Sense and sensitivity


African animals have evolved a battery of sensory adaptations with which to negotiate their surroundings. Some have refined the familiar five senses to impressive extremes; others have developed unique tricks of their own – sixth senses, if you like – that take perception to some surprising new levels. By Mike Unwin.


Sniffing it out 
As befits the owner of the world’s most impressive nose, an elephant has an extraordinarily sensitive sense of smell. Watch a jumbo and you will see that it is constantly raising its trunk to the breeze, or using the tip to explore the ground and the bodies of its companions. This relentless olfactory questing mirrors the focus of its mind, be it on hidden food, impending danger or missing companions. The chemical information obtained is interpreted within the nasal cavity, where layers of sensitive tissue are packed with millions of receptor cells. This super hooter proves especially useful during breeding: one elephant can determine the sexual state of another by detecting hormones or chemical molecules in its urine, dung or glandular secretions. And if it cannot find out enough by simply sniffing, it will collect the substance on the tip of its trunk and lift it to the roof of the mouth, where a natty device known as Jacobson’s organ completes the analysis before passing the message to its brain.

Sounds of silence 
An owl, as you might guess from its big eyes, has powerful eyesight – about 100 times more powerful in darkness than our own, in fact. But, perhaps surprisingly, its most valuable sensory weapon is hearing. Most animals perceive the direction of a sound by sensing the time lag between its arrival in one ear relative to the other – a sound coming from the right, for instance, reaches the right ear before the left and the difference computes its direction. But in owls, one ear sits slightly lower on the head than the other. This unique arrangement serves to exaggerate the perceived time lag in the vertical as well as the horizontal plane, and enables an owl to pinpoint the source of a sound with amazing accuracy. An African grass owl (Tyto capensis), for instance, can catch a mouse in complete darkness using hearing alone. The saucer-like ‘facial discs’ around an owl’s eyes act like feathered satellite dishes, directing sound to the ears and ensuring that the slightest rustle of an unsuspecting rodent seals its fate.

The hard, horny bill of a bird may appear a rather insensitive instrument: the perfect tool for capturing, dispatching or butchering prey, but not – you would imagine – for detecting it. Many wading species, however, have a dense concentration of nerve endings near the tip of their bill that enables them to sense the movement of unseen prey. Such species are known as ‘tactile foragers’. The yellow-billed stork (Mycteria ibis), for instance, habitually feeds with its open bill plunged deep into the mud. At the slightest movement of prey, which includes frogs, worms, crustaceans and small fish, the bill snaps shut in a lightning-fast reflex response. The prey is then lifted to the surface and – since the stork, like most birds, has little sense of taste – swallowed in a gulp.

Pit stop 
The African rock python (Python sebae) – as if not content with its five metres of coils and wicked teeth – has evolved a fiendish extra sense with which to capture a meal. Special heat-sensitive pits on the ‘labial’ scales around its mouth detect the body heat radiated by passing rodents and other warm-blooded prey. These pits, packed with nerve-endings, can perceive heat changes of less than 0.003 degrees Celsius, enabling the snake both to locate its prey and to work out the direction in which it is moving. The sense data pass instantaneously to the brain, so the python can target its quarry with a lethally accurate strike. This ability proves invaluable in darkness, when eyesight is largely redundant, effectively giving the big snake a form of infrared vision.

Current affairs 
Sharks are more sensitive than any other animal to electrical fields, with the great white shark (Carcharodon carcharias) able to detect half a billionth of a volt. And the bad news is that all living creatures – that includes you and me – produce an electrical field when moving through water, which immediately alerts sharks’ attention. The secret lies beneath the sharks’ skin in the form of tiny electro-sensitive organs known as the ‘ampullae of Lorenzini’ (named after the 17th-Century Italian biologist Stefano Lorenzini who first described them). Each ampule consists of a jelly-filled canal that opens to the surface by a pore. But the good news is that scientists have now exploited this acute electro-sensitivity in order to produce a shark repellent for divers: the ‘POD’ (Protective Oceanic Device) produces its own electromagnetic field, which disrupts a shark’s focus on any potential target.

Edition 43: Summer 2008


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