The Great Stinking Clue

In search of a fruit 
called durian

By David Quammen

Outside magazine, February 1993

Fruit is the means that trees have invented for traveling from one place to another. But not every fruit travels as well, or as far, as others. Some kinds are adventurous. Some are more laggard. Some hit the ground unswallowed and don't even roll. So to get to the core of the matter, you'll need to do a little traveling yourself. My advice is: Start with a flight to Bali. Then follow your nose upwind toward a species of tree called Durio zibethinus.

The fruit of that tree is a yellow-green ovoid, big as a rugby ball, heavy as fate, upholstered all over with thorns. It goes by the name durian, from the Malaysian word duri, for "thorn." It's a hard capsule that hangs from a stout stem and God help you if you're beneath when it falls. It looks about as succulent as a stuffed porcupine but splits open along suture lines to reveal its amazing innards. Each inner chamber contains several large gobbets of ivory-white pulp. That's the edible stuff. Inside each gobbet, a seed. The seed itself is as big as a chestnut. Durian is renowned throughout Asia for its luxuriant flavor. its peculiar anatomy, and its indecent stench.

Besides being a delicious fruit, and one of the worlds largest. scientifically it’s among the most interesting. You could think of it as one botanist has done—as the great stinking clue to the evolution of tropical forests.

But by now, let’s say, you’ve arrived at the Bali airport. Bali, your gateway to Indonesia, is an anciently civilized island just south of the vast equatorial forests of Borneo. Some scientists believe that those forests, or others nearby, are where the phenomenon of fruition first arose.

Botanists use the word fruit in two senses. In the more technical sense, it applies to any seed-bearing structure formed from an ovary of a flowering plant and shaped so as to facilitate the seed's dispersal. Whether the seed-bearing structure travels by wind power, or by floating on water, or by catching a ride aboard an animal, is immaterial. According to this strict definition, the downy white parachute of a dandelion seed constitutes fruit. The whirlybird wing that carries a maple seed is a fruit. A leathery acacia pod is a fruit, and so is an acorn, and so is a burr. Scientific professionals, with their uncommon love for jargon, have invented abstruse bits of terminology to designate the various modes of travel, and one of those terms is endozoorhory, meaning dispersal by way of the interior of an animal. That’s the method practiced by what you and I and a plainspoken botanist would commonly call fruit.

This is the word’s more familiar sense: a flavorful and nutritious packet of vegetable pulp that seems made to be eaten. In fact, it is made to be eaten. It’s a bribe of concentrated carbohydrates and water and in some cases fats, offered to animals in exchange for seed transport. The seeds are hidden within the pulp and protected one way or another (either inside a hard sheath or a stony pit, or by their small size and numerousness) from being destroyed in the course of the animal’s chewing and digestion. The indigestible seeds are off-loaded from one end or the other—by regurgitation or by defecation, the latter of which enhances a seed’s growth prospects with a godparent’s gift of manure. Evolution has taught flowering plants that this is an effective way to launch and position their offspring. True, the production of fruit entails some high metabolic costs, but the costs are justified by the rewards. Seed dispersal is that crucial. A plant species doesn’t flourish if it doesn't travel.

So when a thirsty South African aardvark eats a water-rich melon and then buries its own dung in a hole, the benefits to plant and animal have been mutual. When a Galapagos tortoise eats a wild Galapagos tomato and then craps out the seeds a week later on a piece of terrain hitherto uncolonized by tomatoes, it’s a classic instance of endosaurochory (dispersal via the gut of a reptile) and everyone is happy. Not least happy are the botanical jargoneers who make up this sort of terminology, I suspect, while they’re out on a field site waiting for plants to grow.

Why is dispersal so crucial? Because it allows a plant to enlarge the number and range of its own offspring. Why is that crucial? Because it’s the standard on which evolution mediates survival, extinction, and change. A tree that has no means of dispersal, dropping all of its seeds in its own shade, will be especially vulnerable to seed predators adapted for taking advantage of its helplessness. A plague of seed predators, descending on such an improvident tree, can destroy every single seed. And if the predators carelessly let a few seedlings spring up, those young plants will tend to be stunted by competition with the overshadowing parent. The seedlings will also compete with one another, so most of the seed crop will be wasted. And if the local climate goes bad, turning permanently intolerable, the tree and its few clustered seedlings will die without leaving offspring. Dispersal of seeds to distant new patches of habitat obviates all of those problems.

It’s an ingenious biological strategy, this matter of travel by fruit. But how did it originate? To that difficult question, a British botanist named E. J. H. Corner offered what he called the durian theory of plant evolution.

In 1949, Corner published a journal paper titled "The Durian Theory or the Origin of the Modern Tree." He refined it in later publications, including a bold essay that appeared in 1954, "The Evolution of Tropical Forest." His notion was that endozoochory (again, the enticement of animals to transport seeds in their bellies) arose before all other methods of dispersal, and that primitive ancestors of D. zibethinus were the earliest practitioners of that strategy. His supporting argument involved many bits of botanical evidence, most crucial of which was the fact that durian belongs to an unusual category of fruit known as arillate.

