In this next article, I think that those of you who participate the in the evolution/creation debate or who just enjoy learning more about animals will find this particularly fascinating. In our first example, we shall discuss the problems and solutions of navigating in the dark by the mustache bat and we shall analyze the intricate compensatory mechanisms that the bat’s primary prey, the moth, has evolved. We shall also quickly look at how fishes navigate murky waters to get a more holistic understanding of this neuroethology field. Once again, images presented here are adapted from the slides of Proffessor Tom Yin. This article may seem a little more technical but I’ve tried to make everything understandable; plus, there’s even some videos, so try to make it all the way through.
So what is neuroethology? Neuroethology is the study of the neural basis of animal behavior, in particular, behavior in the context of survival. It has been said that for each biological problem which exists, there is a species which can provide a solution. Perhaps then, a better definition of neuroethology is the study of species which have evolved behaviors that are specialized to solve such biological problems. Moreover, this approach emphasizes a comparative analysis across different species of animals to better understand the biological problems and the solutions (i.e. the arms race that develops between predator and prey).
In our first example, we shall examine the sensory system of the mustache bat, an animal that has been very thoroughly studied by Professor Nobuo Suga. In particular, the system of interest will lie in the bat’s echolocation and how the bat has evolved to solve the problems which arise with echolocation. Moreover, we shall examine the bat’s prey, the moth, and how the moth has evolved defensive mechanisms to allow it a fighting chance against such a cunning predator.
There are over 800 known species of bats which comprise about 25% of all mammal species. They are relatively small animals and tend to hunt at night, thus they have relatively poor visual systems. Usually, when an animal has one particularly poor sensory modality, it will compensate by having one of its other sensory modalities be extremely sensitive and highly tuned; naturally, the bat has superb hearing as we shall see. Most bats hunt insects and in a typical night, as many as 500 insects may be devoured. Why hunt at night? The only other major insectivore are birds and they only hunt during the day. Thus, hunting at night provides the bat with no competition at all (save themselves), a major survival bonus. In addition, the bat’s natural enemies, hawks and other carnivorous birds, do not hunt at night, giving the bat almost free reign. But how does the bat solve the many problems of navigating at night?
Echolocation was originally discovered by a neuroethologist by the name of Donald Griffin in 1938. Much of Griffin’s work was built upon the discoveries by an Italian, Lazzaro Spallanzani, who showed that bats were able to navigate using audition. From this, Donald Griffin was able to demonstrate that bats used ultrasonic calls in the range of 20 to 120 kHz, above normal human hearing (the high end of human hearing is the bat’s low end; thus bats can hear an entire order of magnitude above us). Moreover, he was able to show that the bats were able to navigate through wire frames in complete darkness.
A natural question one might ask is, “Why do bats use ultrasonic sound?” First, back to basics; sound is simply energy vibrations propagating through a medium, traveling as a mechanical wave. Waves can be described by their wavelength, their frequency, speed, amplitude, as well as numerous other properties that I’m sure you all remember from trigonometry and physics. According to Merriam-Webster, a wavelength is, “the distance in the line of advance of a wave from any one point to the next point of corresponding phase” or, essentially, from one peak to the next peak. Then, the frequency measures the number of repetitions of the wavelength per unit time (usually in seconds or one Hertz, one cycle per one second). As we can see in the illustration above, if the sound has a high frequency, the wavelengths will become shorter whereas if the sound has a low frequency, it will have a longer wavelength. Therefore, the answer to why bats use ultrasonic sounds (high frequencies) is because ultrasonic sounds have small wavelengths. The smaller the wavelength, the better the bat can detect smaller objects, such as insects. In fact, objects as small as 1 mm can be detected by a bat.
