You feel your way forwards in the tunnel, noticing the coolness of the earth beneath your feet.
You are hungry… There is food here, but it is invisible in this darkness unless you can find it by touch.
You encounter prey, flailing your longer fingers over it, touching it carefully with your short fingers before you tear it to pieces and devour it. The whole thing is over in a fraction of a second.
But where are the fingers that you use to ‘see’? Not on your hands… Instead they are protruding from the end of your nose.
The star-nosed mole’s eyes are tiny, and register only the difference between light and darkness. In contrast, its 22 highly touch-sensitive, fleshy rays arranged symmetrically around its nose act like an eye; this animal touches things in order to ‘see’ them.
We perceive vibrations in air as sounds using our ears, and we may also feel for example an explosion, or loud music, as sensations in our chest cavity. Elephants use fatty pads on the soles of their feet to sense seismic pressure waves, such as caused by other elephants stampeding, from many kilometres away. Moles, and other mammals including rats, mice, and even cats, use their whiskers to feel the movements of prey nearby. Whilst the star nosed mole does have whiskers, the ‘rays’ of its strangely-shaped nose are covered in hundreds of tiny domed structures (known as Eimer’s organs) which detect both touch and vibration. These organs are also found around the nostrils of other moles, as well as some of their relatives.
‘Touch’ involves many sensations, such as contact, pressure, pain, heat and cold. Your hand has around 17,000 touch-responsive nerve endings, mostly on the thumb and index finger. The star nosed mole’s nose contains around 25,000 Eimer’s organs, served by 100,000 myelinated (fast) sensory nerve fibres specialised to respond to light touch, and at least as many slower, (unmyelinated) fibres picking up other types of information. That makes more than 200,000 nerve endings clustering into the mole’s nose, even though it is barely the size of your fingertip.
The mole moves its nasal rays using tendons connected to the skull and facial muscles. When patrolling its network of tunnels, it scans for prey by repeatedly touching the ‘star’ to the burrow floor. To ‘see’ what it has found, it raises its snout and swings the rays backwards before sweeping them forward over the object. These animals forage by moving their ‘star’ back and forth with astonishing speed; they can detect, identify and consume small prey in as little as 120 milliseconds, and on average it takes 230 milliseconds.
Nerve endings in the Eimer’s organs transmit signals to three dedicated regions in the animal’s brain. This neural map has distinct zones corresponding to each fleshy ray. These form three separate ‘star maps’ in the mole’s cerebral cortex, each thought to handle different types of touch-based information. The zone dedicated to the same fleshy ray in all three ‘star maps’ is connected, forming a ‘neural super-circuit’ for each organ. Similarly specialised regions of brain cortex handling tactile signals from individual whiskers are also found in mice and rats; effectively, each whisker forms a separate sensory organ.
In the star nosed mole these star maps span the left and right hemispheres of the brain, and connect along the midline by a collection of nerve fibres that form the corpus callosum. Similar left-right connections are found in the visual cortex of human brains and those of other primates. What is surprising about the star nosed mole’s cortical map is that the relative sizes of the areas devoted to each ray do not correspond with their physical size. The smallest (11th) pair of rays, positioned just above the mouth have fewer Eimer’s organs yet have the largest brain area dedicated to processing their sensory input. This takes up around 25% of the total ‘star map’.
Why are the signals from these short rays given so much of the mole’s brain processing capacity? And might this ‘tactile eye’ tell us about how eyes work?
How does the mole’s nose behave like an eye?
The star nosed mole shifts its nasal star in a series of swift ‘glances’ over the prey, making ten or more contacts per second. Whenever it re-positions the star, the outer rays (ray pairs 1 to 9) gain an overview of the surface whilst the shorter 10th and 11th pairs of rays (just above the animal’s mouth) make a further, more detailed examination.
This activity mimics the ‘saccade and fixate’ movements of our own eyes. When we look at something, our eyes scan the view by shifting from one region to another (the saccade), focussing at each point (fixating) to capture detailed information. When we ‘fixate’, our eye muscles adjust the lens to focus an image onto an area at the back of the eye, known as the ‘fovea’. At this point we see a small area in sharp focus, but are also aware of a wider peripheral vision in which we see less detail.
Our visual fovea is small, but dense in cone cells, containing three types of colour detecting pigments which respond to different wavelengths of light. Each cell has its own dedicated nerve ending, relaying information to the brain cortex. In contrast, most light-responsive cells around the periphery of the fovea are ‘rods’. These respond to low intensities of white light, but capture less detail than the cones. Multiple rods connect to the same neuronal relay. The area of our brain dedicated to processing the sensory input from the fovea is much larger than that from these peripheral cells.
The reason why the mole’s nose rays are so eye-like is that they capture sensory information in a similar manner to our rod and cone cells. Eimers’ organs in the 10th and 11th pairs of fleshy rays house many more free nerve endings than those of other rays. Thus, rays 1 to 9 have around 4 nerve fibres per organ, whilst rays 10 and 11 have on average 6 and 7 fibres respectively. Eimers’ organs in the more elongated nose rays collect lower resolution information, just as rod cells provide our peripheral vision. Organs from the 11th ray pair capture detail, in the same manner as the cone cells in our fovea.
