Bats are a very ancient and diverse group of mammals, evolving over 50 million years ago and making up a fifth of all living mammal species (Simmons et al. 2008). They are notable for multiple distinctive adaptations, including being the only true flying mammals and using echolocation to “see” in the dark.

Echolocation requires several different individual adaptations to function most effectively. These include specialized vocal structures, specialized hearing structures, and a brain that is optimized for processing sound information in a particular way.

Contrary to popular belief, many bats have good eyesight, eat fruit, and don’t echolocate. The present discussion mainly relates to Microchiroptera (“microbats”) which are often insectivorous and typically have less visual acuity. Echolocation is most commonly used for detecting prey and navigating in darkness. (Buckles 2015, p. 282.)

How echolocation works

Echolocating bats periodically emit a high-pitched sound which they can then hear being reflected (echoed) off of nearby objects. This provides a lot of information, including where the object is located and how it’s moving. Distance can be inferred from how long it takes the echo to reach the bat’s ear. Motion can be detected because of the Doppler effect: if something is moving away from the bat, it will take longer for each peak of the sound wave to collide and reflect back from the object. This has the effect of increasing the wavelength of the sound, which is equivalent to lowering its frequency (pitch). Likewise, if an object is moving towards the bat, the soundwaves will be reflected more quickly and the sound will come back being higher pitched.

Identifying the direction of a sound is possible thanks to binaural hearing, which is analogous to binocular vision. The same sound actually arrives at each ear at slightly different times due to the speed of sound in air and the distance between the ears. The brain “lines up” the two sound inputs, but the delay can be used to determine a direction. For example, if a sound is coming from the right, it will hit the right ear first. The special shape of the bat’s ear enables it to get even more precise information about direction. There is much more that could be said about bats’ echolocation, but suffice it to say that all the adaptations combined create a very precise three-dimensional “picture” of the environment around the bat.

“What is it like to be a bat?”

Philosopher Thomas Nagel published an essay with this title in 1974. His main argument is against reductionism with respect to consciousness and ultimately against materialism (aka physicalism).

… fundamentally an organism has conscious mental states if and only if there is something that it is like to be that organism- something it is like for the organism.
We may call this the subjective character of experience. It is not captured by any of the familiar, recently devised reductive analyses of the mental, for all of them are logically compatible with its absence.

Nagel 1974, p. 436

Nagel’s argument, roughly, is that there is something that is “what it’s like,” and that this is so different in bats and humans that the information about what it’s like to be a bat is totally inaccessible to us, one of “facts beyond the reach of human concepts” (p. 441).

I have said that the essence of the belief that bats have experience is that there is something that it is like to be a bat. Now we know that most bats (the microchiroptera, to be precise) perceive the external world primarily by sonar, or echolocation, detecting the reflections, from objects within range, of their own rapid, subtly modulated, high-frequency shrieks. Their brains are designed to correlate the outgoing impulses with the subsequent echoes, and the information thus acquired enables bats to make precise discriminations of distance, size, shape, motion, and texture comparable to those we make by vision. But bat sonar, though clearly a form of perception, is not similar in its operation to any sense that we possess, and there is no reason to suppose that it is subjectively like anything we can experience or imagine. This appears to create difficulties for the notion of what it is like to be a bat.

Nagel 1974 p. 438 (emphasis added)

The problem is that Nagel in 1974 had some incorrect ideas about biology. First, he overstates the significance of echolocation for bats. It is not really analogous to how humans use vision, and bats in fact use a variety of senses to interact with the world (including vision and the other four basic senses, similarly to most other mammals). Second, he overstates the dissimilarity between echolocation and human hearing.

Humans have binaural hearing and can detect the direction a sound is coming from (though sounds directly in front or directly behind us can get mixed up). We can approximate distance and motion through volume and even through the Doppler effect. Indeed, human ears are not simple holes in our heads: we have sizable fleshy protuberances that have a complex shape to focus sound in the most useful way.

To be sure, human hearing and sound processing is not adapted for echolocation. But humans and bats are both mammals with a sense of hearing, and there is no reason to think it is a fundamentally different kind of sense perception in bats.

Is it just a bad example?

The fact that bats are a bad example does, I think, make Nagel’s argument more confusing, but that alone doesn’t really undermine it. Another example Nagel brings up is wasps, which he avoids focusing on since some readers might deny they have conscious experience at all (p. 438). I’d argue that they obviously do, since they obviously have sense perception. Even an insect has senses that are analogous (at least) to human senses. I would not think this was a good example either.

Nagel introduces the idea of hypothetical aliens whose sense perception is radically different from our own (p. 440). This type of thought experiment is common in the philosophy of mind. But is such a thing actually possible? Looking at the diverse forms of life that exist on earth, many of the same sensory adaptations have evolved independently several times. The toothed whales (odontocetes) evolved echolocation independently of bats, for example. It seems that there are only a few basic types of information about the environment that are both A) possible to ascertain biologically, and B) useful for survival and reproduction. These include vision/light, sound/vibration, temperature, pressure, and detecting chemicals that are either toxic, essential, or otherwise informative (taste/smell). There are a few more unique examples, such as sharks’ sensory organ that enables them to detect electromagnetic fields, however most of sharks’ sensory perception is highly analogous to that of humans.

So while there is great diversity in living things, there are also many similarities that large, complex organisms share, and among these are many forms of sense perception. We simply don’t see such radical differences in sense perception in nature.

What is it like to be a human?

I think Nagel’s point could have been made simply by comparing two people, although I understand why he went with Martians. If there is such a thing as “what it’s like” to be someone, then I can never know that information for anyone other than myself. I can imagine what it would be like, and my imagination may or may not be accurate, but it’s impossible to know.

But do I know what it’s like to be me? Is there such a thing as what it’s like to be me? Though it seems obvious on its face, I find this questionable. Do I know what it’s like to ride a roller coaster? I certainly remember riding a roller coaster, but people can remember things that never happened to them. If you have a dream about riding a horse but you’ve never done so in real life, do you know what it’s like to ride a horse?

My point is that there is a difference between the actual present experience of sense perception and the recalling or imagining of something not presently being experienced. If I do know what it’s like to be a human, then that knowledge is instantaneous and fleeting. It is knowledge that can be neither explained nor stored in memory. Knowledge of facts generally isn’t like that.

On the other hand, if I can know what it’s like to be someone else simply by imagining, then I can know what it’s like to be a bat, or indeed virtually any complex organism, at least partially. This type of knowledge can be obtained and explained reductively.

So as I see it, we can either know what it’s like to be a bat or else there is no fact that is “what it’s like to be a bat.” For what it’s worth, I don’t think highly reductive language is good at communicating this type of information (if it exists). I don’t think it makes sense to try, either, any more than it would make sense to try to describe something in structural engineering using quantum physics. We have different levels of abstraction for a reason. That doesn’t mean that a bridge is anything other than an arrangement of elementary particles, nor that consciousness is anything other than sense perception being processed in the brain. There is no need to introduce non-physical entities in order to talk about these things at a high level.

References

Buckles E. L. (2015). Chiroptera (Bats). Fowler’s Zoo and Wild Animal Medicine 8, pp. 281–290.

Nagel, T. (1974). What Is It Like to Be a Bat? The Philosophical Review 83(4), pp. 435-450.

Simmons, N., Seymour, K., Habersetzer, J. et al. (2008). Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature 451, pp. 818–821.