How Did Mistletoe Get Into the Treetops?

It’s unclear which trendsetter hung up mistletoe first. Some blame the ancient Greeks who kissed under the plants during the harvest festivals. Others put it on first century druids who luckily might have decorated their homes with them.

But we may have to look back further. An article published this month in The American Naturalist introduces new potential culprits: tiny prehistoric primates and marsupials who may have first brought the plants – or at least their seeds – high into forest canopy over 55 million years ago.

Almost all types of mistletoe in the world live in branches of trees or bushes, where they reach their hosts via root-like structures and suck off water and minerals. They use the energy they get to grow showy, colorful flowers and drop nutritious leaves with devotion.

You could call this parasitism – but you could also call it the life of the party. Well-done mistletoe plants attract a wide variety of living things, from bees and other insects that pollinate their flowers to mammals and birds that live on their branches. They are “the center of attention in many terrestrial systems,” said David Watson, professor of ecology at Charles Sturt University in New South Wales, Australia and author of the new paper.

Songbirds especially like to eat mistletoe berries. It is believed that prehistoric songbirds helped mistletoe travel the world and land in various types of hosts, where they eventually diversified into the hundreds of species that exist today. It has also been hypothesized that songbirds were instrumental in a crucial step in mistletoe development: moving from the ground – where mistletoe ancestors parasitized the roots of other plants – to the treetops, where they instead tap branches.

However, a recent analysis of mistletoe evolution complicated this story. The date of this crucial step was postponed further than previously thought to 55 million years – long before any of the songbird groups now dependent on mistletoe first appeared, said Dr. Watson.

“It reminded me that there are now other organisms that eat mistletoe,” he said. Perhaps one of them had an ancestor who carried a fateful berry to the treetops.

Dr. Watson, who for decades studied mistletoe in many settings, flipped through his mental Rolodex of contemporary dispersers.

A hamster-sized marsupial, the Monito del monte, came to mind. The species is the only mistletoe disperser in the Andes, he said. “If you see a mistletoe in the Andes, it came out at the far end” of a monito. The ancestors of this animal were well positioned in space and time to encounter and enjoy the fruits of an ancestor of contemporary mistletoe, and to decorate nearby trees with the resulting droppings.

Other tiny climbers, the mouse lemurs of Madagascar, spread a different type of mistletoe through their own ritual. Your local mistletoe has small, clinging seeds. The lemurs “let the seeds adhere to them and tend them as a family,” said Dr. Watson. Your ancestors – or some other primate in the past – may have done something similar and inadvertently ushered in a new mistletoe era.

It is next to impossible to know for sure. The scenarios that Dr. Watson interprets mammalian-driven movements to the canopy are “plausible, but not the only ones,” said Romina Vidal-Russell, a botanist at the National University of Comahue in Rio Negro, Argentina who specializes in parasite plants. Some fruit-eating birds were there at the right time too, she said, and might have helped the plants make the jump.

“Unfortunately we don’t have a time machine to go back and see,” said Dr. Vidal-Russell.

No matter who was involved, it must have been a festive scene.

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