快猫短视频

ELEPHANTS have a set of lungs unique among mammals, but for 300 years no one
has known why. Now we have an answer: they are ideal for snorkelling.

Mammals have a fluid-filled space between their lungs and chest wall known
as the pleural cavity. The cavity lets the lungs expand uniformly like a balloon
within the irregularly shaped chest. But elephants don鈥檛 have a pleural cavity.
Instead, their lungs are surrounded by loose connective tissue. This feature was
first noted in 1682, but has never been satisfactorily explained.

Now John West, a physiologist from the University of California, San Diego,
says the anatomical quirk is an adaptation to life under water. He presented his
theory last week at the International Union of Physiological Sciences conference
in Christchurch, New Zealand.

Elephants can stay below the surface of the water by breathing through their
trunks. If elephants had a pleural cavity, this behaviour would create a large
pressure difference between the interior of the lungs, which is open to
atmospheric pressure, and the interior of fine blood vessels within the
membranes lining the cavity. These vessels would be squeezed by higher pressure
from the surrounding water. The pressure on the body of an elephant swimming 2
metres under water would be about 20 per cent greater than inside the lungs,
says West. This pressure gradient is steep enough to burst the small blood
vessels in the pleural membrane, which could be fatal.

So evolution dealt with this problem by replacing the pleural membranes with
layers of connective tissue minus the fine blood vessels. The loose tissue
allows the lung to slide and expand against the chest wall while the elephant is
snorkelling.

The explanation fits neatly with the idea that elephants were once water
babies. Molecular evidence suggests elephants are closely related to manatees
and dugongs, and recent palaeontology studies imply that elephants evolved from
aquatic creatures to land lovers
(快猫短视频, 15 May 1999, p 25).
Elephant fetuses also have kidney structures, called nephrostomes, which are
found in aquatic vertebrates but not land animals.