Pitcher plants are actually known for their appetite for insects - but now researchers are reporting how successfully some species of these plants have switched their diet to the droppings of small mammals: they turned their traps into toilet bowls, so to speak. The team was able to show that these "toilet pitcher plants" can get significantly more nutrients than their carnivorous relatives in the barren altitudes of the island of Borneo.
Most plants are content with the nutrients that they draw from the soil via their roots. But some plants obtain additional food in a bizarre way: the so-called carnivores catch insects in order to gain growth advantages in barren locations, primarily through the increased supply of nitrogen. Some reps snap, others set sticky traps, and some set pitfalls. The last group includes the representatives of pitcher plants (Nepenthes). Their leaves form tendril-like extensions with bulbous structures at the ends. Their openings have nectar glands that are supposed to attract prey. The victims then find no hold on particularly smooth structures and fall into the cup in which they await digestive fluid.
Specialized in feces
This "standard concept" occurs in different types of pitcher plants in different sizes and shapes. A particularly large number of female representatives can be found on the Indonesian island of Borneo. However, in recent years, researchers have also discovered bizarre specializations there, reports an international research team. The pitchers of some species are therefore less suitable for catching insects - instead they have adapted to small mammals in a more peaceful way: They offer them tasty nectar, which they can lick comfortably while sitting on the pitcher.
The rear end of the small mammals is obviously positioned in a favorable way for her: while they are feasting, they obviously often do their business in the pitcher openings, as observations and traces of excrement show. “Originally, they used colors and scents to attract and capture insects. But these pitcher plant species now invite small mammals to deposit their excrement in the pitchers. Traps have become toilet bowls,” says co-author Gerhard Gebauer from the University of Bayreuth.
In their study, he and his colleagues have now investigated the extent to which this diet can offer the plants advantages over insect trapping. To investigate the nutrient supply, the researchers used the proof of the nitrogen isotope ¹⁵N in the plant material. The higher the levels, the more of the important fertilizer element the plants get from the alternative sources, the researchers explain. They examined tissue samples from eight different pitcher plant species from the high mountain regions of Borneo. Four were known to have animal excrement on their menu, while the others were representatives who adhere to the traditional method of trapping insects. “Common” plants from the region also served as comparison plants.
"Toilet pitcher plants" are better nourished at high altitudes
The analysis results initially confirmed that pitcher plants can usually obtain more nitrogen from their alternative sources than "normal" plants in their vicinity. A comparison between the different pitcher plant species then showed that the ¹⁵N content in the tissues of those who had adjusted their diet to animal excrement was more than twice as high as in the species that only feed themselves by catching insects. “The high proportion of the nitrogen isotope ¹⁵N in plant tissue is a clear indicator of an improved supply of nitrogen and other important nutrients. Our investigations therefore clearly show that switching to faeces as a new food source was worthwhile,” says Gebauer.
But why do only some pitcher plants collect droppings? The researchers explain that the concept probably only has advantages over insect trapping at certain locations. This is because species with adaptations to collect mammalian excretions are only found at high altitudes. It is known that there are significantly fewer prey animals than at lower altitudes. That is why the toilet concept is obviously particularly worthwhile in these areas. "This change in function is a surprising example of plants being able to creatively adjust their diet," Gebauer concludes.
Source: University of Bayreuth, specialist article: Annals of Botany, doi: 130/7/927/6779531