How head related transfer functions, ear sensitivity and outer ear structure are related to predatory and defensive behavior of mammals Maxwell Alexander, Sarah Kaddatz, Colin Seaman, Jess Villasana Introduction to Psychoacoustics Pantelis Vassilakis
Abstract This paper explores the relationship between the nature of mammals’ pinna (specifically the domesticated cat, elephants, deer and polar bear) and their position on the food chain as either predator or prey. Research was primarily derived from scientific journal articles with general information gathered from books and websites. We focused on the head related transfer functions (HRTF): how head size, pinna size and the distance between the pinnae related to the ability of each animal to localize sound. Also researched was how the shape of the pinna changed the spectrum of a sound at the ear. Our initial hypothesis was that the shape of each animal’s pinna would directly relate to their position as predator or prey and that the predators and the prey, respectively, would share similarities in their pinna shape. Whether an animal was predator or prey would be determined by the degree of pinna mobility, pinna size relative to the head and body, the shape of the pinna and whichever band of frequencies the mammal was most sensitive to. Our research revealed that pinna shape does not necessarily connote whether an animal is a predator or prey, but rather it is a reflection of evolutionary and environmental factors. What determines predatory and defensive nature is the pinna functionality as opposed to size and shape.
Discussion The main focus of the research was on different mammalian species and to make connections between the pinna shape and size and the correlating trophic position. The majority of the research done was focused on the sound localization, and HRTFs of elephants and cats, with additional research into the whitetail deer and polar bears. Upon researching elephants, who have a very large and mobile pinna, it became quite clear that they have very acute sound localization, but only in the lower frequencies. The hearing range of the elephant is from 10 Hz to about 12 kHz, the lowest high frequency cutoff of any mammal. A study showed that elephants can accurately localize some sounds within two degrees of change. However, this test was only using white noise, so another test was doing testing different frequency bands. The Elephant was most acute between 125 Hz and 250 Hz, but as the sounds increased in frequency the acuity of sound localization dropped dramatically. Once 2 kHz was tested, the elephant could barely localize the sounds (Heffner and Heffner, 1982). This is probably due to its very large head and interaural distance (the distance between the two ears). Since elephants cannot really hear high frequencies as well as low frequencies, the study leads one to believe interaural phase differences play a major of the role in its sound localization abilities. Cats, just like elephants, have mobile pinnae and would move them to a reference position when localizing sounds. The cat’s upper and lower thresholds of hearing are shifted one octave higher, putting it’s range of hearing between 50 Hz and 22.5 kHz. All of the tests on cats were done on domesticated house cats and largely focused on their ability to localize sounds with
its mobile pinna structure. The research showed that a mobile pinna does not directly contribute to sound localization acuity, except when binaural cues serve no function-when a source is directly behind a subject (Heffner and Heffner, 1988). It was determined that the mobility of the pinna was used for sound coloration (changing the spectrum). Pinna movements allow for a better focus on certain frequencies, while being able to attenuate other frequencies. Frequency response tests done on the cat show that between 4 and 6 kHz there is a fairly large energy peak, and around 8.5 kHz there is a large energy drop (Heffner and Heffner, 1988). It appears that cats would use this to their advantage to improve the signal to noise ratio, allowing for a better focus on a specific sound source. This idea leads to the belief that sound localization is not improved by mobile pinnae. It would appear that a larger interaural distance correlates to a more acute sense of sound localization, but at the expense of the ability to hear higher frequencies. The smaller the interaural distance, the better ability to perceive high frequencies (Heffner and Heffner, 1988). Unfortunately, there is not as much research available on the whitetail deer and polar bears as there is on cats, and elephants. Based on what is available, there are a few useful pieces of information that would positively contribute to the hypothesis. The first is whitetail deer have the most sensitive hearing between the frequencies of 4 and 8 kHz which were found to be the frequencies that the deer use to(D’Angelo et al., 2007). This can lead to the conclusion that animals evolve in a way that is best suited for survival; they need to find a mate in order to reproduce and keep the species going. The second was found while testing polar bears. Sounds of ringed seals (polar bears primary prey) were played to a pair of polar bears in captivity. Once heard they heard the sounds of the seals, their ears perked up, they began to sniff the air, and
