Ernesto Gianoli wasn’t the first person to work out his frustrations with a walk in the woods, but the motivation behind that walk—and its results—were certainly unusual.
Gianoli studies the plants of Chile’s temperate rainforests. When he goes out into the field, he usually works to a tight schedule, involving dawn-to-dusk sampling and measuring. “One day, I felt that while being engaged in these work plans, we were missing the joy of the quiet observation of nature,” he says. “I told my students that I would dedicate some hours to walk slowly across the forest, just observing. And then it happened.”
Gianoli noticed that the leaves on one particular shrub seemed to be growing from two very different stems—one much thinner than the other. He eventually realised that the thin stems actually belonged to a Boquila vine, whose leaves were exactly the same as the shrub’s. He walked on and found Boquila entwined around many different trees; in most cases, its leaves matched those of its host. It looked like a mimic, and one with many guises.
“It was astonishing,” he says. “I was familiar with the vine but I had not noticed this feature before. I walked back to the hut where the rest of my team was waiting, and told my undergraduate student Fernando Carrasco-Urra, ‘Do you want to be famous? I’ve got the idea for your thesis.’ Of course, they mocked me.”
But as Carrasco-Urra and Gianoli collected more data, the scepticism faded. Boquila’s leaves are extraordinarily diverse. The biggest ones can be 10 times bigger than the smallest, and they can vary from very light to very dark. In around three-quarters of cases, they’re similar to the closest leaf from another tree, matching it in size, area, length of stalk, angle, and colour. Boquila’s leaves can even grow a spiny tip when, and only when, it climbs onto a shrub with spine-tipped leaves.
“There are some leaf features that are too hard to copy, such as serrated leaf margins,” says Gianoli. “It is common to see cases where Boquila “did her best”, and attained some resemblance, but did not really meet the goal.”
The same vine can even mimic several trees! If it crosses from one plant to another, its leaves change accordingly.
“Even orchids, the world’s best known plant mimics, just mimic one specific model, or just share the general appearance of several similar flowers,” says Anne Gaskett from the University of Auckland. “This vine seems to mimic many specific models, depending on its host—something we’ve previously only seen in animals.”
Environmental factors like light aren’t behind these similarities. After all, Carrasco-Urra and Gianoli found very different Boquila leaves in areas with very similar light levels. They also showed that the unusual leaves only turn up when there are other plants to mimic. A Boquila vine climbing up a bare tree trunk looks exactly the same as one that’s crawling along the forest floor. It only changes when there’s a leaf around to mimic.
Why? Carrasco-Urra and Gianoli suspect that the disguises protect Boquila from hungry mouths. By climbing, the vine can already avoid plant-eaters on the ground, but the duo showed that it bears even fewer signs of damage if it climbs on a host tree rather than a leafless support. Does it just become less conspicuous, or does it gain an advantage by mimicking distasteful hosts? No one knows yet.
It’s also unclear how the vine mimics other trees, let alone so many. Australian mistletoes can mimic the trees they grow upon, but they are parasites that tap directly into their hosts. By contrast, Boquila can match hosts without any contact.
Carrasco-Urra and Gianoli suggest that they might be picking up on airborne chemicals released by other trees. We know that chemicals like these can act as alarms, which tell plants that their neighbours are in danger and to raise their own defences. Perhaps Boquila taps into these danger signals to work out which disguise to adopt.
Alternatively, the vine might be using genes from its host. There are many cases where genes have moved horizontally from one plant species to another, sometimes via a parasite or microbe. This idea is speculative and unlikely, but it is strange that Boquila takes on the guise of the nearest leaf, even if that leaf doesn’t belong to the tree that the vine has actually climbed.
Carrasco-Urra and Gianoli are now trying to solve these mysteries by testing Boquila’s abilities in experiments. They’re moving the vine from one host to another and exposing it to the smells of different hosts, to see if it changes accordingly. They also want to sequence the DNA of the vine and its hosts to see if any genes could be hopping across.
“The naturalist view should come first and the scientific approach should follow,” says Gianoli. Observation, then understanding. It’s the approach that Charles Darwin, the quintessential naturalist, used to develop his theory of natural selection. It’s the approach that Gianoli wanted to return to when he went for his walk.
Reference: Gianoli & Carrasco-Urra. 2014. Leaf Mimicry in a Climbing Plant Protects against Herbivory. Current Biology. http://dx.doi.org/10.1016/j.cub.2014.03.010