Why the coco-de-mer evolved such giant seeds

The larger the seed, the greater the nutrient supply available to a seedling for its growth. Fellow members of the species that possess lower reserves cannot prevail and thus eventually die off. "The competition for the transfer of their own genes within the species fueled the race for even bigger seeds," says Peter Edwards. (Credit: "coco-de-mer" via Shutterstock)

The fruit of the double coconut palm or coco-de-mer, which houses the tree’s enormous seeds, are the largest among plants. The fruit can weigh up to roughly 39 pounds and can reach a diameter of over one-and-a-half feet.

The heavy seed hampers the plant’s dissemination significantly, as the ripe coconut falls to the ground and remains there. The double coconut palm also grows in soil that is extremely low in nutrients.

The gigantic fruit would appear to be wasteful, an evolutionary contradiction, but researchers working with Peter Edwards, emeritus professor of plant ecology at ETH Zurich, present new findings that suggest otherwise.

The palm has developed a clever mechanism to optimize the provision of nutrients for itself and its offspring—and as a result produces large seeds. It is the only plant species that takes care of its offspring in order to increase their chances of survival.

Funnels and gutters

In order to care for its seedlings, the palm has evolved a system of funnels and gutters that redirects nutrients and water to the benefit of its offspring, as well as the tree itself.

The leaves function like a funnel, efficiently capturing rainwater, which the stalks then divert to the base of the palm tree.

Edwards and colleagues have demonstrated how the nutrient-rich residue of animal feces, pollen, and dead plant material that collects on the palm leaves is directed with the water to the base of the tree, thus fertilizing the soil with phosphates and nitrates.

A soil analysis conducted by the researchers confirmed that the concentration of phosphorous and nitrogen 7-8 inches away from the tree is 50 percent higher than at a distance of 6-7 feet away.

Coco-de-mer also optimizes the use of nutrients. The researchers discovered that the leaves and trunk contain very little nitrogen and phosphorous.

“Both nutrients are low in these plant parts because the palm tree has very few nutrients at its disposal from the surrounding environment, and it has also developed in such a way that all available resources are invested in its flowers, inflorescence and fruit,” says Kaiser-Bunbury.

Growing up in shadow

The double coconut palm’s efficient nutrient use and ingenious enrichment system explain how it can grow oversized seeds, but not the evolutionary meaning behind it. “We must therefore look further back in evolutionary history,” says Edwards, the lead author of a study in New Phytologist.

Early forms of the double coconut palm, which can be traced back to African ancestors, probably depended on animals to disperse their seeds. When the Seychelles split away from India some 65 million years ago, the palm was isolated from the mainland and therefore from the animals that scattered its seed.

From that point on, the seeds fell to the ground near the parent tree and stayed there. As a consequence, the seedling had to adjust to growing in the shadow of its parent’s canopy.

This adaptation and the nutrient supply, which grew over the course of evolution, gave the palm tree an advantage over other plants and it eventually displaced them. The double coconut palm thus became the dominant species, shaping the forest.

Competing ‘cousins’

The researchers interpret the size of the seed as a response by the palm tree to competition between seedlings of the same species. As a rule, most plant species form mobile seeds that can fly, swim, or be transported by animals, so the seedlings do not compete with their parents for light, nutrients, and water.

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The fruit of the coco-de-mer, however, is too big and heavy to be mobile. The young palm tree, therefore, is not only fighting with its parent for the scare resources but also with its “cousins.”

The larger the seed, the greater the nutrient supply available to a seedling for its growth. Fellow members of the species that possess lower reserves cannot prevail and thus eventually die off. “The competition for the transfer of their own genes within the species fueled the race for even bigger seeds,” concludes Edwards.

This suggests that double coconut palm follows an evolutionary pattern found in other species that flourish on isolated islands: colonizing plants on isolated islands develop large seeds over time. This is why island species have larger seeds than their mainland relatives.

Coauthors are Christopher Kaiser-Bunbury from TU Darmstadt and Frauke Fleischer-Dogley from the Seychelles Islands Foundation.

Source: ETH Zurich