepenthes rajah pitcher rim licking nectar off the lid. Photo by: Konstans Wells |
Small mammals sampled the pitchers’ nectar frequently: Wells’s cameras captured a total of 56 visits by tree shrews and 42 visits by rats. On average, each pitcher plant got a fresh fecal snack every 3.4 days.
To learn how the pitchers attract rats and shrews, the researchers analyzed the milky substance secreted by pitcher lids. The team found more than 40 aromatic chemicals, including some commonly found in fruit—the mammals’ usual fare.
Proving that this three-way relationship is a real ecological “mutualism” is a challenge. Each species must benefit to satisfy that test.
“We don’t know what the benefit is to the small mammals,” said Wells in an interview with mongabay.com. “What is the energy gained and how much time do actually they spend on Nepenthes rajah compared to using other resources?”
 Nectar-secreting glands on the inner surface of a Nepenthes rajah pitcher lid. Photo by: Konstans Wells |
Clarke agreed the results are too preliminary to demonstrate a mutualism. “It seems to be one,” Clarke said. “But we would have to do a lot more work on it to find out whether the relationship between the Nepenthes rajah and the rat is as tight or as close.”
The occasional casualty found stewing in a watery grave of pitcher plant juice complicates the mutualism definition.
“What I observed from one drowned tree shrew was that this pitcher plant was at least active for a few more days,” said Wells, “and actually attracting other small mammals.”
CITATION: Wells K, Lakim MB, Schulz S and Ayasse M. Pitchers of Nepenthes rajah collect faecal droppings from both diurnal and nocturnal small mammals and emit fruity odour. Journal of Tropical Ecology. Published online 31 May 2011. DOI: 10.1017/S0266467411000162
Marissa Fessenden is a graduate student in the Science Communication Program at the University of California, Santa Cruz.
