There are some weird combinations of species in the world. For example, ants fiercely guard the thorny acacia tree while the plant provides shelter for the insects. Similarly, the stinging sea anemone provides a home for the clown fish. Now, researchers have found a new mutualistic relationship. They've discovered that an insect-eating pitcher plant teams up with ants to prevent mosquito larvae from stealing its nutrients.

The relationship is odd to say the least. Pitcher plants consume insects, so it's surprising that a certain species of ants, Camponotus schmitzi, thrives within and on the plant. In fact, the ants only live on one species of Bornean pitcher plant where they walk across slippery pitcher traps, swim and dive in the planet's digestive fluids and consume nectar and prey that fall into the trap.

The benefits for the ants are obvious, but researchers have long been puzzled about what the pitcher plant gains from the relationship. After all, the ants apparently steal nutrients from the plant. Despite this, though, pitcher plants with ants are usually much larger than those that don't harbor the insects. This seemed to point to some sort of mutualistic relationship.

In order to investigate this odd couple a bit further, the researchers ventured into the Bornean rainforest. They examined the plants and the ants, making careful observations. They found that, surprisingly, ants appeared to increase the pitcher plant's ability to capture more insects by keeping their traps clean. In addition, the ants protected the plants by actively hunting mosquito larvae that otherwise bred in pitcher fluids and sucked up plant nutrients.

"Kneeling down in the swamp amidst huge pitcher plants in a Bornean rainforest, it was a truly jaw-dropping experience when we first noticed how very aggressive and skilled the Camponotus schmitzi ants were in underwater hunting: it was a mosquito massacre!" said Mathias Scharmann, one of the researchers, in a news release. "Later, when we discovered that the ants' droppings are returned to the plant, it became clear that this unique behavior could actually play an important role in the complex relationship of the pitcher plant with the ants."

It turns out that the pitcher plants benefit from the ants, after all. Now, researchers know exactly how this unique interaction works and why pitcher plants with ants grow so much larger.

The findings are published in the journal PLOS One.