Pitcher plant port-a-potty for the tree shrew

A pitcher plant (courtesy of Wikimedia Commons)

Timeline, 2009: As humans, we are a bit limited in our imaginations. For example, we’d probably never consider climbing onto the edge of a toilet seat and licking the sides while…um…employing the toilet for standard uses. Perhaps one reason—among many obvious choices—is that we’re not tree shrews living in the wilds of Borneo in Southeast Asia.

If you’re now envisioning tree-dwelling rodents enjoying the civilized development of having their own toilet, you’re not too far off. Borneo is home to a number of unusual relationships between species, but none may be stranger than the one that has developed between the tree shrew and the pitcher plant. The pitcher plant is carnivorous, and as its name implies, has a pitcher-shaped structure that it uses to trap its food.

The many uses of the pitcher plant

Normally, a pitcher plant growing on the ground is the perfect trap for hapless animals drawn to its minimal nectar output. For some species, they’re not a death trap but a place to brood offspring—one frog uses the pitcher plant to lay its eggs, where trapped, digested insects may provide some nourishment. The insects fall in because the funnel-shaped pitcher part of the plant has a slippery lip that acts as a deadly superslide for any insect that alights on it. Unable to gain a foothold, the animal slides helplessly into the plant’s interior, landing in a pool of digestive enzymes or bacteria that slowly break it down.

What does a pitcher plant do with digested insect? It does what any organism, plant or otherwise, does with its food—it extracts nutrients from it. One primary nutrient that plants (and everything else) require is nitrogen. This element is part of life’s important building blocks for DNA and RNA and the amino acids that make up proteins. Thus, to grow and reproduce, organisms must acquire nitrogen from somewhere. Some plants form a partnership with bacteria to get their nitrogen. Pitcher plants digest insects for it.

Unless no insects are available. While ground-growing pitcher plants in Borneo can subsist on available ants and other crawly critters, some pitcher plants grow on vines and trees, where ants are largely unavailable. In addition, mountainous environments are not known for harboring lots of ants, so the pitcher plant needed a new plan for getting its nutrients.

Nectar for nitrogen

The plan, it seems, was selection for making more nectar, reducing the slippery factor, and behaving like both a toilet and a food source for an abundant animal in the Borneo mountains, the mountain tree shrew. Using video cameras, researchers based at a Borneo field station captured one of the most unusual mutually beneficial relationships in nature: the tree shrew, while enjoying the abundant nectar uniquely produced by these aerial pitcher plants, also poops into the pitcher plant mid-meal. The plant, perfectly shaped for the tree shrew to park its rear just so while it eats, takes up the feces and extracts nitrogen from it. In fact, these pitcher plants may derive up to 100 percent of their nitrogen from the tree shrew poop.

Researchers think that this friendly relationship must have been in the making for a very long time. The pitcher plant opening is perfectly shaped and oriented so that the nectar collects just at the lip and the shrew must orient while eating so that the funnel-like pitcher collects any poop that emerges. The plant also has developed sturdier and thicker structures that can support the weight of a dining/excreting tree shrew, which isn’t much at less than half a pound, but quite a bit for a plant to support.

As odd as this adaptation may seem, it’s not unique. Ground-dwelling pitcher plants have formed similar mutually beneficial relationships with insect larvae that help themselves to some of the insect pickings that fall in. These larvae excrete any leftovers, and the plant harvests nutrients from these excretions. Interestingly, the tree shrew itself dines on insects, so the pitcher plant is still indirectly deriving its nitrogen from insects even when it uses tree shrew poop. It’s just getting it from the tail end of a rodent intermediary instead.

Tricky little orchids

Orchids attract collectors all over the world. One of the things that draws us to these unusual plants is their Machiavellian approach to life. They unfeelingly employ deception to their benefit, usually practicing their art on unsuspecting members of the insect community. Research has revealed that one species of orchid, Anacamptis morio (or Orchis morio), or the green-winged orchid, lays its bold insect trap in an attempt to avoid a trap itself.

Inbreeding avoidance: not just for royalty

Although plants can do many things that most members of the animal kingdom cannot—self-fertilize or increase chromosome numbers in a generation—they’re still better off when reproductive measures result in an increase in genetic variation. As with most organisms, inbreeding is not a healthy thing for a plant, and many plants have mechanisms to avoid it.

The idea of inbreeding avoidance led researchers to a theory to explain the remarkable behavior of many orchids. These beautiful, much-coveted flowers attract humans and insects with their alluring fragrances and colors. For insects, some orchids add to the attraction by mimicking the female of the insect species, or wafting the scent of eau d’ dung for insects that prefer laying their eggs in such places. But of the 30,000 known orchid species, about 10,000 have nothing to offer the hapless insect in return: their flowers have no nectar.

Why keep coming back for nothing?

Researchers have sought to explain why insects would continue to visit such a stingy plant, and why the plants continue to get away with and employ their nectar-free strategy. The strategy itself seems in violation of so much of our understanding of the natural world, a place typically characterized by tradeoffs. In fact, orchids without nectar are not wildly popular among insects—it is difficult in many cases to witness a bee pollinating a green-winged orchid in the wild—but they still do manage to get pollinated.

Scientists investigated wild-growing green-winged orchids on a Swedish island and figured out why this species cheats insects so mercilessly. It’s about genetic variation. The flowers attract the bugs, but offer the foraging insects nothing, driving them on to explore other plants. Although the orchids have not provided food, they have given the unsuspecting insect a payload of a different kind: pollen. The bug—still on a quest for nectar—forages in other plants, pollinating as it goes along. Voila! No self-pollination. Plants that result from self-pollination are usually weak and unhealthy, and self-pollinating can be a waste of precious pollen.

Interviewing bees

Scientists detected this self-pollination avoidance by interviewing bees. They queried specific bees with plants that had been artificially dosed with nectar or with plants in their natural nectar-free state. The researchers found that bees stayed around the nectar-ful plants twice as long and investigated twice as many flowers on the same plant, which would promote self-pollination. Bees that found no nectar moved along to other plants, promoting cross-pollination.

One thing that could confound the interpretation of these results is that bees can remember how a plant smells. If a bee strikes out with one orchid, it will remember that orchid’s smell and not waste its time foraging around in other flowers that smell the same.

In separate research performed by a team in Switzerland, scientists found that the flowers of a nectar-producing orchid species all smell very much the same. But flowers on different plants of the green-winged orchid all smell different. A bee might have failure at one green-winged orchid and remember the smell, but then fly straight into another green-winged orchid plant because its smell is different. The unhappy bee falls into the orchid’s trap and gets nothing, but the deceitful orchid itself has had a great success: avoiding the trap of self-pollination.

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