visually began to scan their cage. After searching without finding the seals they became very agitated, clawing and chewing at their cage bars (Nachtigall et al., 2007). This observation leads to the idea concerning predatory nature, predators will use all of their available senses in order to locate their prey After research of the relationship of different mammalian pinna shape and size and its correlation as to mammals classified as predator or prey was completed, two things were seen. Firstly, there appears to be no evidence supporting the initial hypothesis of pinna shape and size affects the position as predator and prey; secondly, there can be a clearly seen relationship between the range of hearing and the distance between the two ears (size of the head). When looking at the ability of animals to localize sounds, the initial assumption was that animals with mobile pinnae would have better acuity when it came to sound localization. It was discovered that there was no evidence to support that assumption. When looking at the different mammals that were researched, they all had mobile pinnae of varying sizes and shapes (the largest being the Elephant and the smallest being the domesticated cat). After analyzing the differences in size and shape, it can be determined that animals develop these wide variety of pinna shapes can be attributed to evolutionary necessity (Atwood, 1955) instead of trophic placement. This is likely due to evolution so that elephants can communicate at frequencies that would be too low for their predators to hear (Readers Digest, 1997). All of the animals researched have mobile pinnae, and all of them use that mobility to aid with sound localization. Out of all of them, the elephant has the most acute sound localization and the largest pinnae, but the size of the pinna is not the reason (Readers Digest, 1997). All of the research into sound localization shows that there does not appear to be any advantage of a
mobile pinna with regards to sound localization. The best example of this is that humans, who do not have a mobile pinna, rank very highly in terms of having an acute sense of sound localization (Heffner and Heffner, 1988). Out of all the animals researched (with a possible exception of the Elephant), humans can detect the smallest amount of change of axis (Heffner and Heffner, 1982). So if mobile pinna does not help improve sound localization, what does? All of the research done into the HRTFs (head related transfer functions) of the four different animals examined showed a correlation between interaural distance and sound localization acuity. However, if the mobility of pinnae did not relate to sound localization, then what does it do? When examining the domesticated cat, research shows that having a mobile pinna structure does serve a distinct advantage: while observing tests of a house cat it was shown to move its pinnae asymmetrically so that each pinna would color the sound differently. Therefore it would appear that with more extreme pinna movement, the more the cat would be able to focus in on a specific sound source (Populin and Yin, 1998). After reading the research on the relationship between the pinna shape and trophic level in animals it has become abundantly clear that the hypothesis that started the entire project did not hold true. Much was learned that, while not supporting the hypothesis, did allow for a better understanding of how the pinna shape and size does affect the animal. The size and shape of the pinna are a reflection of how it has evolved to suit its surroundings and because of that, the frequencies that animals are most sensitive to correspond to the frequencies with which their communication is primarily comprised. Also because of the research, it became apparent that the mobility of the pinna allows for a better ability to pick out specific sounds and for better tracking
of movement. The research contributed to a greater understand of sound localization and how it relates to HRTFs. In short, the research initially showed that with a larger distance between the two ears, the more acute sound localization, but it was discovered that this assumption did not hold up. There were many examples of animals with large interaural distances that did not fit in with the assumption. For example horses and cows have similar interaural distances to that of humans, but very poor sound localization (Heffner, 1997). Further research into the discrepancies would lead to a comparison between the width of the best field of vision along with sound localization. When the two were compared it become clear that with a better field of vision came more acute sound localization (Heffner, 1997). This comparison allows for the defining relationship between pinna function and predatory nature. Predators use the mobility of their pinnae to focus on what is in their field of vision to closely track their prey. The prey in turn have less acute sound localization and fields of vision because it is not necessary for them to survive. The prey just need to be able to get a rough idea of where a sound is coming from in order to know that it needs to go the opposite direction. A simple examination of pinna structure through visuals demonstrates an error in our hypothesis. Mountain lions are capable of attacking strong and healthy deer, while the wolf and coyote aim for weak and malnourished deer. They visually have similar pinnae structure, but the coyote and wolf are not as effective because they are smaller animals relative to the mountain lion (Trophyseekers Worldwide, 2011).
