The Ghosts of Evolution

Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms
By Connie Barlow

Basic Books

Copyright © 2002 Connie Barlow
All right reserved.

ISBN: 9780465005529

Chapter One

Ghost Stories

* * *

We live on a continent of ghosts, their prehistoric presence hinted at by sweet-tasting pods of mesquite, honey locust, and monkey ear.

—Paul Martin

"I've got a screwy idea," wrote Dan Janzen in an October 1977 letter to a colleague. "Let's write a paper together. I sit in my deciduous forest in the Pacific lowlands of Costa Rica looking at a large number of fairly large fleshy fruits with hard seeds.... An enormous number of these fruits end up simply rotting on the forest floor. When cattle are introduced, they busily go around and gobble up quantities of these fruits, but even then they sometimes don't get all of them. I think you can see where I'm going."

    Where Janzen was going was toward a profound shift in ecological thinking.

    Dan Janzen is a tropical ecologist based at the University of Pennsylvania. Costa Rica has been his site for fieldwork since the mid1960s. He is the sort of guy who attracts the Crawfoord Prize (biology's highest honor) while still in his forties. It took me most of a morning to read his publications list. At the time Janzen was seeking a collaborator for his "screwy idea," he was thirty-eight years old and solidly established in his field. Plant-animal interactions in the tropics were the focus of his work. In 1977, however, he was beginning to suspect that he—along with every other ecologist working with large tropical fruits of the New World—had been wrong in one very big way. They all had failed to see that some fruits are adapted primarily for animals that have been extinct for thirteen thousand years.

    Janzen was beginning to suspect that forests and savannas where livestock grazed were in some ways closer approximations to landscapes of the prehuman past than were nature preserves where cattle and horses were banned. The clue was rotting fruit.

    What are fruits for, after all? Fruits—big or small, and produced by' all flowering plants—contain the seeds of the next generation. Every seed is made of an embryo provisioned with a sack lunch. The sprouting plant will draw upon the stored energy and nutrients in the seed until it can capture sunlight in its own leaves and acquire minerals through its roots. Yet there is more to a fruit than a seed. Almost all fruits are equipped as well with a way for the kids to leave home. A maple seed is given wings, a dandelion tufts. A fleshy fruit like an apple, orange, grape, or tomato is given a lure. The lure is the pulp, and its mission is to induce a mobile animal of the correct specifications to serve as the vehicle for the seed or seeds within.

    Janzen's letter to his hoped-for collaborator continues, "How hard would it be to make a list of the big mammals that would have eaten the same size and kind of fruits that are eaten by peccaries, tapirs, deer, and cows, and that would have occurred in a Pacific coastal lowland deciduous forest within the last 100,000 years?"

    Peccaries, tapirs, and deer are the largest herbivores native to Costa Rica today. These are the animals that can eat the biggest and the most fruits, but they do not serve as vehicles for many of the seeds hidden within the lures. Some seeds are too big: a tapir or deer will eat around the seed or spit it out before swallowing. Other seeds may be crushed in the molar mill. The piglike peccaries are especially formidable seed predators. As well, too many fruits may fall from a tree all at once. Even though the coterie of local dispersal agents might be inclined to consume the fallen fruits properly, molds and microbes make a meal of them first. Still other fruits fail to find a proper vehicle because they remain out of reach when ripe.

    Janzen's mention of cows may seem odd, because cattle are not native to the Western Hemisphere. They were introduced from Europe. Nevertheless, they now serve the reproductive interests of some tropical plants better than the natives can. Janzen's view of cattle had shifted away from the standard ecological portrayal of livestock as alien invaders. Rather, he had come to think of cattle as proxies for the great beasts that had inhabited not just Costa Rica but the entire Western Hemisphere for all but the final thirteen thousand of the last fifty million years.

