Scientists have recently discovered that bat species producing higher-frequency calls have evolved shorter faces and smaller ear chambers than species calling at lower pitches.
The finding connects the sound a bat makes with the bones that form its head, linking acoustic behaviour to skull evolution across bats.
What museum bat skulls reveal
Hundreds of preserved bat skulls in museum collections show how species have shaped their heads around the frequencies they use for hunting and navigation.
Dr Giada Giacomini of Liverpool John Moores University (LJMU) examined 446 specimens from 219 species, comparing skull measurements with each species’ peak call frequency. She found a consistent relationship between pitch and cranial shape.
Higher frequencies were repeatedly associated with shorter snouts and smaller ear chambers, including in lineages that eat fruit as well as insects.
These repeated relationships indicate that call frequency has constrained skull design across several bat families, raising further questions about the interaction between size and sound.
Skull size and frequency
Larger heads were generally associated with lower pitches, a relationship that influences echolocation: locating targets by timing returning echoes.
Larger vocal folds and longer air spaces in bats tend to resonate at lower notes, whereas very small structures support higher notes.
Among insect-eating species in the study, skull size accounted for roughly one-third of the differences in peak pitch, indicating that body scale played a part.
Some families nevertheless departed from this pattern, suggesting that habitat or hunting style can influence pitch beyond the simple rules of size.
High calls shorten faces
Across most bats, higher pitches were associated with a shorter facial profile, despite differences in diet and hunting style.
A shorter snout alters resonance, the build-up of sound within a cavity, and can favour higher-frequency calls.
The relationship between pitch and face shape was strongest in species that call through their noses, suggesting that nasal passages form part of the system.
As the study compared only living species, it cannot establish when individual lineages changed, but it does identify where changes were concentrated.
Structural limits of nose calls
Even at similar pitches, producing calls through the nose was more closely associated with skull shape than calling through the mouth.
Facial air chambers direct sound forwards, meaning that a small alteration to bone can change the direction in which a call travels.
“Our results suggest that the functional demand of echolocation in nasal emitting species might be stronger compared with oral emitters,” wrote Giacomini.
This stronger influence helps to explain why bats using steady-tone calls often have elaborate nose shapes and rigid skull designs.
Fruit bats break rules
The pattern also applied to fruit-eating bats, although many combine sound with vision and smell.
Even bats specialising in fruit used echolocation to avoid collisions and land safely, so changes in pitch remained important to them.
In these bats, pitch followed changes in skull shape rather than overall size, a finding that surprised the authors.
This overlap suggests that the demands of feeding may shape the head in one direction, while the demands of sound shape it in another.
Ear chambers also change
The effects extended beyond the face: species producing higher-pitched calls had smaller bony chambers around the middle ear.
In some mammals, larger tympanic bullae, the bony shells containing the middle ear, can improve sensitivity to lower frequencies.
In the new bat dataset, species calling at lower pitches retained relatively larger ear chambers, consistent with long-established patterns of hearing.
This detail suggests that the skull responded to both sound production and sound reception, rather than changing only at the face.
Muscles may shape skulls
Producing sound requires muscles, and the muscles responsible for calling also occupy space in the head.
Most bats produce pulses with the larynx, or voice box, which vibrates in the throat; the system depends on rapid, forceful control.
A 2011 paper linked the speed of bat calls to superfast throat muscles, supporting a mechanical role for anatomy.
If muscles and bones evolved together, skull shape might reveal as much about the motor system as it does about hearing.
Trade-offs in bat sonar
A 2013 review described a trade-off: higher pitches produce sharper detail but also narrow the sonar beam.
Because calls weaken over distance, bats hunting in confined spaces often use high pitches for precision while accepting a shorter range.
Bats hunting in open air often put energy into louder, lower calls and narrow their beam, helping them detect insects at greater distances.
Such ecological choices could account for differences in the new skull results between families, rather than one rule applying to every bat.
Clues for future studies
With extensive museum material already digitised, skull shape could help identify unknown bat calls in the field and in archives.
Connecting skull form with calling style may support acoustic monitoring, as many species occupy the same habitats and produce overlapping calls.
The dataset concentrated on species that call using the larynx, so it did not examine bats that echolocate with tongue clicks and follow different rules.
Despite this progress, Giacomini wrote that “A great proportion of bat skull shape variation remains unexplained”.
Linking pitch and anatomy
Call pitch, skull size and the point where sound leaves the head now come together in a single account of constraint and choice.
More detailed research into muscles, ear structures and flight behaviour could reveal how bats balance several sensory tasks within a single skull.
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