Conclusion Much of the initial speculation involved with the initial hypothesis ended up having little scientific basis. In the end, plenty of our speculation did have some merit, but for different reasons. The reason the pinnae of animals are a certain size and shape is a direct result of evolution, due to that both predators and prey share very similar pinna function rather than shape. In order for predators to survive, they need to be very adept at tracking their prey and thus many of them have a mobile pinna structure allowing them to pick out movements of their prey more effectively. The prey, in turn, have to be adept at not getting eaten by their predators, so they must be able to pick out the movements of their predators. We can conclude that a mobile pinna is crucial for survival. The animals that exist today are here because they are best suited to survive in their environment, and the shape, structure, and functionality of their pinna is a direct representation of how they would survive in their environment. Animals that live underground have pinnae that can seal of their ears and keep dirt out. In general, the frequencies most amplified through the hearing process are those of intercommunication with their kin. This allows for effective mating detection and warning of oncoming predators or nearby prey. Communication is key to trophic placement. Humans are on the top of the food chain because of this.
Works Cited Atwood, W. H. (1955). Auditory Organs. In Comparative Anatomy (2nd ed.) St. Louis: The C.V. Mosby Company. “In some mammals, such as elephants, donkeys, and rabbits, the [pinna] may be very large. Elephants fan themselves and drive away flies with them. In burrowing and aquatic mammals the ears are small. Some people are able to wiggle their ears like animals, but in many the vestigial muscles and nerves for the purpose are nonfunctional” (Atwood 257). This suggests that the pinnae of animals have multiple purposes besides sound location. It also suggests that animals have different pinnae because of where they live, not what they eat or get eaten by. D’Angelo, G. J., De Chicchis, Gallagher, G.R., A. R., Miller, K.V., Osborn, D. A., & Warren, R. J. (2007). Hearing Range of White-Tailed Deer as Determined by Auditory Brainstem Response. The Journal of Wildlife Management 71(4): 1238-1242. This journal's goal was to figure out how best to calibrate 'sound fences' so that deer would not run across roads and get hit by cars. What was useful to our research was that they figured out the hearing range of deer and the frequency band at which they are most sensitive. The hearing range of the white-tailed deer tested to be between 250 Hz - 30 kHz with best sensitivity between 4 kHz and 8 kHz. These frequencies were found to be the main communication frequencies that these deer use amongst themselves. Earle, O. L., & Kantor, M. (1974). Animals and Their Ears. New York, NY: William Morrow and Company, Inc. This source talks about the basic structure of various animal’s outer ears. It is great for the pictures presented for the basic look of their pinnae. It also included general measurements for the size of the pinnae associated with the animal. In general, the text notes: “The change in the direction of the pinna may enable the animal to collect sound waves and to funnel them down the channel that connects with the middle ear more efficiently” (7). The following are facts gathered from this book on pinna size of different animals: 1) Each pinna of the African Elephant may measure 3 ft across. The Elephant is approximate 10-11 feet tall. The pinnas normally rest against the elephant’s head and shoulder, but are brought forward upon hearing any sort of noise (10). “Elephants have comparatively small eyes and poor eyesight, but their acute sense of hearing [ makes up for it]” (11). Indian Elephants have considerably smaller pinnas than African Elephants (11). At one point we tried to find the frequency ranges of both African and Indian Elephants and compare them, but we could only find them for the Indian Elephant. 2) Antelope jackrabbit has pinnas up to 18” long while its head and body combines are about 23” (22-23). The eastern cottontail has 3” long or less pinnas while their head and body are approximately 16” long (23). “Hares and rabbits may carry their pinnas pointing backward, though usually they are held erect... But the lop-eared rabbit’s twelve-inch-long ears always droop downward. This animal is never found wild; it is
