Hyper-sensitive hunters
2002-05-16 10:47
Paris - Alligators and crocodiles are snappy hunters thanks to
hyper-sensitive jawline stubble that detects the slightest ripple
in the water from potential prey, a biologist says.
The dome-like bumps, about the size of a nail on an adult's
little finger, are dotted along the creature's top and bottom jaws, the University of Maryland's Daphne Soares says.
The knobbly organs are connected to a hyper-sensitive bundle of
nerves and covered with a thin layer of skin that is only half as
thick as the hide on the rest of the body.
Soares conducted tests with the Mississippi alligator to see
just how fine-tuned the sensors were to a disturbance in the water.
The alligators were able to locate the position of a single
droplet that fell into their lab tank, turning their heads and
lunging at the site in the hope that it was food.
However, the sensors only worked when the reptiles were lying
with their jaws lying along the waterline, with the top part of
their head in the air and the bottom part under the water.
When their heads was fully out of the water, or completely
underneath it, the creature was unable to turn its head towards the disturbance.
Soares got confirmation of her theory by covering up the sensor
spots with a thick layer of plastic jelly, and the alligators were unable to detect any ripples at all.
Reporting her work in Thursday's issue of Nature, the British
science weekly, Soares says the sensors, which she calls dome
pressure receptors (DPR), seem to work by detecting minute
pressures in a wave.
All of the 23 species of crocodilians alive today have DPRs, and
the distinctive, spotty markings in the reptile's jawbone shows that their prehistoric ancestors had them, too.
That is significant, because it shows that the detection system
has enabled these creatures to survive more than 65 million years.
Ancient survivors
"Crocodilians are an ancient monophyletic group that has existed long before the time of the dinosaurs," says Soares.
"Behavioural, anatomical, physiological and palaeontological
evidence indicates that these predatory animals have evolved a
unique sensory organ to mediate orientation towards disruption
of the water surface."
Even though crocodilians are in a field by themselves, they have
one thing in common with other ancient survivors - the duck-billed platypus and the snake.
The platypus detects changes in electrical fields and the snake
spots infra-red. But they and crocodilians all use a similar nerve bundle, called the trigeminal nerve, to signal the detection to the brain stem.
"The trigeminal nerve, which leads to a hypertrophied brain-stem
nucleus, is the largest of all the cranial nerves in the
alligator," says Soares. - Sapa-AFP
- SAPA