do bats echolocate in color?

Tonight I waited by our pond hoping to see a bat.  Spring is finally here and there are more than enough flying insects to tempt a bat, but nothing.  The past few years have been terrible for bat watching - I'm afraid white nose syndrome has taken its toll.  Still, there were a few last year including a fearless little brown bat that would swoop within a few feet.

 

About fifteen years ago I built a few simple bat detectors with simple being the operative word.  A microphone that worked up to about 100 kHz, more than enough for many bat calls, and a  bit of electronics to sample and downshift the signal's frequency by a factor of 32, bringing it to a range where it could amplified and sent to headphones.  You learn a lot about their feeding habits watching and listening, but serious students of the critters were using sophisticated spectral analysis and high speed photography.  In a decade the field was revolutionized.

 

The richness of the signals is wonderful - you can move around in the time and frequency domains and "see" some of what they are up to.  Not only can they measure distance and relative velocity of their prey, but they can determine size and even what it might be to some extent.  You find yourself wondering how the bat experiences this richness.  We perceive rich visual and acoustic worlds, dogs and ferrets have fantastic senses of smell.  Do bats have something like color to sort out the different frequencies of their echoes?  They clearly use doppler shifts - just how do they perceive that?

 

It is easy to imagine what lower levels of perception are, but going the other way is frustrating.  As a teen I came across Edwin A Abbott's wonderful Flatland - a world with sentient beings all living in two dimensions.  I didn't appreciate that it was a commentary on Victorian hierarchy, but imagining what three dimensions must be like to a two dimensional being held me for some time.  Of course it is easy for us to imagine as beings of a higher dimensionality.

 

Most mammals only have two cones to sense color.  Your dog doesn't see the world in black and white, but his color vision is greatly limited from ours.  Although we only have three sensors, our brain sorts out the relative strength of the signals and creates something we call color.  We are sensitive to wavelengths from about 400 to 700nm and can perceive differences of about 1nm in the blue-green and yellow region and about 10nm in blue and red areas.  So although we can only distinguish a few hundred frequencies, our brain mixes these together to give a very large number.¹  But we have an idea what our vision is like, what about our dogs?²

 

Using a few papers on dog vision I cobbled a simple filter together.³  Here are some common colors seen by us and the same objects as seen by a dog. Of course I have no idea what "colors" the dog would imagine, the relative difference is what is important.  Although color vision is limited, you shouldn't feel sorry as your dog has a much better sense of smell than you and probably can't imagine how debilitating it must be to live in such a deprived world.

 

As rich as it seems our view of the world is extremely limited.  We tend to live in the time domain and can only discriminate between events a few tens of milliseconds apart.  Likewise we live more or less in the present and the notion of geological time is foreign. The wavelength range we see corresponds nicely with the output of our Sun, but it it is a tiny bit of the electromagnetic spectrum.  To make matters more interesting we live slightly in the past - our brain synchronizes some signal inputs moving us some number of milliseconds backwards in time and much of our visual perception is averaged over very long periods - we live in a mixture of the fairly recent stretching out to the past ten or fifteen minutes back.  

 

But our brains let us build sensors that give views into the richer universe

 

In order to keep the post short I'll stay away from multispectral, hyperspectral and ultraspectral imaging - at least not here as it is extremely important and deserves attention.  Basically you look at a lot of wavelengths to find any number of interesting things.  My guess is this is why Google was interested in a drone company.  The richness of Google maps is very sparse when you consider what is really available.  There are some rather impressive opportunities to understand the natural world - and one can imagine a number of business opportunities.  Timing and practicality are issues, but it is clear where this is going.  But back to bats and the present.

 

Smartphones give you some sensors, a computer and a connection to the Internet.  The computational piece is now sufficient to do sophisticated signal processing.  If you build certain types of sensors into the phone - or plug them in -  impressive work can be done.  Serious amateur, citizen and real science.  Environmental monitoring could quickly reach a state that has a major impact on pollution law.  There are the beginnings of that in China with very primitive tools.  The legal landscape for polluters may see a large shift in the next decade.  Atmospheric particulate, NO_x, CO, CO₂, and various water tests should be possible, although there are issues of calibration and repeatability that need to be addressed. Carbon dioxide measurements in the office may have a real impact on your own working conditions and even office productivity.

 

It would be fun to watch bats more seriously as an amateur, but the $5k and up for tools is prohibitive.  But there are smartphones...  All you would need is a good ultrasonic microphone and some programming.  The A7 chip in current iPhones has serious number crunching ability in its graphic cores.  It turns out someone has done it - the  price is about a tenth that of serious last generation kit and it is just as capable - potentially more capable.⁴

 

Developments like this are only the beginning of the directions are phones will take us.  Sensors are really important.

 

But what does acoustic imagining seem like to a bat?  Is the the similar to echolocation in whales or ...?

 

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¹ This varies a lot in humans and some of it is sex-linked.  Women are much better than men in the reds.  

² We sort of have a handle on it - there is still much to learn.  I have a neurological disorder that mixes color and sound, so I have no idea what orthogonal senses of hearing and seeing are.  This causes some issues that were probably selected against thousands of years ago in human evolution, but are benign after agriculture was invented.

³ Miller PE, Murphy CJ (1995) Vision in dogs. J. Am. Vet. Med. Assoc. 207:1623-34.  The filter is really simple and a rough hack, but it gives an idea.  Write if you want the mapping.  Here is a rough visual map, normal spectrum on top and filtered on the bottom:

 ⁴ In large quantities this could be very cheap, but this is low volume and they need to recover their costs.   

 

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Recipe Corner

 

Roasted Cabbage Salad

 

Ingredients

 

° half a green cabbage cut into chunks

° one diced apple - I used a Fuji, but almost anything should work

° a handful of pecan halves

 

vinaigrette

° 1 tbl cider vinegar

° 1 tbl balsamic vinegar

° 1 tbl dijon mustard

° 1 tsp maple syrup

° kosher salt and freshly ground black pepper

° 2 tbl extra virgin olive oil

 

Technique

 

° put the cabbage on a baking sheet, sprinkle with a few tsp of the olive oil, add some salt and pepper and spread into a single layer.  Roast at 400°F for about 15 minutes 'til the cabbage starts to brown.

° cool and transfer to a cutting board and dice.  Put into a bowl with the diced apple

° put the vinaigrette ingredients into a jar with a lid and shake to emulsify. Pour over the cabbage/apple and toss. 

° toast the pecans in a dry frying pan over medium heat until you can smell them, but *before* they brown.  Add to the salad and toss.

° I served with dried cranberries.  It was suggested that crumbled goat cheese would be amazing.