Biology Reference
In-Depth Information
controversial hypothesis formulated by Dan Janzen, one of ecology's leading thinkers, and
Paul Martin, a world-renowned paleontologist. They dubbed their idea “the megafaunal fruit
syndrome,” and we speculated that it might explain the phenomenon of latrine groves. Janzen
and Martin proposed that the now extinct New World tropical megafauna—ground sloths,
elephant-like creatures, prehistoric horses, and other giants of the Pleistocene—once played
a major role in the dispersal of the woody flora. They argued that the long coexistence of
neotropical plants and large mammals that ate large fruits influenced the evolution of fruit
and seed traits of some plants so they would be consumed and dispersed by giant mammals.
What is a megafaunal fruit? The short answer: picture an avocado or a mango. In theory,
such a fruit would be eaten by big mammals with large mouthparts that could easily handle
fruits too big to swallow and too hard for smaller contemporary fruit eaters—birds, bats, ro-
dents, monkeys—to crack open. The classic example is a dull-colored large fruit that shields
its big seeds in a thick shell or rind that is hard when the fruit is ripe. Presumably, coevolu-
tion occurred because giant mammals provided a better vehicle for dispersing the seeds of
these plants than did small fruit eaters or gravity alone. Those plants that created fruits with
features attractive to big mammals received more attention and better dispersal to safe ger-
mination sites. The mammals received a highly nutritious and concentrated meal from the
pulp surrounding the seeds, which they digested before passing most of the seeds intact. Such
efficiency in feeding for the big mammals and distant dispersal away from the parent tree
helped to cement this relationship—an evolutionary pas de deux—between two seemingly
unlikely partners. For the rhinos of Chitwan,
Trewia
fruit fit the bill as a megafaunal fruit.
Trewia
is the most common tree in the forests bordering the Rapti River, but no arboreal or
flying creature touched its ripe offerings. Instead, these large green fruits, about the texture
and size of young apples, carpeted the forest floor, and the rhinos vacuumed them up.
To test the hypothesis that the dominant plants that had evolved in Chitwan were those
not only eaten but also dispersed by megaherbivores, I had to look at the situation with and
without the big wild mammals to assess the strength of this interaction. In the absence of rhi-
nos, cattle, deer, or water buffalo—the large fruit eaters—uneaten fruits pile up and rot, their
seeds are attacked by bright red insect seed predators called largid bugs, and there is almost
no germination. But what we had documented so far was the importance of large herbivores
to fruit removal. We didn't know anything about seed dispersal.
To find answers, we needed a volunteer rhino to observe. Sunder Shrestha offered us Kali,
the one-horned rhino at the National Zoo in Kathmandu. We drove up to Kathmandu on sev-
eral occasions to conduct feeding trials. Kali was a willing participant, even though she had
probably never tasted a
Trewia
fruit because she had been taken to Kathmandu as a calf. The
fruits were placed on a ledge in her enclosure so she could accept or reject them. During the
first trial, using her prehensile upper lip as a rake, she ingested all 114 of the fruits placed
