How Did Mistletoe Get Into the Treetops?

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It’s unclear what trendsetter first hung up mistletoe. Some blame the ancient Greeks, who kissed under the plants during harvest festivals. Others pin it on first-century druids, who might have decorated their homes with them for good luck.

But we may have to look even farther back. A paper published in The American Naturalist this month presents new potential culprits: tiny, prehistoric primates and marsupials, who might have first brought the plants — or at least their seeds — high up into forest canopies over 55 million years ago.

Almost all of the world’s mistletoe species live in the branches of trees or shrubs, where they glom on to their hosts via root-like structures and siphon off water and minerals. They use the energy they get to grow showy, colorful flowers, and to drop nutritious leaves with abandon.

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

Songbirds especially like to gorge on mistletoe berries. It’s thought that prehistoric songbirds helped mistletoes travel the world and land in different kinds of hosts, where they eventually diversified into the hundreds of species that exist today. It was also hypothesized that songbirds helped with a vital step in mistletoe evolution: the move from the ground — where mistletoe ancestors parasitized the roots of other plants — up into the treetops where they tap into branches instead.

But a recent analysis of mistletoe evolution complicated this story. It pushed the date of that crucial move further back than previously thought, to that 55 million years range — long before any of the songbird groups that now depend on mistletoe first appeared, Dr. Watson said.

“That reminded me that, well, there’s other organisms that are around now that snack on mistletoe,” he said. Perhaps one of those had an ancestor who carried a fateful berry up to the treetops.

Dr. Watson, who has spent decades studying mistletoe in many environments, flipped through his mental Rolodex of contemporary dispersers.

A hamster-sized marsupial, the Monito del monte, sprang to mind. The species is the only mistletoe disperser in the Andes Mountains, he said. “If you see a mistletoe in the Andes, it came out the rear end” of a Monito. This animal’s ancestors were well-positioned in space and time to encounter enjoy the fruits of a forebear of contemporary mistletoes, and festoon nearby trees with the resulting droppings.

Other tiny climbers, the mouse lemurs of Madagascar, spread a different kind of mistletoe through a ritual of their own. Their local mistletoes have small, clingy seeds. The lemurs “get the seeds stuck to them, and groom them off as a family,” Dr. Watson said. Their ancestors — or another primate of the past — may have done something similar, inadvertently starting a new mistletoe era.

It’s pretty much impossible to know for sure. The scenarios Dr. Watson lays out of mammal-driven moves 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 parasitic plants. Some fruit-eating birds were also around at the right time, she said, and might have helped the plants make the leap.

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

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

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