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The bats that live in carnivorous plants in Borneo

In 2009, ecologist Ulmar Grafe (University of Brunei Darussalam) was busy working in Borneo. He was there to study how tadpoles survive in the digestive fluid of Nepenthes pitcher plants but his project took a turn when a student discovered something out of place in the pitcher of a Nepenthes rafflesiana elongata: a tiny wooly bat (Kerivoula hardwickii). The researchers fished the bat out of the pitcher and were surprised to find that (1) it was alive and (2) that it was clearly using the pitcher as a roost. Surveys of other Nepenthes rafflesiana elongata in the area revealed more bats and it became clear that these findings were more than coincidental.

A pitcher plant, Nepenthes rafflesiana elongata, with a dislodged wooly bat in Borneo. Photo: Holger Bohn
A pitcher plant, Nepenthes rafflesiana elongata, with a dislodged wooly bat in Borneo.
Photo: Holger Bohn

According to Grafe, there five different growth forms of Nepenthes rafflesiana Borneo and the one with the roosting bats, Nepenthes rafflesiana elongata, is a standout – and not just because its the only growth form to harbor bats. Compared to the more common Nepenthes rafflesiana typica, this particular variety of pitcher plant grows aerial pitchers, is four time longer, and is much less successful at capturing insects (it has a 7 times lower capture rate). It is, however, very successful when it comes to enticing this particular subspecies of wooly bat to roost in its pitchers. Very low levels of digestive fluid coupled with a lignified “girdle” at the base creates a snug environment for sleepy bats.

Intrigued by this unexpected association, the researchers set to work attaching tiny, custom-made transmitters to 17 of the diminutive bats (which weigh only 4 grams) that they found in pitchers. They then spent six weeks tracking the bats and monitoring a total of 223 pitchers in the study area. They also analyzed the nitrogen content of pitcher plants which housed bats compared to those that did not. That analysis revealed that the pitcher plants with bats had nitrogen levels in their leaves 33.8% higher than those of their bat-deficient neighbors.

Service benefit provided by N. r. elongata to K. h. hardwickii. (a) Aerial pitcher of N. rafflesiana var. elongata. (b) The same pitcher with the front tissue removed to reveal a roosting Hardwick's woolly bat. (c) The shorter aerial pitcher of N. rafflesiana variety typica. Credit: T. Ulmar Grafe, et. al.
Service benefit provided by N. r. elongata to K. h. hardwickii. (a) Aerial pitcher of N. rafflesiana var. elongata. (b) The same pitcher with the front tissue removed to reveal a roosting Hardwick’s woolly bat. (c) The shorter aerial pitcher of N. rafflesiana variety typica.
Credit: T. Ulmar Grafe, et. al.

Nitrogen is the reason Nepenthes plants have pitchers in the first place. The evolutionary pressures of growing in nutrient-deficient environments (such as Bornean peat swamps) led Nepenthes plants to develop an alternative means of meeting their nutritional requirements. Animals, such as insects, are a fantastic source of nitrogen and trapping/digesting them provides Nepenthes plants with the nutrients they need to grow and survive. Another great source of nitrogen happens to be bat guano and herein lies the key to this ecological “odd-couple.” Through a mutualistic relationship, the bat benefits by having a secure roost that provides both concealment from passing predators and protection from solar radiation and the plant receives a steady supply of nitrogen-rich bat poo…it’s a win-win!

Symbiotic associations between mammals and carnivorous plants are unusual, though not unheard of. “This is the first case in which the faeces-trapping syndrome has been documented in a pitcher plant that attracts bats and only the second case of a mutualistic association between a carnivorous plant and a mammal to date,” according to the paper Grafe, et. al. published following their work in 2011.

Adorable graphic from the Schöner paper on Nepenthes-bat acoustic attraction.
Adorable graphic from the Schöner paper on Nepenthes-bat acoustic attraction.

Since that time, there have been a few important developments. For one, Nepenthes rafflesiana elongata has been reclassified as its own species: Nepenthes hemsleyana. Additionally, researchers have determined that these plants possess an ultrasound reflector attractive to mutualistic bats and that this reflector is the primary mechanism enabling the bats to easily find and identify the plant’s pitchers.

Fascinating.

References and Further Reading:

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