On the evolution of toilets
Just when you think you’ve seen it all, you learn about something completely unexpected. In this case, it’s a new way to get nitrogen, an important nutrient for all living things. Where the soil is poor in nitrogen, various plants have developed ways to trap insects and the like, among them the pitcher plants. Now it seems that a few species have adapted their pitchers to get nitrogen another way.
Several pitcher species are large enough to trap small mammals, but the scientists noticed that this hardly ever happened. Why else might the plants grow such large pitchers? The poo inside was the giveaway. The scientists found that it was from the mountain tree shrew (Tupaia montana), and could be the major nitrogen source for some plants.
The plant needs to do a couple of things to succeed as a toilet. First, it needs to be large enough to accommodate a tree shrew. Measurements of the ‘toilet’ species showed that the size of the pitchers fitted neatly with the length of a tree shrew.
Second, the plant needs to entice the tree shrew into the correct position. The inner surface of the lid produces nectar, attracting the tree shrew to feed from it. Then the shape of the lid is critical: compare the concave, upright lid of a ‘toilet’ species in the top picture to the lid of another species in the lower picture. The best place to get at that lid is sitting on the pitcher itself. The lid even has a partition in the middle, so a tree shrew going in from the side can only get to part of it.
Cameras set up to film the pitchers caught several visits from tree shrews, although only one where it left a dropping. The scientists suggest that the tree shrew may also leave urine (which is rich in nitrogen), but that needs more study.
Of the three species studied, two (Nepenthes rajah and N. macrophylla) also still catch insects, while N. lowii seems to be more specialised. A fourth species (N. ephippiata) looks like it might have the same trick, but wasn’t studied.
If they’re right, evolution has produced the first chemical toilet. Which is pretty amazing. I’m reminded of the bit in The Last Continent, where bizarre plants evolve on spoken demand.
Chin, L., Moran, J., & Clarke, C. (2010). Trap geometry in three giant montane pitcher plant species from Borneo is a function of tree shrew body size New Phytologist DOI: 10.1111/j.1469-8137.2009.03166.x
Giant meat-eating plants prefer to eat tree shrew poo, BBC Earth News