Sunday, March 23, 2014

Plant Story -- Osage-Orange and the Animals of the Pleistocene

Osage-oranges  on the ground
 on the ground
Osage-orange, Maclura pomifera, is a tree endemic to (native only in) North American. Its range. when Europeans first encountered it, was small: the area where Arkansas, Texas and Oklahoma meet. Its habitat is described as woods, forest edges and streams. Widespread planting of Osage-orange for fences in the 1850s-1870s has greatly expanded its distribution.

I talked about Osage-orange's interesting wood previously (link). But Osage-orange has memorable fruits. See Osage-orange fruit They are bigger than oranges. The outer rind is warty. Inside it is more solid than an orange, with a row of small seeds. (Gray's Manual of Botany calls the fruits "disappointingly dry and hard" inside). Not much of anything eats them. People don’t eat them, some horses like them, deer eat a few and determined squirrels will tear them open and eat the seeds, but still the big fruits pile up at the base of the tree. 

Mostly we don't think about what we see and ask "why?" But why does Osage-orange make a huge fruit? Plants are rooted. To get to new areas, something (wind, water, an animal) has to carry the seeds away. Osage-orange fruits seem horribly inefficient at dispersing the plant. 

The current answer is: the fruits evolved to be eaten by animals that have gone extinct.

 Here's the story:
In 1982, Dan Janzen and Paul Martin published an important paper in the journal Science "Neotropical Anachronisms: The Fruits the Gomphotheres Ate," arguing that there are plants in the Americas that evolved to have their seeds dispersed by big New World animals, usually by having the animal eat the seeds and deposit them in their feces. Up through the  Ice Ages, the Pleistocene, which ended about 10,000 years ago, the Americas had ground sloths, horses, camels, mammoths, gomphotheres and more (link). Janzen and Martin proposed that plants with fruits that were dispersed by one or several of these big animals survive even though the animals are gone. They made a substantial list of Costa Rican plants that they thought might have been dispersed by big extinct animals  (also called"megafauna" mega for very big, fauna, animals). (Picture of some proposed fruit Link  or see original papers: Janzen and Martin; Guimaraes et al.
Janzen and Martin didn’t talk much about North America, but the theory should apply. Big animals of the Pleistocene went extinct in North America as well as Central (and South) America. One North American plant they did suggest had lost its dispersers was Osage-orange. 
Osage-orange in hedge row
Osage-orange in
hedge row
Pursuing the question of North American plants with extinct animal dispersers, Connie Barlow wrote the excellent book, The Ghosts of Evolution. She looked for plants in North America that have characteristics suggesting a relationship with a now-absent animal. She  recognizes such plants because they are "overbuilt" for current interactions and have small or patchy or flood-plain distributions because only gravity and floods disperse them now. Like Janzen and Martin, she points to Osage-orange. Its distribution certainly fits. Despite its limited range when first encountered, it grows successfully in all but 7 states of the continental U.S. (USDA plants map) suggesting something other than climate limited its distribution.
Lets look at it from the plant's view for a moment: what does the Osage-orange fruit suggest? The fruits  are as big or bigger than oranges with imbedded seeds. Getting the seeds out isn't easy (try it). This suggests the fruits should work like tomatoes, eaten whole by an animal and dispersed in the feces. Some animal-dispersed fruits contain seeds that encourage the eater to eat the tasty pulp and throw the seed away, but in those, it is pretty easy to toss away the seedy part (eat around it or spit it out). That's not the case for Osage-oranges.

When Europeans arrived, nothing in Osage-orange's range except bears and buffalo had a mouth big enough to eat a whole Osage-orange fruit. I cannot find a report that either of those animals ate them. 

However, it is easy to imagine that some now-extinct animal—a ground sloth (easily 6‘ high at the shoulder) or mammoth or some other massive animal--put its big mouth around the Osage-orange fruit, swallowed it--pulp, seeds and all--and then defecated the seeds as it wandered across North America.  

Horses like Osage-orange fruits (photos from the web: 1 2), in fact one common name for Osage-orange is horse apples. Horses evolved in North America, went extinct in the Pleistocene and were reintroduced by the Spanish. Deer do eat Osage-oranges, but apparently mainly by munching fallen fruit late in the winter. Cows tend to choke on them. Squirrels strip out the seeds and eat them, so only dropped seeds are dispersed. A seed-dispersal system aimed at squirrels would let the squirrels easily carry the fruit away, to drop the seeds away from the tree. Osage-oranges are too big for easy removal by squirrels.

To date, attempts to see if elephants and other living big animals will eat Osage-oranges  have been disappointing, but as Barlow explains, it hasn't been seriously studied. 

So the current answer to the mystery of “why does Osage-orange have these big inedible fruits?" is that the animal that loved eating Osage-oranges went extinct 10,000 years ago. Since then Osage-orange has hung on by sprouting from established roots and a few seeds escaping squirrels, but the tree's distribution shrank down a very limited area.

There's a lot more to the story of extinct Pleistocene megafauna and American fruits: see  links below. 

From Central America, avocados.  Before people, what dispersed them?
And many other interesting characteristics to Osage-orange: google it!

One thing this theory does is to make me look more carefully at my environment. Perhaps trees with lots of undispersed seeds under them are providing ecological and evolutionary clues that I should have noticed. 

Honey locust, Gleditsia triacanthos
Honey locust, Gleditsia triacanthos, another tree Barlow suggest is missing its dispersers

Comments and corrections welcome.

Note on spelling Osage-orange. The USDA plants list drops the capital on Osage, but it refers to an Indian tribe, and so I think it needs to be capitalized. One online source said it should be hyphenized, Osage-orange because it is related to mulberries, not oranges. I can see that would be clearer but almost nobody does that.


Barlow, Connie. Anachronistic fruits and the ghosts who haunt them. Arnold Arboretum. Paper online  

Barlow, Connie. 2006. The Ghosts of Evolution. Basic Books, NY. Check out the video on her website Song

Fernald. M. L. 1970. Gray's Manual of Botany. 8th ed. D. Van Nostrand, NY.

Guimaraeas, Paul0 R. Jr.,, Mauro Galetti and Pedro Jordano. 2008. Seed dispersal anachronisms: rethinking the fruits extinct megafauna ate. PLOS One.  article online See pictures of fruit in Figure 1.

Janzen. D. H. and P. S. Martin. 1982. Neotropical anachronisms: the fruits the gomphotheres ate. Science. 215:  19-27. the paper online

More on Pleistocene animals and modern plants:

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