A recent study has uncovered unexpected similarities in the evolution of the inner ear among various mammals. An international team of researchers, guided by Nicole Grunstra from the University of Vienna and Anne Le Maître from the Konrad Lorenz Institute (KLI) for Evolution and Cognition Research, discovered that a diverse group of mammals known as Afrotheria, along with some distantly related but ecologically comparable mammals, have developed similar inner ear structures independently. The findings were published in the journal Nature Communications.
A recent study has uncovered unexpected similarities in the evolution of the inner ear among various mammals. An international team of researchers, guided by Nicole Grunstra from the University of Vienna and Anne Le Maître from the Konrad Lorenz Institute (KLI) for Evolution and Cognition Research, discovered that a diverse group of mammals known as Afrotheria, along with some distantly related but ecologically comparable mammals, have developed similar inner ear structures independently. The findings were published in the journal Nature Communications.
The inner ear, located within the bony structure of the skull, plays a critical role in both hearing and maintaining balance. The variety of shapes observed in the inner ear among different species has typically been viewed as responses to unique environmental conditions and movement patterns. Additionally, the inner ear’s design often reflects evolutionary lineage, with closely related species displaying more similar inner ear structures compared to those that are more distantly related. This suggests that non-adaptive (neutral) evolutionary processes might significantly influence the structure of the inner ear more than previously recognized. This recent investigation into the inner ears of a diverse array of mammals brings new insights into this subject.
Insights through virtual 3D modeling
The research team, consisting of evolutionary biologists and paleontologists, including experts from the Natural History Museum Vienna, explored the inner ear configurations of Afrotheria. This particular group includes a wide variety of mammals with different anatomical features and habitats, such as aardvarks, elephants, golden moles, hyraxes, elephant shrews, and manatees. They compared the ear structures of these animals to those of other mammals that, while having different evolutionary backgrounds, are similar in anatomy, ecological roles, or movement, including anteaters, moles, rodents, hedgehogs, and dolphins. Utilizing X-ray microtomography, they examined skulls from museum collections to create virtual 3D representations of the inner ear, which allowed them to compare and analyze ear shapes in relation to habitat and locomotion.
Separate evolution under similar conditions
“Our findings indicate that ear shapes are more alike in analogous species compared to non-analogous ones, even when the latter have a closer evolutionary relationship,” remarked Nicole Grunstra, the study’s lead author. For instance, the inner ear shape of manatees does not closely resemble that of elephants or hyraxes (which are closely related Afrotherians) but is more similar to that of a dolphin, a more distantly related mammal. This similarity in shapes correlates with adaptations to living in fully aquatic environments. The research also highlighted similar inner ear shapes across other unrelated species with similar habitats or feeding strategies, such as species adapted to underground living or those found in tree canopies. “Our study demonstrates that mammals sharing ecological niches or locomotion features developed similar ear shapes, which is not just coincidental,” explains Anne Le Maître, a senior author of the study. This strongly supports the idea of convergent evolution, where different ancestral ear structures evolve alike due to similar evolutionary pressures.
Enhanced adaptability of the mammalian ear
This new research appears to challenge recent findings on birds, reptiles, and some mammals that have raised questions regarding the extent of adaptive processes influencing variations in the inner ear among vertebrates. One interpretation is that adaptive differences in ear shapes emerge more prominently when contrasting species with varied ecological strategies, as seen with Afrotheria and other mammals. Another explanation could be that the mammalian ear exhibits higher ‘evolvability,’ or the inherent potential for adaptive evolution. The mammalian ear is unique in its complexity. Unlike birds or reptiles, mammals have developed additional components through the evolutionary alteration of jaw bones, which later became integrated into the middle ear. This adaptation enables mammals to perceive a broader spectrum of sounds, especially high-frequency tones. Crucially, this complexity increases anatomical, genetic, and developmental intricacies of the ear, allowing for a greater variety of ear shapes to evolve. “An increase in the genetic and developmental components of a characteristic gives natural selection more avenues to explore, facilitating the emergence of diverse adaptations,” adds co-senior author Philipp Mitteröcker from the University of Vienna. This enhanced adaptability may have been pivotal in allowing mammals to adjust to new environments and locomotor patterns throughout their evolutionary history.
This study was financially supported by the Austrian Science Foundation (FWF).
