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.

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