An Immense World (II)

This is the second part of my summary of the most interesting bits of An Immense World. In the first part we saw familiar senses being used in unfamiliar ways. In this concluding post, I'll look at the weirder, more exotic senses animals posses. Last time we saw colour being extended to different wavelengths, vision occurring at different speeds, and hearing revealing a world of insect communication hitherto unsuspected. These are fascinating enough, but here let's cover the senses which are completely unlike anything we've ever experienced.

Sounding

Though not the main topic of the book, as we saw last time, Yong gives some examples of how hearing can reveal surprising things about animal cognition. And the use of sound blurs the line between sensing and acting in other ways. Owls hunt using sound (though a directional accuracy of 1 degree hardly seems as impressive as Yong seems to think it is), and Yong speculates that cats might lie down to hunt to increase their sensitivity to ground vibrations. But of course some animals also use sound very much more actively. Sperm whales produce sounds of a truly staggering 236 decibels, which is frankly utterly ridiculous. What the hell are they using this for ?

Lots of things, probably. Sound like this could be used to stun or disorient prey. It could be for long-distance communication across entire oceans (elephants can use infrasound to communicate with other elephants several miles away). It could even be for sonar mapping of the ocean floor, with whales apparently behaving just as though they had such a map to work with. 

Yong explains in far more detail than Higgins just how staggeringly sophisticated animal sonar is. Bats desensitise their hearing out of phase with their chirping sequence to avoid deafening themselves, having to emit ~140 decibel blasts to achieve a decent range. They also focus their calls into narrow beams to extend the range further. Their vocal muscles are the fastest of any animal in order to produce sufficiently rapid calls. They can perceive differences in the return time of the echoes of just a single microsecond. This gives them information about their target which doesn't merely tell them how far away the whole thing is, but an imaging-like capability with literally millimetre precision. To aid this, their chirps aren't at just a single frequency, but sweep across a range of frequencies to get information on different spatial scales. There are even some bats which adjust the frequency based on the Doppler shift of their moving targets. And they adjust all this constantly to account for their own motion.

I find it nigh-on impossible to imagine what this must be like. It seems if anything to be like having a hand-held LIDAR system on you at all times. Does the bat have a mental image of its astonishingly complicated data, or does it perceive it as something else entirely ?

In one sense it doesn't matter. The important thing is the bat does get access to and makes use of this information. It uses the everyday familiar sense of hearing for a world view which is utterly alien to us. But all this is very demanding, so we may forgive the bats their occasional lapse into adorable incompetence :

They can distinguish two grades of sandpaper whose grains differ by half a millimetre, but will also plough headlong into a newly-installed cave door. They can discern flying insects by shape, but will go after a pebble launched into the air. Bats are fully capable of avoiding such errors. They're just not paying attention.

Then there are dolphins. Sound is even more potent underwater and dolphin sonar is astonishingly powerful. They can tell the difference between hollow metal cylinders that are externally identical but differ in thickness by the width of a hair. Once they've scanned an object without seeing it, they can identify it using vision alone - clear evidence that they really do understand the 3D shape of an object (with even better than visual fidelity) even if they're not using sonar to create an image per se.

And dolphins are literally living ultrasound scanners. They can tell if people are pregnant by yelling at them. They can work out the internal structure of their prey and sense their air bladders or if they have metal hooks inside them. 

Trying to imagine what it must be like to have such a sense is probably impossible. It seems very unlikely that sonar-equipped animals can process this immense level of detail in the same way that we process sound. Yong suggests that rather than being a sound-based version of vision, it might be more similar to touch - but none of our senses give us information about the internal structure of objects with anything like the capabilities of dolphin sonar, so no analogy is perfect. Of course, that's what makes it so fascinating.

Electromagnetism

While electric eels are famous for being able to electrocute potential threats, some fish can electrolocate. They create a weak electric field that surrounds them, in effect a weak current flowing through the water between electrosensors on their body. Any disturbances in the field - changes in conductivity due to different materials, changes in salinity - can be detected. For some fish this has become their primary umwelt, and then respond far more to the presence of metal in the water than they do an artificial light. Which makes sense if you live in a murky river where light isn't usually that important. To such creatures, metal shines like a light bulb, while actual light bulbs are nowhere near as impressive.