especially bread and is kept as a pet” (24). Since the lop-eared rabbit is not found in the wild, we took that to mean that it could not survive in the wild. Its lopped ears would act as natural earmuff. It would have a difficult time using its hearing to locate predators. 3) Mule deer have pinnas that are approximately 8” long (25). 4) The common Indian Mongoose has small pinna that can seal off the external ear while they are digging to prevent dirt. Mongooses are good predators. They were able to fight off plague rats and kill venomous snakes (31). Elbert, C. S., Blanks, D.A., Patel, M.R., Coffey, C.S., Marshall, A.F., Fitzpatrick, D.C. (2008). Behavioral Sensitivity to Interaural Time Differences in the Rabbit. Hear Res 235(1-2):134-142. “An important cue for sound localization and separation of signals from noise is the interaural time difference (ITD). Humans are able to localize sounds within 1–2° and can detect very small changes in the ITD (10–20 µs). In contrast, many animals localize sounds with less precision than humans. Rabbits, for example, have sound localization thresholds of ~22°. There is only limited information about behavioral ITD discrimination in animals with poor sound localization acuity that are typically used for the neural recordings. For this study, we measured behavioral discrimination of ITDs in the rabbit for a range of reference ITDs from 0 to ± 300 µs. The behavioral task was conditioned avoidance and the stimulus was band-limited noise (500–1500 Hz). Across animals, the average discrimination threshold was 50–60 µs for reference ITDs of 0 to ± 200 µs. There was no trend in the thresholds across this range of reference ITDs. For a reference ITD of ± 300 µs, which is near the limit of the physiological window defined by the head width in this species, the discrimination threshold increased to ~100 µs. The ITD discrimination in rabbits less acute than in cats, which have a similar head size. This result supports the suggestion that ITD discrimination, like sound localization (see Heffner, 1997, Acta Otolaryngol Suppl 532:46–53, 1997) is determined by factors other than head size” (134). Fay, R. R., & Popper, A. N. (2005). Comparative Mammalian Sound Localization. In Sound Source Localization. New York: Springer Science + Business Media. The results of the HRTF of a domestic house cat shows us were outlined in chapter five of this book. Frequencies below 5,000 Hz are amplified by the pinna structure of the cat. There is an energy peak around 4-6 kHz. This is the range of frequencies that have the most amplification and are the main communication frequencies that cats talk to each other. Midrange frequencies between 5kHz and 20 kHz are shown to have a spectral notch where energy attenuation occurs. Above 20 kHz, the spectrum gains complexity in the transfer function and contains many peaks and notches as the response declines as the frequency is increased (157). Heffner, R. S. (1997). Comparative study of sound localization and its anatomical correlates in mammals. Acta Otolaryngological (Stockholm), Suppl. 532, 46-53. This is by far the source that correlates to our topic the most. It questioned our initial end result that head size was the determining factor in localization. Its findings were that “head size is not a very good predictor of sound localization acuity” (48) and had graphs to demonstrate the relationship between interaural distance (microseconds) and the
sound-localization threshold (in degrees) It also demonstrated that animals of a predatory nature had better visual perception and sound localization acuity. This makes sense because when animals are hunting, they need to be able to pinpoint their prey in order to grab it properly. The animals that are scavengers have a wider range of vision and hearing to determine if something is coming for them. If they hear or see anything that is somewhat dangerous, they run in the opposite direction. Prey animals tend to have eyes closer to the sides of their head, and, for instance, are not able to focus on the point of a pen. They merely see that the pen is there. Their audio localization is directly related to this. “The width of the field of best vision is a reliable predictor of sound-localization acuity” (51) and has a relation coefficient of 0.922. “Vision, of course, is strongly influenced by an animal’s lifestyle, with the result that lifestyle factors can become apparent correlates of auditory abilities. An example of this correspondence is the relation between trophic level and sound-localization acuity predatory species tend to be better localizers than prey species (r=0.643). However, predators also tend to have their best vision directed forward and, more importantly, concentrated in a relatively narrow region; as a result, when the influence of field of best vision is removed mathematically, the correlation between trophic level and sound localization collapses to insignificance (r=0.132)” (50-51). It was also noted that “if we factor out the contribution of field of best vision from the correlation between interaural distance and sound-localization acuity, there seems to be no remaining influence of interaural distance” (51). While increasing the distance between the ears may allow for better localization of low frequencies, overall soundlocalization acuity is more related to field of best vision. This source allowed us to determine characteristics of predator and prey animals and draw the conclusion that while they have defining characteristics, the hearing system by itself is not the main contributing factor to trophic level (the placement on