Why Did These Carnivorous Plants Switch Their Diet From Bugs To Poop?

Carnivorous plants that evolved to feed on animal droppings have a more nutritious diet than their insect-eating cousins

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It was reported a more than a decade ago that some species of tropical pitcher plants, Nepenthes species, changed their diet from insects to animal poops. But thanks to new research, we now know why they did it.

There are roughly 160 species of carnivorous plants known. They all live in nutrient-poor soils and have evolved modified leaves shaped like a pitcher filled with a digestive fluid to capture rare nutrients, particularly nitrogen but also carbon and phosphorus, by digesting the bodies of insects, arachnids and occasionally larger creatures like frogs or even small rodents that fall inside.

“These [pitchers] evolved primarily as the means of attracting, capturing and digesting prey, and occur in the form of so-called ‘lower pitchers’ and ‘upper pitchers’, specialized towards the capture of crawling and flying insects, respectively,” said lead author of the study, botanist and restoration ecologist, Adam Cross, a Senior Research Fellow in the School of Molecular and Life Sciences at Curtin University. Dr Cross is an internationally-renowned authority on the ecology and conservation of carnivorous plants.

“Pitcher shape, size and color are highly variable among species of Nepenthes, but most species produce pitchers with an overhanging lid, a more or less cylindrical peristome surrounding the pitcher mouth (sometimes flattened, markedly flanged or toothed), and interior walls comprising a glandular zone sometimes surmounted by an upper waxy zone”, Dr Cross explained (Figure 1).

Carnivorous pitcher plants use a sweet-smelling nectar to lure in potential prey, which can slip on the smooth surfaces and fall into the fluid-filled tubular pitchers where they become trapped.

“Captured animals drown in an enzyme cocktail fluid that also facilitates the decomposition of, and nutrient-acquisition from, prey”, Dr Cross elaborated.

But some pitcher plants that live in the tropics have evolved to exploit an even richer source of nitrogen than bugs: mammal poops.

“A handful of Nepenthes species have evolved away from carnivory towards a diet of animal scats”, said the study’s senior co-author, a taxonomist and field botanist Alastair Robinson, the Manager of Biodiversity Services at the Royal Botanic Gardens Victoria, who is widely regarded as the world’s authority on the carnivorous plant genus Nepenthes.

These plants live at very high elevations in the tropics of Borneo. Nepenthes lowii, for example, is found only in mountaintop cloud forests at elevations of about 1676.4 meters (5,500 feet) to 2591 meters (8,500 feet) above sea level. This climbing pitcher plant’s stems can reach lengths of more than 30 feet, with individual pitchers about four inches across. N. lowii pitchers are noteworthy for their distinctive shape, which is strongly reminiscent of a toilet bowl, complete with lid (the part of the leaf that can help block out heavy rain).

You may recall that poop-digesting pitcher plants were first discovered in 2009 when it was found that mountain tree shrews would feed on a goopy nectar produced by the lids of the pitchers, and then poop into these plants’ toilet-like ‘bowls’ before departing (ref). Later studies found more of these unusual plants, which can also feed on droppings from summit rats, woolly bats, and even a small bird known as the mountain blackeye. To do this, these poop-eaters lure in animals with their sugary syrups, but instead of tricking their targets, these plants allow the animals to feed on the nectar. As the animals take their time eating their sugary snack, they often defecate directly into the pitchers.

But until now, it wasn’t known precisely how effective this unusual strategy was for plants that were collecting rare nutrients from the environment, nor how nutritious a poop-based diet is compared with the insect-heavy diets of other pitcher plants. So Dr Robinson, Dr Cross and their collaborators investigated.

Dr Robinson, Dr Cross and their collaborators identified how much externally-acquired nitrogen (¹⁵N) and carbon (¹³C) isotopes were present in tissue samples collected from 10 pitcher plants. They then compared the poop-eaters to their insect-eating cousins that live at lower altitudes and also tested locally occurring non-carnivorous plants as controls.

They found that ¹⁵N was significantly enriched in all Nepenthes tested when compared to non-carnivorous plants nearby, but the ¹⁵N levels were more than twice as high in Nepenthes that evolved to capture mammal droppings than insect-eating species. Interestingly, researchers mentioned that bird droppings provided slightly less nitrogen, but even they were more nutritious than a carnivorous diet.

“We found that nitrogen capture is more than two times greater in species that capture mammal droppings than in other Nepenthes”, Dr Robinson surmised.

But what would drive these plants to evolve away from consuming insects?

“Insect prey is scarce on tropical peaks above 2200 metres, so these plants maximise nutritional returns by collecting and retaining fewer, higher-value nitrogen sources, like tree-shrew droppings”, Dr Robinson explained.

This study is a lovely demonstration of the tight relationship between ecology and evolution: the higher a pitcher plant resides on a mountain, the more resourceful they must be to obtain enough nutrients to thrive.

Sources:

Adam T. Cross, Antony van der Ent, Miriam Wickmann, Laura M. Skates, Sukaibin Sumail, Gerhard Gebauer and Alastair Robinson (2022). Capture of mammal excreta by Nepenthes is an effective heterotrophic nutrition strategy, Annals of Botany 130: 927–938 | doi:10.1093/aob/mcac134

Charles M. Clarke, Ulrike Bauer, Ch’ien C. Lee, Andrew A. Tuen, Katja Rembold and Jonathan A. Moran (2009). Tree shrew lavatories: a novel nitrogen sequestration strategy in a tropical pitcher plantTree shrew lavatories: a novel nitrogen sequestration strategy in a tropical pitcher plant, Biology Letters, 5:632–635 | doi:10.1098/rsbl.2009.0311

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