By Aubrey Sanders
On the wet slopes of Borneo’s montane cloud forests, beneath a labyrinthine canopy of prehistoric tree ferns and dripping sphagnum mosses, grows one of the planet’s strangest evolutionary treasures: the plant with an appetite for flesh.
Pitcher plants, named for the elongated leaf structure that forms a saxophone-like throat at the end of a vine, produce a sweet nectar that entices insects to feed. Unable to resist the sugary intoxicant, the insects crawl along the plant’s slippery rim, unaware that a pool of digestive juices lies in a gaping basin below.
The rim of the plant, called its peristome, exhibits tiny ridges that make it difficult for prey to gain secure footing. Inevitably, the insects plunge into the waxy bell of the plant, where its digestive enzymes work like stomach acid to break their corpses into nutrients that the predator can imbibe.
Once a victim falls in, there is no coming back out. With some pitchers growing up to two feet in length and others producing three quarters of a gallon of lethal fluid, these death traps can digest hundreds of insects at a time.
But why develop a practice of carnivory in the first place?
While plants use energy from the sun to transform carbon dioxide and water into the starches and sugars on which they survive, they also need to extract vital mineral nutrients from the soil around them. No mineral nutrient is more essential to plant growth than nitrogen, which is a fundamental component of the molecule that facilitates photosynthesis.
In bogs and marshlands, where soil is highly acidic and deficient in these mineral nutrients, some plants found another way to supplement their diets: they developed an appetite for nitrogen-rich insects.
Darwin adored carnivorous plants for their ability to adapt extreme evolutionary traits in order to thrive under inhospitable conditions. It only takes one glance at the plump, hair-trigger jaws of a Venus flytrap or the sticky grip of a Sundew tentacle to glean that these plants can be formidable, if unexpected, predators. With sweet mouths and venomous bellies, pitcher plants are no exception.
Although researchers have recorded incidents of pitcher plants devouring hapless frogs, mice, lizards, and even rats, these seem to be anomalous events that arose by virtue of the victim being in the wrong place at the wrong time. In fact, pitcher plants have developed astounding mutually beneficial relationships with certain animals.
The plant kingdom is replete with mutualism. Hummingbirds and honeycreepers, for example, pollinate plants when they dart from one tubular flower to the next in search of nourishing nectar. Mammals eat the fruits and nuts produced by trees, and later disperse the trees’ seeds after they have passed through the digestive tract and been stripped of their hard exterior. Long-tongued bats pollinate the ephemeral blossoms of the saguaro cactus, which only remain open for a single, crucial night.
These relationships all follow the same pattern of plants providing animals with sustenance, and animals providing plants with reproductive mobility.
“Carnivorous plants in general have already solved the problem of nutrient deficiency in a very unusual way by reversing the ‘normal system’ of animals feeding on plants,” said Michael Schoner of Ernst-Moritz-Arndt-University of Greifswald in Germany.
Pitcher plants have evolved traits that take this role reversal one step further, allowing them to share in incredibly rare win-win relationships with certain animals.
A few years ago, Schoner, along with his wife and fellow researcher Caroline, followed Ulmar Grafe of the University of Brunei Darussalam into the peat swamps of Brunei. There, they discovered that small, wooly bats (Kerivoula hardwickii) roost inside of the pitcher plant Nepenthes hemsleyana.
N. hemsleyana is less skilled than its many Nepenthes cousins in its ability to trap insects, but it possesses a rounder pitcher and a shallow fluid level, both of which enable wooly bats to roost inside without slipping into the digestive juices. In exchange for a parasite- and competition-free perch, the bats fertilize the plants with their droppings, providing N. hemsleyana with 34 percent of the essential nitrogen it needs to survive.
This discovery led the team to wonder how the little bats could possibly locate the pitchers within the wild tangle of jungle flora. Not only would their ultrasonic calls bounce off of every surface, but the bats would also need to distinguish N. hemsleyana from its more abundant cousins, which are unsuitable for roosting.
In the neotropical forests of Central and South America, flowering plants that rely on bats for pollination have evolved petals and leaves that behave like satellite dishes. When the nectar-hungry bats cry out in the darkness, the reflective petals send back echoes that help to make the plant more discernible within its dense surroundings.
According to a new paper released by the Schoners, Grafe, and primary author Gerald Kerth of the University of Greifswald, the back wall of Borneo’s N. hemsleyana also acts as a reflective structure that renders the pitcher acoustically attractive to bats.
“The bats are clearly able to distinguish their plant partner from other plants that are similar in shape but lack the conspicuous reflector,” said Schoner.
This is the first example of such an evolutionary structure emerging from the jungles of the Old World, and the first time ever that plants have exhibited these traits for non-pollination purposes.
The poetry hidden in the process of natural selection could hardly be better expressed than in the relationship shared by a plant that has evolved to eat flesh and a flying mammal that uses sound to see. Though the struggle of survival may be grueling, Borneo’s beautiful and deadly pitcher plants demonstrate that where life exists, it will not retreat.