Land Mammals

Uses of Hearing in Mammals:

• Terrestrial mammals use their ears for more than just hearing. The semicircular canals of the inner ears are part of the ear’s vestibule system and functions in balance, posture control, and gaze stabilization. The ear’s auditory system, including the cochlea and ossicles, functions in hearing (Maitre et al., 2020).

Location of Ears:

• The mammal’s ears are located within the temporal bone of the skull (Maitre et al., 2020).

Method of Hearing:

Evolutionary Driving Factors:

• What drove the evolution of the mammalian ear is still not completely understood. In Mesazoic mammals, sensitive hearing was probably selected because the mammal’s current predators had forced the animal into a nocturnal lifestyle where sight was limited. Sound shadows were selected so the directionality of sound could be established, and high-frequency hearing allowed intraspecific communication that couldn’t be detected by the mammal’s low-frequency hearing predators. Mammalian ears could also have been an evolutionary result of the miniaturization of the jaw (Warren & Nowotny, 2021).

• It’s also possible that the incorporation of the primary jaw joint bones into the middle ear was originally an evolutionary adaptation for mastication, but the independent and gradual uncoupling of the ossicle bones from the other chewing bones led to specialized adaptations of the ossicles that increased the evolvability of the ear and its functions. In this way, selection could have been for mastication or for hearing, independent but inherently coordinated functions (Maitre et al., 2020).

Evolutionary Overview:

• The main evolutionary adaptation of the mammalian ear was the incorporation of the bones of the primary jaw joint (the stapes, malleus, and incus) into the middle ear (Maitre et al., 2020). This transformation occurred multiple times independently over the course of mammalian evolution. The ossicles bones are separated from the jaw, allowing for the evolution of hearing independent from the function of mastication. These bones can be compared to the current primary jaw joint structure of jawed non-mammalian vertebrates to provide comprehensible evidence of this evolution; the malleus and incus are homologous to the articular and quadrate bones, respectively, and the ectotympanic bone is homologous to the angular bone. Furthermore, there is evidence that the stapes acted as a transmitter for vibrations from the ground to the inner ear in early aquatic tetrapods, essentially functioning as a rudimentary middle ear (Warren & Nowotny, 2021).

• The ossicles can vary greatly in shape, function, and structure depending on the auditory needs of the organism (high or low frequency hearing) and the external environment (aquatic or terrestrial). The evolutionary differences in the ears of marine mammals will be further explored in subsequent sections. Also, the mammals’s specific inner ear morphology, including the semicircular canals, is linked to its locomotion and posture needs (Maitre et al., 2020).

• Some of the novel adaptations of the mammalian ear include the evolution of a sophisticated outer ear including an auditory canal and pinna that can vary extensively in form to accommodate different functions and environments. Pinna in this case refers to the external, visible portion of the ear. Mammalian ears also have the organ of Corti that lives within the cochlea, and its electromotive outer hair cells allow mammals to perceive high-frequency sound (Maitre et al., 2020).

Marine Mammals

Uses of Hearing in Marine Mammals:

• Marine mammals often live in cloudy waters where vision is impaired and hearing becomes crucial to the mammal’s survival. Marine mammals use vocal communication for a variety of social and feeding functions, requiring the ability to hear intraspecific communication. Some of these functions include mating, rearing of their young, and group cohesion (Duarte et al., 2021).

Location of Ears:

• The ears of marine mammals, specifically cetaceans, are not within the skull like they are for land mammals. The middle and inner ears are encased in bones external from the skull, but attached to it by ligaments or bone (“Hearing in Cetaceans and Sirenians,” 2020).

Evolutionary Overview:

• One of the main problems, and most obvious differences, between land mammals and marine mammals is that sound is now traveling through a non-air medium. This creates an impedance mismatch between the external water and the air-filled inner ear (Ketten, 1992). For this reason, cetaceans, such as whales, dolphins, and porpoises, have a non-functioning ear canal blocked with a waxy plug. Instead it is thought that fat layers and pan bones transmit external vibrations to the tympanal membrane of the inner ear. Unlike terrestrial mammals, the middle ear of marine mammals is filled with some soft tissue as well as air and the external pinnae is not present. The inner ears of marine mammals also have a larger number of auditory fibers and a larger auditory nerve. The cochlea of the inner ear has a membrane that is vibrated by the stapes called the basilar membrane. This membrane functions to separate out frequencies so that a thicker and stiffer basilar membrane enables the organism to hear a higher frequency and a thinner and more elastic basilar membrane enables low-frequency hearing. Marine mammals have evolved to have both thicker and thinner basilar membranes than land mammals, depending on the species, allowing them to hear within frequency ranges most advantageous for their environment. Despite the morphological differences of the inner ear compared to land mammals, its function remains the same (“Hearing in Cetaceans and Sirenians,” 2020).

• Since marine mammals don’t walk on land, their need to maintain their balance and control their posture is greatly reduced, leading to a reduced need for the vestibule system of the ear. They have very small semicircular canals compared to land mammals, and these canals may indeed be vestigial. Another theory is that these semicircular canals are used by the mammal as a type of accelerometer (Ketten, 1992).

• Evolution has also led to many marine mammals having notably dense tympanic and periodic bones and large, dense ossicular bones (Maitre et al., 2020; Ketten, 1992). These very strong bones most likely evolved in response to the high external pressure underwater and are able to withstand the pressure associated with diving maneuvers (Ketten, 1992).