Good to the last dropping: pitcher plant evolves to be shrew loo

March 12, 2010 • 8:28 am

We evolutionary biologists have one advantage over other biologists: we’re constantly encountering bizarre features of nature showing the almost unbelievable variety of adaptations that can be produced by natural selection.  This means that we’re constantly getting little frissons of pleasure, and I get at least one of these a month.  The most recent came from a new paper in New Phytologist by Chin et al. These authors make a pretty compelling case that the morphology of three species of pitcher plants in Borneo has evolved to not only catch the droppings of tree shews (and use them as nutrients), but to compel the shrews to defecate (and probably urinate) in their pitchers.  In other words, they’ve evolved to be shrew toilets.

Working on Mount Kinabalu in Borneo, Lee et al. discovered that three species of pitcher plants in the genus Nepenthes (N. lowii, N. macrophylla, and N. rajah, the world’s largest carnivorous plant, shown in Fig. 2), have evolved features that make them attractive to treeshrews of the genus Tupaia.  (Treeshrews are neither rodents nor shrews; they’re a group more closely related to primates than to rodents). The plants have recurved “lids” that produce a sweet substance that the tree shews lap up while sitting astride the pitchers.

The authors found that, depending on the species of plant, between 60% and 90% of the pitchers contained fecal pellets from treeshrews.  Video cameras placed near pitchers of N. rajah captured 7 treeshrew visits, lasting an average of 24 seconds, and one of these showed the beast crapping into the pitcher. Previous analysis (Clarke et al. 2009) showed that these pellets provide a large fraction (58-90%) of the nitrogen needed by N lowii.

Geometric analysis of the three feces-trapping species showed that the leaf morphology has evolved in such a way that forces the shrews to sit astride the plant while eating its exudate (Fig. 2). The three species in which feces were observed all have a combination of lid angle, lid concavity, and pitcher width that forces the shrews (given their body size, which was also measured) to straddle the plant if it wants access to the entire lid surface.  Other species, and pitchers of immature plants in the three “target” species, don’t have this combination of traits, and although treeshrews will sometimes lick their lids, they don’t defecate in them.

Figure 1 (from Chin et al.)  The senior author measuring a pitcher of N. macrophylla.  Bar is 5 cm (about 2 inches).

Figure 2 (from Chin et al.). Images from videocam recordings of N. rajah pitchers.  Left:  T. montana stradding a pitcher and eating its secretions.  Note the animal’s butt inside the pitcher.  Right: treeshrew feces in a pitcher.  Scale: 5 cm.

Now this coincidence between treeshrew size and plant geometry may be a coincidence rather than an adaptation, but it’s a good guess that, given the frequency of fecal-pellet deposition in the pitchers and the important contribution they make to the plant’s acquisition of nitrogen, as well as the fact that the three species of pellet-using pitchers are evolutionary outliers among Nepenthes in their large size and lid configuration, the pitchers have evolved to be shrew loos. In the earlier paper by Clarke et al., the authors posit that, because some treeshrews scent mark the plants by urinating on them, the pitchers could also be using this urine.

The authors describe the whole situation as “an extraordinary example of co-evolution and specialization” but I reserve the the term “co-evolution” for cases, like figs and their fig-wasp pollinators, in which both species have evolved because of their interactions. In this case there’s no evidence that treeshrews have themselves undergone any evolutionary change since they started using pitcher plants, although this is possible.  The shrews, for example, could have a genetically based propensity to seek out pitcher plants. But we don’t know if this is the case.

Fig. 3.  I can haz privacy? A mountain treeshrew doing its business on a pitcher.  Photograph by Ch’ien C. Lee.

Pitcher plants are a remarkable example of convergent evolution: the evolutionary modification of leaves into pitchers that trap insects or other nutrient-providing stuff has occurred three times in three independent groups, the Sarraceniaceae (North and South America), the Nepenthaceae (tropical Asia), and the Cephalotaceae (Australia).

And, finally, there’s one species of ant, Campanotus schmitzi, that dives into the pitcher-plant liquid and retrieves drowned insects as well as living mosquito larvae.  These ants can actually swim, remaining submerged for up to 30 seconds! The authors note that it can then take an ant up to 12 hours to haul its prey out of the fluid and up to the pitcher mouth.

Clarke and Kitching (1995), the authors of the study, suggest that this is a mutualism between ant and plant since the ants prevent too many arthropods from accumulating and putrifying inside the pitcher—something that disrupts the plant’s ability to digest them.

h/t: Matthew “Swammerdam” Cobb


Chin, L., J. A. Moran, and C. Clarke.  2010.  Trap geometry in the giant montane pitcher plant species from Borneo is a function of tree shrew body size. New Phytologist, in press (early view).

