Haunted Fruit – A Tale of Botanical Anachronisms
“Living organisms are beautifully built to survive and reproduce in their environments. Or that is what Darwinians say. But actually it isn’t quite right. They are beautifully built for survival in their ancestors’ environments.”
–Richard Dawkins (1998)
In the spring of 1804, Captain Meriweather Lewis and Second Lieutenant William Clark set out on their famous expedition to explore the western territories of North America, including the newly acquired region of Louisiana. Less well-known, however, is one of the underlying reasons Jefferson had for commissioning the expedition in the first place: to find evidence of mammoths.
Thomas Jefferson was an amateur paleontologist who avidly collected fossils from extinct fauna in the New World. While president, he had a special room in the White House set aside for fossils and even published a paper on the nomenclature of Megalonyx jeffersonii, a giant ground sloth which he mistakenly identified as a large cat (Simpson, 1942). But Jefferson didn’t believe in extinction, writing, “Such is the economy of nature, that no instance can be produced of her having permitted any one race of her animals to become extinct; of her having formed any link in her great work so weak as to be broken” (1784/1999). So when he sent off Lewis and Clark into the uncharted west, he wrote to them specifically to be on the lookout for the remains or accounts of animals thought to be rare or extinct (Jefferson, 1803).
But Jefferson’s ideas were outdated by just a few millennia, because about 12,000 years ago, at the close of the Pleistocene Epoch, giant megafauna all over the world, totaling about 200 genera, began going extinct. Before then, North and South America were home to a menagerie of giant mammals including mastodons, mammoths, giant ground sloths, dire wolves, saber-toothed tigers, and a plethora of giant armadillos, beavers, and peccaries, just to name a few (Martin, 2005). But even though the megafauana were lost during the twilight of the Pleistocene, they left an indelible mark on the plants and animals that did survive. There are some trees that remember the Megalonyx and the mammoths, the glyptodonts and mastadons, and the traits that they co-evolved with these extinct creatures can still be seen today.
The Riddle of the Rotting Fruit
The ecologist Dan Janzen began work on his dissertation in 1963 at the University of Pennsylvania. He was particularly interested in bruchid beetles and the seeds of certain palm trees, on which the beetles laid their eggs. To that end, he chose a site in Costa Rica in order to study the relationship between beetle and seed. It was here, deep in the tropical rainforests of Central America, that Janzen would uncover a mystery he’d spend the next twenty years trying to solve.
The fruits of Scheelea rostrata are about the size of eggs and fall to the forest floor in the thousands when ripe. There are a few animals, such as agoutis and peccaries, that will eat these fruits and discard the seeds, but the trees produce far more fruit than these animals can consume, so the majority of it is left to rot. There are two problems with this, the first being that any seeds that try to grow beneath their parent plant will often be easily outcompeted by their established progenitor and die. This is why many plants, especially trees, have evolved mechanisms, including animal dispersal, to transport their seeds a certain distance away form their place of origin. But bruchid beetles are also the natural enemy of Scheelea seeds. After the decomposition of the thousands of rotting fruits, the hard seeds within become exposed, and bruchids quickly begin laying their eggs inside them. Once they hatch, the larvae will feed on the endosperm and embryo, killing the seed (Wilson and Janzen, 1972). This is the fate of virtually all of the Scheelea fruit left untouched by mammals. So why would a tree put forth the resources to produce so much fruit when only a few will be effectively dispersed? No one could really come up with a good explanation.
Meanwhile, Scheelea species in Africa didn’t seem to have the same problem. This is because the fruits were being eaten by elephants. Once the fruits are consumed by these large mammals, the seeds pass through the digestive tract unscathed (much like apple seeds in humans), are transported away from their parent tree, and are then deposited in a new environment safely protected from bruchid beetles by a thick layer of elephant dung, which incidentally makes a great fertilizer for germinating seeds. But the Americas don’t have anything nearly the size of an elephant to aide in seed dispersal…at least, not anymore. But the megafauna of the Pleistocene had to be eating something, and they were around long enough (several million years, which is much longer than Homo sapiens have been on the scene) that they almost certainly coevolved with the plant species around them. And these mammals have only been extinct for about 12,000 years, which, evolutionarily speaking, is a mere blink of an eye and not nearly enough time for plants to respond to the loss of their partners. These trees are therefore anachronistic in the truest sense of the word, out of place and time, and haunted by species the world lost in the not too distant past.
