Pitcher Plants: "the most wonderful plant in the world"

OK, Charles Darwin actually said that about another carnivorous plant, the unrelated North American Venus Fly Trap Dionaea muscipula, but I'm sure he'd have held the same high opinion of pitcher plants! Certainly his contemporary, the equally brilliant biologist Alfred Russel Wallace, referred in The Malay Archipelago to the "wonderful Pitcher-plants". And wonderful they certainly are! In fact I hope I'm about to be able to tell you some new stories that will delight you. When I first came across the pitcher plant story - and at regular intervals since - "what?!" and "really?!" came to my lips with embarrassing frequency.

Raffles' Pitcher Plant Nepenthes rafflesiana, near Telupid, Sabah.

There are some 150 species currently recognised in the genus Nepenthes (the sole genus in the family Nepenthaceae), a number which is rising all the time (eg one book on carnivorous plants cited 70 species in 1983, another expert on the genus knew of 135 in 2012*).

N. chaniana, Crocker Range, Sabah (named for Datuk Chan Chiew Lun, prominent Malaysian naturalist)
was only described in 2006.

You will note that all my pitcher pictures here were taken in Malaysian Borneo, but that's not inappropriate as Borneo - along with neighbouring Sumatra and the Philippines - is the world hot spot for them. The seeds are wind-distributed however, and there are also species (albeit only one to three for each) in north Queensland, New Caledonia, India, Sri Lanka and across the Indian Ocean in Madagascar.

Like other carnivorous plants - and there are at least ten groups of them, all unrelated and thus evolved independently to the lifestyle - they live in situations, often boggy, which are low in nutrients, especially nitrogen. For this, meat is a good, albeit unlikely-sounding, solution.

Pitcher plants are climbers, the key element being of course the pitcher, which forms from a tendril extending from the mid-vein of the leaf.

Fanged Pitcher Plant N. bicalcarata, Batang Ai Reservoir, Sarawak,
in the first stages of forming a pitcher from the extended leaf vein.

N. macrovulgaris, near Telupid, Sabah, showing the developed pitcher attached to the
leaf vein.

Fully-formed, it is an erect bulbous structure with waxy internal walls and a slippery lip around the rim, often with downward-pointing ‘teeth’. There is generally an umbrella-like lid to prevent rain from flooding the trap; this lid, like the rim, is generally coloured to attract prey and often also produces nectar for the same purpose.

Fanged Pitcher Plant at Batang Ai (above)
and Slender Pitcher Plant N. gracilis (below) with fully-developed pitchers;
note coloured lids to keep rain out and attract victims.

 

Common Swamp Pitcher Plant N. mirabilis, near Telupid.
This is the most widespread of all pitcher plants, growing from China to northern Australia.

Overall the pitcher is indeed a deadly trap for small animals. It contains a liquid produced by the plant with a detergent-like surfactant to reduce the surface tension and prevent insects from floating on the surface and potentially taking off again. The liquid also often contains sugars to attract the prey. It used to be said that there are no digestive enzymes in it (such as are employed by sundews for instance) and that the prey simply decomposes naturally by bacterial action, but this thinking has changed, and recent work has revealed up to 30 different digestive proteins in some pitchers. Moreover there are actually also bactericides and fungicides to reduce 'waste' in the digestive vat.  Nutrients are absorbed by glands in the lower part of the pitcher.

In fact it's time to clarify the loose term 'the pitcher', as the plant usually produces two types of pitcher – stout lower pitchers are formed first and sit on the ground, often with flanges to direct ground insects to their doom, while slighter, often more colourful upper pitchers on vines, illustrated above, form as the plant grows, to attract flying insects.

Raffles' Pitcher Plant lower lobe, near Telupid, Sabah.

Close-up of the above lobe, and of N. fusca (Crocker Range, Sabah, below)
showing the nasty down-pointing 'teeth' round the rim which,
with the slippery waxy coating on the inside wall, make it almost impossible for
a victim to climb out.

 

Fanged Pitcher Plant lower lobe, Crocker Range, Sabah.
(Compare with upper lobe in first picture above.)

N. ampullaria, Batang Ai Reservoir, Sarawak.
The ground can be covered by these deadly little pots; however while this species readily grows
extensive vines, it is most unusual in rarely producing upper pitchers.

The tendrils of the upper pitchers often wrap around stems of other plants for support.

Raffles' Pitcher Plant upper lobe supported by tendril, Bako NP, Sarawak.
Note the colour variations in the colours of pitchers, even within the same species.

As well as a wide range of invertebrates, the larger species are known to kill vertebrates including lizards, rats and reputedly even small monkeys on occasion. Had Wallace know what was going on in the pitchers he may have been less enthusiastic about quenching his thirst from them! Flowers are produced on a long raceme, with separate male and female plants.

