This observation supports the hypothesis that a now extinct megaherbivore may have been the mutualist. It is at times eaten by horses
(Turner, 2010, p. 48), but the seeds do not seem to survive the equine
gut passage (Boone et al., 2015), making dispersal by native, pre-Columbian horses unlikely. No benefit of (though also no damage
resulting from) passing through a proboscidean gut was shown in a
feeding experiment carried out with the help of an Asian elephant
(Elephas maximus) acting as a surrogate (Boone et al., 2015). The giant
ground sloth (Megalonychidae: Megalonyx jeffersonii) has also been
discussed as a possible mutualist (Bronaugh, 2010).
Tracking the Elusive Creature
The exercise described below was part of a dendrology module and
a forestry and forest ecology module for university students in
teacher’s training (M.Ed.) in biology, agriculture, and horticulture.
The location was the Berlin Botanical Garden’s arboretum, which
holds several specimens of the species.
To avoid the artificial context of post hoc methods such as
interviews, which harbor a risk of reactivity (Cotton et al., 2010)
and do not capture the authenticity of the first encounter with
the object, the work was evaluated by observation, aided by field
notes on students’ contribution to the discussion.
When we approached the tree, I opened the exercise by offering a
few details on the tree’s distribution and past uses (see above), avoiding mention of aspects that might influence the students’ own observations and perceptions. I initiated and supported discussion with a
few guiding questions, as students on their own are unlikely to come
to conclusions that are equivalent to results generated in long-term
research (Hammer, 1997). The questions were intended to both trigger engagement with the object and induce hypothesis formation.
The first guiding question was “What does this remind you of?”
Not surprisingly, mostly large, roundish fruits such as apple, orange,
and even pomegranate were named. To students comparing it to an
apple, I cited the common name “hedgeapple.” To those likening it
to an orange, I pointed out that (apart from the obvious reference in
the name Osage-orange) an old synonym, M. aurantiaca, refers to this
similarity in shape and that the fruit was sometimes fraudulently sold
as a type of orange to unsuspecting customers (N.N., 1836). One student compared it to a strawberry, for its rugose textured surface. One
student saw a resemblance to breadfruit and was told that the plant
was, in accordance with the taxonomy, occasionally referred to as
“breadfruit’s cousin” (Thone, 1936). Several not-too-surprising nonbotanical comparisons were brought up. A student who, in jest, compared the fruit to a “brain” for its furrowed surface was told that it
is indeed sometimes called “monkey brains” (Weeks et al., 2010,
p. 300). Students who likened it to horse droppings were quoted lines
by poet Ernest Kroll (1914–1993), who saw “The Osage orange drop
its hideous fruit/ Upon the cracking walk like a green / Excrement. . .”
(Kroll, 1955, p. 57).
When asked to describe the fruit’s scents, students mostly
struggled for words that would fit and typically just referred to a
“citrus-like” note or chose “green” and “fresh” as descriptions. All
perceived it as a pleasant aroma.
To draw attention to the problem of dispersal, I pointed out
that all the fruit were found within a rather short radius around
the foot of the stem and asked the students how this plant might
disperse in nature to avoid intraspecific competition between
seedlings and the parent tree. “Animals” was an obvious answer.
One student offered “water,” which is plausible as an alternative,
though probably not very efficient, way of dispersal.
Finally, I asked the students which animal they thought might be
a likely disperser, considering the size and firmness of the fruit.
I pointed to the story of the praedicta moth as a case in which the mor-
phology of a plant gave a clue to the morphology of an elusive animal.
Suggestions included “deer,” “roe deer,” “cattle,” “horses,” “elephants,”
“elk,” “moose,” “large birds,” “mice,” “squirrels,” “wolves,” and “bears.”
One student jokingly offered “dinosaurs.” To narrow the list down to
potential dispersers, I asked the group to test the individual hypothe-
ses using their knowledge of the respective animals and considering
the morphology of the fruit. In this way, for example, the wolf
hypothesis was dismissed because that animal is carnivorous. The
rodent hypothesis was rejected because mice are likely to eat and thus
destroy the seeds. The same reasoning excludes most birds, which are
also likely to eat the seeds; moreover, the fruit lacks the bright colors
of typical bird-eaten fruits. Many large birds that might be able to
carry away the fruit, such as eagles or vultures, are carnivorous.
Studying the fruit, it appears that an ideal disperser would
swallow it without too much chewing. This excludes further candidates. It is too big for deer to swallow. Native equids of the Americas are extinct, and Old World horses and donkeys – potential
“surrogate species” – were brought across the Atlantic only in the
early 16th century. In addition, the seeds do not survive the equine
gut passage well (Boone et al., 2015). On the mention of dinosaurs,
I commented that this hypothesis could not be rejected on the basis
of animal anatomy, but solely in light of the timescales. As for the
elephant, I agreed that the fruit had the shape and size of tropical
“elephant fruits.” Also, elephants have a rather inefficient digestion,
and Maclura seeds were shown to survive passage through their
gut. The sound of dropping Osage-oranges may also be thought
of as similar to that of some large elephant fruits that cause a quite
perceivable sound and thus, like a “talking drum,” call in the disperser. However, it had to be noted that elephants were few and
far between in the wild in the Americas.
Eventually a student, half in earnest, suggested “mammoth.”
Students were invited to search for a reason to reject this hypothesis. None was found. I further reminded them that there were
a number of very large herbivorous and frugivorous animals in
Pleistocene America. I added, in the words of Wood (2003,
p. 228), that “the osage orange evolved in a world of club-tailed
glyptodonts, shovel-tusked gomphotheres and imposing giant
ground sloths who consumed it with gusto and propagated its
To understand present-day ecosystems in most parts of the world
requires awareness of the fact that, as phrased by Alfred Russel Wallace (1876, p. 150), “we live in a zoologically impoverished world.”
On the question of why the tree was, with its disperser gone, not
extinct now, I referred back to the much-reduced post-Pleistocene
and pre-Columbian distribution and the more recent expansion due
to human use. I also added that the time span of about 10,000 years
since extinction of the putative megaherbivore mutualist was too short
for evolutionary changes in a tree that can propagate vegetatively and
whose lifetime can span a few hundred years. Also, I pointed out that
changes in population structure and population genetics as a result
of extinction of dispersing animals are understood to be an ongoing
process (Guimarães et al., 2008) and mentioned that in the recent