A Test of Potential Pleistocene Mammal Seed Dispersal
in Anachronistic Fruits Using Extant Ecological and
Physiological Analogs
Madison J. Boone, Charli N. Davis, Laura Klasek, Jillian F. del Sol, Katherine Roehm, and Matthew D. Moran
Southeastern Naturalist, Volume 14, Issue 1 (2015): 22–32
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M.J. Boone, C.N. Davis, L. Klasek, J.F. del Sol, K. Roehm, and M.D. Moran
2015 Vol. 14, No. 1
22
2015 SOUTHEASTERN NATURALIST 14(1):22–32
A Test of Potential Pleistocene Mammal Seed Dispersal
in Anachronistic Fruits Using Extant Ecological and
Physiological Analogs
Madison J. Boone1, Charli N. Davis1, Laura Klasek1, Jillian F. del Sol1,
Katherine Roehm1, and Matthew D. Moran1,*
Abstract - Using Elephas maximus (Asian Elephant) and Equus ferus caballus (Domesticated
Horse) as ecological analogs to extinct Pleistocene mammals, we tested the effect of
gut passage on 3 proposed anachronistic fruits: Diospyros virginiana (American Persimmon),
Maclura pomifera (Osage Orange), and Asimina triloba (Paw Paw). We found that
elephant-gut passage of persimmon seeds increased their germination success and decreased
their time to sprout, while Osage Orange seeds showed no benefit to gut passage. Neither
American Persimmon nor Osage Orange seeds survived gut passage through horses. Both
mammals refused to consume Paw Paw fruits. Assuming a similar physiology and behavior
compared to our modern analogs, we suggest that extinct North American elephant species
could have been important seed dispersers for American Persimmons but were unlikely to
be effective for Osage Orange or Paw Paw, while horses would have been poor dispersers
for all plant species tested.
Introduction
An anachronistic fruit is defined as one that lacks any apparent seed-dispersing
mechanism (Barlow 2001). These fruits tend to be large and fleshy with high nutritional
quality, characteristics that make them likely candidates for dispersal by
endozoochory, in particular dispersal via ingestion by large vertebrate animals
(Janzen 1982). Between 10,000 to 12,000 years ago at the end of the Pleistocene
epoch, many lineages of North American mammals went extinct. This extinction
event included 35 genera and was particularly severe among large mammals (over
100 kg; Faith and Surovell 2009). Researchers speculate that these megafauna may
have had profound ecological effects (Johnson 2009), including serving as seed
dispersers for many species of plants (Barlow 2001, Janzen 1982, Peterson 1991).
Although North America still has a diverse assemblage of large mammals that could
be potential seed dispersers, most survivors are either ruminants or carnivores.
Today, there are no large, native, non-ruminant herbivorous mammals over most
of the temperate portions of the continent (Pecari tajacu, L. [Collared Peccary]
ranges into small parts of temperate North America). Ruminants, since they chew
their cud and have a fermenting stomach, destroy most seeds they consume and are
therefore likely poor potential seed dispersers (Barlow 2000, Cosyns et al. 2005,
Prasad et al. 2006; but see Janzen 1982). Carnivores typically have guts designed to
1Department of Biology, Hendrix College, 1600 Washington Avenue, Conway, AR 72032.
*Corresponding author - Moran@hendrix.edu.
Manuscript Editor: Scott Markwith
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rapidly process food and with harsh conditions to defend against pathogens (Janzen
1977, Rosenblatt et al. 2014, Stevens and Hume 2004), so while they occasionally
supplement their diet with fruit, they may not be ideal seed dispersers. We suggest
that large non-ruminant herbivorous mammals would have been much better
Pleistocene dispersers, much as they serve that role in locations where they survive
today (Donatti et al. 2007, Henry et al. 2000, Janzen and Martin 1982).
While it is impossible to assay the effects of extinct animals’ digestion on seeds
from anachronistic fruit, two formerly prominent and widespread North American
mammals, Mammut americanum Kerr (American Mastodon) and Equus spp. (North
American horse), have living analogs. Elephas maximus L. (Asian Elephant) is
related (diverged about 27 million years ago; Shoshani et al. 2007), and therefore
presumably physiologically and behaviorally similar to the American Mastodon
(Aguirre 1969, Thomas et al. 2000). Equus ferus callabus L. (Domesticated Horse)
was derived from Asian horse ancestors in recent times. That lineage diverged from
the North American horse about 5 million years ago (Weinstock et al. 2005), has
similar tooth morphology (Wang et al. 1994), and presumably still resembles its
evolutionary relative in physiology and behavior (Janis 1976).
