Biology Reference
In-Depth Information
I continued to study this remarkable species for several more years in Nepal, and my fas-
cination with rhinos continues to this day, even as they barely hold their own or, in some
cases, hurtle toward extinction. In the 1700s millions of rhinos were alive, but by 2011 the
numbers of the five living species together totaled slightly more than 28,000 wild individu-
als. Most common are the 20,140 southern white rhinos, followed by the 4,840 black rhinos,
all in Africa. In Asia, there are 2,900 endangered greater one-horned rhinos. The Sumatran
rhino, of which there are about 250 left, and the Javan rhino, with a population of fewer than
50, are among the rarest mammals on Earth.
A species can have a narrow range and low density and yet, as we saw with birds of para-
dise, bowerbirds, and even Kirtland's warblers, can still be easy to spot if you look in the right
places. The same is true for three of the rhinos, the white, black, and greater one-horned. No
such “good viewing habitat” exists for the other two species. Would that statement have been
true two thousand or even fifty years ago? To a conservationist, the question might seem aca-
demic. They are rare now, so it is better to address the immediate, pressing issues of poaching
and habitat loss. Yet, to craft a longer-term strategy, biologists need to know under what con-
ditions rhinos might once have flourished, in order to understand how it might be possible for
them to flourish again.
A little paleontology greatly aids our understanding. The fossil record shows that rhinos
first appeared in the late Eocene epoch (35 million years ago), the earliest known rhinoceros-
like mammals having been described from fossil deposits in Asia, North America, and
Europe. They looked nothing like the giants of today. Instead, these primitive rhinoceroses
resembled early horses and tapirs, rhinos' nearest living relatives. They were diminutive and
delicate, were probably abundant, and lacked horns. Between the late Eocene and Oligocene
(about 33 to 23 million years ago) perhaps the most dramatic change was an increase in body
size from the ancestral forms to larger, more recent body types, a phenomenon frequently
observed in mammals. This is an example of Cope's rule—that species within a lineage tend
toward gigantism over evolutionary time. Another, more recent development was the appear-
ance on the rhino skull of a unique, boneless horn. Two traits helped the rhinos to spread
widely across the landscape. The evolution of broad feet with three toes allowed them to
adapt to marshy habitats and expand their range. Of greater ecological importance were in-
creases in the size and height (from the gumline) of high-crowned molar teeth to better handle
a diet of coarse grasses.
Growing so large was probably a defense against natural predators. After rhinos reached
a certain size, there were no longer any natural enemies large enough to kill any except the
very young calves. Rhinos filled every herbivore niche possible except the arboreal, although
the previously mentioned giant giraffe rhinoceros browsed the limbs of trees. The animal's
feeding strategy was to eat a great deal but process the bulk quickly in its giant digestive sac.
And being giant herbivores meant they were capable of making a living by eating lush, coarse
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