Auditory

• Sound is measured in frequency in Hertz (Hz) which is the number of vibrations per second.
• Intensity of sound is measured in Decibels (dB), which is a logarithmic scale that indicates loudness.
• The human ear is capable of detecting sound waves with a range of frequencies from 20-20000Hz, with infrasound being below 20Hz and ultrasound being above 20000Hz.
• Dolphins can detect frequencies as high as 200,000Hz.
• The auditory system of some marine mammals allows them to live between land and water due to differences in how sound travels and how sound reacts.
• Sound travels through water at 1.5km/sec, which is 4X faster than travelling through air.
• Lower frequencies travel further than higher frequencies.
• Odontocete cetaceans need sound for echolocation for hunting and determining surroundings.
• Echolocation is the detection of objects by means of reflected sound.
• Echolocation requires various organs to produce, receive and decipher sound and meaning sound is double edged, allowing marine mammals to survive successfully underwater but also is a cause of mortality.
• Human hearing organ consists of the external ear, middle ear, and inner ear.
• Cranial nerve 8, or Cranial nerve VIII, brings messages to the brain from the hearing organ.
• The external ear consists of the pinna which is attached to the external auditory meatus, and the tympanic membrane which forms one wall of the middle ear cavity.
• No vibrations go into the skull and mix signals, allowing for better directional localization of sounds or echos.
• Killer whales in Argentina have beached themselves to catch seals.
• Bottle nose dolphins in South Carolina drive fish on to shore banks and grab them there.
• Bottle nose dolphins in Florida use mud nets in shallow areas.
• The fat pad extends backwards to a thin walled section of the middle ear, the ectotympanic bone.
• All adaptations allow for both ears to be independent from each other and to act as separate receivers.
• Mysticetes use bubble netting to hunt and capture, with no sound involved.
• The posterior part of the mandible, beyond the teeth, is thin walled and is called the pan bone.
• Bones vary between cetaceans, sirenians, and carnivores and vary from species to species within orders.
• In marine mammals, the bones of the middle ear are totally different in shape and size from those of terrestrial mammals.
• The ear complex is integrated into the skull, more similar to terrestrial mammals.
• The pan bone is concaved to form a hollow that contains tissue called a fat pad or channel.
• Sound is conducted to the middle ear through the mandibles or jawbone.
• The ectotympanic bone is made up of two separate bones, which are very limited in connection to the rest of the skull.
• Odontocete whales have sinuses that surround the ear complex, effectively making the ear complex isolated from the skull.
• Odontocetes use loud blasts during hunting which temporarily stun or disorientate their prey.
• The middle ear cavity is housed in the ectotympanic bone and contains the malleus, incus, and stapes bones which form a chain attached to the inner ear.
• The inner ear is encased within the petrosal bone and is composed of a number of cavities within the bone.
• The cochlea, the most critical cavity for hearing, is a long hollow rolled up organ with 3 ducts.
• The neural membrane, which has a long row of neurons, transmits signals to the vestibulo-cochlear nerve which signals to the brain.
• Sound reaches the brain when the hair cells in the neural membrane fire a signal to the brain.
• Sea otters and polar bears, both terrestrial mammals, have no obvious specializations for hearing or vocalizing.
• Sirenians, such as manatees, have no external pinna and a narrow auditory canal, lack sensitivity and directionality, and have no ultrasonic capabilities, possibly making them more susceptible to boat hits.
• Pinnipeds, such as phocidae and odobenidae, have no pinna, a large tympanic bulla or bone where the middle ear is, and some have a plexus of arteries and veins in the middle ear, adaptation for diving.
• Cetaceans, such as dolphins and whales, have a distinct auditory system where sound reaches the middle ear differently, with odontocetes having no pinna but a very narrow external auditory meatus, and mysticetes having a small bit of cartilage connecting the ear complex to the skull.
• Odontocete cetaceans need sound for echolocation for hunting and determining surroundings
• echolocation = detection of objects by means of reflected sound