Mammals evolved similar inner ears despite different ancestry

When you think about evolution, the ear might not be the first thing that springs to mind. But it turns out that our ears, the little organ tucked away in our skull plays a surprising role in how mammals adapt to their environments.

An international team of scientists has discovered something quite astonishing. Mammals with little to no genetic connection but sharing similar lifestyles have evolved similar inner ear shapes.

Mammals and their inner ears

One could argue that few parts of the animal body are as vital yet overlooked as the vertebrate inner ear. This intricate structure has two vital tasks: maintaining balance and preserving hearing.

It’s interesting to note that its complex shape varies among animals. One might think this diversity speaks to the modifications needed for different environments and locomotor behaviors.

Yet, closely related species seem to have more comparable inner ear shapes than those distantly related.

Does this mean neutral (non-adaptive) evolution plays a larger role in shaping the inner ear’s morphology than we initially suspected? This latest study on the inner ears of a diverse group of mammals provides some new hints.

Virtual 3D models bring clarity

In order to better understand the evolution of the inner ear in Afrotheria, an international team of evolutionary biologists and paleontologists, including researchers from the Natural History Museum Vienna, studied the inner ear’s shape in Afrotheria.

This group consists of related mammals that drastically differ in their anatomy and habitats: aardvark, elephants, golden moles, hyraxes, rodent-like elephant shrews, and sea cows.

They compared these ear shapes to those of analogous but distantly related mammals, like anteaters, ‘true’ moles, rodents, hedgehogs, and dolphins.

The team relied on X-ray microtomography on skulls stored in museum collections to construct virtual 3D models of the inner ear.

The aim was to compare inner ear shapes between Afrotherians and their analogues and to analyze ear shape in relation to habitat and movement.

Shared selection pressure

“We found that the inner ear shape is more similar between analogous species than between non-analogous ones, even when the latter share a more recent common ancestor and are therefore more closely related”, explains Nicole Grunstra, first author of the study. 

For instance, dolphins and sea cows display greater ear shape similarity due to adaptations to a solely aquatic environment, even though the latter is more closely related to other Afrotherians such as elephants or hyraxes.

The study found similar tendencies among other distantly related species with similar environment or nutrition methods: subterranean species and tree-dwelling ones share similar ear shapes.

“We were also able to show that eco-morphologically similar mammals evolved similar ear shapes as an adaptation to shared ecological niches or locomotion, rather than by chance”, interprets senior author Anne Le Maître from Konrad Lorenz Institute (KLI).

Evolving ears of mammals

This research seems to conflict with recent studies on birds, reptiles, and certain mammals, which have questioned the importance of adaptive mechanisms in shaping inner ear variation.

One explanation might be that adaptive differences in ear shape surface more when comparing species with diverse ecological strategies.

Another explanation revolves around the concept of ‘evolvability’, or the inherent capacity for adaptive evolution.

The mammalian ear, particularly sophisticated among vertebrates, can detect a wider range of sounds, especially high pitches.

The anatomical, genetic, and developmental complexity of the ear potentially widens the range of possible ear shapes that can evolve.

Adaptations of mammals

“An increase in genetic and developmental factors of a trait gives natural selection more knobs to turn, which facilitates the evolution of different adaptations,” adds co-senior author Philipp Mitteröcker from the University of Vienna. 

Perhaps the increased evolvability of the ear played a notable part in adaptations to new environments and locomotor behaviors during mammalian evolution.

In the ever-evolving tapestry of life on Earth, the surprisingly convergent evolution in the inner ear of mammals serves as a potent reminder of the marvels of adaptation and evolution.

The study is published in the journal Nature Communications.

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