This graph illustrates the spectrogram of a mustache bat’s call with time on the x-axis and the frequency on the y-axis. Each call is composed of a constant frequency (CF) component followed by a frequency modulated (FM) component with three different harmonics (H1, H2, and H3) of each CF/FM call. Many bats will have different variations on this CF/FM combination with some bats having only FM whereas others use both CF/FM. The CF component is well suited to detecting Doppler shifts and the FM component has a wide range in frequency and is well suited to detecting various details of the target. The first harmonic is at 30 kHz, the second at 60 kHz and the third harmonic at 90 kHz; the second harmonic is also the strongest harmonic, slightly bolded.
When the bat makes a call during the search phase, there will be a slight delay before the echo returns and it is the delay that tells the bat the distance of the target; the longer the delay, the farther away the target is. Looking at the graph above, one obvious question should pop out: if the call is sent out at 30 kHz, why is the echo returning at a slightly higher frequency? As the bat approaches a target, the frequency of the echo will increase due to the Doppler effect; the frequency of the call stays constant but since the bat is moving towards the target (assuming a stationary target), the echo returns more quickly than the call is going out, resulting in a higher return call frequency than an outgoing call frequency. In addition, the relative Doppler shift also tells the bat the relative speed of a moving target relative to the moving bat, another useful advantage (discussed later below).
As we can see above, when the bat is searching, the duration of its call tends to be longer whereas when the bat is approaching its target, the duration of its call shortens considerably and the repetition rate increases. When the bat nears its target during the terminal phase, the call is shorten even more considerably.
Now that we have dissected the physics of the bat’s echolocation, let’s see it in action. The following videos were captured by Professor Cynthia Moss and demonstrate the exquisite and amazing ability of echolocation. Dr. Moss studies bats and has a large room with numerous audio receivers and video cameras to study all the different angles of the bat’s movement and echolocation; she then is able to create a spatial computer map and track the bat’s echolocation. In this first video example, we can see that the red dot represents the target; watch as the bat nears the target, you can hear an increase in the number of vocalizations. In the next video example, we see a bat make a run at a moving target and miss and fly around to make another pass. In this last video example, we can see a top-down view of the bat chasing a moving target and we can see the rate of its vocalization; again, watch as the bat nears the target, the rate increases. For more videos like these, visit Dr. Moss’s website.
So how has the bat’s brain evolved to deal with this wide array of sounds? The next solution to this problem lies in the neurology of the bat brain. In the above picture, we can see a schematic of the mustache bat’s brain (Figure A) with much of it devoted to the auditory cortex (Figure B), the part of the brain which is specialized for processing auditory information. Many people would argue that more is known about the mustache bat’s auditory cortex than any other animal’s auditory cortex, although it tends to be one of the lesser publicized subjects in neuroscience (as compared to the visual system). As you can see, the bat’s auditory cortex is divided into functional areas. As with other sensory systems, the bat’s auditory cortex is organized in a particular fashion, in this case, its tonotopically organized, that is, it’s mapped by frequency with various parts dedicated to processing sound of one particular frequency.
You can see that in the lower-right part of the diagram, the bat’s cortex is processing sound from 10 to 50 kHz and then in the central circle region, there is a large portion of the cortex devoted to processing sound from the 60 kHz range. If you remember previously, 60 kHz is the second harmonic of the bat call and is the strongest of the three harmonics and therefore, it requires more cortical processing. In fact, this portion of the bat auditory cortex is the equivalent to our visual fovea, the part in our eye that is needed for strong, focused vision. Thus, this acoustic “fovea” (or in layman’s term, sweet spot) of the bat is responsible for strong, focused audition. Going further to the left of this central region of the auditory cortex, we reach the portion of the cortex that processes sound in the 90 to 100 kHz range.
