The vertebrate ear is a remarkable structure. Tightly encapsulated within the densest bone of the skeleton, it comprises the smallest elements of the vertebrate skeleton (auditory ossicles) and gives rise to several different senses: balance, posture control, gaze stabilization, and hearing. Nowhere else in the skeleton are different bones and functional units packed so close together jointly embedded in their anatomical environment, which hampers the independent evolution of the ear components. Furthermore, the inner and middle ears have already achieved their final size around birth in mammals, creating further challenges for evolutionary change.

All this makes it puzzling how mammals, a predominantly nocturnal group reliant on hearing, were able to occupy such a vast diversity of environments in the aquatic, terrestrial, subterranean, and aerial realms that require an amazing variety in hearing abilities, locomotion and posture. How could the different, tightly connected parts of the ear adapt independently to these diverse functional and environmental regimes?

Despite its similar function, the ear is composed of different bones in mammals, birds, and reptiles. In birds and reptiles, the lower jaw and its joint are composed of multiple bones, and they have a single auditory ossicle that transmits the sound. Modern mammals, by contrast, have three ossicles (malleus, incus, stapes), all of which are separate from the jaw. This evolutionary transformation of the primary jaw joint into the mammalian ear ossicles is one of the most iconic transitions in vertebrate evolution, but it is not clear why this complex transition has happened.

Recently, my colleagues and I suggested a new hypothesis: This substantial evolutionary change increased the "evolvability" (capacity for adaptive evolution) of the ear and its associated sensory functions in mammals, in addition to any direct enhancements of mastication or hearing. The incorporation of several jaw bones into the mammalian ear has considerably increased its genetic, regulatory, and developmental complexity which, in turn, has increased the evolutionary degrees of freedom for an independent adaptation of the different functional units of the ear. Despite the tight spatial entanglement of functional ear components, the increased evolvability of the ear in mammals may have contributed to their evolutionary success and adaptive radiation in the vast diversity of ecological and behavioural niches observable today.

For my project at the KLI, I will test this hypothesis by comparing the variational properties, the macroevolutionary adaptation and the evolutionary rates of inner and middle ear shape across birds and mammals by high-resolution 3D imaging and cutting-edge multivariate biometric methods.