Seed dispersal in the rainforest canopy: mistletoe and the Lovely Cotinga

One of the tallest trees in the Neotropical rainforest, the mighty nutmeg, depends on toucans and rodents to disperse its seeds to places where they can grow. Here is a tree that can reach 150+ feet (45+ meters) in height, and without the help of rainbow-colored birds and bucktoothed mammals its seeds are doomed. What happens to small plants that grow in the forest canopy 100+ feet off the ground and don’t even have roots in the soil? Do they also need wildlife to disperse their seeds?

Mistletoe is great example of a plant that grows high up in the forest canopy and never touches the ground. Never touches the ground?? How does a plant do that? Epiphytes are those plants that grow on the surface of other plants. In a shady forest all the plants are competing for light. Some plants like the nutmeg tree win this little battle by growing head and shoulders above the rest. Other, smaller plants win by groing on the branches of trees, high up in canopy where they get all the sunlight they need. Orchids. Bromeliads. Cacti. These are just some examples of plants that bask in the sunlight on the branches of trees high in the rainforest. They get sun from above, water from the rain, and when leaves decompose around the roots that they grow along the surface of branches, they get their food. But what do these plants do with their seeds? And how did that puny plant get way up in the tree anyways? For a mistletoe, and lots of other plants, the answer is birds again.

Here is one of my favorite birds of the Neotropical rainforest, the Lovely Cotinga. This bird oozes cool! It’s a day-glow, neon turquoise blue with a plum purple throat. I bet astronauts can see these dudes from space.  And yes, the actual name of the bird is “Lovely Cotinga.”

A male Lovely Cotinga perches like a jewel atop the rainforest. Photo © James Adams of the Lodge at Pico Bonito.
A male Lovely Cotinga (Cotinga amabilis) perches like a jewel atop the rainforest. Photo © James Adams of The Lodge at Pico Bonito.

 

And here is the object of cotinga desire, Psittacanthus rhyncanthus, a species of mistletoe. There is nothing a cotinga loves more than to chow down on mistletoe fruits.

 

Mistletoe berries (purple fruits lower left) are the gas that light up a male Cotinga like this one. Photo © James Adams of the Lodge at Pico Bonito.
Mistletoe berries (purple fruits lower left) are the “juice” that light up a male Cotinga like this one. Photo © James Adams of The Lodge at Pico Bonito.

And herein lies our rainforest intrigue. The mistletoe, like the nutmeg, is a trickster. It puts out hundreds of small fruits that birds like the Lovely Cotinga love to eat, but this time the joke is on the birds. After a cotinga or other unsuspecting bird goes in for a tasty meal of fruits it finds out that its gullet is full of seeds, seeds that are so sticky, so gooey, it’s like having a mouthful of glue globs. To rid itself of these seeds the cotinga has to literally wipe its face across the surface of a tree branch until the seed sticks to the branch. Voila! Just like that a mistletoe is born. From branch, to bird, to branch, the mistletoe never leaves the forest canopy and is transplanted by an agent in electrified blue feathers. Stranger than fiction? That’s life in the rainforest.

Don’t believe it? You can see for yourself at places like The Lodge at Pico Bonito, Honduras, where Lovely Cotingas show up by the flockful every year during the rainy season from January to March. James Adams, a manager at the Lodge, witnessed an interesting dispute over some mistletoe between two cotingas.

An adult and a juvenile male Lovely Cotinga dispute a favorite perch. The tree branches are being killed by mistletoe plants spread by the contingas themselves. Photo © James Adams at the Lodge at Pico Bonito.
An adult and a juvenile male Lovely Cotinga dispute a favorite perch. The tree branches are being killed by mistletoe plants spread by the contingas themselves. Photo © James Adams at The Lodge at Pico Bonito.

