Coffee tree saplings, like this one, were probably a more common site in the past when mastodons roamed North America. © AMNH

The tree's large pods contain several seeds embedded in a green pulp that's rumored to be deadly enough to drop a cow. The seeds themselves are so poisonous that no animals, other than humans, are known to eat or disperse them in any way. Nor does the wind carry the seeds any appreciable distance. This shortage of dispersal mechanisms is probably the main reason why the Kentucky coffee tree is so rare. Although it's quite capable of thriving in a variety of habitats, the tree is usually found near waterways. There it can rely on the eroding action of moving water to abrade its very tough seed coats—the Kentucky coffee tree's seeds can't germinate without first having their seed coats breached.

The pods and seeds of the Kentucky coffee tree. © AMNH

From an evolutionary point of view, it makes no sense for an organism to evolve characteristics that offer no reproductive advantage. One might ask, "Why does the Kentucky coffee tree seem to be so poorly adapted to its native ecosystem? What 'went wrong' with this peculiar plant?" Probably nothing, say botanists who have been attempting to recreate this unusual plant's ecological history. The tree seems to have adapted quite well to its environment—at least that was the case 12,000 years ago.

Circumstantial evidence suggests that the Kentucky coffee tree may be waiting for a mastodon—its likely partner from the past—to come by. Though biologists don't know what the long-extinct mastodons ate, they can make inferences by looking at the diet of elephants, the mastodon's closest living relative. The similarity of the coffee tree to certain foods that modern elephants love, coupled with the many ways elephants cope with plant poisons, makes a case for the coffee tree being part of the menu of their extinct mastodon relatives.

The grinding molars of the mastodon reunited with one of its likely food items. © Connie Barlow

Plants often develop seed poisons to protect against "seed predators" like rodents, who completely destroy seeds by chewing them to bits. But large fruit-eating animals typically swallow seeds whole, in part because the toxins in the seeds deliver a bitter surprise if the fruit-eater chomps down. The molars of the fruit-eating mastodon would abrade the seed coat to some degree, and then its gastric acids would further abrade it, making germination possible. The seed, still viable, would eventually be ejected from the animal's gut in a heap of fresh fertilizer. The theory is that the Kentucky coffee tree, having rewarded the mastodon with a snack, ensured its dispersal via the animal's digestive process.

Could the Kentucky coffee tree be what's called an ecological anachronism? Is it a survivor from a distant past in which it partnered with animals less fortunate than itself? Has its retention of poisons—developed to thwart lesser would-be dispersers—kept it from adapting to a world without mastodons? These questions are hard to answer on the basis of the fossil record provided thus far by the players in this drama; the record is far from complete. Traditionally, past extinctions have not been given much consideration in ecological theory, but a growing number of scientists are considering this scenario for the ecological history of the Kentucky coffee tree, as well as for other possible ecological anachronisms.

With no modern seed dispersing animals to collect the seeds, the pods of the Kentucky coffee tree lie where they fall and rot. © Connie Barlow

The tree persists mainly because it has a backup seed dispersal system: the relentless action of streams and rivers. Its highly poisonous leaves are virtually pest- and nibble-free, good protection for the tree's survival in an uncertain future. A shortage of seed dispersal mechanisms doesn't mean that the Kentucky coffee tree is doomed to extinction—it merely means that this tree seems to have lost an edge it had several millennia ago. And although plants evolve more slowly than the animals with which they often partner, the Kentucky coffee tree may "realize" that it's time to find another cohort to help it reclaim its ancient haunts.