Abstract

Sound is important for all marine mammals for finding food, for communication, for navigation, and in some cases avoidance of predators. Sounds that animals hear (i.e., supra-threshold sounds) may influence their behavior in various ways. For example a mammal might approach to investigate or swim away to avoid the sound. Animals might emit a sonic response or fall silent. Intense anthropogenic sounds might mask echolocation, communication, or other auditory cues important to the individual and its group. Harmful effects of the most intense sound exposure could include deafness. Deafness can occur when the level of exposure exceeds the dynamic range of the ear for a sufficient period to cause irreversible damage to hair cells. Dynamic range of the ear may vary depending on hearing threshold.

The ear can be divided into three parts, inner, middle, and outer. When predecessors of mammals came from an aquatic environment onto the land, a primitive inner ear had already developed. The fluid-filled inner ear originally evolved in the aquatic environment and presumably is insensitive to hydrostatic pressure changes as the animal changes depth; however, to be an effective receiver for sound in the aquatic environment, the primitive inner ear must have been sensitive to the rapid pressure changes of a sound wave in water. For sensitive hearing in air, mammals had to evolve a middle ear providing sufficient amplification of sounds received at the outer ear to overcome the enormous impedance mismatch (a ratio of almost 3600 times) between inner ear fluid, which is more like seawater, and air. Three suspended bones or ossicles (malleus, incus, and stapes) within the middle ear connect the ear drum to the oval window of the inner ear and convert low-pressure, high-volume velocity excursions of sound waves in air to high-pressure, low-volume velocity waves in the perilymphatic fluid of the inner ear. Hair cells along the basilar membrane of the inner ear convert these high-pressure, low-volume velocity waves into neural signals that the brain perceives as sound.

Research to date indicates that dolphins, among marine mammals have the most highly developed auditory sense. Dolphins as with all cetaceans, are conceived, gestated, born, and live their entire lives in the water. During the long gestation period in the uterus, the developing dolphin is exposed to abundant sound from the environment because its mother's body tissues, including the amniotic fluid that bathes the developing fetus, are well matched to seawater in acoustic impedance. Othello Langworthy, a neurobiologist who made many early observations on the brains of dolphins and whales, suggested that the cetacean auditory system, and indeed the great expanse of cetacean neocortex, reached its high state of expansion on the basis of this early auditory input during brain development. Although there have been no specific studies on hearing by the cetacean fetus, clearly other mammals have demonstrated a capability to hear in the womb well before birth. For example, the human fetus responds to acoustic stimulation. In a group of 31 women, roughly 50 percent of fetuses responded to acoustic stimulation by acceleration of the heart beat at 24 weeks after conception and 100 percent responded to sound by week 28. Bottlenose dolphins, (Tursiops truncatus) with a gestation period of roughly 52 weeks compared to 38 weeks in the human, are born at an advanced stage of development. At birth, the neonate will be 100 to 125 cm in length and may weigh 15 to 20 kg. Most likely, the ear is functional and the dolphin auditory system is receiving acoustic input for a long period in the womb.