Some North American examples of proposed anachronistic fruit include Diospyros
virginiana (L.) (American Persimmon), Maclura pomifera (Raf.) (Osage
Orange), and Asimina triloba (L.) (Paw Paw). Researchers have argued that these
fruits have characteristics—either 4–10 cm in length with up to 5 large seeds
(American Persimmon and Paw Paw) or greater than 10 cm with numerous small
seeds (Osage Orange) (Guimaraes et al. 2008)—that are adaptations for megafaunal
dispersal (Barlow 2000). At the time of European settlement, these species
appeared to have contracted ranges (Berry 1916, Burton 1990, Murphy 2001, Skallerup
1953), perhaps because they were not able to recolonize their former territory
as the glaciers retreated at the end of the last ice age. Previous studies examined
the ability of Canis latrans Say (Coyote) and Procyon locor L. (Raccoon) to disperse
American Persimmon and Paw Paw seeds (Cypher and Cypher 1999, Roehm
and Moran 2013). Seeds collected from Coyote scat germinated at the same rate as
whole fruit and dissected-seed controls but showed significant reduction in plant
quality (Roehm and Moran 2013). Seeds ingested by Raccoons had significantly
higher germination rates than controls (Cypher and Cypher 1999), but there was
no measure of plant quality in that study. While researchers have speculated on potential
seed dispersers for Osage Orange, and anecdotal evidence indicates horses
may be dispersers (Schamback 2000), no direct investigations have been published
addressing this species. We cannot completely discount, and it has been suggested,
that human populations in North America selected for large fruits in more recent
times (12,000 years ago to present; Peterson 1991), but there is little evidence for
cultivation of these 3 species, and Osage Orange is not edible to humans.
In this study, we fed the 3 anachronistic fruits to Asian Elephants and Domesticated
Horses and collected seeds from dung. We grew these collected seeds in a
common garden experiment with appropriate controls and measured germination
success, days to sprout, and plant quality.
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Materials and Methods
Fruits from American Persimmon and Osage Orange were collected from the
ground under several trees in Faulkner and Conway counties in central Arkansas
during mid- to late October and late October, respectively (2012 and 2013 for
American Persimmon: 35 08'46"N, 92°53'54"W and 35°08'09"N, 92°29'25"W;
2011 and 2012 for Osage Orange: 35°14'48"N, 92°28'25"W). Fruits of Paw Paw
were purchased from Earthy Delights in Okemos, MI, in late September 2013.
The fruits were fed to 2 female Asian Elephants on Riddle’s Elephant and Wildlife
Sanctuary in Guy, AR, and 2 Domestic Horses (quarter horse and thoroughbred
mix) from a private ranch, also in Guy, AR.
For the American Persimmon experiments, 40 fruits were fed to each elephant
on 2 separate occasions: 16 October 2012 and 31 October 2013. Twenty fruits were
fed to each horse during a single trial on 31 October 2013. For the Osage Orange
experiments, the elephants were each offered 1 Osage orange fruit on 25 October
2012 in a preliminary feeding trial to determine if they would eat the fruits. Anecdotal
information (Schambach 2000) and personal observations had indicated
that horses would consume Osage Orange fruit. For the official seed-collection
experiments for Osage Orange, elephants were offered 2 fruits each on 29 October
2012, and the horses were offered 1 fruit each on 11 November 2011. For the Paw
Paw experiment, we offered 15 fruits to each of the elephants and 10 to each of the
horses on 27 September 2013, but none would consume the fruit. We attempted the
Paw Paw feeding with 5 additional horses and 1 Equus africanus asinus L. (Donkey)
on the same date. In all trials, we fed horses fewer fruits compared to elephants
because of their smaller size and concerns for their gut sensitivity to unusual foods.
After feeding, both elephants and horses were kept in enclosures so that all their
dung could be collected. We periodically removed the dung from the enclosures
and searched it carefully to recover any intact seeds. Dung was collected for 54
hours, well past the average time for gut passage in elephants (Hackenberger 1987)
and horses (Van Weyenberg et al. 2005). We monitored dung for an extended time
(14 days) in the 2011 feeding trial of Osage Orange to horses. Intact seeds were
removed and stored temporarily (i.e., less than 3 days) in the refrigerator at 3 ºC.