the food chain). Heffner, R.S., & Heffner, H.E. (1982). Hearing in the Elephant (Elephas maximus): Absolute Senstivity, Frequency Discrimination, and Sound Localization. Journal of Comparative and Physiological Psychology, 96(6): 926-944. This article goes very in depth about the sound localization of elephants, focusing in on tests involving both clicks (100 microseconds) and white noise (100 milliseconds) as well as pure tones from 125 Hz to 8 kHz. From the basic results of the tests, Heffner showed that elephants where most accurate at localizing white noise over clicks, over 70 percent accurate down to 2 degrees. And in pure tones it was most accurate at locating the lower frequency bands off 125 and 250 while at frequencies above 2 kHz is virtually cannot localize the sounds. (394-397) The article then takes that information and makes a direct comparison between head size and ability to accurately localize sounds, basically the larger the head the more accurate localization (941). It also makes a comparison between low and high frequency hearing limits, where the basic idea is the lower the low frequency hearing capability extends, the lower the high frequency capability extends (939-940). It also recognizes the draw backs of the tests most notably saying that none of the animals tested for localization where real carnivores that hunted, they where mainly
omnivores that did not really need that accurate sound localization to survive. Another note is made that only one elephant was tested during the experiments and the authors hypothesize that with further testing elephants would show to have an even higher level of acuity when localizing sounds. One very interesting note in the article is that the elephant would actively extend the large pinnae during the localization tests, and during frequency discrimination tests it would leave them relaxed, from this observation alone, it can be concluded that its mobile pinnae play a large role in the elephants ability to localize sound (936-937). Heffner, R. S., & Heffner, H. E. (1985). Hearing range of the domestic cat. Hearing Research, 19 (1), 85-88. "In mammals, high frequency hearing ability is correlated with functional inter-aural distance such that mammals with small inter-aural distances are better able to hear higher frequencies than larger mammals" (87). This tells us that animals with smaller heads have a higher sensitivity to higher frequencies than those with bigger heads. This is what lead our research to discuss the different frequency sensitivities of the different animals. Heffner, R. S., & Heffner, H. E. (1988). Sound localization acuity in the cat: Effect of azimuth, signal duration, and test procedure. Hearing Research, 36(2-3), 221-232. When comparing mobile to non-mobile pinna, there does not seem to be any indication that mobile pinna are superior in localizing sound sources. Humans and cats have nearly identical thresholds when localizing sounds at 90 degrees away from the center axis, in the so-called ‘cone of confusion’, where the subject cannot use binaural cues. Humans have superior sound localization at all angles where binaural cues are available. The difference that mobile pinna seems to make is in how quickly the localization threshold gets worse the more off-axis the sound source becomes (the 90 degree threshold for cats is only twice that of their 0 degree threshold). “A simpler explanation is that the proportionally smaller increase in the lateral localization ability of cats is due to their poorer frontal localization acuity and not to any advantage of their pinnae for lateral localization” (231). It is possible to conclude that the difference between the ability of cats and humans to localize is in the head size. While cats and humans have fairly similar pinna sizes, humans have much larger heads which may serve to enhance both binaural and monaural cues. Because of HRTF’s and head shadowing, there is a greater difference between what each ear ‘hears’, making localization acuity better. “During tests of frontal sound localization acuity, an Indian elephant was observed to extend its pinnae nearly perpendicular to its head at the beginning of each trial and then return them to the relaxed position against its head following presentation of the sound. This behavior was specific to sound localization since the elephant did not extend its pinnae during tests of absolute or frequency-difference thresholds. ... Further, the elephant made more errors on those trials in which it did not extend its pinnae, indicating that pinna extension may be necessary for accurate localization.” (231) This activity seems to indicate that for animals with mobile pinnae, the accuracy with which sound is located depends very much on how the pinnae are moved. It is also possible that the auditory system of animals with mobile pinnae cannot fully compensate for variations in
