Adapted to kill: How the pitcher plant traps its prey

The trap of a pitcher plant
Nepenthes, always happy for you to drop in. Photo: Angela Sevin / Flickr

When it comes to carnivorous plants it’s Venus Flytraps that get the most attention, with their snapping jaws. Bladderworts have stunningly fast traps. Sundews glisten and coil around their prey. Pitcher plants like the Nepenthaceae in contrast don’t seem to do much. It looks like they’re just sitting there, waiting for gravity to do the work, almost like couch potatoes. In fact there’s a lot going on, as a paper from next month’s Annals of Botany shows.

Jonathan A. Moran, Laura K. Gray, Charles Clarke and Lijin Chin have written a paper Capture mechanism in Palaeotropical pitcher plants (Nepenthaceae) is constrained by climate (you can read it for free) that not only looks at what pitcher plants do, but also where they do what do. And they’ve looked at a lot of plants – almost 2000 populations of over 90 different species. So what is it that pitcher plants are doing?

The basic pitcher mechanism is well-known. It’s a specialised leaf holding liquid in it. The prey fall into this pool and are digested. What varies is not just how the prey fall in, but what they fall into.

The liquid is a juice that dissolves the body of the victim, but not all pitchers use the same liquid. In some the liquid is viscoelastic. It’s a liquid that resists the more you push becoming like treacle. Gaume and Forterre showed how this could be used to aid trapping. Their paper, at PLOS One, has video showing how much harder it is for a fly to escape from this fluid than water. It’s clearly an aid in trapping prey, but if that’s the case why don’t all pitcher plants have it?

Further up there are differences too. Around the rim of the pitcher is the peristome. In the image at the top that’s the ridged entrance to the pitcher. This is where you find the nectaries that entice visitors to the plant. Microscopic ridges on the peristome make it much easier to move into the pitcher than out. When this gets wet it becomes even harder to keep a foothold. Different pitcher plants have different widths of peristome.

Pitchers can also have wax around the upper part of the inner wall. Oddly, this can work in the opposite way to the peristome, with the wax making the surface unwettable. For some insects that rely on moisture for a foothold, this becomes a very slippery surface. If the wax detaches from the wall then that adds to treacherous nature of the surface.

Moran et al. note that different pitchers use different trapping mechanisms to different degrees. A pitcher that uses wax a lot isn’t likely to have much of a peristome. This makes some sense because the peristome and wax work in opposite ways, but what is it that causes a pitcher to have viscoelastic fluid or not?

The authors decided to look at the local environment for the various species of plants. They measured all sorts of climactic factors and saw how suitable the local climates were for various factors of pitcher plant. They compared the habitat suitability for plants with small peristomes compared it against plants with large peristomes, then viscoelastic fluid against watery fluid and the waxy plants against the non-waxy plants. They also defined two syndromes. A plant with a large peristome, viscoelastic fluid and little or no wax was wet syndrome. If on the other hand a plant had a small peristome, wax but no viscoelastic fluid were dry syndrome. If the authors were right and climate was a major factor in the distribution of the plants, then the wet syndrome pitcher plants should be found in the more humid regions.

Probability of pitcher plant distribution.
Probability of habitat suitability dry syndrome and wet syndrome Nepenthes prey capture mechanism groups, modelled from 19 bioclimatic variables using Maxent. Probability is mapped by colour, from grey (0–0·1) to dark blue (0·9–1). Image by Moran et al. 2013.

Sure enough the wet syndrome plants are found in the humid climates of Sumatra and Borneo. The dry syndrome plants have a much wider distribution.

Humidity would certainly explain the distribution of large peristome plants, they work better in damp conditions, but why the viscoelastic fluid too? Moran et al. point to other research that shows pitchers with viscoelastic fluid are found in montane environments with a large amount of flying prey. This sounds good to me because one of the problems with catching flies on a slippery surface is that they can fly. Having a syrup at the bottom of the pitcher would make any momentary fall much more serious.

The study makes a good case for why wet syndrome pitchers are where they are, but the dry syndrome plants are found everywhere. Moran et al. recognise this is a puzzle and while they don’t have any firm answers, they have some ideas. It’s down to economics.

Building parts of plants has a cost in terms of energy and resources. Leaves are relatively simple to build, but traps are a lot more work. This is why carnivorous plants don’t grow in good soils. It’s simply easier to get the nutrients through the roots. For pitchers building a big peristome is a cost, because it needs stiffening and reinforcing. If you have a big peristome you have to get a payback for the extra cost, otherwise you’ll be better off with a small peristome. It’s the same for the fluid. A viscoelastic fluid is a complex chemical soup. It needs a lot more work than usual, so there’d better be a good reason for it.

It seems that where there is a benefit to losing wax and building better peristomes then pitchers do this. If there’s a need to grab flying food, they’ll do that too. So instead of simply being passive, it seems that pitcher plants are constantly adapting and refining their killing technique.

Images

Pitcher Plant by Angela Sevin / Flickr. This image licensed under a Creative Commons by-nc licence.

Pitcher plant distribution by Moran et al. 2013. © the authors.