Skull morphological adaptations to acoustic emissions: peak frequency in bats

Chiropteran species developed diverse sound emission systems (i.e. oral or nasal) which impose different structural and functional demands on the skull. Because peak frequency (FP) reflects key aspects of echolocation performance-such as spatial resolution, call attenuation and beam directionality-it provides a continuous, biologically meaningful proxy for sensory adaptation. To investigate how sensory adaptation influences cranial evolution, we examined the relationship between skull morphology (size and shape, quantified using three-dimensional geometric morphometrics) and FP across a macroevolutionary dataset representing approximately 65% of extant bat genera. Phylogenetic comparative methods were employed to assess skull morphological variation associated with FP. We predicted that species relying on multiple sensory strategies (e.g. frugivores) would exhibit weaker associations between skull morphology and FP compared to insectivorous species that depend more heavily on echolocation. Unexpectedly, we found that frugivorous nasal emitters presented significant skull shape (but not size) adaptations to the frequency emitted. In both insectivorous and frugivorous species, high frequencies were associated with a relatively short rostrum. Moreover, FP exerted stronger constraints on skull shape in nasal emitters than in oral emitters. Our results highlight FP as an important factor shaping skull morphology across bats, even in groups that rely on other sensory modalities.