pinnae position when localizing sound and must place them in a ‘familiar’ position (perpendicular to the head) in order to accurately localize sound sources. The size of the head and the ability to perceive frequencies of smaller wavelengths (high frequencies) are directly related: smaller the head/distance between the pinnae, the higher frequencies the animal seems to be able to perceive. Also, there is a relationship between the size of the head and the ability of an animal to accurately localize sound. It seems that the greater the distance between the pinnae (larger the head), the more accurately an animal can identify locations (humans, elephants). Knowing this we can conclude that the smaller the head, the less adept a creature will be at localizing specific sound sources, but it will be able to perceive higher frequencies. Conversely, the larger the head the more accurate the creature will be at localizing, but the highest perceivable frequency will be relatively lower. Nachtigall, P. E.*, Amundin, M., Møller, T., Mooney, T. A., Röken, B., Supin A. Y., Taylor, K. A., and Yuen, M. (2007). Polar bear Ursus maritimus hearing measured with auditory evoked potentials. The Journal of Experimental Biology, 210(7): 1116-1122. *Author for correspondence regarding this article. First, they played sound ringed seals make (they are polar bears' primary prey) to polar bears in captivity and observed their actions. "The bears erected their ears, lifted their heads, visually scanned the room and then began sniffing. As the ringed seal calls continued to be played the bears became active, paced their cage, groaned and chuffed, then pawed and chewed at their cage bars. ... the bears responded to their primary prey's underwater vocalization, presented in air, in a manner that indicated some importance of in-air hearing in detecting and locating their under-ice prey." This can be used as proof in that land-based-mammal predators may initially locate prey by hearing, but then switch over to more powerful senses like smell. The other part of this article was actually giving hearing tests to three anesthetized polar bears. They measured from 1kHz to 22.5 kHz and found that the most sensitive band was between 11.2 and 22.5 kHz, the threshold of hearing in that section being less than 27 30 dB (very good). This means that polar bears have an acute and wide-frequency range sense of hearing. Plack, C. J. (2005). The Sense of Hearing. Mahwah,NJ: Lawrence Erlbaum Associates, Inc. This source goes in depth into discussion about localization through the Head Related Transfer Functions, bi-aural cues, and even pinna structure. Surprisingly, it even compared the pinna function of humans to animals, which is quite essential to our topic. These cues allow us to further pinpoint our target, or prey. “The pinna are more important in other animals (bats, dogs, etc.) than they are in humans. Our pinnae are too small and inflexible to be very useful for collecting sound from a particular direction, for example. They do, however, cause spectral modification (i.e., filtering) to the sound as it enters the ear, and these modifications vary depending on the direction the sound is coming from. The spectral modifications help the auditory system determine the location of a sound source” (63). This quote suggest that flexibility and size of the pinnae play a role in collecting sound from that particular direction. Larger pinna that are mobile can help to collect sound from different directions, and therefore do
not need the various cavities as much for spectral modification cues to help with localization. “Much of the ambiguity about sound location can be resolved by moving the head. If a sound source is directly in front, then turning the head to the right will decrease the level in the right ear and cause the sound to arrive at the left ear before the right ear. Conversely, if the sound source is directly behind, then the same head movement will decrease the level in the left ear and cause the sound to arrive at the right ear first. If the head rotation has no effect, then the sound source is either directly above or (less likely) directly below. By measuring carefully the effects of known head rotation on time and level differences, it should be possible (theoretically) for the auditory system to specify the location of any sound source, with the only ambiguity remaining that of whether the source elevation is up down. If head rotations in the median plane are also involved (tipping or nodding of the head), then the location may be specified without any ambiguity” (184-185). According to a study conducted by Thurlow, Mangels, and Runge, turning the head is only useful for sounds that transpire over an extended period of time. This way the listener can hear the same sound with multiple pinnae positions. If a sound is fleeting, the listener will not be able to make this head movement before the sound has already decreased to levels at or near inaudibility. An example of this is a twig snapping (185). It is also important to note that the cavities present on the pinna can cause certain spectral modifications to the sound (amplification of frequencies whose wavelengths match the resonant frequencies