Sentient What Animals Reveal About Our Senses

Animals are frequently celebrated for their exceptional senses, being able to hear, see, or smell things that we cannot. In Sentient, zoologist and television documentary director Jackie Higgins shows that some of this research has taught us more about how humans perceive the world, while other research reveals that we are less of a pushover in the sensory department than we previously thought. And do we really have only five senses? We need to talk about Aristotle.

I admit that I initially misjudged Sentient, thinking it was going to be another book about the wonderful world of animals senses à la Martin Stevens's Secret Worlds. There are plenty of fascinating critters in this book but the clue is in the subtitle – Sentient is as much about human senses and their neurological and psychological underpinnings. This research, as Higgins points out at the start, is upending the classic notion of "the five senses" of touch, taste, vision, smell, and hearing that Aristotle posited. We have many more sensory receptors, with the total anywhere from twenty-two to thirty-three, depending on who you ask. Sentient is a smart survey of twelve such senses with each chapter juxtaposing research on animals with that on humans.

Several chapters deal with the classic five senses and introduce exceptional animals. The eyes of the mantis shrimp contain some twenty different types of photoreceptors, perceiving not just colour but also properties such as polarization. According to neurobiologist Justin Marshall, "400 million years ago, one of them got hold of an optics text book and now they are a physics lesson on a stick" (p. 12). The nose of the star-nosed mole looks like a hand and provides a sense of touch, but acts more like an eye, having taken over the visual cortex in the mole's brain. And catfish have a skin that is covered in taste receptors, to the point that they "do not possess a tongue, as we know it. Their whole body is their tongue" (p. 127).

Other senses are more contested. Classic research by Adolf Butenandt showed pheromone communication in moths. By identifying and then synthesizing the chemicals in question, it "created the model for how we should go about pheromone analysis" (p. 183), according to zoologist Tristram Wyatt. Despite tantalising research in humans by way of lap dances, smelly t-shirts, female menstrual cycles*, and speed-dating on (androgen) steroids, we have yet to close the circle Butenandt-style and identify the substances involved. Some have thus rejected the notion of human pheromones. Wyatt takes a more moderate position: "pheromones have been identified in almost every animal you can think of [...] if the entire kingdom is awash with airborne aphrodisiacs, why should we be any different?" (p. 184). Animal navigation and sense of direction is a huge topic on which much has been written. Higgins limits herself to bird migration and details the two rival theories for how birds perceive Earth's magnetic field. Might it be by crystals of the iron mineral magnetite found in the body, or rather by light-sensitive proteins in the eye dubbed cryptochromes?

Finally, several fascinating chapters deal with less obvious senses. Cheetahs are known for their speed, but it is their sense of balance that steals the show. Their vestibular organs in the inner ear are exceptionally well developed compared to other cats. Walking on two legs the way humans do is similarly "one of the most daring balancing acts in the animal kingdom" (p. 203), and we have similarly enlarged balance organs for a primate. Pioneering studies in which people retreated to caves, shut away from daylight, revealed an inner sense of time and established the field of chronobiology. A third type of photoreceptor diffusely present in mammalian retinas, neither rod nor cone, has been implicated in this. Finally, mechanosensory neurons give us a sense of body awareness or proprioception, telling us where our body is in space. A patient who suffered from a rare form of body-blindness has shown parallels with the octopus whose arms are only partially controlled by the central nervous system.

Next to animal research, Higgins shows how we have learned much about human senses from those unfortunate souls who were afflicted by neurological disorders or suffered accidents. Drawing on the work of the late Oliver Sacks and others, she describes cases of blind people who visualise their environment through sound or touch, those whose colour vision is enhanced or completely absent, those whose pain receptors make them either immune or hypersensitive to pain, those who have no sense of smell or suffer from olfactory hallucinations, and so on. Time and again, neuroplasticity plays into this, with brain regions normally dedicated to one sensory modality rapidly being usurped by another when one sense goes missing. And this is not unique to humans. Regarding vision, one researcher quoted here puts it like this: "It's provocative, but we're arguing that the brain may not be organized into sensory modalities at all [...] The striate cortex is visual only if you have vision" (p. 94). It does answer that old chestnut of whether a tree falling in the forest makes a sound if there is nobody to hear it. On one level, yes, pressure waves are being produced; but on another level, no, perception happens in the brain.

One recurring theme is that, when put to the test, humans perform surprisingly well. Higgins describes experiments in which subjects perceived single photons, heard sounds down to zero decibels, or smelled substances at concentrations normally reserved for dogs. This might leave you with the impression that humans are, perhaps, supersensers after all. The epilogue puts this into perspective by introducing, arguably rather late, Jakob von Uexkülls's concept of the umwelt, "the slice of the surrounding environment sensed by an organism". David Eagleman writes that "Our brains are tuned to detect a shockingly small fraction of the surrounding reality" (p. 288), something that will be further explored in Ed Yong's upcoming book An Immense World.

Sentient stands out for explicitly building a bridge between the fields of animal sensory biology and human neurology and psychology. My impression is that one usually plays second fiddle to the other in many other books. Higgins, however, balances her coverage well and includes incredibly interesting studies from both fields, making this book appealing to readers interested in either field.

* Supposedly, women living together would synchronise their menstrual cycles. Higgins takes this to be true, though my impression is that this was subsequently shown to be an artefact. A good write-up of the arguments pro and contra can be found on the Modern Fertility blog.