Subtle Traps in the Borneo Jungle
Whatever you may think about cats, there is no denying that they are beautifully designed. No, I do not mean their attractive appearances and charming personalities. I mean their design features which allow them to catch mice. The cat, of course, is an expert in the strategy of the hunt. She lies in wait, preferably in a spot where she is not too conspicuous. Then she stalks her prey, slinking along ever so quietly. Finally she winds up for a mighty spring through the air with a precise landing spot… right where the unsuspecting victim hesitates. Then voila! Here she comes to show the victim to you. Naturally you are very impressed with this cat’s achievement. If her strategy were not perfect, there would be no victim since mice are notoriously cautious.
There is no doubt that the catching of prey requires strategy. A mouse trap may do the trick, but only if it is suitably set and also baited to attract the victim. Normally one would not think of plants as mouse traps. In the tropical wetlands of southeast Asia however there are vines which grow leaves shaped like deep, deep cups or pitchers. These special leaves serve as traps and some of the really large traps (as tall as 60 cm or 2 ft) occasionally catch small frogs or mice. Ugh! Can you imagine a plant digesting a mouse? Some of the Nepenthes hanging pitcher plants can do it.
Recently a well-known species of Nepenthes was in the news because of some newly discovered talents. About 70 species are known of these woody vines from tropical habitats in Malaysia, Indonesia, Madagascar and Sri Lanka. These plants develop ordinary green leaves as well as brightly coloured pitcher shaped structures which range from 5 cm to 60 cm tall, depending upon the species.
Small pitchers typically trap flies, beetles and ants. Larger traps are able to catch larger animals. These plant traps do not move. They trap their prey by means of clever design features. Usually the rim around the opening is brightly coloured. Sweet nectar also perfumes the rim. Typically an insect, attracted by the bright colour and sweet scent, lands on the rim of the pitcher. When the nectar proves good tasting but sparse, the insect crawls over the edge in search of more. The inside walls however are extremely slippery. The insect skids downward but recovers its footing on hairs (which unfortunately all point downward). The insect thus follows the route of least resistance. Soon it falls into a pool of rain water and drowns in the bottom of the cup. The plant thereupon sends digestive juices into the water and the nutrients are absorbed back into the plant where they are put to good use contributing to more plant growth. Only the exoskeletons of the insect victims remain in the cup as a testimony to the pitcher’s past treachery and success at exploiting animal victims.
Such is the story of most Nepenthes vines. There is one species however that grows in swampy areas of Borneo, Malaysia and Sumatra. The plant is considered very rare today, but in Victorian England it was a popular choice for toasty warm greenhouses called “stove houses.” The pitchers in
Nepenthes albomarginata grow at most 22 cm (9 inches) tall. These structures tend to be light green or reddish green. The only remarkable feature of these pitchers is the strikingly white “minister’s collar” on the outside of the cup just below the rim. This has always been that plant’s claim to fame and its species name means white (albo) edge (marginata). Nobody suspected that this white band represented anything more remarkable than an attractive decoration. It now transpires, however, according to a brief communication in Nature (January 3, 2002 pp. 36-37) that these traps function differently from other Nepenthes species. These traps are even more cunning than we might heretofore have imagined possible among plants.
Some zoologists (animal specialists) found themselves the sad job of observing the predatory activities of Nepenthes plants in the tropical forest of Brunei (on the Island of Borneo). Most species of Nepenthes in the wild, boasted a varied collection of insect victims in their pitchers, but this was not exactly the case for the plants with the minister’s collars. For these plants it was either feast or famine. Some pitchers bulged with the exoskeletons of literally thousands of termite victims. Other pitchers of the same species contained only a few ants, beetles or flies (usually fewer victims than other species of pitcher plant in the area) These observations caught the attention of the biologists for two reasons. Firstly, all the termites in the really full pitchers were from the same species. It was actually a surprise to see any termite victims at all. Termites are almost never caught in the pitchers of other Nepenthes species, and the large number of victims seemed to have all been caught about the same time, judging from their state of decomposition. Why termite victims and why so many, the scientists asked themselves. Also they noticed that apart from termites, these Nepenthes cups seemed comparatively unsuccessful at attracting victims. The scientist therefore decided to find out what was going on. They would carry out some experiments in nature.
The scientists soon found that Nepenthes albomarginata did not seem to attract victims by means of any kind of odour. Even the favourite species of termite could pass a mere 1 cm away from a pitcher and not realize it was there. However, when these researchers placed intact pitchers in front of a marching column of suitable termites searching for food, the lead individuals — upon encountering the white rim of the pitcher — then turned back to call the attention of their termite friends to this interesting find. It so happens that the white band is made up of tiny white hairs (trichomes). Something about those trichomes excited the termites, both worker and soldier termites alike. They began to jostle about for position in order to gorge on the white hairs and to make food pellets from this material. As these insects pushed and shoved for position on the pitcher rim, thousands fell into the open cup. The scientists recorded as many as 22 victims falling into a cup per minute. The end result was a pitcher bulging with thousands of struggling, dying termites. A typical number was 1000 victims, but 2000 were common too and more than 4000 have actually been documented in one cup. By now all the white trichomes were gone but that pitcher could not accommodate more victims anyway. The plant would have to grow more pitchers for further meals.
Thus the Nepenthes species with the white collar has been found to offer its own living tissue as bait for insects, specifically certain termites. No other carnivorous plant baits the trap in such a manner. Scientists do not yet know why the termites are so excited about the white plant hairs. The termites which are most commonly victimized are unusual in that they search for food in large numbers and over considerable distances. These particular termite species eat living threads of fungus and algae from the soil. Termites superficially resemble ants in their appearance and social organization but apparently in their biology they more closely resemble cockroaches! Their social organization is rigidly structured and includes a reproductive king and queen (with supplementary sexual individuals in case the royal couple dies), as well as plenty of workers and soldiers. Termites typically feed on dead wood or dry leaves and grass, or even fungal gardens cultivated in their huge nests.
We thus see that the preferred termite victims of Nepenthes albomarginata are unusual in their foraging habits and uncultivated fungal diet. The plant predator too is unusual in that its white hairs mimic the appearance of the termites’ favourite food and the hairs seem to taste like that favourite food too.
This is the first case which we have discovered where a carnivorous plant has been shown to bait for preferred insect victims. In addition, the plant appears to use taste as its bait, a very subtle means of attracting prey. No sweet smelling nectar advertises the presence of these traps. The preferred victims moreover almost never find their way into other Nepenthes traps. It is not as if they are in the habit of falling into open cavities. It does indeed appear that Nepenthes albomarginata traps were specially designed for their particular victims. A cat, as we saw, has a hunting strategy. Plants cannot follow a plan, they simply grow. Clearly God, the Great Designer, conferred appropriate anatomy and a trapping strategy on these plants. These remarkable specimens certainly add to the beauty and diversity of their natural communities. Whereas all carnivorous plants demonstrate evidence of “clever design”, the plants which seem so demure, rimmed in white, are actually the most cunning predators of all. Truly nobody can say that biology is boring. The creation continues to amaze us and to inspire awe. What will they discover next?
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