While parts of the bat’s auditory cortex is specialized for processing certain frequencies, it also is specialized for processing different CF/FM components of the call (in the upper left hand corner of the diagram). These are areas that contain cells which are sensitive to different combinations of CF and FM calls; one part of this auditory cortex is sensitive to the CF1 component of the call while another is sensitive to the CF3 of the return. Thus, the bat is keeping track of individual components of the call and the return echo and different harmonics of them. Moreover, there are cells which are sensitive to the delay or to the frequency difference (Doppler shift difference) of the CF1/CF3 etc. So we see here that having evolved a night life, the bat has evolved an elaborate echolocation system to help it navigate and it has evolved a highly specialized brain to process all of these sounds. Not only is its auditory cortex sensitive to all of the different frequencies, it is sensitive to all of the different harmonics, to the delays, to the different CF/FM components of the call and echo, to different Doppler shifts, to the volume, range, amplitude, velocity and a whole plethora of other properties! But there’s still more.
As mentioned, the bat’s auditory cortex has cells which are sensitive to different combinations of frequencies, CF/FM components, delays etc. Illustrated above are cells which are sensitive to a particular combination of pulses/calls (P) and echoes (E). In these experiments, a bat call and echo was simulated. In Graph A, we see that a call combining the three different harmonics is presented with no echo and we see that the cell is mildly responsive (a small spike in the neuronal activity), therefore, this particular cell is partially sensitive to a call in the given frequencies. In Figure B, we see that the three different harmonics of the echo are presented to the same cell but this time, we see no activity present and thus, the cell is not sensitive to the echoes when it is presented alone. However, when the simulated bat call of the three harmonics, followed by a short delay and then the three harmonics of the echo, are presented together (Figure C), we see high activation of this particular cell; this the cell in the first row is most sensitive when the call and echo of the three different harmonics are presented together. In the other examples (Figure D), the call of the second and third harmonic is presented with, followed by a short delay of the right length, the echo of the first and third harmonic; here we see no activation at all. But when the first harmonic of the call and the second harmonic of the echo are combined, we see strong activation of this cell (Figure E). In Figure F, the cell is shown to also be sensitive to the first harmonic of the FM1 component of the call and to second harmonic of the FM2 component of the echo. Thus, there are cells that are most sensitive when the right combination of the 6 different pulses (3 CF and 3 FM), delay, and the right combination of the 6 different echoes are presented in the right order, resulting in 36 different combinations. And now we understand how it is that having different cells which are sensitive to the right combinations of frequencies, components, and harmonics is the key to allowing the bat to maintain order and allow it to process and understand all of these different sounds.
Most bats live in very large colonies, containing tens of thousands of bats. One has to wonder just how do the bats avoid cross talk between their own call and their neighbor’s call. The solution to this problem arises from a Doppler shift. As mentioned earlier, if a bat is approaching a stationary object, the echo will return at a slightly higher frequency since the bat is moving towards the object. If the bat is chasing a target that is flying away faster than the bat, the echo will return at a lower frequency and if the bat is flying towards a target that happens to also be flying towards the bat, the echo will return at a even higher frequency. Recall also that in the bat brain, they have an expanded region that is highly sensitive to the 60 kHz range (the second harmonic) and in particular, each bat has its own unique expanded region for one particular band. For example, there might be a bat which has an expanded region for 60.1 kHz while another bit might have an expanded 60.15 kHz and another for 60.2 kHz. So each bat seems to develop a target frequency that is their favorite frequency (so to speak). Thus, each bat is capable of modulating their call so that the Doppler shifted echo falls within that preferred region and this is one method in which the bats can avoid confusing the echo of another bat with their own.
In the graph above, a bat was tied to a swing that swings back and forth with a stationary target in front of the bat; the bat is freely echolocating. The red dots indicate the second harmonic of the calls of the bat. When the bat swings forward towards the stationary target, we can clearly see that the bat has modulated its call so that the Doppler shifted frequency (black line) falls within its preferred range, ~61 kHz. This behavior is known as Doppler-shift compensation. So they’re adjusting their call so that the Doppler-shift of the echo falls right into their hot spot. Note that the bat doesn’t do this Doppler-shift compensation during the backward swing; it continues to call in the normal frequency range (you don’t see dots laying elsewhere do you?). Why? Because either the bat is flying backwards (unlikely) or the target is flying faster than the bat and the bat will not expend unnecessary amounts of energy when it might be able to catch an easier target.