Ancient Alliances and Their Modern Analogs
Scheelea rostrata conjures apparitions of ancient gomphotheres, creatures related to elephants that had four tusks, the lower two attached to a conspicuously protruding jaw that strongly resembled a shovel (Ferretti, 2008). The fruit may also have been a favorite of giant ground sloths, the largest of which (Megatherium americanum) weighed about four tons, more than twice the size of an adult giraffe (Fariña et al., 1998; Hal-Martin, 1977) and towered a looming 13 feet when standing on its hind legs (Argot, 2008). But Scheelea aren’t the only plants with anachronistic traits; in fact, there may be several species we come in contact with on a daily basis that share what Janzen termed the megafaunal dispersal syndrome. Gourds, squashes, avocados, and even chocolate trees, as well as several other species literally in our own backyards, may have evolved for megafaunal dispersal (Kistler et al., 2015; Janzen and Martin, 1982). But these species obviously managed to escape extinction themselves since they’ve stuck around to modern times, the reason being that they found new dispersers to spread their progeny: us!
Take gourds and squashes (cucurbits), for example. The fruit of wild cucurbits contains bitter triterpenoid compounds (Tallamy and Krischick, 1989) that are unpalatable to us and other small mammals. However, extinct megafauna appeared to be undeterred by the bitter taste, as Cucurbita seeds have been found in the fossilized remains of mastodon dung (Newsom and Mihlbachler, 2006). It appears that the megafauna helped these plants disperse their seeds. Not only that, but cucurbits are weedy plants that prefer disturbed habitats, the exact type of environment that giant animals make when they rummage around a landscape. So when the megafauna began going extinct, the cucurbits followed suit, with the extinction of some species and the fractionation of the geographic range of others (Kistler et al., 2015).
Ironically, the same thing that led to the demise of the megafauna played a key role in saving the cucurbits. Multiple groups of humans began domesticating these plants, beginning in Mexico around 10,000 years ago (Smith, 1997). These groups selected and bred varieties that lacked the bitter taste of wild gourds, and since then, we’ve managed to create hundreds of new cultivars and varieties around the world (Nee, 1990; Ferriol et al., 2015).
Those plants that lost their megafuanal dispersals and weren’t subsequently favored by humans likely would have gone extinct, or were able to persist by virtue of other traits allowing for their reproduction. Osage orange (Maclura pomifera), which is actually more closely related to figs than it is to oranges, is a large, thorny tree with dull orange bark and large bright green fruit that, when sliced open, exude a white, milky sap. These fruits were one of the first exchanges Lewis and Clark made on their expedition, Lewis having written to Jefferson that, “So much do the savages esteem the wood of this tree for the purpose of making their bows, that they travel many hundreds of miles in quest of it” (Jackson, 1963). This species has been suggested as another anachronism, the fruit having been tentatively observed in mastodon dung as well (Newsom and Mihlbachler, 2006) and was once common throughout parts of North America (Martínez-Cabrera and Cevallos-Ferriz, 2006). But Osage orange was never cultivated by humans the way avocados and cucurbits were, and its once broad range has subsequently been reduced to a small native* region near the Red River Gorge in Texas, Oklahoma, and Arkansas. It’s been able to avoid complete extinction by its prodigious production of root sprouts, by means of which several trees can be produced from the same clonal parent. Other putative anachronisms that never received the attention of humans are capable of this as well, including Kentucky coffeetree, honeylocust, and persimmon.
There is a wealth of evidence that suggests humans played a big role in the eventual demise of megafauna all around the world (Martin, 1973). In fact, wherever humans go, we tend to carry waves of extinction with us, a trend that is perhaps most forcefully illustrated by what is now being called the sixth mass extinction, caused by our current pattern of overpopulation, emission of greenhouse gases, and the urbanized development of our planet (Kolbert, 2014). But, for better or worse, there is a very real possibility that some of the giant beasts of ages past might make a comeback. Recent efforts have focused on sequencing the genome of fossilized mammoth remains, the first attempt having been published in 2008 in the journal Nature, in which the authors were able to obtain approximately 50% of the genome from mammoths remains that were thousands of years old (Miller et al., 2008). One of the goals of this work is to eventually clone a mammoth, or even to infuse modern elephants with modified mammoth genes to allow them to expand their range further north, into environments where anachronistic plants have long waited for the return of their missing partners.
*Illustration by Marlin Peterson: http://marlinpeterson.com/
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