Mimicry in animals is not altogether uncommon, nor is it that rare for them to mimic a more dangerous species, known as "Batesian mimicry". When the goal is not getting killed and/or eaten by predators, looking a bit like a very dangerous animal or plant is advantageous, so although it might be surprising to find a chick that looks like a toxic caterpillar, we shouldn't be too surprised that they evolution nudged them in this clever direction.
The ability to mimic multiple species is a far rarer thing, but there are several animals out there that have inherited this useful trait. For example, the mimic octopus ( Thaumoctopus mimicus ) is able to mimic at least 16 different animals, including mantis shrimp, ghost crabs, cuttlefish, stingrays, jellyfish, and sea snakes.
But there is only one species of plant that humanity knows of that is able to mimic multiple other species; Boquila trifoliolata, also known as the "chameleon vine", found in the temperate rainforests of Chile and Argentina. When not climbing up a host tree, the vine's leaves were all found to be the same in appearance, but once it starts to climb, it has an unusual and difficult-to-explain ability to match the appearance of its host's leaves.
"Field observations indicate that B. trifoliolata often mimics the leaves of its supporting trees in terms of size, shape, color, orientation, and vein conspicuousness, among other features. This phenomenon includes the display of a mucronate leaf apex (a small spine at the leaf tip) when twining around a tree with such mucronate leaves; the botanical description of B. trifoliolata does not include this feature," a 2014 paper on the topic explains, adding that this vine has been found climbing several different species of tree, where its leaves adjust to mimic the host.
"Moreover, when traversing different hosts, the same individual vine changes its leaf morphology accordingly," the authors add.
Maybe the weirdest thing about this plant is that contact does not appear to be necessary in order for it to begin its chameleon schtick, making the mechanism behind it tricky to explain.
"Because leaf mimicry is observed even when the model is not directly in contact with the vine, we might hypothesize that host plant volatiles trigger specific phenotypic changes in neighboring vine leaves. Volatile organic compounds have been shown to elicit specific responses in neighboring plants, including induction of secondary metabolites, increased expression of defense-related genes, and, overall, numerous changes of the plant transcriptome," the team explains.
"However, research on airborne plant-plant signaling has not reported specific morphological changes in leaves, and nothing is known about the involvement of volatiles of the model in leaf mimicry via induced reprogramming of genes in the mimic."
Alternatively, though they deemed it less likely, they suggested it could be the result of horizontal gene transfer between plants, mediated through airborne microorganisms, potentially explaining mimicry without contact.
In one particularly controversial study published in 2021, researchers suggested that Boquila trifoliolata has "plant vision". In the study, the team grew the plant attached to a wooden trellis. At the top were artificial leaves, whilst the bottom half of the plant was unable to "see" the fake, plastic leaf. According to the team, the real leaves near the top showed some signs of mimicking the artificial leaves, whilst the leaves at the bottom did not.
Controversially, and not to much interest from other biologists, the team suggested that the plants displayed "ocelli-based plant vision" to explain their results.
Not stopping there, they also suggested it could form memories.
"It appears that over the months, [ B. trifoliolata ] plants improved their mimicking of the plastic host plant significantly. The mimic leaves doubled in size from one analysis to the next (first analysis December 2020, second analysis June 2021) and the form factor has reduced significantly, approaching the form factor of the plastic leaves having slender shapes," the team behind the 2021 study wrote.
"This improved ability of [ B. trifoliolata ] plants to mimic shapes and sizes of plastic leaves implicates learning and memory processes in plant mimicry."
Whilst a fun idea, science isn't always about fun. The small study has been criticized by other scientists, including a horticulture expert, who explained several plausible reasons why the leaves may have changed shape in the experiment, especially noting that the authors' experiment did not work in the winter.
"Summer months are hotter and brighter than winter months," she explains for the Garden Professors blog. "The experimental leaves were exposed to increasing heat and water loss compared to the shaded control leaves. Newly expanding leaf morphology changes in response to changing environmental conditions."
As well as this, plastic plants have been shown to release volatiles in warmer temperatures, which can then affect leaf morphology.
Criticizing the study and a second journal article based upon it, she concludes "this is a deeply flawed article based on a poorly designed experiment and reflects significant author bias in the interpretation of the results".
More recently, another team has attempted to explore the horizontal gene transfer (HGS) hypothesis more thoroughly, finding some support for it. They compared endophytic bacterial communities on three different types of leaf samples: those on a Rhaphithamnus spinosus tree, those on Boquila trifoliolata that were mimicking the tree's leaves, and those on Boquila trifoliolata that were not mimicking the leaves, though they were on the same vine.
"We found that mimetic Boquila were closer to the model tree Rhaphithamnus spinosus than non-mimetic Boquila in terms of the composition of endophytic bacterial communities," the team explains in their paper.
"Our results suggest the involvement of bacterial agents in leaf mimicry by Boquila, yet we are still far from proving the HGT hypothesis. Thus, here we validate — and promote further research on — the role of bacteria in this unique case of leaf mimicry."
More study is needed on these highly unusual plants.