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Comparative Hearing Julia Nussbacher Melissa Walker Katherine Cummings Kyle Rosenblad Reptiles -Julia Nussbacher Reptiles!!! Reptiles can either have an inner ear or an outer ear, though not in the sense that humans have ears. The external ear of a reptile has a visible tympanic membrane (in humans it is known as the eardrum), which is basically a thin membrane that acts as a barrier between the external portion of the ear and the middle ear. This membrane is either directly next to the skin of the reptile or located deeper in the head of the reptile. The function of the tympanic membrane is to cover the middle ear cavity. On the side of the middle ear cavity adjacent to the inner ear are two distinct openings. One of the openings is circular and is covered by a thin membrane. A membrane does not cover the second opening, which is shaped like an oval and is located closer to the neck of the organism[1]. Reptile auditory organ systems also include stapes, which are bones shaped like stirrups located in the middle ear (in humans it is the smallest bone in the body!!!). The stapes of a reptile cross the middle ear cavity and the inner end is situated in the oval opening while the outer end has a cartilage cap that is adjacent to the tympanic membrane[1]. This particular cartilage cap is also known as the extrastapes and is attached to the quadrate, which provides the majority of the support for the lower jaw. If we look beyond the middle ear we come to the inner ear cavity, which contains the organs, which enable the organism to maintain balance, and also contains the organs associated with reptilian hearing, more specifically, the cochlear duct. The inner ear contains a fluid known as the perilymphatic fluid in which is suspended the saccule and the cochlear duct[1]. There is also perilymphatic fluid within the cochlear duct itself. The duct has two distinct regions, which are called the papilla basilaris and the macula lagenae. These regions are composed of multiple sensory cells with cilia-covered membranes. The cells eventually connect to the auditory nerve. So now that we know about the parts of the reptilian hearing system, lets look at how they work together to allow reptiles to hear! When a sound is made, it produces vibrations that travel through the air and eventually come into contact with the tympanic membrane. Reptiles also have a bone in their body called the quadrate, which can also detect vibrations in the surface, which they are on such as the ground. When the sound vibrations contact the tympanic membrane or the quadrate it causes them to vibrate as well. Vibration of the tympanic membrane causes the extrastapes and therefore the stapes to vibrate as well. The result of this is the movement of vibrations through the middle ear cavity, through the openings and into the inner ear cavity where the cochlear duct is located. The vibrations cause the sensory cells of the cochlear duct to stimulate the auditory nerve facilitate the transmission of the information from the external world to the brain. It is interesting to note that many reptiles lack the tympanic membrane and so instead ‘hear’ with the quadrate. Furthermore, the ability to hear with the tympanic membrane varies among species of reptiles which is the result in variations of membrane thickness, the depth at which the organs are located in the head, and also the relative sizes of the structures which make up the auditory system. Now lets look at the auditory system of some specific categories of reptiles: crocodilians, snakes, and lizards Crocodilians: Relatively speaking, crocodilians are most responsive to lower frequencies of sound. They hear best within the range of 50-1500 Hz[1]. What makes crocodilians unique is that they can hear sound by using organs other than those that are strictly sensory (the ears and jaw bone). Crocodilians have what are called apical pits, which can be found on the scales that cover their bodies. These apical pits can detect vibrations due to sound while the animal is submerged in water. Snakes: As it turns out, snakes do not have external ears like many other reptiles. The skin of snake bellies has tiny mechanisms called mechanoreceptors, which are sensitive to vibrations in the ground. The mechanoreceptors then transmit the vibrations to the quadrate by way of the spinal nerves. The quadrate is a bone located on the side of the head which aides in transmission of sound to the cochlea. After the vibrations pass along the spinal nerves they make their way ultimately to the inner ear. Snakes are most sensitive to sound frequencies around 200-300 Hz[1]. It is interesting to consider here the idea of snake charming. It turns out that the snakes are actually mesmerized by the sight of the flute rather than the sound. That is not to say, however, that the snakes are not aware of the music. They may not hear the music the way we do, but they can sense the vibrations. Lizards: Lizards are capable of hearing sounds ranging from 500 to 4000 Hz[1]. However, they hear best at a frequency of 700 Hz. The papillae (a protrusion at the base of a hair follicle) of lizards’ ears have two distinct types of hair cells. One type has larger diameters at the base, is more numerous, and contains larger afferent (brining towards an organ) nerve cells and efferent (conducting away from an organ) innervations. The other type is smaller in basal diameter, its afferents are of a smaller size and a lesser number, and they are completely lacking in efferent innervations. In addition, the afferent nerve fibers are much more frequency-selective[2]. 