In realms of the senses

We have all been taught in school that human beings communicate through only five senses: touch, taste, sight, smell and hearing.

American science writer Ed Yong’s spellbinding new book An Immense World: How Animal Senses Reveal the Hidden Realms Around Us proves to us just how woefully limiting that definition is. It not only disregards other human sensory faculties, but also those possessed by the many species we share the planet with.

We refer to people who show a supernatural ability to communicate as having a ‘sixth sense’, but Yong weaves a fascinating tale of the scores of other ways that organisms perceive — charting their environment through echolocation, magnetic and electrostatic fields, vibration, air and water currents, and many other stimuli.

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After reading An Immense World, we will not think of ourselves and fellow animals great and small in the same way again.

‘Senses that seem paranormal to us only appear that way because we are so limited and so painfully unaware of our limitations,’ Yong writes, describing the mantis shrimp’s eyes that brim with 20 types of photoreceptors, allowing it to ‘see’ polarised light.

Other animals smell what they see, hear what they touch, and see the earth’s magnetic field. They use one or many senses to make sense of their world, their Umwelt.

The dog we worship at Kukur Tihar sees its marigold garland and the vermilion as green and yellow because of its dichromatic eyes, compared to the human trichromatic vision. We know dogs can hear sounds in a frequency range inaudible to humans, but they are probably also able to detect seismic waves that precede earthquakes.

The dog’s nose scoops up odour molecules, sniffing six times a second through nostrils with side-slits that allows it to smell even while exhaling. Dogs' moist noses may even have inherited some of the infrared detection ability of their wolf ancestors. 

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Ed Yong talked to hundreds of scientists around the world doing cutting-edge research on sensory perception. Besides our five senses, we humans also have the sense of balance (equilibrioception), awareness of our own bodies (proprioception), as well as sensations of pain and pleasure.

Other animals either have enhanced senses that we already know of, or means to perceive their surroundings that defy belief.

A rattlesnake uses its forked tongue to smell in stereo and turn its world ‘into a map and a menu’. We know bats use ultrasound echolocation to intercept insects mid-air, but scientists have found they use Doppler compensation of radar signals to chase moths.

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Whales use infrasound sonar to communicate with each other across vast oceans, greenback turtles return to the exact beaches where they were born navigating with the earth’s magnetic field for precise geolocation. Migratory birds like bar-headed geese make nonstop trans-Himalayan flights memorising genetically-encoded magnetic maps. Sea birds also have an ultra-sensitive olfactory sense with which they steer their oceanic migration routes.

Elephants use infrasound rumbles to communicate with distant herds, but can also sense ground vibrations, listening with their feet to recognise individual fellow-elephants. The human sense of touch is so sensitive we can detect texture at a molecular level on surfaces. Like the peacock, the colours of the Impeyan pheasant (डाँफे) probably hide additional ultraviolet colours that we cannot see. The feather crest on its head is likely not just a mating ornament, but sensors that ‘feel’ the air, just like the filoplume of gliding birds act as anti-stall warning systems.

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The Aedes aegypti mosquito can detect carbon dioxide, heat, and the odour that the human body gives off, a trait the dengue virus uses to propagate itself.

Odomos creams do not drive away mosquito vectors, they only repel mosquitos after the taste receptors in their feet find DEET disgusting. The insect’s eyes also send signals to its brain in time-lapse, so everything appears in slow motion -- which is what makes it so difficult to swat mosquitoes and flies.

Animals use receptors to make sense of the stimuli in their surroundings. Chemoreceptors in the nose and fingertips detect molecules that trigger smell and taste. Mechanoreceptors sense texture and vibrations through touch. Photoreceptors make sense of light.

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Eyes send electric signals to the brain through the optic nerve, and it is the occipital lobe that assigns wavelengths of light a colour. So, there is nothing really ‘green’ about a leaf, Yong explains: ‘Colour, then, is fundamentally subjective.’

There is so much more awe and wonder in An Immense World. Ed Yong devotes his last chapter to how human activity in this bright and noisy world is threatening the planet’s sensescape. His plea is to ‘save the quiet and preserve the dark’, advice that the government and tourism planners in Nepal may want to take to heart.