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Secret Worlds review: Immerse yourself in amazing animal senses

The ogre-faced spider remakes its photoreceptors by day to hunt at night

Gerhard Koertner/Avalon.red/Alamy

Secret Worlds: The extraordinary senses of animals
Martin Stevens
Oxford University Press

ECHOLOCATING bats use ultrasound to map out their surroundings at night. That information is fine-grained, helping them distinguish between the wing cases and body of a beetle and the wings of a moth. The high frequency of ultrasound generates clearer sonic images than sound in our hearing range would. Be glad bats use it, because they are seriously noisy. A single bat out for a meal screams at about 140 decibels. A human, shouting a metre away, generates only 90 dB.

This is the kind of extraordinary sensory ability celebrated in Secret Worlds by Martin Stevens, at Exeter University in the UK. It is a popular version of his textbook Sensory Ecology, Behaviour, and Evolution, but with important extras. While paying its dues to those amazing abilities of animals, Secret Worlds has some very intriguing things to say about the evolution and plasticity of the senses – and above all, the cost of acquiring them.

“Rather than seeing countless species all around us, each with every single one of their senses being a pinnacle of what is possible,” he writes, “we instead observe that evolution and development has honed those senses that the animal needs most, and scaled back on the others.”

Stevens presents startling data about the expense involved in sensing the world. For example, a full tenth of the energy used by a blowfly (Calliphora vicina) at rest is spent maintaining the photoreceptors in its eyes and associated nerve cells.

He also highlights some remarkable energy-saving strategies. For instance, the nocturnal ogre-faced spider (Deinopis subrufa) from Australia has such large, sensitive eyes that it must break down its photoreceptors and membranes during the day, and regenerate them to hunt at night.

Senses are too expensive to stick around when they aren’t needed, so they disappear and reappear over evolutionary time. In evolutionary terms, their genetic mechanisms are surprisingly parsimonious with the same genetic pathways cropping up often, and in unrelated species.

While Stevens’s main point is nature’s parsimony, it is the wonderful extremes that will stick in most minds. For instance, the whiskers of a harbour seal (Phoca vitulina) help it find a buried flatfish using nothing more than the water flow created by the fish’s breathing.

Even more arresting are the chapters devoted to senses wholly unfamiliar to us. Using their infrared thermal receptors, vampire bats select particular blood vessels to bite into. Many marine species detect minute amounts of electricity, allowing them to hunt, elude predators and attract mates.

As for animals’ ability to use Earth’s magnetic field, Stevens reckons “it is no exaggeration to say that understanding how [it] works has been one of the great mysteries in biology”. There are two main theories to explain it: one relating to the presence of crystals in the body that react to magnetic fields; the other to light-dependent chemical processes occurring in the eyes in response to magnetic information. The robin complicates the picture because it seems to boast both systems, one for use in daylight and one for use in the dark.

And what of satellite images of cows and deer that show herds lining themselves up along lines of magnetic force, their heads pointing to magnetic north? One can only hope that Stevens will return with fresh insights in a few years.

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