1. Kaplan, Melissa. (2002). Reptile Hearing. Herp Care Collection. < http://www.anapsid.org/ reptilehearing.html> 2. Manley, G. A. (2000) Proc. Natl. Acad. Sci. USA 97, 11736-11743. Fish -Melissa Walker If you walk along a stream, the term ‘babbling brook’ will certainly make a great deal of sense. Turbulent water like this can be quite loud when perceived by our ears and by those of other creatures as we walk by on land. Imagine, though, how the many species of fish living within the stream itself must deal with the constant noise of running water. Even areas underneath grand waterfalls are home to an abundance of fish, all of whom share similar ear structures for hearing. These ears, however, are quite different from our own due to the environment that shapes them. Sound for fish, of course, does not travel in waves through the air as it does for us. Instead it (quite literally) travels in waves through the water. Since air density is much less than that of water density, the mechanics of these waves change. Sound actually moves much faster through water than through air, and this leads to its wavelength also being exceptionally longer. This means that noises at certain frequencies may have longer wavelengths than the fish’s body itself! Typically, this constrains fish to hearing a range of low frequency noises roughly between 40-1,000 Hz. As far as perceiving these waves is concerned, density plays a large role. For fish, whose flesh density is roughly equivalent to the water itself (lest they sink or float), these sound waves would pass through them without disturbing any tissue. Their auditory structures, appropriately then, are designed to be different densities than the surrounding environment. Fish boast a great diversity of hearing organs and hearing methods among themselves, but are basically made of the same components. Two inner ear assemblies, made of bone, contain structures such as the otolith, saculus, lagena, and cranial nerve. The otolith itself is covered in more than 100,000 hair cells, which are shifted by the sound wave. This directionally specific change displaces the otolith, since it has a greater density than water. The cranial nerve perceives this shift, and the message then travels to the brain. Since fish live in such diverse environments, some have more advanced auditory methods than others. Fish in streams, for instance, benefit from simpler ears, since the background noise they experience overpowers quiet signals. Fish in the deep ocean, on the other hand, have extremely complex systems, since their environment is so quiet and sensitive to the smallest noise. Lack of other sensory, since there is no light to see by, also puts strain on having good ears at such depths. In general, this divides fish into two broad groups: hearing specialists or hearing generalists. Hearing generalists typically live in loud stream environments that necessitate a high threshold for noise. Hearing specialists- the fish that live in quieter waters- often use other organs to enhance their hearing sensitivity, such as their swim bladders. These gas filled sacs are sensitive to sound vibrations because they are much lighter in density than the fish’s flesh or the water surrounding. By connecting this bladder to the inner ear, most successfully through Weberian ossicles, these vibrations can be transmitted and perceived. Fish with this adaptation can hear a much greater range of frequencies than those with simpler systems. In general, fish can be just as sensitive to auditory stimulation as other vertebrates. They can also distinguish between a sound’s intensity, frequency, and direction, as well as pick out certain tones among white noise. Since these capabilities are so similar to that of humans, it is thought that fish may have laid the evolutionary precursor to our own ears. Sources: http://www.parmly.luc.edu/parmly/fish_aud_psych.html http://jn.physiology.org/cgi/content/full/77/6/3060 http://www.life.umd.edu/biology/popperlab/research/deepsea.htm http://www2.biology.ualberta.ca/jackson.hp/IWR/Content/Anatomy/Inner_Ear/index.php Dolphins -Katherine Cummings Sound travels five times faster in water than air, making a dolphin’s sense of sound its most important sense. Without it, a dolphin would be unable to communicate or locate objects in its environment. Dolphins use echolocation, a process so precise it could be referred to as “seeing with sound.” First, a dolphin generates a clicking noise with its nasal sacs, located behind its forehead (or “melon.”) The melon consists of fatty tissue and fluid and acts as an acoustic lens, focusing each click into a narrower, more direct path. When the sound reaches an object, some of the energy bounces back to the dolphin in the form of an echo. The echo reaches the panbone of the lower jaw, and is transmitted to the middle ear by the fatty tissue behind the panbone. From the middle ear, the sound travels to