The incredible relationships between ants and pitcher plants have been in the news lately, so it might be time to summarize some of the most recent discoveries.
Ant drinking nectar from the peristome of an upper pitcher of Nepenthes rafflesiana. Bako National Park, Sarawak, Borneo. Photograph from Wikimedia.
Pitcher Plants – An Introduction
Pitcher plants are part of a group of carnivorous plants that are known to capture arthropods, particularly insects. There are many different species, and some are not all that closely related.
The pitcher plants are named for their bottle-shaped structures that hold fluids. Insects and other arthropods roaming nearby often slide into the fluid and drown, not because they are particularly clumsy, but because the inside upper rim is very slippery. The slipperiness may be due to waxes or due to special hairs (trichomes), as with the example below.
Some pitchers serve simply as passive traps that capture anything wandering by, whereas others produce nectar at the lip (called the peristome) to attract even more prey.
Ants and Pitcher Plants
Unlike most other arthropods, ants have some special relationships with pitcher plants. Many of these relationships are not well understood yet.
In this video we can see the ants can run around on this particular pitcher plant without falling in, although a sawfly has already been captured. A related video (link comes up onscreen towards the end of this video) shows a woodlouse falling right into the same trap. The ants do not appear to be feeding on nectar in this case.
Dr Ulrike Bauer from the University of Bristol, UK and her colleagues have been studying how insects are captured by certain pitcher plants which supply nectar. They found that when the traps of these pitcher plants are dry, ants can walk on them easily. When the traps are wet, then the ants fall in and drown.
Bauer and her colleagues have recently taken their work a step further further and suggest that pitcher plants may benefit from being alternately wet and dry. More ants are recruited to pitcher plant nectar when the pitchers are dry. When the traps become wet again, the ants fall into the traps in greater numbers than if the pitchers had been constantly wet (2015).
Probably the most intriguing discovery has been the relationship between Camponotus schmitzi carpenter ants and the fanged pitcher plant, Nepenthes bicalcarata (object of the famous photograph by Mark Moffett).
These tiny carpenter ants nest in the tendrils of the pitcher plant. Remarkably the carpenter ants are able to swim through the fluid in the pitcher plant that drowns other insects. In fact, the worker ants swim around in the pitcher to remove insects as food for themselves and also catch pupae of a species of fly that lives in the pitcher plant fluids.
Some early workers thought the ants might be ripping off the plants by taking their food, but later work has shown the carpenter ants provide a number of services to the pitcher plant.
- Clean the rim of the peristome, making the pitcher a better trap.
- Chase away some herbivores that might attack the pitcher plant
- Capture fly pupae, which helps keep valuable nutrients available to the plant.
- Knock arthropods into the trap when they are protecting the pitcher.
- Remove large carcasses that might rot/putrify the pitcher.
All of these services mean that Nepenthes bicalcarata pitchers are long lived and so not need to be replaced as frequently.
This video shows Camponotus schmitzi carpenter ants in action. It is based on the work of Thornham et al. from their 2012 paper in Functional Ecology.
Have you ever seen ants around or in pitcher plants? Do you know what kind?
Bauer, U., M. Scharmann, J. Skepper, W. Federle. 2012. ‘Insect aquaplaning’ on a superhydrophilic hairy surface: how Heliamphora nutans Benth. pitcher plants capture prey. Proceedings of the Royal Society B: Biological Sciences, 280 (1753): 20122569 DOI: 10.1098/rspb.2012.2569 (free .pdf available)
Bauer U, Federle W, Seidel H, Grafe TU, Ioannou CC. 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: 20142675.
Downloaded from http://rspb.royalsocietypublishing.org/ on January 15, 2015
Scharmann, M., D.G. Thornham, T.U. Grafe & W. Federle 2013. A novel type of nutritional ant-plant interaction: Ant partners of carnivorous pitcher plants prevent nutrient export by dipteran pitcher infauna. Plos One 8(5) e63556. (Article available)
Thornham, D.G., J.M. Smith, T.U. Grafe & W. Federle 2012. Setting the trap: cleaning behaviour of Camponotus schmitzi ants increases long-term capture efficiency of their pitcher plant host, Nepenthes bicalcarata. Functional Ecology 26:11-19. (Article available)