Like sonar, electrolocation can penetrate barriers. It's sensitive to both conductance and capacitance, is unaffected by turbulence. And it's instantaneous and omnidirectional, a big advantage over sonar, and may even afford something analogous to colour perception. The penalty is that creating a strong field is costly, so its range is far more limited : typically centimetres rather than metres, let alone anything further. The fish have compensated by evolving extreme agility and omnidirectional swimming capabilities, apparently oblivious to whether they're swimming forwards or backwards. What does your orientation matter if you can perceive everything in high resolution in all directions at once ?

However, while generating a field is difficult, sensing the fields deliberately generated by other fish is relatively easy. For communication purposes, electric fish can indeed perceive on scales of a few metres. And as with birdsong, the relevant information to them occurs on incredibly short timescales - microseconds. Nor is electroreception necessarily confined to fish, with evidence for a similar sense now being examined in bees and spiders. Like the ability to see ultraviolet colours or hear ultrasound, the "ultra" prefix is only a reflection of our own bias; it may, for many animals, just be normal.

What it's like, of course, is damned hard to say. Yong quotes some as likening it to touch. But the ability to have a direct sense of capacitance and conductivity is, as with sensing internal structure, really not something which has any obvious comparison.

Likewise with magnetic fields. It's well-known that some animals can navigate using the natural field of the Earth, and Yong says that turtles can sense both the inclination angle of the field and its intensity. They appear to be born with something akin to a magnetic map, enabling them to immediately navigate long journeys. It might not literally be a map as such, but a simple set of instructions : if the field is like this, go this way. But it serves the same purpose.

Higgins described two of the ways the magnetic sense could work but Yong offers a third : induction. Swimming induces a weak electric current in the water around it, and that current is affected by the magnetic field. So the animals may not be sensing the magnetic field itself but rather its affect on their electroreceptors. This would avoid much of the controversy around the other explanations, and could even apply to birds, which have conductive fluid in their inner ear which may respond to the Earth's magnetic field as they fly. Are they hearing magnetism ? We don't know. There's also evidence it's involved with their vision, so they may well be seeing it instead, or as well.

Yong closes this chapter with a look at the uncertainties, but comes across in a rather lazy footnote as having considerable skepticism about whether humans have an unconscious magnetic sense. Here I think Higgins does a much more thorough job in examining the work that's been done on this area, and it's a bit surprising to me that Yong would have kind of a problem with the notion of unconscious senses, let alone that they might not work well in all circumstances. Higgins addresses all these points quite thoroughly.

Hydrodynamics

This was the most interesting section of the book to me. While the sonar stuff is incredible, it's at least vaguely familiar. But while in What A Fish Knows (mainly concerned with cognition rather than senses) the author mentions the lateral line of fish that senses pressure changes in the water, Yong does a much better job of explaining just how amazing this is. It appears to be based on the sense of touch, but it's at least as different from our sense of touch is as sonar is from our sense of hearing. I'd go so far as to stay it's more like a direct sense of hydrodynamics.

But first, some more familiar uses. Like Higgins, Yong covers the star-nosed mole. He points out that touch can be sensitive at the molecular level, and notes that the mole in particular has an astonishingly rapid sense of touch - faster even than vision. So in stark contradiction to our intuition (at least mine), touch can be a sense that delivers as much information as vision does; after all, we don't receive the image we perceive directly, but the brain constructs it as our eyes continuously dart around. The mole can do the same with its nose, so it's entirely plausible that it creates the tactile equivalent of an image.

More familiar furry friends might be doing the same. Rats, mice and others can continuously move their whiskers several times a second, an action "delightfully known as whisking", which may be doing something very similar. Whiskers are of different lengths and orientations, and each one connects to different parts of the cortex. So again, building up something like a tactile image seems credible. 

