Our Pumpkin Problem


'Tis the season of the jack-o'-lantern, pumpkin pie, and the pumpkin spice latte. Pumpkins, unlike turkey and cranberries, were likely on the menu of the first American Thanksgiving in the autumn of 1621. So given its place in our history and culture you might think that it should be pretty evident what a pumpkin is. You would be wrong.

The mental picture of a pumpkin that most people, including me, conjure up is the basketball-size orange fruit that we carve into scary faces around Halloween. Yes, I said "fruit." Pumpkins contain seeds -- a defining trait of fruits.

Fleshy fruits like pumpkins are meant to be eaten - by nonhuman animals. Fruits are nature's way of providing animals a nutritious reward for swallowing their seeds and dispersing them far and wide when they emerge from the animals' other end.

For this plant strategy to work, the seeds need to be fully developed when the fruit is eaten. Plants manage this by making fruits taste awful prior to the seeds being ready as anyone who has bitten into an unripe fruit can attest. Also, plants need to discourage animals from chewing up - that is, killing -- the seeds before swallowing them. Plants attempt this by packing seeds with foul tasting, potentially toxic chemicals which can be largely avoided if the seeds are swallowed whole. So for you campers eager to try subsisting on wild fruit, first be very careful - some of those fruits are flat poisonous to people -- and second when in doubt spit out or swallow the seeds, don't chew them.

By now you might be wondering what sort of animal would (or could) eat something the size of a pumpkin without chewing up their dime-size seeds which brings us to mastodons.

Mastodons you may recall were North America's elephants. They looked a lot like a modern Asian elephant except they were thickly furred. They appeared around 5 million years ago and disappeared about 10,000 years ago, not long after people first arrived on this continent. Mastodons loved to dine on pumpkins.

Well, not pumpkins exactly. They dined on pumpkins' wild ancestor, the avocado-size wild squash. We know this because biologists who study the ecology of ancient times love to pick through ancient dung whenever and wherever they can find it. Mastodons left around a lot of dung which has been picked through extensively and intact wild squash seeds have been found there in abundance.

Wild squash like any self-respecting plant protects its seeds with bitter tasting, toxic chemicals, but recent research suggests that mastodons lacked the taste buds to sense those bitter-tasting chemicals. Also, the mammoths' enormous size would have protected them against eating enough wild squash to make them really sick.

Here is our first pumpkin problem. Pumpkins are a domesticated version of the wild squash. Pumpkins though have had the bitter, toxic chemicals bred out of them by ancient farmers. But there are numerous other domesticated versions of the same wild squash species, including summer squash, several types of winter squash, spaghetti squash, acorn squash, and zucchini. All of which have also had the toxins - at least those toxins resistant to cooking - bred out of them. A jack-o'-lantern pumpkin is a breed of squash in the same way that a Great Dane, Labrador retriever, or pug is a breed of dog.

Because all these squashes are a single species, they can all interbreed. If you pollinate a pumpkin flower with zucchini pollen or vice versa, you will end up with a fruit that is something in between the two. Maybe call it a pumpk-ini? Worse, if you pollinate any of these domestic squashes with pollen from a wild squash, you will end up with a fruit that is bitter-tasting and toxic like wild squash, which is why we owe mastodons a bit of gratitude for going extinct.

When mastodons roamed the American forests, wild squash were abundant. So abundant that if people had tried to domesticate them into something edible, their pollinators -- native American bees (honey bees did not arrive until the first European colonists) -- would have no doubt often dusted them with the pollen from wild squash, keeping them inedible. Squash are one of the oldest domesticated crops, predating the Pilgrims by thousand of years. At that time, people had no inkling about pollination biology, so they would never have figured out why despite their best efforts to selectively breed sweet-tasting squash, it continued to have that foul taste. However, wild squash numbers fell dramatically when mastodons, their chief seed disperser, vanished. Once the wild squash was rare enough, the chances of wild pollen-carrying bees landing on plants that people were selectively breeding for their size and taste became remote. Domestication of all those squash varieties could proceed.

So now you may think you know what a pumpkin is - a large orange domesticated breed of wild squash that we carve into scary faces. Not so fast. That is the jack-o'-lantern pumpkin. The canned pumpkin filling that we use to make pies and spice lattes is not that at all. It is a completely different species, South American rather than North American in origin! The pie-filling pumpkin is the same species that includes another breed we call butternut squash. So a zucchini is more closely related to your jack-o'-lantern than is the filling in your pumpkin pie. What's more, those giant pumpkins that farmers are always winning prizes for growing are yet a third species, also South American.

So, what is a pumpkin? As the physicist John Wheeler has reputedly said about quantum mechanics, "if you're not completely confused then you cannot possibly have understood it."

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