Happy Holidays, Everyone!
There are perhaps 4,500 species of parasitic woody plants, and mistletoes (~1,500 species) are unique among these for their chosen microhabitat on the host; mistletoes attach in the canopy, rather than the roots. This evolutionary innovation likely gives aerial mistletoes several advantages over plants on the forest floor, including access to light, escape from some natural enemies, and in some circumstances, increased access to water. Canopy life is pretty sweet.
Canopy life is so great that mistletoes have evolved independently from root parasite ancestors five times. These leaps into the canopy all happened 55-75 million years ago, according to the most recent estimates. But this presents a berry sticky problem: if the animal-dispersed mistletoes evolved 55-75 million years ago, they appeared long before the songbirds that currently eat and disperse their seeds. Mistletoes haven’t evolved wings or legs (yet), so how did they get into the canopy?
It may be that different birds dispersed mistletoe seeds 55-75 million years ago. But Watson (2020) thinks this is unlikely. Instead, Watson (2020) suggests that mistletoes were dispersed by tree-climbing marsupials, like the colocolo opossum (Dromiciops gliroides), which still eats mistletoes in South America today.
Now, I know this post is supposed to be about mistletoes, but do you know what a colocolo opossum looks like? Google it. It’s freaking adorable! Why isn’t this little seed disperser on every Christmas card? Hallmark, call me.
Though birds might not have assisted the mistletoes’ assents to the canopy, birds did play a role in subsequent mistletoe diversification; as songbirds radiated, so too did mistletoes, many of which are now pollinated and dispersed by many bird species. There are more details left to be worked out before we understand how the ~1,500 mistletoe species evolved, and considering bird and marsupial dispersers might help piece together this puzzle. Watson (2020) suggests that we could learn more about mistletoe evolution by considering the functional traits of the different species; sticky green berries are likely to be stuck in marsupial fur and later groomed off, colorful berries are likely to be consumed, etc.
As I’ve explained via poem in a previous post, mistletoes are keystone species in forests, providing habitat and food for many species. ‘Tis the season to give and be berry merry.
Watson, D.M. (2020). Did Mammals Bring the First Mistletoes into the Treetops? The American Naturalist, 196, 769–774.