Sensory

The Star-nosed Mole and Our Sense of Touch

Guest post, by Jackie Higgins (@JackieHiggins_)

This extract has been adapted from Jackie Higgins’s new bookSentient: What Animals Reveal about our Senses’. Out now in the UK with Picador and in the USA on 22 February 2022 with Atria.  Higgins considers the human senses through the ears, eyes, skins, noses, tongues and more of other animals: exploring Aristotle’s five senses, from hearing with a great grey owl to taste with a goliath catfish, as well as lesser known senses, such as those of balance and of body, using a cheetah and then an octopus.

The accolade of fastest killer in the animal kingdom goes not to the cheetah, but to a species from the wetlands of the north-eastern United States and eastern Canada. Guinness World Records hail it as the most rapid and voracious mammalian predator. The entry reads, ‘It takes humans 650 milliseconds to respond to a red light when driving’; an eye-blink lasts at most 300 milliseconds, yet this creature can identify, capture and consume its prey in under half this time. One wonders whether its unfortunate quarry even has time to register its fate.  To compound its sinister reputation, it has been compared to H. P. Lovecraft’s famous extra-terrestrial Cthulhu – ‘whose face was a mass of feelers’ – on account of twenty-two blood-engorged, fleshy tentacles that radiate from its nostrils. Yet this furry fiend barely fills a human hand, seldom ventures above ground and poses little threat unless you are an earthworm. Indeed, it is a rather bewitching mammal whose unusual snout gives it the name the star-nosed mole.

Star-nosed mole. Photo by Ken Catania

Nashville, Tennessee is home both to country music and star-nosed research. Ken Catania is the world authority on these moles and based at Vanderbilt University.  ‘It’s hard to imagine a stranger animal,’ he told me, ‘like a creature you might picture emerging from a flying saucer to greet a delegation of curious earthlings.’

As this mole advances down a tunnel, the twenty-two tips of its star move so fast that Catania decided to film them with a highspeed camera.  This enabled him to see the mole identifying and catching prey in as little as 120 milliseconds. ‘A phenomenally short amount of time,’ he told me. ‘I don’t know of any other mammal that comes close to this.’ However, the implications of the footage went beyond mere record-breaking.  Played back in slow motion, it revealed the star’s rays feeling their way through the darkness, moving quickly and independently, touching up to a dozen objects every second. Moreover, whenever one of the first ten pairs encountered a morsel of food, the eleventh, somewhat shrunken pair, would always investigate before passing the prey to the mole’s mouth and its tweezer-like front teeth. The footage showed that the mole’s nose is highly attuned to the sense of touch.

Under a scanning electron microscope, the star emerges as a whole new topographical world. ‘The skin surface becomes a cobbled landscape of tens of thousands of epidermal domes. A beautiful anatomy,’ Catania explained.  Further investigation ascertained that each dome hides various mechanoreceptors. ‘I did a careful check to see if there were any other types of sensors but found nothing else.’ No evidence of electroreceptors, chemoreceptors or any other sensory abilities beyond touch.  Setting himself the unenviable task of counting the domes on a star, Catania tallied on average 25,000: five times more than on a European mole’s nose.  Given each dome is served by around four myelinated nerve fibres, he reached a conservative estimate of 100,000 nerves.  On every count the star-nosed mole surpassed its European cousin by some margin.

We humans pale further in comparison.  According to Catania, ‘The nerve count on the mole’s snout alone adds up to many more touch fibres than one finds on the whole human hand’ – 100,000 to the mole, 17,000 to us – ‘and yet the entire star is only a centimetre across. Imagine having six times the sensitivity of your entire hand concentrated in a single fingertip.’ He concluded that no other mammalian organ known to science is as densely innervated and acutely primed for touch as this snout. ‘Humans may have more diverse ways of feeling with their fingers, but the star-nosed mole has the most sensitive touch organ of any mammal yet discovered.’

The star-nosed mole teaches us much about our sense of touch through an organ we normally associate with our sense of smell. Its celestial nose may be the most sensitive appendage among mammals, but it relies on the very same cells found in our skin and at our fingertips: sensors that bring us the feel of the world beyond. Bach-y-Rita famously said that we do not see with our eyes, but with our brains. Similarly we do not hear with our ears, smell with our noses, taste with our tongues or feel with the sensors in our fingers. Even by this line of logic – with our brain as our body’s all-important sense organ – the mole confounds expectation.  Catania has uncovered the way in which its star is etched across its brain means that its nose, which looks like a hand, in practice acts more like an eye. He said, ‘Some might suggest that the mole’s visual cortex was taken over by touch but there is no line in the brain that says, “Here is where the visual cortex should be.” Think of it another way, somewhere there is a star-nosed mole writing a book on humans and wondering if our strangely large eyes took over all the area that is supposed to be for touch.’  In De Anima Aristotle declared, ‘Man is inferior to many of the animals, but in delicacy of touch he is far superior to the rest.’ Clearly, he was unacquainted with this stellar mole.

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