N. ampullaria inflorescence, Klias Peat Forest, Sabah.

To my embarrassment I seem not to have recorded which species this belongs to; Crocker Range, Sabah.

But there are even more extraordinary twists to the tale too, based on the fact that many animals have associated themselves with pitcher plants in roles other than that of lunch! A red crab spider Misumenops nepenthicola lives in the pitcher of several lowland species, diving into the brew to hide among the insect corpses at the bottom when threatened. A bubble of air enables them to stay under for 40 minutes. To escape they secure very fine silk safety lines around the edge of the pool, leading out to the lip of the pitcher. However they also take advantage of the pitcher’s other amenities. The female lays her eggs under the lip of the pitcher; even newly hatched spiderlings dive in to escape danger. And they hunt by hiding under the lip, grabbing the plant’s lunch before it falls in, or even going down and (temporarily) ‘rescuing’ it.

Misumenops nepenthicola. Photo by Nepenthes out There.

Remarkably, quite a few other animals live in the pitcher too, including a species of crab. Some of these, such as some mosquito larvae, are Nepenthes specialists, unable to live anywhere else; such animals are known as Nepenthebionts! Others, such as the spider and the crab (Geosesarma malayanum), are often found there but can live elsewhere and are called Nepenthephiles; the crab tends to roam from basal lobe to lobe, sampling the contents.

Ants are common prey items – but not always. Nepenthes bicalcarata actually provides a living space in the enlarged leaf stem for a colony of Campanotus schmitzi to live; the ants scavenge food from the pitcher, and somehow manage to drag it out again, though the climb of 5cm may take 12 hours. (On consideration, spurred by an astute reader's comment, I do wonder if that was an observation of an accident on the ant's part? It doesn't make much energetic sense.)

Fanged Pitcher Plant, Batang Ai Reservoir, Sarawak, showing entry hole to
ant accommodation in tendril.

The nature of the benefit to the plant has long been unclear, though there is some evidence that plants without the ant colony are more susceptible to weevil attack. However more recent work suggests that the ants help break down the prey and return it to the plant in a more digestible form, as scraps and faeces. Rather than dive for the food, I'm pretty sure they mostly hang around the rim of the pitcher, either hauling in what they can reach, or even nabbing it before it gets to the liquid; by eating on the edge of the pitcher, their valuable debris ends up in the plant's system.

Finally, at least three species of pitcher plant endemic to Borneo are remarkable in that they seem to have moved on from a carnivorous lifestyle, to surviving on the dropping of birds and especially tree shrews, or tupaias, attracted to their specialised nectaries. The lid in these species is bent back, not covering the pitcher at all. Among the bristles covering it are nectaries producing a sugary secretion. While feeding on the sugar, the tupaias sit on the pitcher, into which they are encouraged to defecate. Measurements have shown that the dimensions of these pitchers are precisely those required to fit the squatting tupaia. How amazing is that?!

N. lowii, Crocker Range, Sabah; this pitcher is starting to dry out.

Mountain Tree-shrew Tupaia montana finding relief on Nepenthes lowii.
Photo by Chien Lee.

But wait - amazingly there's even more! I am grateful to my friend, artist Peter Marsack, for bringing this one to my attention in response to my posting. A recent (2015) publication has revealed yet another extraordinary pitcher plant story, in parallel vein to the previous one, this time involving bats. Nepenthes hemsleyana lives in coastal northern Sarawak and Brunei, and was described in the early 20th century but subsequently confused with Raffles' Pitcher Plant until very recently. (In 2011 it was redescribed as N. baramensis, but the original name takes precedence, though N. baramensis is used in key books such as Field Guide to the Pitcher Plants of Borneo, see below.)

It has been known for some time that  Hardwicke's Woolly Bat Kerivoula hardwickii roosts in the upper pitchers of the plant, apparently finding a more suitable bedroom in terms of stable microclimate than it does in pitchers of closely related species. But how does it find and recognise these desirable residences? The answer, it has now been revealed, is that the bats can hear them! There is a long concave surface to the back of the pitcher which reflects the bat's sonar from all angles, so the bat can home in and find somewhere to spend the night (ie the day, in their case). This is known as a strategy by a few South American rainforest flowers, but never for a purpose other than pollination and never in the Old World. But why? Same answer as for N. lowii; the bats inevitably defecate in the pitcher, as a result of which the plants need produce less chemicals to make the water a deadly trap, and produces less nectar and scent. It catches fewer insects of less variety than do other pitcher plants - it really only needs them as back-up.

I continue to be delighted by the amazingness of nature, but the unexpected becomes the norm after a while...

I'm sure there are still many more stories about the fabulous pitcher plants still waiting to be told, but I hope these are enough to go on with for today.

(* Gordon Cheers Carnivorous Plants and Stewart McPherson and Alastair Robinson's Field Guide to the Pitcher Plants of Borneo respectively.)

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