In late 2010 I was fortunate to be able to join a team of researchers investigating animal interactions with the giant pitcher plant Nepenthes rajah. Endemic to the cool and mist-shrouded slopes of Mount Kinabalu and Mount Tambuyuon in northern Borneo, this is not only one of the world’s largest Nepenthes but also the source of the original stories of “rat-eating” carnivorous plants. There exist a number of confirmed incidents of this plant consuming small mammals which are unfortunate enough to fall within the voluminous pitchers. Recent research however, has shed light on a far more interesting and complex animal/plant relationship, and prior to our 2010 work it had been already shown that the Mountain Treeshrew (Tupaia montana) visits the pitchers of N. rajah (as well as several other Bornean Nepenthes) to feed on the sweet nectar exuding from beneath the pitcher lid. The plant benefits from these visits by frequently collecting the treeshrew’s scat which fall inside the pitcher and are a valuable source of nutrients.
During the field work it was observed that some of the animal droppings found on and in the pitchers appeared different than those left behind by the Mountain Treeshrew. However, despite many days of watching the plants during daylight hours, treeshrews were the only mammals seen to visit the plants. It wasn’t until camera equipment was left in place to monitor the plants during the night that we ascertained the presence of another animal, the Summit Rat (Rattus baluensis) at the pitchers. This is the first instance confirming a rodent/Nepenthes relationship, and it is presumed that it matches the treeshrew/Nepenthes mutualism albeit nocturnally. The photo here, the first ever to illustrate this interaction, shows an image I obtained at night by use of an infrared camera trap positioned adjacent to the pitcher and illuminated by multiple strobes.
Unlike the related Nepenthes lowii, another Bornean pitcher plant which has lost its means to trap insects and depends entirely on the treeshrew droppings for its nutrients, N. rajah retains its carnivorous apparati, a slippery peristome and smooth pitcher walls, and still consumes prey. It is quite likely that the records of rats and treeshrews having drowned in N. rajah pitchers were not instances of these mammals seeking out a source of water (as has been assumed in the past), but rather the result of unfortunate animals falling into their “toilet”.
Chin, L.J., J.A. Moran, C. Clarke (2010) Trap geometry in three giant montane pitcher plant species from Borneo is a function of tree shrew body size. New Phytologist 186: 461–470.
Clarke, C.M., U. Bauer, C.C. Lee, A.A. Tuen, K. Rembold (2009) Tree shrew lavatories: a novel nitrogen sequestration strategy in a tropical pitcher plant. Biology Letters 5: 632–635.
Greenwood, M., C. Clarke, C.C. Lee, A. Gunsalam & R.H. Clarke (2011) A Unique Resource Mutualism between the Giant Bornean Pitcher Plant, Nepenthes rajah, and Members of a Small Mammal Community. PLoS ONE 6:6.