    This vast chunk of evolutionary time is the Cenozoic era, commonly known as the Age of Mammals. Many tropical fruits of the Americas evolved their size, color, odor, seed casing, and other adaptive features during millions of years of association with big mammals. We all know that the Age of Dinosaurs (correction: the Age of Nonavian Dinosaurs) ended sixty-five million years ago, when one or more big meteors struck the planet. It is a matter of taste whether one wishes to regard the Age of Mammals as over now too. There should be no disagreement, however, that the Age of Great Mammals has indeed ended everywhere except southern Africa and patches of tropical Asia.

    The Age of Great Mammals ended long before chainsaws and internal combustion engines evolved. In Europe and nontropical regions of Asia, it petered out in steps between fifty and fifteen thousand years ago, when the straight-tusked elephants, woolly mammoths, rhinos, and other great beasts of the Pleistocene epoch vanished. Throughout that vast continental mass, a quarter of all genera of animals regarded as megafauna—those weighing more than a hundred pounds, or forty-five kilograms—were lost to extinction. Europe lost all six species of herbivores weighing more than a thousand kilograms. In Australia the Age of Great Mammals ended sometime between forty and thirty thousand years ago, when giant kangaroos, enormous wombats, and rhinolike marsupials (as well as the most formidable crocodiles, lizards, and snakes) were purged from the landscape. This extinction catastrophe stripped Australia of all but one of its sixteen genera of megafauna. In the Western Hemisphere, the Age of Great Mammals came to an abrupt end thirteen thousand years ago, when the mastodons and mammoths, the ground sloths and glyptodonts, the native horses and large camels, and a beaver and an armadillo both as big as a bear all disappeared forever. North America lost 68 percent of its generic richness of Pleistocene megafauna (32 of 47 genera), and South America lost 80 percent (47 of 59 genera).

    Outlying islands were hit even harder, though several thousand years later, following advances in sailing technologies. Not until seven thousand years ago, for example, did Cuba lose its half dozen species of sloth, including a ground dweller as big as a black bear. Just four thousand years ago, while the Egyptians were building pyramids, the last mammoths on Earth expired on an island off Siberia. Madagascar lost all of its biggest lemurs within the past two thousand years, along with both native species of hippopotamus, a strange carnivore, and the giant elephant birds unique to that island. Throughout the Cenozoic, New Zealand had no mammals other than bats, but less than a millennium ago colonizing peoples handily exterminated all dozen species of flightless moa.

    Most of us live in impoverished places. Each year throngs of visitors ogle the wildlife in America's national parks, unaware that for tens of millions of years the land has never been so bereft of big creatures The bison and elk and moose of Yellowstone are a remnant of what came before. We can still experience the great beasts that were lost, however, if we learn to read the plants.

    The Age of Great Mammals may be over, but the plants have not yet caught on. Those that depended upon mammals to swallow big fruits, as well as those that deployed armaments to deter soft snouts from stripping foliage, are still doing what they have always done. Fruit rotting on the ground is the most obvious sign. Later, Janzen would notice other examples of adaptations mismatched in time. But in his initial letter, tropical fruits commanded his attention:

    "The point is that if we were to take an East African coastal forest community and dump into it the fruits that fall into a Costa Rican deciduous forest community, it is my certain prediction that all of these fruits would be immediately eaten.... Now if we were to remove all the animals from an East African deciduous forest and leave only two species of pigs, one deer, and one tapir, how long would it take before the trees lost their mammal- dispersed fruits or otherwise adjusted themselves to the shortage of seed dispersal agents?"

    Janzen continued, "I don't even know where to begin to try to work out a list of the sorts of animals that would have been available in these forests up until, say, ten or a hundred thousand years ago. That's what I am hoping you could do, perhaps right out of your head."

    The you was Paul Martin. Martin is a Pleistocene ecologist, now emeritus at the University of Arizona in Tucson, and one of the leading experts on the Ice Age ecology of North America. Other scientists may know more about mammoths or ground sloths or glacial climates or vegetational shifts. Martin's specialty is Pleistocene generalist. His work on the ecological interactions between the megafauna of the Western Hemisphere and the first human colonists brought him notoriety a decade before Dan Janzen sought his assistance. Janzen wanted to draw upon Martin's knowledge of the Pleistocene bestiary and to solicit his opinion about likely megafaunal interactions—not with humans but with fruits.