It's marine animals where this difference becomes most apparent. Possessing a similar set of whiskers, blindfolded seals can sense the wake of a single fish from maybe as much as 200 m away, several minutes after the fish has passed. This, to a human, is altogether weird. It's a sense that provides as much information as vision, is similar to smell in that it provides information about the recent past, but it works more like touch. Seals can tell from hydrodynamics alone the size and shape of a fish, the direction it was swimming, and are so sensitive that, at close range, they can even feel the water pushed by the motion of their gills. Fish themselves, with their lateral line sensors that similarly detect the flow of water, can still form schools even when blind. Crocodiles too have special bumps on their snout that can detect flow, and apparently use this for detecting prey, mates, and their own young.

But hydrodynamic senses may not be limited to the water. Birds too have feathers that appear to be specialised for detecting airflow, while bats have hairs that do the same thing. And the most sensitive case of all is found in a spider, which can detect airflows as slow as a centimetre per minute. It can detect the absurdly small flows produced by its tiny prey, which itself may have similarly extreme powers of sensitivity. And they are, truly, extreme : "They can be deflected by a fraction of the energy in a single photon. These hairs are a hundred times more sensitive than visual receptor that exists, or could possibly exist." Why they have this outrageous level of sensitivity remains a mystery.

Conclusions

A running theme of the book is that sensory organs can be used for multiple purposes. In his conclusions Yong goes further, noting that different sensors can be unified much like in synaesthesia. For example, the platypus' bill contains both electrical and touch sensors, but with the neurons in its brain receiving signals from both together - suggesting it has a combined sense of "electrotouch". Mosquitoes have sensors that response to both temperature and chemistry, while ant antennas respond to touch and smell.

Sensors are also integral to cognition. We've seen examples of how behaviour can only be fully understood by trying to appreciate the umwelt even of very familiar animals, but it can go further than that. The brain is to an extent a prediction engine, generating what it expects to perceive and comparing expectations with reality, but how it does this - how, for instance, it can fill in details like missing or scrambled words, or remove repeated words - is a mystery. More on this when I eventually review Livewired.

While Yong doesn't go into the history of research in as much depth as Higgins, he more directly conveys the sense of frustration of certain researchers more clearly. Although he doesn't ever really got for the "lone genius" myth, it's clear that a lot of what's now taken as dogma was one dismissed within the scientific realm as quackery. Now this goes quite against my own instincts of academia but the book is so replete with examples of this that it has to be taken seriously. Perhaps there's something peculiar in the life sciences ? Then again, Yong only presents anecdotes. Someone should do a rigorous statistical history to see just how often apparently silly claims turn out to be true : what fraction of lunatic claims are just that, what fraction of the scientific community dismisses the results, how this manifests itself in papers compared to more popular writings, etc. But I digress.

I'll end with two points. First, Yong is a materialist, noting that stories about transferring our minds to other beings are simply impossible : 

"An animal's sensory world is the result of solid tissues that detect real stimuli and produces cascades of electrical signals. It is not separate from the body, but of it. You simply can't imagine how a human mind would work in a bat's body or an octopus's, because it wouldn't work."

This is a view I personally find to be extraordinarily mad. After making it clear in such detail that our perception of the world is not a "real" thing but dependent wholly upon arbitrary sensors, to then insist that our senses are directly perceiving reality itself is a truly strange claim. No, there is something out there that induces something in here, but to imply you've got a full picture of that whilst simultaneous admitting that other entities will experience something radically different is just silly nonsense, frankly. More on that here.

Finally, I began this summary/review with butterflies. As I said, Yong doesn't consider these explicitly, but there seems enough here to speculate as to how they perform such incredible feats of agility. First, their vision may not be very high resolution, but it's likely extremely fast, so they have in effect plenty of time to work out what's going on. They might have somewhat sharper vision than we guess, considering they can probably sense UV - which might at least help them further in distinguishing other butterflies from everything else, along with hypersensitive smell. And they can probably sense the tiny airflows from each other's wingbeats. The key lesson here is that there's so much more to the familiar world around us than we would ever normally suspect. It really is just as the title says : an immense world.