Clarke,  C. M. and R. L. Kitching. 1995.  Swimming ants and pitcher plants: a unique ant-plant interaction from Borneo.  J. Tropical Ecology 11:589-602.

Clarke, C. M., U. Bauer, C. C. Lee, A. A. Tuen, K. Rembold, and J. A. Moran. 2009. Tree shrew lavatories: a novel nitrogen sequestration strategy in a tropical pitcher plant. Biology Letters 5:632-635.

23 thoughts on “Good to the last dropping: pitcher plant evolves to be shrew loo

  1. Note the animal’s butt inside the pitcher.

    It would make my day if this phrase appears in the paper, especially in the figure caption for Figure 2.

  2. Now, it is a fact (at least on par with a christian fact, as it were) that if the christian god idea existed beyond the scope of a notion, it would have, at the very least, provided a paper hanger and a roll to go with it.

  3. Any chance the substance the shrews are consuming is acting as a laxative? The food source is enough, I’m sure, but that’d be pretty interesting as well. It’d definitely provide an incentive like homo sapiens reaching for the morning coffee.

    1. Yeah, of course it would be good if the plants could do something to INDUCE the shrews to defecate, but I doubt that, given the 24-second length of an average visit, it’s a laxative! There would be no evolutionary advantage to making a shrew do his business in the pitcher of a different plant.

      1. Unless the plant population was spatially structured such that nearby individuals were often close relatives. This would be the case if these plants reproduce clonally, for example by vegetative runners* or their seeds tend to disperse short distances.

        * not a Botanist; I don’t know the correct terminology.

        Some materials have a nearly-instantaneous laxative effect on humans, though the only examples I can think of off the top of my head are also highly toxic.

      2. Well, that’s assuming the laxative would have to act immediately and from my limited knowledge of natural laxatives they can sometimes take hours (e.g. senna leaf). I’m not sure how often they’re stopping by for a drink.

        TheBrummell brings up a possible explanation of how a laxative effect could benefit both but that’d mainly rely on how the pitcher plants are dispersed and how far the shrews travel.

        It’s fun speculating anyways!

  4. Do tree shrews normally just defecate wherever they happen to be, or do they have specific places they tend to go to in order to do their business?

  5. That’s f’ing hilarious. The tree shrew in the second photo is so cute. Now all the plant needs to do is evolve a bidet nozzle and a flush handle, and the shrews will start planting them in their 5 star tree-hotels.

  6. Gradual evolution is clearly ruled out. The pitcher plant has to have been an exact fit the first time a shrew tried to use it. Also, no shrew would have even tried if there hadn’t already been shrew toilet oil on the lid. How could a shorter shrew, for example, have reached all the way across without falling to its death? And don’t try to sell me that old saw that the short shrews went extinct!
    Obviously a case of Intelligent design. It is, after all, more intelligently designed than the toilet in this house.

    1. You aren’t quite getting the role of coincidence in evolution. If shrews couldn’t sit astride it, some other animal would take on the job. The plant, originally evolved to catch insects, has plenty of time to wait for one animal or another to figure it out.

      You’re also making the false assumption that the shrew is the one that changed size. You must consider how the plant could change, and that it’s not necessary for everything to work the first time. If the plants were too small, shrews would still go to eat the nectar, but they would just crap over the edge. Advantage: shrew. If the bowl was too large, they’d just hang on to the side, and again, crap over the edge. However, if the plant is exactly the right size, the shrew can still eat from it, but it craps into the bowl. Advantage: shrew & pitcher plant. Not only is the shrew now feeding the plant, but the plant has more nutrients, which it can use to proliferate more and produce more nectar.

  7. We now have a new metaphor to explain why creationists are attracted to public school boards – and what they do when they get there.

  8. Premonitory, Dr Coyne? You were right: Good to their last droppings. The NYT reports a short while ago on TX board of “education”: …’ changes that will put a conservative stamp on history and economics textbooks, stressing the superiority of American capitalism, questioning the Founding Fathers’ commitment to a purely secular government and presenting Republican political philosophies in a more positive “ligt” (sic). Are you aware of the german evangelical couple that sought political asylum, here, to freely home school their children? (In fact this is such a disgusting comedy that I will spare the pain)

  9. I’m disappointed that the botanists discovering the shrew loo wasn’t named Eta Oin.

    Someone please remedy that.

  10. Might be interesting to know Tupaias habitually secrete their crap to avoid giving away location to predators — would be lovely preadaptation.

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