of the cavities). For human ears, these spectral modifications consist of high frequencies. “The precise pattern of peaks and notches depends on the angle at which the sound wave strike the pinna. In other words, the pinna imposes a sort of directional ‘signature’ on the spectrum of the sound that can recognized by the auditory system and used as a a cue to location” (186). These cavities are thought to help with the localization along the medial plane. The overall pinna will tend to have a shadow effect that allows us to help distinguish if a sound is coming from in front or behind us. If it is behind us, the higher frequencies are slightly attenuated because of this shadowing effect. In order to know that the sound is in fact behind us we would need to know what the sound would sound like before this attenuation takes place to know that the high frequency content is being attenuated. This means, that for the shadow effect to help us localize sounds, we have to be very familiar with the sound we are trying to localize (185-186). Populin, L. C., & Yin, T.C. (1998). Pinna Movements of the Cat During Sound Localization. Journal of Neuroscience 18(11): 4233-4243 The experiments outlined in this article attempted to determine the direct role of pinna during sound source localization in the domestic house cat. There was speculation that pinna mobility could help the animal process more than one sample of a sound, and that it might separate the spectrum of a sound source from the HRTF. This source provided information about: the significance of symmetry and asymmetry of positioning when an animal moves pinna during localization, pinna movement in response to acoustic stimuli was done more rapidly than with visual stimuli alone, and the observation of what could
possibly be a ‘starting position’ when an animal anticipates a sound that gives an acoustic advantage to the listening animal. “It appears that as it prepared to localize sound, it pulled its pinnae to a position that provided some acoustical advantage. Bringing the pinna to a standard position when certain of having to localize the source of a sound could be a simplifying strategy that could facilitate localization” (4242) Three types of pinna movements were observed: the difference between movements in visual targets and acoustic targets, asymmetrical pinna positioning for horizontal and vertical sound sources, and a complex series of rotations and displacements (2) “These series of images illustrate (1) that the pinna moved in the general direction of the target; (2) that the movements of both pinnae were not necessarily symmetrical for horizontal or vertical targets; and (3) that in some instances, the pinna response was the result of combined complex rotations and displacements” “The broadband noise stimulus also evoked pinna movements in the direction of the sources... The major differences [between the pinna movements to visual and auditory targets] is that acoustically evoked movements have shorter latencies than those evoked by visual targets” (4238). Researchers were able to conclude that the pinna served such functions as sampling an acoustic source multiple times within a very short time period, and extracting the spectrum of a sound source from the HRTF. Also, the conclusion was drawn that larger pinna movements could improve the signal to noise ratio by focusing the pinna more on the object (4243). Rahn, J. E. (1984). Ears, Hearing & Balance (1st ed). New York: Atheneum. This source explains the function of the pinna and HRTFS in very general terms. It helped us to understand the exact function at its basics. The basics here were very important to grasp if we were going to compare different species based on these ideas. “The function of the pinna is to reflect sound waves to the ear canal. Most of us can move our pinna only slightly, if at all; but many other animals with pinnae (mostly mammals) can move them a great deal more... in this way they collect sound from several directions without moving their heads or bodies. This may enable them to detect where danger or prey lies without revealing their positions. Humans fall into the few species of mammals with stationary pinnae” (30-31). This provides evidence to one of our ideas that mobile pinna can help in localization in a similar fashion that moving our head does. This allows for different colorization of the sound and allows for a more stealthy defense from prey, as well as a more sneaky approach for predators. “The difference in the intensity of the sound that reaches each ear depends in part on the exact location of the sound source. If it lies on a line that goes through both ears... then the difference in intensity at the two ears will be greater than if the source is almost directly in front of you and only a little to the right or to the left. The difference in intensity helps you to determine the direction from which the sound came. If the sound lies directly on the midline of the head...the intensity of the sound reaching the two ears will be the same” (96).
