Crocodiles are the closest living relatives
of dinosaurs which perished
about 65 million years ago, says Nutan Shukla
Reptiles that
are useful to man
THE expression crocodile tears is
commonly used to denote the false display of emotions in
human beings. In fact, tears do stream down the
crocodiles eyes though we can be reasonably sure
the cold-blooded creature feels no twinge of emotion. In
fact, tears are the work of salt glands which work to rid
the animal of excess salt as it does not sweat like we
do. For many years it was believed that this was the only
method by which crocodiles secreted salt.
Now comes the discovery
that salt is also discharged along with the
crocodiles tongue fluid.
Australian biologists
Laurence Taplin and Gordon Grigg were the first to
confirm the existence of salt glands in the tongues of
crocodiles. They washed the mud and brine of 29 salt
water crocodiles, rinsed their mouths and dried their
membranes. They then administered shots of methacholine
chloride into their bellies and later siphoned the
secretions that welled up from their tongues which
confirmed the presence of salt.
Crocodiles are the closest
living relatives of dinosaurs which perished about 65
million years ago. Adult crocodiles carry stones,
weighing approximately one per cent of their total body
weight, in their stomaches.
It is believed that the
stones act as a ballast, enabling the crocodile to adopt
the familiar, almost submerged, log-like pose. The stones
may also serve another purpose. These creatures have a
two-chambered stomach, the larger part of which is very
muscular and usually contains a few kilos of pebbles.
Crocodiles cannot chew,
and so swallow their food in large lumps. As the stomach
churns, the action of the pebbles help grind the food,
facilitating efficient digestion.
Crocodiles, universally
loathed by humans, are, in fact, very useful to man.
These large water-living reptiles keep waterways clean by
feeding on decomposed carcasses, in addition to their
diet of fish. Though their fearsome-looking jaws seem
powerful enough to take a chunk out of almost any animal,
the truth is crocodiles must rip the meat off their prey
thrusting their whole bodies underwater after drowning
their victims.
Fisherman once killed
crocodiles because they thought the reptiles ate
prodigious quantities of fish and were competing for
catch. It is now known that in spite of its bulk, a
crocodile possibly consumes even less fish than a stork !
The secret lies in the reptiles
"cold-blooded" fuel-efficiency. As it uses
solar energy to warm its body, it consumes very little
food.
These creatures are
exclusively adapted to an aquatic existence, and are
generally found in lakes, rivers, swamps and estuaries.
They are divided into two families, namely, Crocodylidae
(crocodiles, alligators and caymans) and Gavialidae (with
one member, the Indian gharial).
While, the families
display similar habits, they are externally quite
different. True crocodiles, caymans and alligators all
have enlarged fourth teeth in their lower jaws. In true
crocodiles, these theeth are visible even when their
mouth is closed.
In caymans and alligators,
however, the teeth remain hidden. The gharials most
distinctive feature is, obviously, its long, slender
snout and males have ghara shaped (pitcher or
pot-like protuberance) nostrils because of which the gharial
has got its name.
All crocodiles possess
raised nostrils which allow them to breathe with the rest
of their body submerged. They can also feed while
submerged, thanks to an air passage with a valve, which
can exclude water from the windpipe.
Crocodiles are first
recorded from Triassic rocks (from 225 million years to
40 million years ago) and were the only "ruling
reptiles" to survive the Mesozic Era (It lasted 160
million years, beginning with the Triassic 225 million
years ago and ending 65 million years ago at the start of
the Tertiary).
Crocodile females are
protective mothers. However, maternal instincts are more
likely to be associated with mammals than with reptiles.
After mating is over she will carefully select a nest
site preferably near water, in the shade of some
overhanging vegetation.
Using her powerful
forelimbs she will now excavate a pit into which around
50 eggs may be laid. Then earth is thrown over the eggs
and for the approximately three-month incubation period
she will zealously guard her eggs from predators such as
mongooses, lizards and birds.
It is only the saltwater
crocodile which makes a nest. When it is time for the
female to lay eggs, she sweeps up leaves, twigs and earth
with her tail to build a dome-like nest. The nest may be
as much as 1 metre high and 2 metres wide at the base in
which she lays her eggs.
|