Do supermarkets and parks hide Pleistocene ghosts

By Gary Raham
Nature Writer and Illustrator

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Ever felt haunted in the grocery store produce department? Perhaps felt the brush of shaggy fur on your arm near the papayas or heard muffled snorts while picking out an avocado for the chip dip?

If not, such imaginings might cross your mind after reading some of the theories of paleoecologists like Paul Martin, Dan Janzen and Connie Barlow. The concept is simple: Many plants have evolved to attract seed dispersers that no longer exist. The evidence lives on in "over-built" fruit.

Our relatively short lives tend to skew our perceptions. Our parents or grandparents are the measure of old age, Thomas Jefferson is ancient history and anything over 6,000 years old is, by many, unthinkable.

But evidence of much older times lies all around. Thomas Jefferson, in fact, became fascinated by mammoth and giant sloth fossils and helped demonstrate through the explorations of Lewis and Clark that these huge beasts no longer existed, even in the wilds of North America. They were, in fact, extinct - a novel concept in the 18th century.

Since Jefferson's time, paleontologists have found that North America was home to an entire zoo of megafauna (animals weighing more than 100 pounds) that included primitive horses and camels, an American lion, saber-toothed cats, wolves, various members of the elephant clan (proboscideans), glyptodonts (think furry armadillo), toxodons (something like hippos), giant sloths and more. These creatures all became extinct about 13,000 years ago at the end of the last ice age.

And they all had to eat something.

Granted, carnivores for the most part ate the veggie eaters, but the latter munched on plants. Very few flowering plants have become extinct since the glaciers melted away, although some have had their ranges severely restricted.

Plants have a long history of developing ways to attract animals to disperse their seeds. Plants like wild ginger attach lipid-rich food packets called elaiosomes to their hard-coated seeds. These attract ants, which eat the lipid gift and unintentionally bury the seed.

Others surround their seeds with fairly small but pulpy, easy-to-pierce berries, which stand out from the green foliage in reds, blues and purples. The plant's seeds get physically or chemically roughed up while traversing a bird or mammal's gut, then are deposited at some distance from the parent plant to sprout.

But there are other seed-bearing fruits that have no obvious means of dispersal. Large, sometimes bound by a hard rind, often with strong odors, these are the fruits that plants have created to woo large mammals.

Consider melons and gourds. In the American Southwest, big-fruited gourds in the genus Cucurbita commonly grow along dry washes. These are commonly called skunk gourd because of the smell of their leaves. The fruit tastes bitter. Yet these viney plants are closely enough related to domestic summer squash that they could, and sometimes do, cross-pollinate with them.

Typically, the large fruits rot in place. Rodents will sometimes bury gourds in their burrows, but only a fraction. They don't appear to be an effective disperser. But in Florida, mastodon fossils have been found in association with Cucurbita seeds. In Africa, the seeds of squashes in the same family can routinely be found in elephant dung. It doesn't seem like much of a leap to surmise that mastodons may well have been an important disperser for these vines.

In "The Ghosts of Evolution" Connie Barlow describes many other examples of North American plants with large fruits, often with smelly pulp, and seeds that need to be cut (scarified) or chemically treated to germinate properly, unless they have traversed a mammal's gut.

Barlow was inspired by the research of botanist Dan Janzen and paleoecologist Paul Martin, who teamed up to merge their scientific insights and propose a "megafaunal dispersal syndrome." The "wasted, rotting fruit" that Janzen found for many plant species in South America made sense when viewed in the light of the kinds of animals in their habitat, not only 13,000 years ago, but continually during the previous 30 million years or more. Their paper, "Neotropical Anachronisms: The Fruits the Gomphotheres Ate," was published in the January 1982 edition of Science.

Gomphotheres were oddly tusked elephants that, along with giant sloths, could rear up 18 feet and may have contentedly eaten papaya in Central and South America. The avocado comes from fruit originally designed to attract glyptodonts and toxodons.

In North America, Osage orange trees produce huge fruits that deer and cattle can't handle, but horses love them. Horses were native to North America before they became extinct here at the end of the ice age. They were only reintroduced 400 years ago. Had they survived, Osage orange might not have become restricted in the wild to a few river valleys in eastern Texas. It grows well in many upland habitats but can now only be dispersed by rolling or floating downhill.

Barlow said that once she became attuned to looking for vegetative anachronisms, every hike or walk in an urban park turned into an adventure. The hard, coiled seedpods of honey locust trees conjure mastodons. Tall species of southwest prickly pear cactus (Opuntia) cry out for their ancient, ice age camel dispersers.

Even a routine trip to the grocery store can be an adventure filled with ghosts from deep time--once you know how to read the messages encoded in fruits made by plants that still remember the ice ages.

References:

Barlow, Connie. "Ghost Stories From the Ice Ages" in Natural History, Volume 110, No. 7, September 2001.

Barlow, Connie. The Ghosts of Evolution, Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms, New York: Basic Books, 2000.