We then grew recovered seeds with appropriate comparison groups in a common
garden experiment in a greenhouse. For the American Persimmon experiment, we
established 4 treatments: 3 digested seeds per pot with 20 g of elephant dung (WD;
n = 26), 3 digested seeds per pot without dung (WO; n = 26), 3 manually dissected
seeds per pot controls (DS; n = 30), and whole-fruit controls (WF; n = 55). For the
Osage Orange experiment, we established 3 treatments: one animal-digested seed
per pot without dung (WO; n = 94), 1 manually dissected seed per pot controls
(DS; n = 50), and whole-fruit controls (WF; n = 5). Because of the relatively small
number of seeds recovered from the Osage Orange, the refusal of elephants to consume
more fruits, and the subsequent limited amount of replication possible, we
did not plant a treatment group with elephant dung. Seeds were planted 1 cm deep
in 200-ml plastic pots filled with GardenPlusTM all-purpose potting soil. Because
of their large size, the Osage Orange whole fruits were planted in 1000-ml pots.
We consider whole fruit treatments to be the most natural in representing uneaten
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fruits in the wild (Roehm and Moran 2013, Samuels and Levey 2005). However,
the manually dissected seed controls are also important since these comparisons
differentiate effects of digestion from effects of removal from fruit. Many fruits
have growth inhibitors that delay sprouting (Robertson et al. 2006), so the manually
dissected control removes that confounding variable. Since Paw Paw was not
consumed by either herbivore, there were no further experiments performed with
that species.
After planting, the pots containing American Persimmon and Osage Orange
seeds and fruits were placed in a refrigerator at 3 ºC for 60 days to cold-stratify the
seeds, a requirement for both species to germinate (Burton 1990, Halls 1981). After
cold-stratification, the seeds were placed in a greenhouse, watered as needed, and
observed every day for 96 days. We determined germination success and emergence
times using the first day that a sprout was observed above the soil. At the end of
the experiment, the seedlings were cut at soil level, dried for 24 hours at 50 ºC, and
weighed to determine aboveground dry biomass. To determine whole-fruit treatment
germination success, we dissected the fruits from the pots to measure how
many seeds had not sprouted.
We analyzed germination success by chi-square analysis. Since the measure for
the chi-square analysis was germination success (positive or negative) and some
treatments had multiple seeds per pot, there was inevitable pseudoreplication. We
calculated mean seedling emergence time and mass per pot to avoid pseudoreplication
(although many seeds came from the same fruit, particularly the Osage Orange
experiments) and then analyzed these data by one-way ANOVA, followed by a
Tukey post-hoc test if significance was found.
Results
For the American Persimmon trials, both horses and elephants eagerly consumed
the fruits. In the elephant trials, all but 1 fruit was consumed (one was
dropped by the elephant and perhaps unnoticed), while in the single horse trial, all
were consumed. From the estimated number of seeds in persimmon fruits (Roehm
and Moran 2013), we recovered 36.4% of seeds fed to the elephants (126 of the
346 estimated number of seeds). Germination success was significantly affected by
treatment, with elephants seeds planted in dung (WD), elephant seeds without dung
(WO), and manually dissected seeds (DS) all sprouting at significantly higher rates
compared to whole fruit (WF) controls (Table 1). No intact seeds were recovered
from the horse trials, although we did recover numerous fragments.
Persimmon seeds that had passed through elephant guts had generally faster
sprouting times than those that did not (one-way ANOVA: F3, 86 = 7.09, P < 0.001;
Table 1. Proportion of seeds sprouting for two species of anachronistic fruits fed to elephants. WD =
seeds with dung, WO = seeds without dung, WF = whole fruit control, and DS = manually dissected
seeds control. Letters indicate significantly different groups
Species WD WO WF DS χ2 P
Diospyros virginiana 0.53a 0.54a 0.200b 0.61a 67.25 less than 0.01
Maclura pomifera n.a. 0.18a 0.001b 0.63c 751.42 less than 0.01
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Fig. 1A). Mass of the plants at the end of the experiment was not significantly
different between treatments (one-way ANOVA: F3, 86 = 0.53, P = 0.63; Fig. 1B),
although there was a trend for lower final mass for the whole-fr uit controls.
For the experimental feeding of Osage Orange, the 2 horses readily and eagerly
consumed the fruit during the 2011 experiment. However, we recovered no seeds
from the dung after monitoring the horses for an extended period (14 days), well
past the expected time for gut passage. Compared to horses, elephants reacted differently
to the Osage Orange fruit offerings. While both elephants consumed an
Osage Orange fruit during the preliminary trial, they appeared much less interested
in subsequent attempts. One elephant refused to consume the fruit at all while the
second elephant consumed 3 fruit, though she appeared somewhat disinterested
and required multiple offering attempts to convince her to ingest them. In further
trials, neither elephant would consume additional Osage Orange fruits. From the
one elephant that consumed the Osage Orange fruits, we recovered 94 intact seeds,
Figure 1. Effect of gut passage
on (A) days to sprout
and (B) seedling mass at the
end of the experiment for
the Diospyros virginiana
(American Persimmon).