the brain. A dolphin can tell how far away the object is by emitting more clicks, evaluating the length of time between each click and echo to determine the distance. Depending on which side of the panbone receives the echo, a dolphin can also determine the exact direction of the object. This system is so precise, a dolphin is capable of locating a pingpong ball- sized object that is a football-field away. Dolphins can hear 7.5 times more accurately than humans; their range is from 0-150 kHz, and humans can only detect sound from 0.2 kHz to 20 kHz. They can echolocate on distant and proximate objects at the same time in a noisy area, while simultaneously whistling to communicate with other dolphins. DOLPHIN COMMUNICATION Every dolphin has its own unique whistle, which aids in identification and efficient communication. These whistles, known as signature whistles, can be thought of as dolphins’ names. Dolphins emit their whistles when looking for prey, in danger, or trying to locate members of their family. They’re social creatures, acting together to find prey or protect each other from danger. These whistles allow dolphins to find one another in bad visual conditions over long distances. Without their fine-tuned sense of sound, dolphins would be solitary and wholly unable to navigate the sea. http://www.dolphins.org/ http://www.inkokomo.com/dolphin/echolocation.html http://www.dolphinsplus.com/dolphin-information.htm#echolocation Bats -Kyle Rosenblad Echolocation in Bats Basics: Echolocation is a sensory system used by a variety of animals—such as bats, porpoises, toothed whales, and a few species of birds and shrews—in which they emit sounds and listen to the returning sound waves that bounce off of objects in their environments to locate those objects. The brain of an echolocator is able to process fine details of the returning sound waves to determine the distances, shapes, and relative directions of surrounding objects (“echolocation”). All known bats of the suborder Microchiroptera, or small bats, are echolocators, whereas no known bats of the suborder Megachiroptera, or large bats, except those of the genus Rousettus use echolocation (“sound reception”). Ear Structure: Bat ears are adapted for echolocation. Their large pinnae make optimal sound collectors, and they are able to very precisely manipulate their ears in order to focus on specific auditory stimuli. Bats are thus adept at “funneling” desired sounds toward the inner ear (“sound reception”). Frequencies: The cries of most echolocating bats range from 80,000 to 30,000 hertz (“sound reception”). They use such high frequencies because in order for a bat to gain an accurate “auditory image” of its surroundings, the sound waves it emits must be small in relation to the objects off of which they will bounce and return. This helps the bat better discern differences in shape, distance, and direction of surrounding objects. The finer the sound waves it receives, the more detailed the spatial information it can glean (“sound reception”). In addition, animals with smaller heads must use higher frequencies because the distance between ears is an important factor in distinguishing details in echolocation. When the ears are close together, the differences between the waves returning to one ear and those hitting the other are less pronounced, so the bat must “counter” by emitting cries of a high frequency (Heffner et al). Predation and Coevolution: Many bat species use echolocation for hunting (“bat”). Often, natural selection engenders an auditory/vocal “arms race” between echolocating, predatory bats and their prey. For instance, some species of echolocating bats in Canada and Côte d'Ivoire prey primarily on moths. Many of these bats use frequencies between 20,000 and 40,000 hertz for echolocation. Unfortunately for these bats, natural selection has thus favored moths that are able to hear well in this frequency range and thus anticipate bat predation. However, natural selection has given some bat species another leg up—these species have developed echolocation strategies that use lower frequencies that are much more difficult for moths to detect (Fenton and Fullard). In this way, natural selection shapes the strategies that predator and prey use to achieve their respective goals of eating and escaping. Those who manage to eat or escape are given the best chance at surviving to reproduce and pass their advantageous traits on to subsequent generations. References "bat." Encyclopædia Britannica. 2006. Encyclopædia Britannica Online. 22 Oct. 2006 <http://search.eb.com/eb/article-252422>. "echolocation." Encyclopædia Britannica. 2006. Encyclopædia Britannica Online. 22 Oct. 2006 <http://search.eb.com/eb/article-9031903>. Fenton, M. Brock and James H. Fullard. “The influence of moth hearing on bat echolocation strategies.” Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology. Volume 132, Number 1 / March, 1979. Pages77-86. Heffner, H.E., R.S. Heffner, and G. Koay. “Hearing in large (Eidolon helvum) and small (Cynopterus brachyotis) non-echolocating fruit bats.” Hearing Research Volume 221, Issues 1-2 , November 2006, Pages 17-25. "sound reception." Encyclopædia Britannica. 2006. Encyclopædia Britannica Online. 22 Oct. 2006 <http://search.eb.com/eb/article-64827>.