The middle ear contains the incus, malleus, and stapes, all of which are instrumental in detecting timbre, loudness, and pitch. Neil Shubin mentions that these bones are instrumental to hearing through air but vaguely references that "hearing in water is different from hearing on land...". (164) Discuss the differences in hearing by comparing acoustic structures from land mammals and aquatic mammals. Use the diagram on chapter 50 of the Campbell textbook to review the role of incus, malleus, and stapes and compare specific functions of these bones to similar structures in aquatic mammals.
Yekaterina Khavkhalyuk (kittykatx93x@yahoo.com)
When looking at the difference between hearing in marine and land mammals, it is important to consider the medium in which they are hearing in. Water is denser than air, so sound waves in water travel faster than sound in air. Because of this, marine mammals hear in a range of 10 Hz to 200 kHz, while humans hear in the 20 Hz to 20 kHz range.
ReplyDeleteThe basic structure of marine and land mammals are very similar, with inner ear structures that resemble each other. The external ear in marine mammals is absent and their middle ear is also modified. Another difference is that the inner ear of marine mammals is made “tougher” and less susceptible to hearing loss, since there is constant noise in the sea that occurs naturally. Therefore the structure of the ear or marine mammals matches the function, since hearing is so important to marine mammals such as dolphins that rely on sonar to detect objects around them. Their ear structure helps them survive in their natural habitat, which is water. Specifically in dolphins, they have twice as many cochlear fibers in their auditory nerves, which aid in carrying the sound signals from the ear to the brain. In organisms, such as cats and dolphins with more nerve fibers than humans, their sense of hearing is more acute. Another structure that makes dolphins’ hearing better than humans is their auditory bulla, a structure in their inner ear that connects that skull with fibrous tissue. Since the auditory bulla is isolated from the skull, sound is able to travel more efficiently through the jaw and middle ear, where dolphins hear through. Dolphins collect sound through their lower jaw, which has replaced the terrestrial outer ear pina, which served little purpose in underwater hearing, since sound does not reflect in water mediums. Also, since dolphins often dive underwater and experience different levels of pressure, they have highly vascularized tissue in their middle ear cavity in order to help the ear adjust to pressure changes, by adjusting the supply of blood to the ear. This enables the dolphin to still receive an adequate supply of blood to its ear, so it can keep hearing even at very high pressures deep in the ocean. All of these various structures enable the dolphin to focus underwater sounds that are too difficult for humans to hear.
In humans, the function of the malleus incus, and staples, which are all a part of the middle ear, is to transmit vibrations to the oval window. The cochlea, which is the structure that actually enables us to hear, is also found in dolphins. Human cochleas are around 3 cm long, while mammalian cochleas are just a little bit longer. The length of a cochlea corresponds to the frequency that can be heard through it; longer cochlea can hear higher frequencies. This makes sense, since we know dolphins can hear frequencies much higher than humans can hear. The structures that make up dolphin ears serve different purposes than the structures in our ear, so there aren’t many comparisons that can be made; many structures have been replaced with structures that aid dolphins to hear better in water.
Sources:
http://www.seaworld.org/infobooks/bottlenose/sensesdol.html
http://www.infj.ulst.ac.uk/~pnic/HumanEar/Andy's%20Stuff/MScProject/workingcode_Local/EarChapter.html
http://www.nap.edu/openbook.php?record_id=10564&page=87
http://www.ncbi.nlm.nih.gov/pubmed/20091313
Anna Leng (annaissbananas@yahoo.com)
As Anna said, one of the main differences in hearing above and below water is that there is a difference in hearing medium. Water is five times denser than air, so if a human tries to listen under water, it is nearly useless because human ear is filled with air. Marine mammals, for example a baleen whale, to overcome this ‘acoustic impedance mismatch’, has a wax plug that fills the external ears, and transmits sound from the water to the inner ear of the whale, because the density of the plug is similar to water (http://www.whoi.edu/science/B/people/kamaral/hearing.html). Also, another difference that set aside terrestrial mammals to aquatic mammals is the overall inner ear structure. The aquatic mammals have basilar membranes that are thinner, longer, and broader at the end, as well as stronger support of the membrane for odontocetes, but weaker support for mysticetes, while the terrestrial mammals have thicker, shorter, and narrower ended basal membrane, indicating that aquatic mammals have a larger range of sound wavelengths that they can hear (http://www.dosits.org/animals/soundreception/mammalshear/hearingincetaceans/). The lack of outer ear, or the pinnae, is present in aquatic mammals, as this is the adaptation of them to become a more smooth shape for better efficiency in swimming.
