The Elements of Evolution (44-15) Vocalization, Hearing

Vocalization

As animals vocalize, their vocal organ transforms motor commands into vocalizations for social communication. ~ Danish zoologist Coen Elemans et al

Sound is a vibration propagating as an audible wave of pressure. Vocalization is sound production by an animal through its respiratory system. Audition is sound perception.

Humans and other mammals vocalize via their vocal cords, which are a folded membrane across the larynx. Sound ushers forth when muscles regulate the voice box as air from the lungs rushes past.

Birds vocalize through their syrinx, which has no vocalizing membrane. Instead, avian vocalizations are made by muscling the walls of the syrinx during airflow.

The syrinx is a tiny box of cartilage. It reinforces the airway, and when air passes over the folds in it, it produces a sound: birdsong. ~ American zoologist Chad Eliason

Structurally, the syrinx is highly diverse across bird species. Some birds, such as condors, lack a syrinx. Their communications are restricted to throaty hisses.

Whereas the mammalian larynx is above the trachea, the syrinx is below the windpipe. Because the syrinx is located where the trachea forks into the lungs, songbirds can produce more than 1 sound at a time (lateralization): separate sounds from each bronchus.

Despite using disparate organs independently evolved, mammals and birds converged on the same mechanism for vocalization, corresponding with the myoelastic-aerodynamic theory.

Expiratory airflow is mechanically converted by vocal folds into pulse-like airflow, which causes air pressure disturbances constituting the acoustic excitation of the system. The mechanical properties and recruitment of different layers of vibrating tissues affect their resonance properties, which in combination with aerodynamic driving forces determine the frequency and mode of oscillation. ~ Czech biophysicist C.T. Herbst et al

Vocalization involves physiological structures and regulatory systems outside (ex vivo) the sound-producing organs themselves (in vivo). The entire vocalization system in birds and mammals is an instance of convergent evolution.

There is substantial redundancy in the control of key vocal parameters ex vivo, suggesting that in vivo vocalizations may also not be specified by unique motor commands. ~ Coen Elemans et al

 Infant Distress

The infant distress calls of many mammal species – from deer to marmots, bats, cats, dogs, fur seals, and humans – are sonically similar: sharing acoustic elements that make them recognizable across species.

A deer understands what a human baby’s cry means. This suggests that the emotional makeup of animals is selfsame.

These mammals are separated by over 90 million years of evolution. The basis for infant cry characteristics may have originated cladistically but it is also an instance of convergent evolution, as the vocal production apparatuses for these diverse creatures are quite different.

 Singing

The capacity to modify vocal syntax to changes in social context is an important component of vocal plasticity and complexity in adult vertebrates, especially in human speech. ~ American zoologist Kirsten Bohn

Flight is not the only thing that birds and bats have in common which each independently evolved.

The free-tailed bat is a fast-flying species, able to flit up to 9 meters per second. A male may only have 1/10th of a second to attract a female to his roost for some romance.

The male free-tailed bat sings beautifully: as well as any songbird in quality and creativity. Once a female is attracted, a male changes his tune: displaying versatility so as to enamor her into mating. Flying is not the only thing that free-tailed bats are fast at.

Songbirds are not born knowing the songs they will sing as adults. Like human infants picking up language, birds listen and imitate to learn the tunes they croon.

Songbirds and hominids descended from a common ancestor 300 million years ago. Birds and hominids independently acquired the ability to vocalize.

~80 genes create similar pathways in the brains of songbirds and humans; pathways which are active when imitating sounds and singing. This genetic activity is not resident in birds that cannot learn songs or mimic sounds.

(All biological functions are controlled by a mind within. The mind may be associated with a molecule, a cell, or an organism. Any reference to physiological activity, such as brain pathways, is merely artifactual to the generative mentation.)

Hearing

Though variations are possible, some biological imperatives have an optimal vector of solution. Audition is exemplary.

Hearing universally involves 3 stages: sound collection, impedance conversion, and frequency analysis. In mammals, the eardrum, middle ear, and cochlear serve those respective functions.

Katydids, which have some of the smallest of ears of all organisms, have the same tripartite structure for audition, albeit with much different components. Each part performs the identical function as in mammals.