The Seedier Side of Science
By Jacalyn Willis, Ph.D.
February 18, 2004
As far back as I can remember I have been fascinated by seeds. Perhaps always. I remember when I first discovered that a little brown, dry, hard capsule could spring to life and become a succulent green plant that might bear flowers and fruit. The notion was intoxicating to me. That I could possess the power to unleash this series of events by adding appropriate watering, soil, and light was what helped keep me a happy kid living in a small apartment in a big, barren city on a street with no trees. In late winter I dreamed of the seeds I would buy from the Brooklyn Botanic Gardens at 2 cents per packet, and the small jungle that I would grow on the window ledge.
Now I study what happens when the seeds of rainforest trees are eaten by various "seed-predators." The seeds of some trees are not much larger than those seeds I planted in cut-up milk cartons when I was a kid. But the seeds that interest me, that are of special interest to the mammal seed-predators (such as squirrels), are quite large and heavy. Think of beans about 5 to 8 centimeters (2 to 3 inches) long, or tiny coconuts the size of a ping-pong ball. The bean-like seeds are from trees (Dipteryx or almendro) that are often 50 meters (150 feet) tall. The coconut-like seeds grow up to be palm trees (Astrocaryum or black palm) about 7 to 20 meters (20-60 feet) tall.
These seeds are from fruits that have a thin, sweet pulp that covers the seed and is very tasty to many species of birds, mammals, and insects. The species that eat these fruits come in two kinds. There are those that eat only the pulp and do not have the kind of teeth necessary to eat the nutty seed within. These are mammals such as spider monkeys, howler monkeys, capuchin monkeys, tayras (large weasels), coatimundis (raccoon relatives), and opossums. There are also fruit flies, big morpho butterflies, and beetles that like the pulp. You can imagine what a party takes place when a big tree is covered with loads of ripe, fragrant fruits.
Birds such as toucans and guans eat the pulp, often swallowing the smallest of these seeds, and passing them out in their droppings. Since they can fly some distance between swallowing, digesting, and pooping, they may drop a seed very far from the "mother tree." In this case the animal has acted to help distribute the seeds through the forest, and we call them "dispersal agents." "Dispersal" means "spreading around." Dispersal is very helpful to the mother tree, because baby trees survive better when they are not near the mother. Almost no baby trees can survive the trampling of mammals feeding under a tree covered in yummy fruits. Baby trees also die from diseases that they catch from adult trees when the babies sprout next to Mama Tree.
When trees of the same kind are close to each other, it is easy for diseases to spread and for leaf-eating (herbivorous) insects to get from one tree to another. The only defenses that trees have are to spread their seeds far away, and to produce chemicals to kill the insects or disease organisms. The production of seeds requires energy in the form of sunlight, which is "packed into" the nutritious pulp and seeds. Any chemical production process, such as making toxins to poison insect pests, requires energy. Both seed and toxin production require lots of energy that a tree might not be able to spare. So it helps a tree to have animal dispersal agents that carry the seeds to safer places.
The other sort of animals that use fruits are those that eat the seed itself. They have teeth that make it possible to break through the tough seed coats. Squirrels and agoutis are rodents, and have big incisor teeth that they use to gnaw (chisel out) the seed coat. Peccaries are wild pigs with big jaws and heavy teeth.
They can hold the seed in their jaws and press hard enough to burst the seed coat. I tried doing that in a mechanical vise, and it took a lot of pressure to make it happen. I don't ever want to be bitten by a peccary! We find lots of freshly split seed coats under the fruiting trees after a night of a peccary "party" under the tree. Take a look at the Dipteryx seeds in the photo. You can see a fruit unharmed on the left, then one that has the pulp eaten off by a monkey, one that has been split by a peccary, the next with a hole gnawed in the middle by an agouti, and another with a hole in the end from a squirrel gnawing.
In these ways the lives of trees and their leaves, seeds, and fruits are interrelated with the lives of many species of animals that feed on them. The seeds are especially important because they contain rich packages of energy that can be used by the new seedling or by an animal predator. The more we understand the ways in which these seeds are used, the more we will understand about how a forest ecosystem works.