Arillate fruits are rare. In the tropics, arillate-fruiting trees account for about one species in a hundred. Most fleshy fruits are considered drupes (such as a peach, with its single hard pit) or pomes (such as an apple, with its accessory flesh surrounding a seedy core) or berries (which category includes also grapes and tomatoes). In an arillate fruit, the aril itself is a fleshy outgrowth of the seed's own integument, which wraps down around the seed like an octopus hugging a basketball. In many cases, the aril is both highly nutritious and brightly colored. Red is common. When such an arillate fruit falls to the ground and splits open, the wink of the aril beckons animals to a rewarding meal. Nutmeg is an arillate fruit on exactly this pattern: the aril is a little scarlet octopus wrapped down around a dull brown seed. The durian, according to E.J.H. Corner, is also an arillate fruit. That creamy white pulp is nothing other than aril gone spectacularly amok. In lieu of a flashy color, it advertises itself with a garish smell.

Corner loved durian trees with a passion that strengthened (and may also have mis-led) his botanical judgments. The durian, in his view, provided an important clue to the evolution of tropical plants. Furthermore, one of the lessons of what he wryly called durianology was that plants of the temperate zones, too, should be understood in the light of this tropical model. Evolutionary botany in the tropics has remained especially obscure because the primeval species have left almost no fossil record, but in D. zibethinus and its closest relatives Corner found what he took to be living fossils. The durian illuminates tree evolution. he believed, in somewhat the same way that the coelacanth Illuminates the evolution of land-walking vertebrates.

At the heart of Corner's argument is that delectably gooey durian aril. Because the arillate condition is so rare, yet so broadly scattered among many different tree families, and so similar anatomically wherever it occurs, it is most likely a vestige of the primitive form shared by all early fruits, rather than a recently evolved derivation. The bright red coloration seems to be primitive too. Although the pulp of D. zibethinus is yellow-white, Corner noted, at least one other species of Durio produces a red fruit with red pulp. Based on these facts and others. he wrote: "The first, and overwhelming, postulate of the durian theory, therefore, is that a red durian fruit exemplifies the primitive fruit of flowering plants." He envisioned iguanodon and other herbivorous dinosaurs gobbling those red globs of pulp and performing the service of dispersal—which is good news for any of us who have been waiting for a chance to use endosaurochory in a casual sentence.

Big seeds dispersed by big animals gave the proto-durians an evolutionary advantage that revolutionized plant ecology, according to Corner. A big seed provides the young seedling with a nutritional head start, and therefore a competitive edge, as it begins life as a pioneer in some new patch of habitat. But big seeds embedded in generous bribes of aril require big fruit capsules to contain them, and big fruit capsules impose their own structural requirements on the trees in which they hang. Thus the second postulate of Corner's theory: Although the earliest flowering plants were probably shaped like a tree fern—with a thick columnar trunk that rose to a single point, not much firm wood, and virtually no limb structure—the necessity of supporting a sizable crop of heavy fruit capsules drove plant evolution toward the development of stronger trunks and stout, woody branches. Trees became bigger and sturdier. They spread their limbs. The tropical canopy blossomed. Insects and birds and arboreal reptiles and mammals evolved to inhabit it, feeding on nectar and leaves and more-advanced sorts of fruits and one another.

This evolutionary upheaval began, if we can trust Corner's guess, in the vicinity of what now is Malaysia. From there. the new plant and animal forms and relationships spread to other regions, including Borneo, southwestern Asia, and tropical Africa, During subsequent episodes of climate change, then, the forests in some of those regions receded for lack or moisture. Zones of savanna opened. and big herds of grazing mammals began to appear. Recapitulating that much, Corner added: "The ‘durian theory’ can contemplate the hair-less arboreal mammal that lurched from the forest in its increasing austerity, to slaughter the wild herds, to harvest the wild grain, and to hack down the testimony of his origin." He meant us.

So there’s not much that his durian theory can’t contemplate. The evidence may be tenuous, the logic may be disputed by some other botanists, but no one can complain that E. J. H. Corner’s thinking lacked scope.

IF you’re a lucky traveler, as I was, you’ll arrive at the height of what Corner called durian-time. "Usually the fruits detach when ripe and crash to the ground, where the pulp turns rancid in a day or two. In Malaya the smell of fruiting trees in the forest attracts elephants, which congregate for first choice; then come tiger, pigs, deer, tapir, rhinoceros, and jungle men. Gibbons, monkeys, bears, and squirrels may eat the fruit in the trees; the orangutan may dominate the repast in Sumatra and Borneo; ants and beetles scour the remains on the ground."—E. J. H. Corner. (...) You check into a little hotel near the town market and, dropping your bags, notice a sign on the inside of your door: it is forbidden to eat durian in the room. But one stroll down the corridor has told your olfactory radar that, this time of year, everyone’s doing it. (...) You accept a handful of pulp. It’s creamy and slightly fibrous, like a raw oyster that’s been force-fed vanilla ice cream. There’s also a hint of almond. It tastes strange, rich, wonderful. It smells like a jockstrap. It doesn’t remotely resemble any substance that you've ever touched, let alone eaten. Inside your gobbet is the chestnut-like seed. Sucking that clean, you toss it away through the forest. So the D. zibethinus tree, with your help, has sent its genes on a journey. The fruit has fulfilled its destiny.