Carnivorous plants have always captured people’s imagination: the Krynoids in Dr. WHO, Cleopatra in Addams Family, Snargaluff in Harry Potter… to name but a few. And perhaps the excitement isn’t that surprising considering that typical plants for most of us are but a picture in the background of our daily lives.
There are many different ways, each perhaps more ingenious than the last one that carnivorous plants have evolved to catch their prey. The infamous Venus flytrap uses small electrical currents, that are induced by an insect brushing against the tiny hairs on its trap, to snap close its “jaws”; bladderwort uses vacuum to suck its victim into a tiny cell that will end up being its final resting place. Perhaps, compared to these exotic examples, the pitcher plats with their puddles of liquid to drown the insect in don’t seem as exciting but, as ever, the looks may be deceiving. Pitchers often have distinct colorations to attract insects, they secrete nectar on the rims of their pitfalls to lure the poor creatures for a “free meal”. When an insect attempts to land on the rim it turns out that the rim is made up of tiny downward pointing scales and is covered with liquid which makes the landing platform very slippery and by the time the mistake can be registered the insect is already falling down into its doom. At the bottom of each pitfall a pool is liquid solution, which consists of various digestive enzymes and bacteria, that will eventually and inevitably digest the insect leaving the plant to absorb all of its nutrients.
Interestingly, some pitcher plants are better at catching the insects than others and indeed, some pitchers grow in areas that have far too few suitable insects to make the carnivorous life-style a sustainable one. So how do they survive? Well, it turns out that what may be one’s grave can be another’s home. Namely, a type of pitcher plant called Nepenthes rafflesiana elongate, which grows in the jungles of Borneo, has evolved special characteristics to accommodate the living of Hardwicke’s woolly bat inside it. The pitcher of Nepenthes r. e. is larger and longer than some closely related species and the level of digestive liquid at the bottom of the plant is usually significantly reduced. Indeed, in some cases pitchers have been found that are large enough to comfortably accommodate inside a mother and its juvenile baby bat. In addition, the slender shape and a special lignified girdle of the pitchers allow the bats to settle head first inside the trap without slipping down to the bottom.
One may wonder what’s the benefit for the plant to have a bat living inside it if it blocks the insects from being lured into the traps. It turns out that the plants benefit from its inhabitants by being used as lavatories! The faeces of bats are rich in nutrients such as nitrogen, which can be absorbed by the plant to be used in its own metabolism. In addition, some pitcher plants are also pollinated by the bats. Bats are actually not the only animals that make use of pitcher plants, a similar mutualistic relationship between tree shrews and another species of pitcher plant, Nepenthes lowii, also exist (those interested see here).
It turns out, that not only the shapes some pitfall traps are especially accommodating for the bats to live in, but also the trap has special reflective properties to make it easy to find. Bats use echolocation to track and catch insects they feed on, however, this ability has also been adapted to find the suitable pitchers for nesting. Recently, it has been shown that the back walls of the pitchers that have mutualistic relationships with bats, also have significantly better acoustic signal reflection compared to those that don’t. Consequently, in the cluttered jungles the echolocation signal is more likely to come back from pitchers that are suited for roosting making them easier to find.
Grafe, T. Ulmar, et al. “A novel resource–service mutualism between bats and pitcher plants.” Biology Letters 7.3 (2011): 436-439.
Schöner, Michael G., et al. “Bats Are Acoustically Attracted to Mutualistic Carnivorous Plants.” Current Biology (2015).