A Poop-Eating Pitcher Plant

The aerial pitchers of Nepenthes hemsleyana (http://bit.ly/2hC94fO) are quite unique in that they are not intended to catch insects. Instead, they have evolved as specialized roosts for Hardwicke's woolly bats (Kerivoula hardwickii). This incredible mutualism is quite unique among these tropical carnivores. The bats get a safe place to roost and in return, they deposit nitrogen-rich feces. 

This mutualism is quite remarkable in that the upper pitchers of N. hemsleyana have pretty much forgone insect capture altogether. Despite the obvious benefits of this evolutionary relationship, no one had bothered to quantify the benefits gained by turning insect catching pitchers into bat roosts. That is, until now. 

A team of researchers based out of University of Greifswald in Germany utilized some cunning methods to demonstrate exactly how much N. hemsleyana relies on bat droppings. What they found what quite remarkable. Plants offered only insects not only had fewer leaves, they also exhibited slower growth, reduced photosynthetic capacity, and reduced survival. It would seem that this mutualism has evolved to the point of being obligate. 

It is estimated that around 95% of the nitrogen needs of this plant are met by bat feces alone. As it turns out, nitrogen bound up in insect tissues were mostly unavailable to the plant. This is not the case for nitrogen in bat poop. Nitrogen deposited by bats comes mostly in the form of urea, which degrades into ammonium and is readily absorbed by the pitchers. 

Essentially Nepenthes hemsleyana now relies on bats to capture prey for them. This "ecological outsourcing," as it has been termed, frees the plant from the rigors of having to capture and digest insects on its own, thus saving valuable energy reserves that can be allocated to structures such as leaves, stems, and flowers. Why this species has evolved this strategy is anyone's guess. Perhaps it has to do with the deep shaded forest understory in which it grows. 

Photo Credit: Merlin Tuttle (www.merlintuttle.smugmug.com)

Further Reading: [1]

When a Mutualism Becomes Obligate

Mutualism. The word invokes this warm and fuzzy "you scratch my back and I'll scratch yours" feeling. It is easy to grasp how a mutualism would develop and be maintained. But, in any system, there are bound to be cheaters. Cheaters reduce the fitness of one of the partners so to avoid such things, some species up the ante by resorting to some interestingly "sinister" methods.

Acacias and ants have quite the relationship. Acacias protect themselves by offering ants hollow spines and branches where their colonies can live. They even sweeten the deal via extrafloral nectaries. These are glands on the stems that secrete nectar that the ants eat. In some ant species, this is their only source of food. Needless to say, the ants become highly protective of their acacia trees. They readily attack herbivores and even go as far as to prune away vegetation that may interfere with their host. This seems like a pretty straight forward mutualistic relationship, right?

Ah, but it goes deeper. To make sure that the ants will solely rely on the acacia and are thus completely tied up in the well being of their host, the acacia alters the ants phenotype at birth. Normally these ants have no issues digesting sucrose. Researchers found that the nectar in the extrafloral nectaries contains a protein called "chitinase." Chitinase inhibits the ability of the ants to digest sucrose. When ant eggs hatch into larvae, their first meal is nectar from the extra floral nectaries. Once the larvae ingest this protein they are no longer able to feed on anything other than their hosts nectar. Thus their very survival is completely tied to the Acacia.

I am positive that more examples of such obligate mutualisms abound in nature. We only have to ask the right questions to discover them. It is also interesting considering what we are finding out about our own behavior and how it relates to the microbiome living on and within us. What about human behavior could be described in the context of a relationship similar to ants and acacias?

Photo Credit: Tony Rodd

Further Reading: