As readers of this blog may know, I’m fascinated by carnivorous plants and, in particular, the question of how their extraordinary adaptations for capturing and digesting organisms such as insects might have arisen. Were these plants designed to be carnivorous originally or is their carnivory an adaptation to the post-Fall world?
Back in 2007, Roger Sanders and Todd Wood suggested that the remarkable mutualistic relationships that pitcher plants sometimes exhibit with other species might offer some important clues to their original habits.
Consonant with this idea, a remarkable study in The New Phytologist has recently demonstrated that at least three pitcher plant species in the highlands of Borneo do not feed primarily on insects. Instead, they obtain their nutrients from faeces deposited in their pitchers by tree shrews feeding on nectar exuded by glands on the inner surfaces of the pitcher lids (Chin et al 2010). The three plant species – Nepenthes lowii, N. rajah and N. macrophylla – have very large pitchers with concave lids held approximately at right angles to their openings. Significantly, the distance from the front of the pitcher’s mouth to the nectar-exuding glands precisely matches the head-to-body length of mountain tree shrews (Tupaia montana). As Charles Clarke, one of the researchers explained, “In order for the tree shrews to reach the exudates, they must climb onto the pitchers and orient themselves in such a way that their backsides are located over the pitcher mouths.” The tree shrews then defecate into the pitcher in order to mark their feeding territory.
According to this BBC news report, the investigators suspect that two other highland species, N. ephippiata and N. attenboroughii, feed on faeces too. Furthermore, there are suggestions that some lowland Nepenthes species may also feed on the faeces of bats that roost in their pitchers.
Although the researchers seem to favour the hypothesis that insect-digesting pitchers secondarily evolved this mutualistic relationship with tree shrews, I’m wondering if it was the other way round? Perhaps pitcher plants were originally designed as toilets for small mammals and only started feeding on insects and other arthropods secondarily? Wouldn’t that be amazing?
Chin L., Moran J. A., Clarke C. 2010. Trap geometry in three giant montane pitcher plant species from Borneo is a function of tree shrew body size. The New Phytologist, advance online publication.
Sanders R. W., Wood T. C. 2007. Creation and carnivory in the pitcher plants of Nepenthaceae and Sarraceniaceae. Occasional Papers of the BSG No. 10, pp.21-22.