Letters above bars indicate
significantly different
groups as determined by
Tukey post-hoc analysis.
WD = seeds planted with
elephant dung, WO = seeds
planted without elephant
dung, WF = whole fruit
control, and DS = manually
dissected seed control.
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which represents an estimated 11.1% recovery rate. Of these, only 18.1% sprouted,
compared to 62.5% that sprouted in the manually dissected control treatment (χ2 =
36.10, P < 0.001). For the 5 whole fruits that were planted, only 2 seeds sprouted
from the estimated 1542 seeds (Bonner and Karrafalt 2008), representing an extremely
low germination success (Table 1). Osage Orange seeds that had passed
through elephants and manually dissected seeds sprouted in the same amount of
time (Fig. 2A) and had the same final mass (Fig. 2B). Whole-fruit controls took
longer to sprout (F2,65 = 21.36, P < 0.001) and had lower mass (F2, 63 = 3.99, P =
0.023), but since we only had two replicate sprouts for that treatment (because of
extremely low germination success), this statistical result is of limited value.
Surprisingly, both elephants and horses refused to consume fruits from Paw Paw.
Both elephants tasted the fruits, somewhat reluctantly, but immediately dropped
them and refused to ingest them, even if put directly in their mouths. All 7 horses
Figure 2. Effect of gut
passage on (A) days to
sprout and (B) seedling
mass at the end of the experiment
for the Maclura
pomifera (Osage Orange).
Letters above bars indicate
significantly different
groups as determined
by Tukey post-hoc analysis.
WO = seeds planted
without elephant dung,
WF = whole fruit control,
and DS = manually dissected
seed control.
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and the 1 donkey eagerly attempted to consume the fruits but immediately dropped
them, after which they refused any further attempts. They also exhibited the flehmen
response after tasting the fruit.
Discussion
Our results indicate that the response of the seeds to ingestion depends upon
the fruit species and potential disperser species. We recovered a substantial fraction
of American Persimmon seeds from elephant dung. These seeds appear to be
affected positively by gut passage through elephants in that they had higher germination
success compared to whole-fruit controls. They also sprouted faster than
both whole fruit and manually dissected control groups. Seedling quality, measured
as mass of seedlings, was similar for all treatment groups. These results together
suggest that elephant ingestion effectively removes seeds from fruits, releasing
them from fruit inhibition (Robertson et al. 2006), without damaging the seedlings.
These results differ from previous experiments with Coyotes (Roehm and Moran
2013), whose ingestion does not increase germination success and damages seeds.
Racoons are also known to increase sprouting success (Cypher and Cypher 1999),
similar to our elephant results, although their effect on time to sprout and plant
quality is unknown.
The elephant results for Osage Orange differed from the persimmons. We recovered
proportionally one-third as many intact seeds from elephant dung as compared
to persimmons. Although the ingested seeds sprouted more successfully than whole
fruits (which basically failed to sprout), they had much lower success compared
to manually dissected seed controls. The time to sprout and seedling quality were
almost exactly the same between elephant-ingested seeds and manually dissected
controls. These results indicate that although it is important for the seeds to be
removed from the fruit, which apparently inhibits germination, elephant passage
damages the seeds severely enough that very few survive gut passage intact and
even fewer sprout.
Although horses readily consumed American Persimmon and Osage Orange, the
seeds of neither survived gut passage. Seed fragments were common in the dung
samples, so we conclude that horses masticate their food more thoroughly than elephants.
Some additional published evidence indicates that horses are particularly
destructive to seeds when feeding on fruits (Janzen 1982). Horses are known to
consume persimmons (Cummings et al. 1997), and anecdotal evidence indicates
that horses are very fond of Osage Orange fruits (hence the common name “horse
apple”; Schambach 2000). Authors have speculated that because of their apparent
affinity for Osage Orange fruits, horses coevolved to become important dispersers
(Barlow 2001). We observed that they indeed are very fond of the fruits, consuming
them much more eagerly than elephants. However, based on our data, horses do
not appear to be effective dispersers. Considering their destruction of all the seeds
in our trials and their apparent morphological and behavioral similarity to extinct
North American horses (Wang et al. 1994), we therefore suggest that Pleistocene
horses were also poor dispersers of the plant species we tested.
