Wednesday, September 25, 2013

Thorn Trees, Avocados, and VLS's (Very Large Sloths)

Photo by Greg Hume, Wikimedia Commons
This weekend I went for a walk with two of my friends through a swamp outside New Orleans. We saw plenty of waterbirds, three young alligators, some burly swamp rabbits, and a total of seven snakes. But what impressed my companions most was a honey locust tree we came across. Those are fairly common where I'm from in Arkansas, but they aren't common here, and they had never seen one before. Hearing them talk about it, I started seeing it with new eyes. The honey locust really is a stunning tree, with its clusters of icepick-sized thorns along its limbs and trunk. It's clearly not a tree to be trifled with.

Then my friends asked a question that I had never thought to ask: why does need such huge thorns? What assailant is it protecting itself from? Is it just paranoid?

None of us had any ideas, so we kept walking and forgot about it. But the next night, in one of those weird little jolts of serendipity, I stumbled across a possible answer. I was leafing through a picture book about natural history, and saw the following passage in the section on plants: "Species with fleshy fruits use animals to disperse their seeds: many of them are swallowed whole and then scattered by birds. In prehistoric times, wild avocados may have been dispersed by giant ground sloths."

That's right, giant ground sloths. They really existed. One of the most jaw-dropping things I have ever seen is a skeleton of one of these in the Smithsonian. I had seen the dinosaurs and pterodactyls skeletons and been duly impressed, but then, in the Ice Age hall, I came across the towering skeleton of Eremotherium; one of the largest of the ground sloths. The thing is just gigantic--as big as an elephant standing on its hind legs. You could build a small tree house between its pelvis and its ribcage...and it's a sloth. I stood there and stared at it like I was three years old.

Photo by Postdlf, Wikimedia Commons
Ground sloths were superficially built like long-armed bears, and like bears, they could stand on their hind legs. Some of them could reach nearly 20 feet into the trees to browse on buds, leaves, and fruit. And some of these fruits may have co-evolved with ground sloths and other big ice age mammals, as a way to disperse their seeds.

Plants can do many things, but what they can't usually do is get up and move around. That means they have to find ways to reproduce while being stuck in the same spot all their lives. They have to get their pollen to each other somehow, and then they have to scatter their seeds so that their offspring won't grow up right on top of them. Pollen is mostly spread by the wind, or by bribing or tricking flying animals like insects, hummingbirds, and bats.

Plants scatter their seeds in equally inventive ways. Dandelions and cottonwoods send them aloft on little cottony parachutes. Maple and sycamore seeds whirl like helicopter blades as the wind catches them, and the Javan cucumber's seed has wings like a hang glider. The seeds we call burs hitch rides on animals, and on our pant legs. Coconuts disperse by floating in the sea, sometimes drifting hundreds of miles from their parent plant. Some fruits dry up and explode. Impatiens are a familiar example--their fruit may burst if you touch it, which is why they are also called Touch-Me-Nots. But even more impressive is the Sandbox Tree, AKA the Dynamite Tree. Its fruits explode with a deafening blast, launching seeds over 150 feet away.

Of course, many fruits have evolved to entice animals to eat them. Plants don't put all that energy into producing sweet, tasty fruit because they have benevolent spirits. They do it to spread their seeds. Some seeds can't even germinate unless they've been through the gut of an animal. This often weakens their tough coating, and as a bonus, the seeds end their intestinal journey in a little dollop of fertilizer on the ground.

And that brings us back to avocados. Avocado seeds are too big for most modern creatures to swallow regularly, but huge prehistoric creatures like ground sloths might have gulped them down without even thinking about it, and then deposited them later in their poop, the way birds deposit blackberry seeds. Today, the ground sloths are gone, and avocados might have dwindled away too if people hadn't come along and started cultivating them. Perhaps that's only fair, since we may be what killed off the ground sloths in the first place, along with a host of other Pleistocene creatures of the Americas, including giant bison, mammoths, mastodons, and four-tusked gompotheres; as well as the predators who relied on them--massive dire wolves, sabertoothed cats, and 1800-pound short-faced bears. The fauna of the Americas was as impressive as the African savanna until a few thousand years ago--right around the time the first humans seem to have arrived. Of course, the climate got a lot warmer at the same time, so the jury hasn't declared humans guilty beyond a reasonable doubt. But they're eyeing us pretty suspiciously.

Anyway, it may be that avocados weren't the only plants left without partners by the extinction of the big mammals. Another tree I grew up around, the Osage orange, may have co-evolved with mammoths, mastodons, and their relatives. These trees once had a wide range across North America, but today they are confined mostly to Texas, Oklahoma, and Arkansas. Some biologists think this may be because their giant dispersers disappeared. In most places, Osage orange fruits simply pile up and rot under the tree. Most modern native animals ignore them, but horses eat and distribute them now. That has a certain symmetry, since Osage oranges would have evolved with native horses of the Americas, which went extinct at the end of the ice age. The reintroduction of horses from Europe may have reintroduced two old friends.

And what about the honey locust, with its menacing spikes? It's another tree that may have co-evolved with giant mammals. As prickly as it is, it's called the honey locust for good reason--its seed pods are filled with a sweet-tasting pulp that probably evolved to attract large animals, who would eat the seeds along with the pulp. If the seed pods aren't eaten, they just fall to the ground, and the seeds inside will be destroyed by insects. But if they get eaten by a large herbivore, they will pass through its gut unharmed. This is actually what happens with relatives of the honey locusts in Africa, the acacias. Elephants help disperse some acacia seeds by eating the pods. The insects attacking the seeds are killed, but the seeds do just fine; and are actually much more likely to germinate if they've made a trip through pachyderm innards. But elephants and other large mammals can be hard on acacias, too--stripping their bark, pushing them over, and browsing too many of their leaves. That's why acacias have thorns--to deter this sort of thing. Their cousins, honey locusts, have even bigger thorns. And maybe that's the answer to my friends' question. Why does that tree we looked at just this weekend have such huge thorns? Maybe it's still trying to protect itself from giant creatures that disappeared thousands of years ago. Those great beasts may seem almost mythical to us, but they were very real, and museums are full of the bones to prove it. They were certainly real to the honey locust, and it still has its thorny daggers ready, in case they ever return.


Anachronistic Fruits and the Ghosts Who Haunt Them / Connie Barlowe

The Trees that Miss the Mammoths / Whit Bronaugh

Saving the Seeds (David Attenborough clip on acacias and elephants)