    Janzen's letter continued: "Obviously I haven't thought the thing all the way through, but I'm curious as to whether you would be interested in trying to put something together with me? For my part, I'll be happy to put together a list of the plants and what their fruits are like and documentation on their acceptability and edibility by deer, peccaries, and a tapir (I have a corralled tapir whom I have been feeding things to). I'll be happy to write a first draft of the manuscript as well. What I'd hope for from you would be some kind of a list of the animals that could have been involved in the evolution of these fruits, their supposed dietary habits, perhaps a statement of what extant animals they resemble most closely, and some high quality skepticism about the harebrained ideas I am sure will come to light as this thing actually gets organized on paper."

    The letter concludes, "I think the whole thing that started me thinking about this was frustration over trying to figure out who the dispersal agents are for Cassia grandis seeds. The fruit is about a half meter long, hard as a rock, cylindrical, and an inch and a half in diameter, and contains large seeds (about 2 cm long by 1.5 cm wide by .5 cm thick). These seeds are embedded in a sweet molasses-like pulp, which is probably highly desirable to any animal that can break the fruit. The fruits hang on the tree long enough so that bruchid beetles and moths kill all the seeds. The inference is that under the circumstances of the evolution of this plant, some big mammal came along and picked the fruits out of the tree before the insects got all the seeds. A giant ground sloth comes immediately to mind as a possibility. You can see why I thought of you. Onward into battle. Sincerely yours. Dan."

    Paul Martin responded promptly: "What a fun letter and idea. I'll invoke the ghosts of some hungry extinct herbivores and you will see if they eat up the fallen fruit. You write the paper, I propitiate the extinct spirits, and off it goes to some unsuspecting editor. Am I reading you OK? Despite your disclaimer, you have thought the thing through very well."

    Right at the outset Martin spoke of the Pleistocene megafauna as ghosts. Ghosts are what Martin sees when he is interacting with plants, and other scientists have since referred to the phenomenon of missing partners in the same way. Janzen, however, has steadfastly refrained from using that word. He prefers to focus on the living rather than the dead. By the time he wrote the first draft of their joint paper, Janzen would have an equally compelling way to characterize the plant side of the partnership: anachronisms.

    Paul Martin's initial response to Dan Janzen is worth quoting at length because it is a fine introduction to the Pleistocene menagerie that will appear throughout this book. Like Janzen's letter, Martin's is delightful for its playfulness and unalloyed enthusiasm.

    "You want a scenario for 100,000 B.C.? That should be the last interglacial. I won't apologize for the bum fossil record and poor dating in lowland tropics. Without consulting the literature, I come up with the following hungry mouths:

• Giant ground sloths, 18 feet tall, heavier than an African elephant ... and bipedal at least some of the time. Genus Eremotherium. Teeth are like pruning shears.

• Some smaller ground sloths, either a Mylodon, a Scelidotherium, or both. Their teeth would be less well adapted for chopping than Eremotherium.

• Some kind of mastodont forest elephants, either Cuvieronius or Stegomastodon, or both. If the giant sloth didn't, these guys did slurp up the fallen fruit with their trunks and beat the bruchid beetles to the huge Cassia pods. The Cassia seeds get a fine, big turd of elephant manure to germinate in, just like Acacia tortilis in Africa.

• A rhino-sized monstrosity: Toxodon, the last of his order. Doris Stone, or somebody, once showed me Toxodon bones from the museum in San Jose. His mouth is full of big buck teeth incisors, like a huge mule. Great for bark shredding?

• One or more species of Glyptodont, their heads never far from the ground and their teeth too small for grinding. The largest were big enough to miscegenate with a Honda Civic.

• Some extinct peccaries, cf. Platygonus, maybe twice the weight of the white-lipped and even more rangy. Catagonus in Paraguay, their nearest living relative, isn't quite extinct yet.