James tells us: “Evidenced by the numbers of mistletoe stuck to this branch [note the little green leaves under the branch – that’s them], Cotingas have favored perches. And apparently they don’t like to share with just anyone. When this young male (note the maturing purple and blue plumage) flew in and tried to accompany this bright blue male, the older male would have none of it. And so a war of silent beak gaping ensued, with both parties opening and closing their beaks at one another, until the younger bird flew off.”

In other words, it appears from watching these colorful birds that they need the mistletoe just as much as it needs them.

Parting shot: Gotta love these guys, right?

Lovely Cotinga (Cotinga amabilis) in Pico Bonito National Park, Honduras. Photo © Roy Toft
Lovely Cotinga (Cotinga amabilis) in Pico Bonito National Park, Honduras. Photo © Roy Toft
Share

Seed dispersal in the tropical rainforest canopy. How does your garden grow?

Seeds of the wild nutmeg (Virola kochnyi).  The red coating is a thin layer of fruit over a huge seed. Photo © David L. Anderson.
Seeds of the wild nutmeg (Virola kochnyi). The red coating is a thin layer of fruit over a huge seed. Photo © David L. Anderson.

Question: What do pumpkin pie, rainforest trees, and toucans all have in common? The answer, quite obviously, is nutmeg! Nutmeg is one of my favorite spices. I use it in pumpkin pie, hot chocolate, apple pie, and some Indian curry recipes. I don’t buy it in powder, though, but in whole seeds, which I grate into the foods I’m cooking. When I want a hot, wintery spicy dish, nothing satisfies better than nutmeg.

You might be surprised to find out that toucans agree with our liking for nutmeg. The seed in the photo above – that’s a wild nutmeg seed growing high in the canopy of the rainforest. When the fruit ripens the husk pops open exposing a plump seed covered in delicate tendrils of fruit. It’s the fruity layer that the toucans are after. A toucan will reach out with its long bill, pluck a seed, swallow it, and fly off through the forest, where it digests the fruit and spits out the seed. If you are lucky enough to find a big nutmeg tree with fruit it will literally be dripping with toucans.

Yellow-eared Toucanet (Selenidera spectabilis) getting ready to swallow a wild nutmeg seed.  Photo © James Adams.
Yellow-eared Toucanet (Selenidera spectabilis) getting ready to swallow a wild nutmeg seed. Photo © James Adams.

Now for the cool part. This toucan thing, flying away and spitting seeds in the forest, is all part of the nutmeg tree’s secret plan. If all those seeds were to fall directly under the parent tree and sprout, the seedlings would wither and die. They won’t get sun under a huge tree, and thousands of seedlings will choke each other out in competition for sun, nutrients, and water, and rodents will feast on seeds by the cheekful because, like toucans, they know where to find a good meal. In the battle for survival that happens every day in the rainforest, the tree needs to send its seeds far and wide if any are to become the next generation.

 

Down the hatch!  After the toucanet flies off it will cough up the seed.  With luck a new nutmeg will sprout.  Photo © James Adams.
Down the hatch! After the toucanet flies off it will cough up the seed. With luck a new nutmeg will sprout. Photo © James Adams.

 

Seedling of a wild nutmeg, "planted" by a toucan or other seed dispersal in the rainforest.  Photo © David L. Anderson.
Seedling of a wild nutmeg, “planted” by a toucan or other seed disperser in the rainforest. Photo © David L. Anderson.

In a natural arms race, nutmeg trees produce the thin fruity arils on their seeds to attract seed dispersers like toucans. Growing all the fruit costs energy, hence the arils are so thin that they are barely there. But it’s enough to lure in toucans by the droves. A nutmeg can’t grow its own garden of seedlings, raising them to nice young trees and planting them in the sweet spots in the forest where one day they’ll become living towers in their own right. Instead, they rig the game and get birds to help. A little bit of fruit for you, dear toucan, and seed dispersal for me, thank you very much.

 

In a future blog I’ll discuss how seed dispersal affects the biology of the entire forest community of trees, mammals, and birds.

Share