As a kid I used to get a kick out of a couple trees without ever giving any thought towards what it was. My friend's neighbor had a some Osage orange trees (Maclura pomifera) growing at the end of his driveway. Their houses were situated atop a large hill and the road was pretty much a straight drop down into a small river valley. After school on fall afternoons, we would hang out in my friends front yard and watch as the large "hedge apples" would fall from the tree, bounce off the hood of his neighbor's car (why he insisted on parking there is beyond me) and go rolling down the hill. I never would have guessed that almost two decades later the Osage orange would bring intrigue into my life yet again. This time, however, it would be because of the evolutionary conundrum it presents to those interested in a paleontological mystery...
The fruit of this tree are strange. They are about the size of a softball, they are green and wrinkly, and their insides are filled with small seeds encased in a rather fibrous pulp that oozes with slightly toxic white sap. No wild animal alive today regularly nibbles on these fruits besides the occasional squirrel and certainly none can swallow one whole. Why then would the tree go through so much energy to produce them when all they do anymore is fall off and rot on the ground? The answer lies in the recently extinct Pleistocene megafauna.
The tree is named after the Osage tribe who used to travel great distances to the only known natural range of this tree in order to gather wood from it for making arrows. It only grew in a small range within the Red River region of Texas. When settlers made it to this continent, they too utilized this tree for things like hedgerows and natural fences.
What is even stranger is that recent fossil evidence shows that Maclura once had a much greater distribution. Fossils have been found all the way up into Ontario, Canada. In fact, it is believed that there were once 7 different species of Maclura. It was quickly realized that this tree did quite well far outside of its current natural range. Why then was it so limited in distribution? Without the Pleistocene megafauna to distribute seeds, the tree had to rely on flood events to carry the large fruit any great distance. With a little luck, a few seeds would be able to germinate out of the rotting pulp. Botanists agree that the Red River region was a the last stronghold for this once wide ranging species until modern man came on the scene.
Another clue comes from the toxicity of the fruits. Small animals cannot eat much of it without being poisoned. This makes sense if you are a Maclura relying on large animals as dispersers. You would want to arm your fruit just enough to discourage little, inefficient fruit thieves from making a wasteful meal out of your reproductive effort. However, by limiting the amount of toxins produced in the fruit, Maclura was still able to rely on large bodied animals that can eat a lot more fruit without getting poisoned. Today, with the introduction of domesticated megafauna such as horses and cows, we can once again observe how well these fruits perform in the presence of large mammals.
Finally, for anyone familiar with Maclura, you will notice that the tree is armed with large spines. Why the heck does a large tree need to arm itself so extravagantly all the way to the top? Again, if you need things like mammoths or giant ground sloths to disperse your seeds, you may want to take some extra precautions to make sure they aren't snacking on you as well. It takes energy to produce spines so it is reasonable to assume that the tree would not go through so much trouble to protect even its crown if there once wasn't animals large enough to reach that high. The Pleistocene megafauna went extinct in what is evolutionarily speaking only the blink of an eye. Trees like the Osage orange have not had time to adapt accordingly. As such, without the helping hand of humans, this tree would still be hanging on to a mere fraction of its former range down in the Red River region of Texas.
Further Reading:
http://plants.usda.gov/core/profile?symbol=MAPO
http://www.americanforests.org/magazine/article/trees-that-miss-the-mammoths/
http://www.plosone.org/article/info:doi%2F10.1371%2Fjournal.pone.0001745