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Surprisingly, neither elephants nor horses would eat Paw Paw fruits. Both species
tasted the fruits but refused to consume them. The horses exhibited a flehmen
response, which although usually associated with sexual behavior, is also known to
occur due to odors they find objectionable (Saslow 2002). This result was surprising
given that Paw Paw fruits are known to be non-toxic and high in nutritional quality
(Peterson et al. 1982), though there is some evidence the skin may have toxic compounds
(Layne 1994). We currently have no explanation for the negative response
to the fruits by both of our test species.
Our results suggest that North American Pleistocene proboscidians (e.g.,
American Mastodon) could have been important seed dispersers for the American
Persimmon. The results also indicate possible coevolution, but results from living potential
dispersers (e.g., Raccoon; Cypher and Cypher 1999) show that the plant did not
necessarily coevolve exclusively with one disperser. This interpretation assumes that
extinct North American proboscidians were similar in behavior and physiology to extant
elephants. There is no opportunity to test this assumption directly, but published
data indicate that the American Mastadon was a generalist, feeding by grazing and
browsing (Green et al. 2005, Newsom and Mihlbachler 2006), and had similar digestive
function (Haynes 1993). Living elephants are known to target fruiting trees and
are well-known seed dispersers in their native habitats (Campos-Arceiz and Blake
2011), further supporting our hypothesis. Mastodons almost certainly traveled long
distances (Hoppe et al. 1999), which would have made them particularly good potential
dispersers compared to living species (e.g., raccoon). Fossil remains of mastodon
dung have contained evidence of the fruits and seeds of many species, including at
least one instance of American Persimmon (Newsom and Mihlbachler 2006).
It has been suggested that because of its extremely large size, fruit of Osage Orange
coevolved exclusively with elephants (Barlow 2001). However, we do not find
evidence in our experiments for that premise. Both elephants ate the Osage Orange
fruits during the preliminary feeding trials but appeared less interested in them in
further attempts (1 rejected them, and 1 ate them somewhat unenthusiastically).
Few seeds survived gut passage and even fewer sprouted. Osage Orange fruits have
an extremely large number of seeds, which some argue is an adaptation to ensure
some seeds survive animal dispersal (Guimaraes et al. 2008), and indeed a small
percentage did survive gut passage through the 1 elephant that would consume
them. However, the fruits do not appear particularly palatable since they possess
copious amounts of milky sap, a strong unpleasant smell, and a fibrous texture (Burton
1990). Conversely, the seeds are highly nutritious (Saloua et al. 2009), unlike
those of persimmons and Paw Paw, which are protected with toxins (Vines 1960,
Woo et al. 1999). Squirrels are known to feed heavily on the seeds of Osage Orange
(Korschgen 1981), but they are probably not effective dispersers since they likely
severely damage the seeds they eat and have not been observed caching any. We
therefore suggest that the Osage Orange fruit may actually be a protective structure
to deter herbivory and has not evolved to attract potential seed dispersers. How the
seeds are ultimately dispersed from the parent plant remains a mystery.
This study is important for understanding the ecological and evolutionary relationships
between extinct mammals and extant plant species. The high rate of recent
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North American large-mammal extinctions but low rate of plant extinctions may
have terminated numerous plant-animal interactions, with likely consequences for
distribution and abundance patterns and ecosystem function. Various conservation
groups have suggested a “rewilding” of temperate habitats across North America
and Eurasia with ecological analogs (e.g., Asian elephant in place of the American
Mastodon), and recent technological advances have even raised the possibility of
“de-extinction” of Pleistocene mammals with the goal of eventual reintroduction
to the wild. While there is much debate on the benefits and risks of such rewilding
programs (Donlan et al. 2006), it seems likely that research teams will proceed if
the technological hurdles can be overcome. If such experiments are undertaken, it
will be important for ecologists to understand the potential interactions of these
species with the native environment and how they will affect ecosystem function.
Acknowledgments
We wish to thank Heidi and Scott Riddle and the staff of Riddle’s Elephant and Wildlife
Sanctuary for providing access to their elephants. Christy Coker allowed us generous access
to her horses and farm. McKenna Raney assisted with the fieldwork for the project.
This research was supported by a BBB student grant to L. Klasek and the Hendrix College
Odyssey Program. Two anonymous reviewers provided valuable comments on an earlier
version of this manuscript.
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