The most efficient trap isn’t always the most deadly trap

Pitchers plants are carnivorous. They catch small animals, usually insects to gain nutrients like Nitrogen. You’d expect that they’d evolve their traps to be as effective as possible. If an insect gets away, that’s one less meal, but that’s not what happens for Nepenthes rafflesiana

N. rafflesiana is a plant that grows in sunny parts of the forests of Borneo, Sumatra, and the Malaysian peninsular. It grows a couple of traps, though the upper traps lack the waxy crystals of the lower traps. Both traps have a peristome, a lip that is very slippery when it’s wet to encourage insects to fall in. But often is isn’t wet. In fact the change in humidity through the day means that it can be dry for eight hours or more. Dr Ulrike Bauer from Bristol decided to have a closer look at what was going on.

She and her team examined plants in Brunei, in the north of Borneo. At first they examined the traps of plants to see how they worked normally. They found the plants ate ants. They ate other things too. There were a few termites, along with some bees, beetles and spiders, but the bulk of food was ants in young pitchers.

Next she tried something simple but clever. She rigged up some pitchers so that they were moistened by a drip from a bottle. The aim was to find out if the peristome drying out during the day badly affected the plant’s ability to capture ants. Each ‘wet’ pitcher on a plant had a companion ambient pitcher rigged up in the same way – to counter the effect of the equipment. Half way through the experiment the pitchers were switched so the ambient pitchers were moistened and the wet pitchers left to dry in the ambient environment.

They found that wet traps captured more flies, so it seems that a wetter trap is more deadly. So do the traps drying out indicate a plant at the limit of it’s range? Maybe slowly becoming more deadly? Bauer thinks that something different is happening, the pitcher plants are using ant behaviour against the ants.

Ants search for nectar, which makes the pitchers effective traps. But if you kill an ant that visits you have just one ant. If that ant can go back home and bring her friends back then you have a party of ants coming to your trap. Bauer et al. found that traps were making batch kills, which suggests this is what’s happening. It’s a good explanation, but it bothered me. They also point out that mass kills are comparatively rare events. That’s fine if you have lots of traps, but a bigger gamble if you have just a few. However, Bauer et al. have an answer for that.

As I mentioned above N. rafflesiana doesn’t produce just one kind of trap. The lower traps have waxy crystals, so while the peristome isn’t always effective, the wax means the lower traps are always working. Young plants are close to the ground to begin with, obviously, so they start by building conservatively with always-on traps and then build more effective batch-kill traps when they can afford to play a longer game.

This is the kind of science I like. The basic idea is simple and easy to explain, but it still takes observation and some careful thought to work out what the observations are telling you. You can also tell that Bauer and her co-authors are confident of their findings, because the paper is written in a way that’s easy to understand. The fact they’ve found the most deadly trap isn’t always the most efficient trap is a bonus.

You can pick up the paper for free, it’s Open Access, at Proceedings of the Royal Society B.

Bauer U., Federle W., Seidel H., Grafe T.U. & Ioanou C.C. (2015). How to catch more prey with less effective traps: explaining the evolution of temporarily inactive traps in carnivorous pitcher plants, Proc. R. Soc. B, 282 (1801) DOI: http://dx.doi.org/10.1098/rspb.2014.2675