“Therefore sound waves of longer wavelengths bend around the head easily and there is very little difference in the intensities of sound at the two ears. With sounds of higher frequencies - at least 1000 Hz; 4000 Hz is better - we usually localize sound easily by intensity differences” (97). Reader's Digest Association. (1997). Intelligence in Animals (105). London: Reader's Digest Association. “[African Elephants] rumble to each other at sound frequencies well below the spectrum of human hearing. The sounds... fall below 20 Hz. Even at 30 Hz the sounds must be intense for us to hear them” (102). This allows them to communicate to each other and warn of oncoming predators such as poachers at a frequency we cannot hear, or even acknowledge without special equipments. In this case, their hearing acts as a defense warning system amongst them instead of detecting oncoming predators. “Echolocation is used both by animals with large pinna in proportion to their head-size and animals with no pinna at all; both bats and dolphins use peak frequencies from 14 to 100 kHz. This lets us assume frequency sensitivity is not necessarily reliant on size of pinna of animal.” (105). This sentiment that frequency sensitivity is not necessarily based on the pinna size lead our research into studying the head related transfer functions when we could find them on various mammals. Trophyseekers Worldwide. (2011) Predators of the Whitetail Deer. huntingnet.com Retrieved from http://www.huntingnet.com/staticpages/staticpage_detail.aspx?id=248. This source was chosen to see what animals hunt the whitetail deer. We used this to discover what animals we could research as predators, relative to the whitetail deer. The natural predators of the whitetail deer are the mountain lion, wolf, and coyote. The bear, bobcat, wild dog and alligator are “predators which do not pose a huge threat to the deer.”. Whitetail deer are also hunted by humans. From this source, we can relate a cat’s ear structure to that of the mountain lion and consider the whitetail deer a prey animal and the cat as a predator animal. This source does disprove our theory in the sense that the wild dogs do not pose a significant threat whereas, wolves and coyotes do. Their ear structures are similar, as discussed on other sources. This source also disproves our theory that ear structure plays a major role in the food chain because mountain lions are capable of attacking strong and healthy deer, while the wolf and coyote aim for weak and malnourished deer. They visually have similar pinnae structure according the pictures present on the page, but the coyote and wolf are not as effective because they are smaller animals relative to the mountain lion. Walker, W. F., & Homberger, D. G. (1998). Nervous Coordination: Sense Organs. Anatomy & Dissection of the Fetal Pig (5th ed.). New York: W.H. Freeman and Company. This book goes through the proper procedure on how to dissect a fetal pig. It notes the different organs present as the dissection commences and notes how they are similar to the human body. It does not go into any detail about the pig’s auricle except to say that it “gathers and focuses the sound waves on the eardrum” (95) and that it needs to be cut off during the dissection process. However, it states: “The human auricle does not normally move, but you may have noticed on a dog or cat that the auricle is turned and directed toward the source of a sound the animal is
interested in� (95). This informs us that members of the cat and dog family have mobile pinnae that are used to help with localization of a sound source.