Why did this tactile eye evolve?
Most moles have touch-sensitive whiskers. Star nosed moles also have whiskers, suggesting that selection for other factors has driven the evolution of their ‘tactile eye’. They are the only mole species to occupy a wetland habitat. Their damp burrow systems are prone to flooding, and are often open along some sections. This means the moles compete for food with shrews and other insect-eating mammals that can gain access to their burrows. The mole’s high metabolic rate requires high energy foods; what they consume must be worth the time they spend searching for and handling prey.
These animals have semi-aquatic habits, and use their starred nose to forage in the same manner in both water and air. These efficient underwater predators have the fastest prey-processing times known amongst mammals. The star nosed mole’s front incisor teeth act like tweezers, allowing them to efficiently handle aquatic insect larvae and other small, high reward prey. Their tactile nasal fovea allows them to discern the distinctive ridges and spines of many of their favourite prey items.
Perhaps even more surprising is that these animals are able to detect scent under water. These moles extrude air bubbles during foraging dives, grasping them using their nose rays, and then ‘sniffing’ the bubbles back into their noses to sample the scents of the water.
Whilst all moles also use their noses for both scent and touch, the star nosed mole is a specialist in these two sensory modes. Its strange nasal star and tweezer-like front teeth are adaptations for finding and grasping tiny insect larvae and other ‘high reward’ underwater prey. The high quality of these food items offsets the metabolic cost of making the mole’s super-fast nasal ‘saccades’. These animals are however obliged to remain semi-aquatic. In the drier tunnel environments of other types of mole, the delicate skin of the rays would be prone to abrasion and drying out. Stiff whiskers and bristles are much more resistant to scouring, and provide more generalist, terrestrial moles with spatial information using touch.
Naked skin itself can provide a sensitive sensory surface. The naked mole rat lives in an extremely dry, enclosed underground environment. Vision is useless underground, and in addition eyes are metabolically expensive. Mole rats fail to develop eyes and their ears are also greatly reduced. They rely instead on touch-sensitive naked skin and bristles scattered along their bodies. The arrangement of the brain cortex of these animals suggests that skin sensations provide an equivalent to ‘peripheral vision’. Most of their neural processing is however devoted to tactile information obtained from their enlarged front incisor teeth. These teeth therefore mimic the tactile fovea in the brain cortex of star nosed moles, the visual fovea of humans and other primates, and the acoustic fovea of echolocating bats.
What does the mole’s tactile nose tell us about our eyes?
Although our eyes work like cameras in terms of how they focus light, the way we perceive the outside world is not continuous. The saccade movements of our eyes allow us to ‘sample’ colour and detail from the view before us. After each fixation, the light-sensitive cells need time to ‘reset’ before they can respond again. The saccade, during which our eyes are not focussing on anything, provides our light-responsive cells with recovery time. The ‘sight we see’ is therefore constructed by us in our mind; we fill in the gaps between sample points, compiling colours, textures and form from these cues, informed by what we already understand about our world.
The star nose mole’s nasal saccades, and its use of the 11th ray pair as a fovea, allows its tactile nose to work as an ‘eye’. Nerves in the Eimers’ organs sample the external surface and capture several types of information about contours and vibrations, which is processed by the three ‘star maps’ in its cerebral cortex. It seems very likely that the mole’s brain combines these samples of information into a detailed three-dimensional understanding of their environment.
The high speed of their nasal saccades allows these moles to snap up and process small invertebrate prey very quickly. When these animals are supplied with a high prey density, however, their saccade speed does not increase; they still last for around 25 milliseconds, during which each prey item is examined several times by the 11th ray pair before being consumed. This suggests that, just as our light responding cells need recovery time, the speed of this ‘tactile eye’ operates close to the working limits of the mammalian sensory system for processing tactile information.
The star nosed mole sees using touch. Echolocating bats use hearing, and most vertebrates (including humans) use vision. All of these animals deploy the senses available to them in their environment to gain an understanding of objects in space. All scan the scene using saccades, and fixate upon points to ‘sample’ details in the view. This saccade sampling shows us that the neural regeneration time in our sensory cells set the limits of what is available to our perceptions.
The saccade-fixate pattern, whether for eyes or noses, is a learned behaviour. We learn to saccade, focus on objects and track movements with our eyes during our first few months of life. Seeing colours develops more slowly; we also have to learn to discern and recognise these. Seeing then, is an acquired skill. It is a product of how we choose to use the eyes we have, whatever form they may take.
This saccade-fixate behaviour reveals that the sensory information which we and other animals use to understand our world, is only a fraction of what is there. There are also realms of sensory information that lie beyond the perceptions of any animal except us. We use tools to extend and improve our senses, ranging from the simple use of lenses which correct our eyesight to the telescopes and other signal-receiving devices which allow us to see deep into space, and listen in on the ‘music of the spheres’.
Text copyright © 2015 Mags Leighton. All rights reserved.
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