How Do Animals Hear? Many Different Solutions to Catching a Vibration
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Detecting sound means detecting tiny pressure changes, and animals have arrived at strikingly different ways of doing it. Some use ears on their legs, some feel vibration through the ground, some hear frequencies far above or below anything a person can perceive, and the variation maps closely onto how each one lives.
The basic problem
Sound in air carries very little energy, and an animal must convert a faint pressure fluctuation into a nerve signal without losing it. Vertebrate ears solve this in three stages. An outer structure collects sound and funnels it to a membrane. A middle section, in mammals a chain of three tiny bones, transfers the membrane's motion to a fluid-filled inner chamber, and because fluid is much harder to move than air, that transfer requires a mechanical advantage or nearly all the energy would reflect away, which is exactly what the bones provide by concentrating force from a large membrane onto a small window. The inner chamber then separates frequencies, in mammals along a coiled membrane that is stiff and narrow at one end and floppy and wide at the other, so different positions respond to different frequencies and hair cells at each position report what they detect. That arrangement means the ear performs a frequency analysis mechanically before any nerve signal is generated.
The range of solutions
Hearing has evolved repeatedly, and the anatomical diversity is substantial:
- •Insect tympanal organs, membranes that can be located on the thorax, abdomen, wings or forelegs, with crickets famously carrying their ears on their front legs
- •Antennal hearing in mosquitoes and fruit flies, where fine hairs are deflected by air movement rather than by pressure, which works only at short range
- •Fish lateral lines, rows of sensors detecting water movement, alongside inner ears that detect sound directly since water transmits it efficiently into the body
- •Snake jaw conduction, picking up ground vibration through the jaw bone resting on the substrate, since snakes have no external ear
- •Bird ears without external structures, with asymmetrically placed openings in some owls that allow vertical as well as horizontal sound localisation
- •Elephant and mole-rat detection of seismic vibration through feet and bone, extending communication over distances where airborne sound would be lost
Frequency ranges and what they are for
Human hearing spans roughly twenty hertz to twenty kilohertz in youth and contracts substantially with age. Other species occupy quite different bands for reasons connected to body size and lifestyle. Small animals hear higher, partly because localising sound using the difference between two ears requires the head to be large relative to the wavelength, so a small head can only localise short waves. Bats and toothed whales extend far into ultrasound because echolocation requires wavelengths short enough to resolve small targets, with some bats using frequencies above a hundred kilohertz. Elephants, whales and cassowaries produce and detect infrasound below human hearing, which travels much further because low frequencies are absorbed less, permitting communication across many kilometres and in the case of baleen whales potentially across ocean basins. Moths evolved ultrasound-sensitive ears specifically to detect hunting bats and respond with evasive dives, which is one of the clearest cases of a sense evolving in response to a predator's sense.
What noise does to them
Anthropogenic noise has become a recognised pollutant with documented ecological effects. Birds in noisy cities sing at higher pitch and greater amplitude to avoid being masked by traffic, which has measurable consequences for mate attraction and territory defence. Species differ in tolerance, so noise filters which birds can occupy an area and changes community composition along roads. Marine noise is the more severe case, because water carries sound extremely well and shipping has raised background levels in the ocean substantially over the past century, compressing the range over which whales can communicate. Military sonar has been connected to mass strandings of beaked whales, apparently through behavioural responses causing decompression injury rather than direct acoustic damage, and seismic air guns used in oil exploration produce repeated extremely loud pulses over wide areas. Mitigation measures include quieter ship propeller designs, speed reductions, seasonal restrictions and exclusion zones, and the pandemic reduction in shipping produced a measurable drop in ocean noise that supplied a natural experiment.
The takeaway
The hard part is moving faint airborne pressure into fluid without losing the energy, which the middle ear bones solve by concentrating force onto a small window. Ears sit on legs, wings and abdomens in insects, and snakes hear through the jaw resting on the ground. Small heads can only localise short wavelengths, which is part of why small animals hear high. Ocean shipping noise has compressed the range over which whales can communicate.