So given this incredible machinery in the bat, one has to wonder why moths are still around, the primary target of the bat. How do the moths avoid being eaten? As one can expect, the moths have evolved some elegant defensive mechanisms to the bat’s offensive array.
Many species of moths have auditory neurons that are ultrasonic detectors, capable of detecting the bat’s ultrasonic calls. In a few particular species of moths, they have two different types of auditory cells: one that is tuned to low intensity calls (A1 cells, threshold of ~40 decibels) and one that is tuned to high intensity calls (A2 cells, data not shown). These different types of auditory neurons result in two very different behaviors in the moths.
In the figure above, the leftward image is the flight pattern of a moth with no simulated bat call. When a loud bat call is presented (middle image) the A2 high intensity auditory cells of the moths are activated and the moth suddenly dive bombs, presumably to avoid the bat. The high intensity calls signal to the moth that a bat is nearby, presumably chasing the poor moth. However, when a low intensity call is presented, the moth simply changes its flight path into the opposite direction. Presumably, the moth thinks that a bat is in the general vicinity and it simply flies in the opposite direction to avoid running into the bat.
Another defensive mechanism that some moths have developed is a jamming behavior. The moths are capable of producing a noise, like that of crickets, at a particular frequency. The moths attempt to jam the echolocating signal of the bats. The above graph illustrates a moth click versus the echolocating sound of a bat. As you can see, the clicking noise of the moth is similar to that of the echolocation. When the moths detect a bat, they can produce this noise to confuse the bat. One would intuitively think that producing noise might attract the bats to their location but not so; the cortex of the bat has all of these combination sensitive neurons as we talked before and once the moth begins to introduce these jamming sounds, the bat can no longer function at its optimal performance.
So this evolutionary game that the moths and bats have developed has been going on constantly with the other constantly evolving a more refined method of predation and escape. But what about other animals? In the so called weakly electrical fish, they have electroreceptors, capable of detecting and generating weak electrical fields. These fish are similar to bats in that they live in murky, dark environments and navigation can be difficult. In order to navigate, these fish have evolved electroreceptors that can generate weak electric fields. These electric fields are then distorted by objects in the fish’s path, telling the fish where to navigate. But here arises a problem: how do the fishes avoid jamming other fish’s electric field?
Like the bat, these fish have evolved mechanisms to avoid jamming one another. They try to change the frequency of the pulse that they’re emitting to avoid jamming their neighbor. One interesting aspect of this jamming avoidance response is that the mechanism by which they do it is similar to that of sound localization in mammals. Sound localization in mammals involves differences in timing to the two ears and looking at the interaural timing difference. So the jamming avoidance response is quite similar to that of mammals; the fishes are looking at differences in the electric field across different parts of their body and detecting small tiny phase differences.
So here are two typical fish species with both of them generating an electric field from their EOD (electric organ discharge) at a particular frequency. When you put in an artificial frequency, either 2 Hz above or below that of the fish, the fish will discharge a frequency to try to avoid the frequency of this disturbance. As you can see in the graph above, when the experimenter oscillates the frequency of the discharge, the fish will switch frequency to avoid jamming the electric probe. This is a very common avoidance strategy that many animals employ.
As you can see from the above examples, as animals move into new ecological niches, they encounter new problems that they may previously not have. However, thorough evolution, these animals eventually were able to evolve the necessary solutions to help overcome such problems. We have seen how the bat has evolved into an uncontested predator at night and how it has evolved the necessary and technical abilities to overcome nocturnal navigation. We have seen how the bat’s primary prey, the moths, has evolved the necessary defensive mechanisms to help it survive against the bat. And we have seen how some fishes can use electrical fields to help them navigate what would otherwise be just murky waters. If you find this interesting, a good place to learn more about neurethology is here.