• Possibly some tropical horse.

• Probably giant tortoises (genus Geochelone).

    "My hunch is that the mastodonts got 70 to 80 percent of your fruit, and that the extinct megafauna had a carrying capacity in your area comparable to the Lake Manyara Reserve, which I think is over 100 animal units per section. [Lake Manyara is in Tanzania; an animal unit is a measurement developed by the livestock industry to indicate herbivory equivalent to one cow plus calf—about a thousand pounds, or 450 kilograms.]

    "While you start the first draft," Paul Martin concluded his letter, "I'll dust off some books on who really lived in Costa Rica or nearby. At least you've solved the Cassia grandis problem. I'm assuming those pods are within easy elephant reach, say, 10-12 feet, and of course with Eremotherium you can go higher. As ever, Paul S. Martin."

Four years would pass before Janzen and Martin's joint effort would appear in the prestigious journal Science. During that time Martin was aggregating fossil information as to which beasts had in fact lived in Costa Rica toward the end of the Pleistocene. He also ventured into the countryside of Sonora, Mexico, where he had long been conducting ecological fieldwork. He wanted to look at the plants with new eyes, now that ghosts were a possibility It was there that he began to develop an idea that would prove useful not only for ecologists working with Pleistocene ghosts but also for studying African elephants. He called this concept "the megafaunal dispersal syndrome."

    Meanwhile, Janzen conducted field experiments in Costa Rica that would provide fruit-specific and other empirical grounding. His studies of tough-husked guanacaste and jicaro fruits and the beneficial or predatory passage of their seeds through the digestive systems of horses, cattle, tapir, and mice were reported in several journals commencing in 1981. This fieldwork and experimentation provided crucial support for the groundbreaking theoretical paper.

    "Neotropical Anachronisms: The Fruits the Gomphotheres Ate" was published in Science in January 1982. Neotropical anachronisms are anachronisms of the New World tropics. The term applies to plants that some time in the past thirty or forty million years evolved fruits intended to attract very large mammals. The big beasts are gone, but the fruits remain. Year after year in the American tropics (and temperate climes too), trees and vines produce fruits that make little sense today. Some fruits simply rot on the ground beneath the parent plant. Others are raided by seed predators or plundered by pulp thieves. Whether rotted, raided, or plundered, viable seeds are rarely dispersed.

    The plants not only remember the great mammals of the Pleistocene and before; they expect gomphotheres, ground sloths, toxodons, and their ilk to show up any day now. Thirteen thousand years is not enough time for plants to notice and genetically respond to the loss.

    Twenty years ago Dan Janzen and Paul Martin launched what has become a new research program in the biological sciences. Their work has also contributed to our awed enjoyment of the living world. Nature enthusiasts as well as scientists who encounter the anachronism concept will have their experience of the plant—and animal—realm transformed.

Haunting the Wild Avocado

Grocery stores are excellent places to encounter ghosts. They lurk in the fruit section, feasting on anachronisms. Paul Martin, who has been honing his occult skills for a quarter century; thinks he's spotted ghosts among the apples and pears. I'm a neophyte, however, so I head straight for the tropical fruits.

    Papaya is haunted by spectacular ghosts. Most impressive are the gomphotheres and ground sloths. The forty species of genus Carica are native to South and Central America. Only four are deemed edible by humans. The kind sold in grocery stores, Carica papaya, probably originated in Mexico. All species of Carica, including those shunned by humans, are haunted by ghosts with gapes large enough to take in the soft fruit whole, rather than bite by bite.

    The avocado bin attracts ghost glyptodonts and toxodons as well as gomphotheres and ground sloths. Because almost all fifty species of genus Persea are native to the tropics and subtropics of the Americas, one can surmise that the avocado genus honed its form in the Western Hemisphere Not all of these species evolved with megafauna in mind. The kind of Persea that thrives along the Gulf Coast of the United States bears a fruit not much bigger than a blueberry.

