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| Pygmy Hippo - Zoe Bianchi |
The pygmy
hippopotamus and the common or Nile hippopotamus are thought to have evolved
from a common ancestor that lived some 10 million years ago. This common
ancestor belonged to a group of creatures known as anthracotheres –
semi-aquatic even-toed ungulates belonging to the order Artiodactyla. The hippo
is placed the same order as giraffes, cattle, goats, pigs, camels and
peccaries.
The Relationship Between Hippos, Anthracotheres and Whales
This
proposition has been supported by fossil comparisons, which show similarities
between the teeth and skulls of anthracotheres and the hippopotamus. For the
past 20 or more years, however, molecular studies have also suggested a
relationship between the hippo and whales. This has been something more
difficult to determine using fossil evidence, as the two groups superficially
bear no physical resemblance.
Recent
discoveries of ancient whale remains in Pakistan, however, have finally made a
physical connection between cetaceans (whales and dolphins) and artiodactyls.
In 2000, Professor Philip Gingerich and his co-workers uncovered the 47 million
year old remains of a whale ankle bone that clearly placed it in the
Artiodactyl order. The overall structure of the ‘leg’ bones of the whale also
suggested that it could use them to prop itself up on land for brief periods.
Indohyus and Raollids
This finding,
combined with immunological, blood protein and DNA evidence (due to the work in
1985 of Vincent Sarich, of Berkely University), appears to place the
hippopotamus as the closest living relative to the whale. The fossil species
Indohyus, an even–toed ungulate resembling a small deer, has even more in
common with whales, possessing structural similarities in the skull and ear
regions.
A research
team led by Dr. Hans Thewissen has also concluded from bone density and dental
studies that Indohyus was aquatic. The fact that Indohyus (a member of the
extinct order Raoellidae) was a vegetarian has also led them to surmise that
the carnivorous nature of whales evolved after they returned to the water.
Jean-Renaud
Boisserie and colleagues at the University of California believe the
anthracotheres to be a common ancestor of both the hippos and whales. Thewissen
and co workers , however, prefer the idea that the ancestors of whales (which
include Indohyus and other Raollids) belong to a side group that branched away
from other artiodactyl groups before the anthracotheres appeared.
How Closely Related Are Hippos, Horses and Pigs?
At any rate,
the ancestry of the hippopotamus seems clearer. Although the word ‘hippopotamus’
is derived from the Greek word meaning ‘river horse’, there is no direct
relationship between horses and the hippopotamus, as the ancient Greeks
believed. Horses in fact belong to the Perissodactyla order, consisting of
odd-toed ungulates with more simplistic digestive systems than the
artiodactyls. Other perissodactyls include the rhinoceros and the tapir.
While pigs and
peccaries are indeed artiodactyls, they are not as closely related to the
hippopotamus as scientists have previously thought. Most current researchers
now combine DNA and fossil evidence to derive evolutionary relationships
between different species. As a result, most, including Jessica Theodor and
Jonathan Geisler of the University of Calgary, have concluded that hippos are
in fact more closely related to whales than to pigs.
The Relationship Between the Pygmy Hippo and the Common Hippo
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| Pygmy Hippo |
Although both
are aquatic, the pygmy hippo is nocturnal and less gregarious than the Nile
Hippo, and possesses less webbing between the toes. It is also much smaller in
size (having a mass of between 180-720 kg as opposed to up to 2000 kg in the
case of the common hippo), with a different diet.
Pygmy hippos
are in fact threatened with immediate risk of extinction, due to a combination
of hunting and habitat destruction in Sierra Leone and other areas of Western
Africa. Thankfully, they are breeding quite successfully in captivity, which
may help to ensure these unusual artiodactyls, along with the common hippos,
remain with us for a little longer.
Sources
•
Arbor, A., 2001, ‘New Fossils Suggest Whales and Hippos are Close Kin’,
ScienceDaily.com. Accessed May 15, 2011.
•
Celeskey, M., 2010, ‘Indohyus and Cetacean Relations’,
VladimirKorsakov.Blogspot.com. Accessed May 15, 2011.
•
Robinson, P.T., ‘River Horses and Water Cows’, University of California,
San Diego, ML.Duke.edu. Accessed May 15, 2011.
•
Sanders, R., 2005, UC Berkeley, French Scientists Find Missing Link
Between the Whale and its Closest Relative, the Hippo’, ScienceDaily.com.
Accessed May 15, 2011.
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