ReplyDeleteThe functions of malleus, incus and stapes are to transmit vibrations to the cochlea where it can be transmitted to the auditory nerve and to the brain. These smallest bones in the body are situated in the middle ear. Malleus is connected to the eardrum, incus to malleus, and stapes to incus and the cochlea (Campbell 1093). Aquatic mammals have pretty much the same bones with same functions, except their middle ear is filled with both air and soft tissues contrasted to terrestrial mammalian middle ear filled with air only (http://www.dosits.org/animals/soundreception/mammalshear/hearingincetaceans/).
Overall, the structures of the aquatic mammals’ ears are adapted to hear well in water, with the plugs, etc. We do share similar structures but with slightly different shapes and sizes, with the usage still the same, with some different structures to help both mammals in their respective natural habitats.
John Park (wisejsm@yahoo.com)
This comment has been removed by the author.
ReplyDeleteShubin states that our water-to-land transition has led to a modification of our ears; “hearing in water is different from hearing on land, and the small size and position of the stapes makes it ideal for picking up vibrations in the ear.” (164) The middle ear is the portion of the ear internal to the eardrum and external to the oval window of the cochlea (between outer and inner ear). The main function of the middle ear is to transfer the acoustic energy from compression waves in air to the fluid-membrane waves in the cochlea (auditory portion of the inner ear). The outer ear collects and directs the sounds, the middle ear filters and amplifies acoustic energy, and the inner ear then transforms acoustic energy to electrical signals. This is very related to the unit we are learning in class right now, the nervous system. Neural impulses are to become processed by the brain from electrical signals in the peripheral system. The process begins with vibrations to trigger the ear drum, which triggers the malleus (hammer), incus (anvil), and then stapes (stirrup) in order. After that, it vibrates hairs on the cochlea to send electric signals through the auditory nerve to the brain.
ReplyDeleteBoth modern land mammals, including humans, and marine mammals have gone through phases of evolution with air-adapted ears. Terrestrial and aquatic mammals both share very similar ear structure. Some marine animals such as whales or manatees can hear very well in water, but very poorly in air. Some marine animals have amphibious like ears and can hear in both medias.
However, in water, sound travels a lot differently than it does do air. Because of the water being relatively denser, sound is able to travel more easily (move about 5x faster compared to atmosphere) (at a temperature of 20 degrees Celsisus, sound travels through ocean water at 1,450 m/s compared to 334 m/s in atmosphere) Velocity also increases with the increased salinity and pressure.
In terrestrial mammals, the ear is divided into three sections. The outer ear collects and directs sound, the middle ear filters and amplifies acoustic energy to the inner ear, and the inner ear is what sends neural impulses to be processed by the brain. The outer ear consists of external pinna and the auditory canal, which collects sound waves. The middle ear in terrestrial mammals has an air filled space, containing series of small bones (ossicles) called incus, malleus, and stapes. The bones connect the tympanic membrane (ear canals) and the oval window (opening of the ear). In the inner ear, sound intensity is changed into electrical signals. In terrestrial animals, we are able to determine not only a scalar sound quantity, instead rather a vector. We are abler to localize and determine direction where sound is coming from, which is a great advantage to determine distance and sound intensity.
Unlike terrestrial mammals, aquatic mammals such as whales have no external pinnae (ear flap). The middle ear is also filled with soft tissue instead of complete air. Ear canals in such marine mammals such as whales are also much narrower and are plugged with dense wax. Instead of being connected to the tympanic membrane for a sound canal, the lower jaw called the pan bone is thought to conduct sound. Instead of having ears attached to the school, middle and inner ears are encased by bones in cavities outside of the skull. Research is still being developed of the connections between ear bones and skull in marine life.
The anatomy of the aquatic and mammalian ears share very similar structures, and both types of ears have allowed us to hear in our designated medias (water or atmosphere). Like both how John and Anna have stated, the main discrepancy between the two are the environmental median they live in. Sound is also an important aspect of communication amongst animals, and vocalizations can attract potential mates.
Kyle Kim, piece847@gmail.com
Sources:
ReplyDeletehttp://www.whoi.edu/science/B/people/kamaral/marinemammalacoustics.html#hearing
http://www.dosits.org/animals/soundreception/mammalshear/
http://en.wikipedia.org/wiki/Middle_ear
Campbell AP Biology Textbook