For Elephants, It’s Not Just Their Ears And Trunk That
Make Them Unique On Land
For the majestic pachyderm, the real story is what
happens when they are under water
San Diego, CA – Each in their own way, Disney’s
Dumbo, Tarzan’s Timba, and Barnum’s Jumbo captured the public’s imagination
during their era. In San Diego, famous pachyderms include Empress and
Queenie, the first elephants at the San Diego Zoo, brought to American in
1923 by the famous hunter Frank Buck. The elephant is the largest of all
terrestrial mammals, as well as the symbol for the political party of the
current White House. Despite so much versatility, some in the public do not
realize that the elephant is also the only land-based mammal that can remain
far below the surface of the water while snorkeling.
Snorkeling Elephants
What the overwhelming majority of the animal’s numerous
fans are unaware of is that the elephant is the only mammal whose pleural
space, the potential space between the lung and
the chest wall, is obliterated by connective issue.
What is the connection between these two unique
elephantine attributes? Some recent studies have some scientists believing
that elephants have an aquatic ancestry and these physical and behavioral
characteristics are the result of evolution. This theory has not been
proven.
The Presentation
There is, however, a physiological reason for the lack
of space around an elephant’s lungs that may support the evolutionary
hypothesis of the animal’s origin. Explaining why nature works the way is
does is one of America’s most renowned respiratory physiologists.
John B. West, of the University of California, San
Diego’s Department of Medicine. Dr. West is the author of “Why Doesn’t the
Elephant Have A Pleural Space,” and “Snorkel Breathing in the Elephant
Explains the Unique Anatomy of the Pleura.” He will be discussing his
research at the upcoming meeting of “The Power of Comparative Physiology:
Evolution, Integration and Application” an American Physiological Society (APS)
intersociety meeting being held August 24-28, 2002, at the Town & Country
Hotel, San Diego, CA.
In his presentation Dr. West will highlight the
following:
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The anatomy of the elephant: Dr. West has
attended three elephant autopsies and confirms that the animal’s two
pleural layers are firmly adherent although it is possible to separate
them with some difficulty by pushing a finger or blunt instrument through
the soft connective tissue joining the two layers. No satisfactory
explanation has been advanced for the peculiar anatomy of the elephant
pleural space.
-
Snorkeling behavior in the elephant:
Snorkeling at any substantial depth creates very large pressures around
the animal’s lung. In spite of these very large pressures around the
lungs, alveolar pressure is essentially atmospheric, because the lung is
connected to the atmosphere by the open proboscis, or trunk. The result is
that the systemic vascular pressures are very high, whereas the pressure
inside the thoracic cavity remains low. This inequality of pressures has
dramatic effects at the interface between the lung and the rest of the
body that is in the pleura. Other mammals such as humans cannot survive
anything like this degree of ‘‘negative pressure breathing’’ (one reason
why it is impossible to buy a snorkel longer than about 30 centimeters).
-
Vulnerability of the pleura during snorkeling:
During snorkeling, all of the elephant’s body tissues including the
head, neck, chest wall, abdomen and limbs are exposed to a high pressure
because of immersion in the water. The only exception is the lung, because
it is connected to the air by a tube. Consequently, a region just
outside the lung has a large pressure differential. This is the reason
why the pleura is so vulnerable. The structure apparently at greatest
risk is the parietal pleura, which lines the thoracic cage and diaphragm.
Since the venous pressure in the systemic system must be very high in the
snorkeling elephant (150 mmHg if the tissue is 2 meters below the
surface), the microvessels of the parietal pleura must have an even higher
pressure inside them.
-
The problem is highlighted by the small blood vessels in
the elephant’s parietal pleura. In a normal mammal, such as a sheep,
there is a single layer of mesothelial cells, and the microvessels are
very close to the pleural surface. The transudate or fluid from these
microvessels enters lacunae and is then discharged into the pleural space,
thus providing the lubricant for the two pleural surfaces so that they can
slide over each other.
-
It is easy to see that this anatomic arrangement would be
impossible in the snorkeling elephant. The microvessels of the parietal
pleura are supplied from the systemic circulation, in which the venous
pressure exceeds 150 mmHg Therefore, the pressure inside the pleural
microvessels must exceed that. However, the pressure in the pleural space
(if one exists) will be very close to alveolar pressure (that is,
atmospheric), only differing from this by the elastic recoil of the lung.
In other words, the microvessels of the parietal pleura will have a
transmural pressure approaching 150 mmHg. Clearly, they would either
rupture or the great imbalance would cause an excess flow of fluid.
-
How evolution answered the problem: Nature’s
answer was to replace the delicate pleura with dense connective tissue.
The visceral pleura is also at risk and thickened. A layer of loose
connective tissue allows some sliding of the two pleural surfaces. With
this anatomic arrangement, the normal production of pleural fluid to
lubricate the surfaces no longer exists. However, in the elephant, the
layer of loose connective tissue between the two dense connective tissue
plates is very extensible; therefore, some sliding of the two pleural
surfaces across each other can occur.
Conclusions
As a result of his work, Dr. West has made a number of
observations. Among them is the fact that the primary evolutionary response
to the vulnerability of the pleural membranes is to replace these with
plates of dense connective tissue. The obliteration of the pleural space by
loose connective tissue, which is the most obvious anatomic peculiarity
appears to be a secondary response brought about because the normal
mechanism for providing lubricating fluid for the pleural membranes no
longer exists, and there is apparently some advantage in allowing the
membranes to slide over each other.
He also notes that the mechanical problem facing the
elephant’s diaphragm during snorkeling. This is because the pressure
difference across the diaphragm is about 150 mmHg, and this has to be
sustained for many minutes while the animal is walking under the surface of
a river or lake. This is a far greater trans-diaphragmatic pressure than can
be sustained by the human diaphragm.
There are relative pressure changes that occur when the
elephant raises water in the trunk for drinking or washing. For example, if
f the water is raised through 200 cm, the alveolar pressure must be 200 cm
H2O below atmospheric pressure, and the relative pressures are essentially
identical to those during snorkeling. Since the elephant can raise water in
its trunk with its mouth open, it is known that the low pressure is not
developed by the buccal (near its cheek) muscles. It could be argued that
this behavior, which is frequent in the elephant, is an additional
evolutionary pressure for the anatomic changes in the pleural space.
However, raising water in the trunk only takes a few seconds, whereas
snorkeling lasts for many minutes and is therefore a much greater potential
problem. As indicated earlier, the snorkeling behavior may have developed
when the animal lived in water, and this would provide a very strong
evolutionary pressure.
Finally, Dr. West notes that
many evolutionary biologists believe that the elephant’s ancestors were
aquatic, i.e., lived in the water, and the trunk may have developed at that
time. There is evidence that the elephant’s closest living relatives are the
manatee and dngong.
-end-
The
American Physiological Society (APS) is one of the world’s most prestigious
organizations for physiological scientists. These researchers specialize in
understanding the processes and functions by which animals live, and thus
ultimately underlie human health and disease. Founded in 1887 the Bethesda,
MD-based Society has more than 10,000 members and publishes 3,800 articles
in its 14 peer-reviewed journals each year.
***
EDITOR’S NOTE: For further
information or to schedule an interview, contact Donna Krupa at 703.967.2751
(cell), or by email at djkrupa1@aol.com.