    Like papaya, the avocado found in grocery stores (smooth and rough-skinned varieties of Persea americana) has been haunted for thirteen thousand years. Many living frugivores, omnivores, and even carnivores are attracted to the oily pulp, but only an animal with a massive gullet will swallow the huge seed along with the flesh. The cultivated varieties of Persea americana have far thicker pulps surrounding the seed than does the ancestral stock, but the seed itself is virtually unchanged in girth. From a functional and evolutionary perspective, avocado intends its fruits to be swallowed whole. That's how the species disperses its seed. The oily flesh is simply the lure. A parent tree could wish for no more desirable fate for its offspring than to have its seeds plopped into the world within steaming heaps of dung.

    Whether growing in commercial orchards of southern California or forest fragments of the neotropics, domestic and wild avocado trees still expect giant mammals to stop by for the harvest. Wave upon wave of Cenozoic megafauna faithfully harvested avocado fruits, season upon season, for tens of millions of years. The identities of the dispersers shifted every few million years, but from an avocado's perspective, a big mouth is a big mouth and a friendly gut is a friendly gut. The passage of a trifling thirteen thousand years is too soon to exhaust the patience of genus Persea. The genes that shape fruits ideal for megafauna retain a powerful memory of an extraordinary mutualistic relationship. Embellished by our own scientific understanding, that memory would look something like this:

    The scene is a tropical forest in Central America fifteen thousand years ago, and a giant has just arrived. Perhaps attracted by the scent of ripe pulp, a three-ton mother and her bear-size toddler approach a tree that shed its fruit crop a few days before. The visitors are ground sloths, whose closest living relatives are South American tree sloths, anteaters, and armadillos. Eremotherium looks like nothing alive today. Think of a bear crossed with a prairie dog or marmot and endowed with the bulk of an elephant. The adult sloth begins to sniff the carpet of fruits for the ripest specimens. Her agile offspring climbs a nearby tree for safety and also because, at this age, climbing is not only possible but irresistible. In a few years, the young sloth's tree-climbing days will be over. By then, an enormous bulk and powerful clawed forelimbs will suffice to ward off all but the most determined predators.

    The mother finds a fruit that smells acceptable and tests it for softness between frontally toothless jaws. The whole fruit is then mashed between tongue and palate. The slippery seed slides easily down the animal's gullet. along with the nutritious pulp. Laxatives in the pulp ensure that the seed will complete its dark journey before digestive juices do it harm.

    Other seeds follow. Before she is satiated, the sloth and her young depart, The adult sloth will balance the oily meal with leafy browse, thus keeping microbes happy in the vast fermentation vat of her gut. Tomorrow the pair will return to the same tree, dispersing seeds along the way. Or perhaps Eremotherium will choose a papaya tree instead. To feed on papaya, the great sloth will sit up on her haunches, using her sturdy tail for a third point of balance. She will choose the ripest pendulous fruit—all of which are borne on the trunk of the small tree. Her reach may exceed four or even five meters.

    The sloth's limbs still show signs of arboreal ancestry. In shuffling from plant to plant, Eremotherium walks on the sides of her paws. The awkward gait may owe to phylogenetic inertia—an inability to evolve away from an established form. Perhaps, too, it owes to the survival advantage of inturned paws. An enhanced ability to climb when young should more than offset an inability to run later on. Or perhaps the anatomical quirk is necessary for the sloth to walk at all. Eremotherium's front feet bear exceptionally long claws, as do the front feet of a relative that will survive the end-Pleistocene extinction: South America's giant anteater. The anteater walks on its knuckles, claws behind and curled skyward.

    Meanwhile, in another part of the forest, one that is especially rich in avocado trees, a small herd of gomphotheres (genus Cuvieronius) approaches on an ancient trail. The herd has traveled tens of kilometers in the past three days, munching greenery along the way. The matriarch remembers the route. She remembers this avocado-rich valley and others throughout a vast region, as well as good places and times to find papaya, cherimoya, sapote, Cassia grandis, and many other treats. She learned these sites while following the lead of her mother, the former matriarch.


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