When Portuguese and early Dutch colonists began to inhabit the small tropical island
of Mauritius in the Indian Ocean, from the early 1500s onwards, they found the island
to be the home of a very unusual bird. This bird was as large as a turkey, of ungainly
build, with short curly tail feathers and tiny wings. It had a strong black beak
with a horny hook at the end. It was so unafraid of man that the Portuguese named
it doudo, meaning ‘simpleton’, because it was so trusting they
were easily able to hit it on the head and kill it for food. Hence the English name
Dodos were slaughtered in large numbers by sailors and settlers, and pigs which
were introduced to the island voraciously ate the dodo eggs. The last dodo was killed
in 1681—less than 180 years after it was first described.
Also on Mauritius at the time was a tree known as Calvaria major. At that
time it was quite common on the island, but by the 1970s only 13 of these Calvaria
trees were left. All 13 were more than 300 years old, and though they produced healthy
looking seeds each year, none ever managed to germinate. The trees had puzzled botanists
for centuries, for their numbers had strangely begun to decline and no new trees
were taking root. Calvaria major seemed destined to go the way of the dodo,
but the tree was dying out for no perceptible reason.
While studying the ecology of Mauritius in the mid-1970s, American ecologist Stanley
Temple came up with an ingenious connection between the decline of the Calvaria
major trees and the disappearance of the dodo. After considering many factors,
Temple concluded that the tree’s large fruit had in times past been eaten
by the dodo. The tree’s seeds are encased in a thick-walled protective coat,
but the dodo’s stone-filled gizzard was able to exert a powerful crushing
pressure on them. The bird’s gizzard (a second stomach for grinding food)
would pound away at the seed’s coat, weakening it and cracking it a little,
but not enough to damage the seed inside. When eventually deposited by the dodo,
the seed was able to germinate.
Without the grinding of the dodo’s gizzard to weaken the thick protective
wall, the seed was trapped inside its hard case. When the dodo became extinct just
over 300 years ago, Calvaria major’s seeds had no way of germinating.
So no new trees grew.
The dodo’s relationship with the Calvaria major is just one fascinating
example of the wonderful balance between animals and plants in God’s creation.
Today the Calvaria major seeds are encouraged to germinate by being fed
to turkeys or by turning them in a gemstone polisher. This, it is hoped, may help
preserve the rare trees for future generations to enjoy.
Post publication note by Martin Tampier (24 August 2011)
More recent findings have cast doubt on the claim that the dodo was necessary for
the Calvaria major trees (now renamed to Sideroxylon grandiflorum)
to germinate. The decline of the trees, while happening after the disappearance
of the dodo, can be better explained through environmental factors, such as large-scale
deforestation and introduced plants and animals (Hershey 2000, Horn 1978, Owadally
1979). The seeds will germinate by themselves, without any abrasion of the protective
coat (endocarp). During germination, the hard endocarp simply splits along a distinct
fracture line as in walnut, peach, and cherry. In addition, other animals, such
as tortoises or parrots, also eat the seeds and would be more likely candidates
than the dodo to spread them.
On the other hand, seeds of many tropical fleshy-fruited plants seem to benefit
or require removal of fleshy layers by animals for germination (Witmer and Cheke
1991), but they never depend on just one single animal species. In that sense, a
similar relationship does indeed exist between some plant and animal groups, testifying
of a complicated interrelationship that does not suit the evolutionary model well.
In the case of the calvaria tree, the relationship with the dodo and other animals
can better be described as a design feature called symbiosis: animals benefit from eating the seed and the plant benefits because
its seeds are then dispersed by the animals. In addition, although only parts of
the seeds actually leave the digestive tract intact, in existing similar cases these
were found to germinate better than the ones that have not been eaten. Early germination
may assist the seed in escaping predation or fungal attack, or allow it to take
advantage of unusual conditions, such as the early arrival of rain. As such, seeds
eaten by animals can diversify the germination strategies available to the plant
More detail on this issue can be found in
David R. Hershey’s article.
Hershey, D.R. (2000). The truth behind some great plant stories, American Biology
Horn, B.K.P. (1978). Dodo apocrypha, Science News113:19.
Moore, Peter D. (2001). Ecology: The guts of seed dispersal, Nature
Owadally, A.W. (1979). The dodo and the tambalacoque tree, Science
Witmer, M.C. and Cheke, A.S. (1991). The dodo and the tambalacoque tree: An obligate
mutualism reconsidered, Oikos61:133–137.
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