‘Lazy’ pitcher plants catch more ants

Who says working long hours will earn you a bigger payoff? It turns out that part-time carnivorous pitcher plants snare twice as many ants as those working around the clock. The secret of their success was published in Proceedings of the Royal Society B.

Named for their funnel-shaped traps, pitcher plants grow in inhospitable, nitrogen-poor places such as rocky cliffs and even on top of other plants. So they eat insects for their nutrients, dissolving their victims in a pool of digestive juices at the bottom of each trap.

But one species of pitcher plant intrigued ecologist Ulrike Bauer at the University of Bristol, lead author of the latest research.

In 2006 she reported that the traps of Nepenthes rafflesiana, a species of pitcher plant found in the lowlands of Borneo, have an exceptionally slippery rim (called the peristome) when wet. This causes any ants trying to get to the nectar inside to plunge to their doom.

But instead of keeping their peristomes moist with extra nectar when there’s no rainfall or dew, the plants allow them to dry out for up to eight hours a day. N. rafflesiana seemed to have evolved a counterproductive habit: surely part-time traps would deliver fewer insects?

Bauer and her colleagues decided to compare the performance of part-time and full-time traps. They forced traps from 30 N. rafflesiana plants to work all day and night by rigging them up with hospital drips, ensuring they stayed wet all the time. After eight days they compared the amount of prey caught by pitchers on a drip to natural, “untreated” pitchers.

To their surprise, the natural pitchers captured twice as many ants. While the trapped prey wasn’t evenly distributed – some pitchers captured huge numbers of ants and others captured none – only the naturally wet/dry plants managed to capture ants in batches.

A pitcher plant can have hundreds of traps growing on it at any time, so a mass capture in a single pitcher can provide an essential shot of nitrogen and phosphorus to the entire plant.

“Pitcher plants maximise their efficiency by being inefficient, if that makes sense,” explains Charles Clarke, an ecologist at James Cook University.

Bauer explains the apparent contradiction by considering the ant-plant relationship.

Ants are drawn to pitcher plants by the deliciously sweet nectar oozing from the inner margin of a mature pitcher’s rim. An ant unlucky enough to be walking across a wet peristome will slip and fall into the waiting pool of digestive juices at the bottom of the trap. But when N. rafflesiana’s peristome is dry, ants can traverse it without peril.

And being social insects, when a scout from an ant colony comes upon a meal as bountiful as a pitcher plant’s nectar, it returns to the nest to bring back a gang of worker ants to help carry home the spoils. This works in the ants’ favour when the peristome is dry, but if even a little bit of dew or rain falls while the scout is rounding up some helpers – and it often rains in tropical Borneo – the peristome turns into a slippery slide sending the foraging crew plummeting en masse into the waiting digestive goo.

“So for a trap, it might make sense to not be super-efficient all the time,” says Bauer.

But the ants don’t necessarily lose from this arrangement either.

“What superficially looks like an arms race between nectar robbers and deadly predators could in fact be a sophisticated case of mutual benefit,” Bauer says. “As long as the energy gain from eating nectar outweighs the loss of worker ants, the ant colony benefits from the relationship just as much as the plant does.”

This part-time strategy seems to work well only for pitcher plants that mostly feed on social insects. Most of the 150 pitcher plant species are found in mountainous areas, rather than lowlands where N. rafflesiana lives, and aren’t visited by ants as much.

Mature N. lowii plants, which grow at high altitudes in Borneo, get their nitrogen and phosphorus from a more familiar kind of fertiliser, Clarke says: “They’ve evolved to interact with tree shrews”, which leave droppings in the pitchers when they visit to feed on their nectar. “It’s pretty cool! Now it’s become apparent that while they do mostly catch insects, some species have these amazing interactions with mammals.”