In this science article we look at the evolution, pollination, growth and seed dispersal of the Coco de Mer (Lodoicea maldivica) palm tree, also known as the ‘double coconut’.
This is one of those few examples of how seed dispersal occurs by gravity alone.
In the 18th century (and in many preceding centuries) people used to find enormous Coco de Mer seeds (or ‘nuts’) washed up on the beaches….
… of the Maldive islands in the Indian Ocean.
All the washed up nuts immediately became the property of the sultan, who either sold them for a high price or gave them away as royal gifts.
Some ended up as beautifully carved storage containers…
….while others became perfume bottles. This perfume bottle is made of one half (or ‘lobe’) of a Coco de Mer nut.
No one knew where the Coco de Mer nuts came from. Local legend would have you believe that the nuts grew inside fruits which hung off trees growing on the bottom of the sea. When the fruits ripened they fell off the trees and rose to the surface.
Fishmen sometimes believed that they saw Coco de Mer fruits growing on trees on the sea bed. By the time they dived into the sea to investigate further the trees had mysteriously disappeared!
The mystery was finally solved in 1768 when French explorer Marion du Fresne discovered Coco de Mer nuts growing on dry land.
He found them growing inside fruits….
…of a previously unknown species of palm tree. This new species of palm (named the ‘Coco de Mer’) grew in isolated valleys on Praslin Island in the Seychelles. The trees towered above all other vegetation.
Botanists soon realized that all the giant nuts found washed up on beaches in the Maldives had actually grown on the Seychelles islands some 2130 kms (1324 miles) away!
The Coco de Mer nuts had reached the Maldives after drifting eastwards in the prevailing ocean currents.
Despite the large number of nuts that were often washed up on the beaches of the Maldives, not a single one had ever germinated; 1,200 islands and atolls in the Maldives Archipelago and not a single Coco de Mer palm tree to be seen anywhere!
We now know that the Coco de Mer nuts that floated on ocean currents from the Seychelles to the Maldives were the rotten and infertile ones that could never germinate.
Healthy nuts became rotten and infertile after their fruits fell off the trees and landed in the sea.
Healthy fruits that landed in the sea were very dense and heavy; as a result they were incapable of floating on water and sunk to the bottom.
Gases now formed inside the nut making it much lighter. The nut then rose to the surface; it could now float but was no longer fertile.
So when the ocean currents washed the nut onto the distant beaches of the Maldives several months later the nut could never germinate.
By way of contrast the Coco de Mer’s distant relative, the coconut, (Cocos nucifera) could germinate after spending months in sea water.
The Coco de Mer is unique among palm tree species in that there are distinct male and female palms.
The female palms grow the fruits. The male palms are taller, more slender and have catkins growing on them.
The small flowers on the catkins of the male palm have a strong scent, produce large amounts of nectar……
….and are very small.
The flowers on the female tree produce a similar scent to the male flowers, but the scent is less strong. Only one flower is active on a female tree at any one time and then for only for a few hours each day.
There has been some debate about how Coco de Mer palms are pollinated.
It is now believed that their primary pollinator is a long legged fly, the Ethiosciapus bilobatus.
J. Gerlach, palms.org/palmsjournal/2003/
Coco de Mer palms grow the largest fruits and the heaviest nuts of any tree in the world.
To answer this question we need to go back in geological time some 184 million years. It was 184 million years ago, during the Jurassic period, that the supercontinent of Gondwana started breaking up. At that time the Seychelles Islands formed part of what we now call the Indian sub continent.
75 million years ago, as the Indian subcontinent was drifting northwards, the 45 rocky Seychelles islands broke away and inched slowly towards their present location in the Indian Ocean.
As the 45 rocky islands moved northwards their climate changed from dry and arid to wet and tropical.
The islands remained geographically isolated; all the plants and trees that continued to colonise the islands were descended from plants and trees that had evolved on Gondwana.
As a result of this geographical isolation, species of plants and trees living in distant lands that were well adapted to hotter, more moist conditions were unable to migrate to the Seychelles. This continuing geographical isolation gave species of palm tree already resident in the Seychelles plenty of time and space to adapt and evolve to meet the new challenges they faced.
It is now believed that the Coco de Mer palm evolved from a Borassus-like palm tree. Today palm trees of the Borassus genus are native to drier regions of Africa and the Indian sub continent.
These Borassus flabellifer palms are from Mumbai, India.
As the climate of the Seychelles became warmer and wetter the vegetation on the islands became taller and more dense. These conditions encouraged the growth of tropical rainforests.
In the newly formed tropical rainforests competition to access sunlight above the forest canopy grew more intense. Proto Coco de Mer seedlings had to adapt or die.
Evolution by natural selection favored Coco de Mer palm trees with larger nuts. It was the larger nuts that grew into taller juvenile palms. The taller juvenile palms had a better chance of competing for light and surviving.
After germination a Coco de Mer nut provides enough nutrients to sustain a juvenile palm for up to four years!
Juvenile palms grow incredibly fast; they can grow leaf stems to a height of ten meters in no time at all. The leaves of young palms can reach a length of 14 meters or more. Trunks do not appear until the palm is 15 years old.
Palms reach maturity when aged between 20 and 40. They can live anything between 200 and 400 years!
A Coco de Mer fruit contains one nut; after reaching maturity the fruit it falls to the ground near the base of the trunk beneath the parent plant.
With most tree species the worst place to grow is underneath the parent tree where juveniles have to compete with their parents for light, food and space to grow.
The evolutionary solution to fixing this problem is ingenious. If the nut is too big to move, then why not move the seedling!?
After falling to the forest floor beneath the parent plant, the hard fruit wall (‘husk’) takes 6 months to disintegrate. The nut then sends out a stem that ‘digs’ a tunnel beneath the surface of the forest floor.
This stem (or ‘rope’) can sometime extend for a distance of 10 meters before a seedling starts to grow. The ‘rope’ is the seedling’s pipeline to the foodstore in the nut.
A larger nut can grow a longer ‘rope’; a longer ‘rope’ means more effective nut dispersal away from the parent palm.
It is worth noting that the ‘crown’ (all the branches and leaves) of a Coco de Mer palm is very narrow compared to the crowns of many other species of tree. Narrow ‘crowns’ block out less sunlight than broad ‘crowns’.
With the benefits of larger nuts comes significant evolutionary costs. It can take a decade for a nut to reach maturity growing inside a fruit. The number of nuts any one female palm can grow is also limited- most female palms produce under one hundred in their lifetime.
Contrast this with a coconut palm which often produces more than a hundred nuts in a single year.
A female palm may carry half a tonne of developing fruit on branches in her crown. In high winds branches holding the heavy loads of fruit sometimes break off, resulting in the loss of the immature fruit and often the loss of the female palm.
As a consequence of the loss of so many female palms, the ratio of female to male trees in many areas is 1 to 2; one female palm to every two male palms.
Today the Coco de Mer only grows naturally in the wild in the Seychelles in national parks on the islands of Praslin and Curieuse. The species is critically endangered as a result of habitat loss.
The harvesting of nuts is strictly controlled and firebreaks have been introduced to prevent fires sweeping through the forests and destroying the precious palms.
There are now very few, if any, nuts falling into the sea to be swept along on ocean currents from the Seychelles onto the beaches of the Maldives.
Fishermen no longer see Coco de Mer trees growing at the bottom of the sea….or do they!?