Big ears and deformed skulls are a recipe for survival.
Evidence of Charles Darwin’s obsession with domestic pigeons can be seen throughout his work, including his groundbreaking book, On the Origin of Species. Perhaps less well known is the extensive research that Darwin also conducted on domesticated ‘lop-eared’ rabbits — the breeds with floppy ears that fall vertically from their heads rather than point up.
His fascination with these rabbits stemmed from his observation that their lengthy ears were linked to an extreme deformation in the side of their skull, in the form of misshapen ear bones. He knew the bony distortion was related to the ears because it was common practice for breeders in the 19th century to select for rabbits that featured an over-sized floppy ear on only one side of the skull — the so-called ‘half lop.’
“Herein lies the key observation made by Darwin: he noticed that the side without an unusually large ear exhibited the normal skull morphology of rabbits, and the ‘lop-eared’ side was characterized by a deformed skull morphology,” Gerardo Cordero of Iowa State University tells The Science Explorer.
“Thus, Darwin concluded that variation in skull morphology was an indirect consequence of selection for longer ears.”
Those observations have lent support to what is known as phenotypic accommodation theory, which posits that traits have the flexibility to change, or compensate, when organisms are exposed to unusual pressures during development. Cordero gives a human example: “we know that physical demands on the skeleton of a tennis player are structurally accommodated by an increase in skeletal mass of only one side of the body – the arm used to strike tennis balls is relatively larger.”
The idea of phenotypic accommodation is of interest to evolutionary biologists because the ability to developmentally alter structures in the body (e.g. the ear bone) to accommodate selective changes in another trait (e.g. the ears) could help boost trait evolution by lending support to the newly changed feature. But until now, very few of Darwin’s examples of exaggerated trait variation in domestic animals have been described in nature.
Cordero, along with co-author Chelsea Berns of Utica College in New York, set out to test the hypothesis that Darwin derived from his lop-eared rabbits, but this time in a natural system — the bushy-tailed woodrat, known for its large ears.
Small rodents rely on their hearing to detect predators and mates, but as the researchers write in their study, published in the Journal of Evolutionary Biology, a slew of environmental conditions, including wind, moisture, and plant cover “render sound perception and thus survival challenging in desert environments.”
Woodrats in northern California live along a gradient ranging from lush coastal forests to arid deserts, and the researchers found that those living in the dryer climates, which are likely to impede sound travel, had longer ears — an effective way to maximize sound collection.
On further analysis of museum specimens, it became clear ear bones of longer-eared woodrats had undergone deformation, similar to that observed in Darwin’s lop-eared rabbits. With longer ears comes greater soft tissue mass, so the altered shape of the ear bones may help support the load, thereby pushing along the evolution of yet longer ears.
Although further study is needed to clarify the precise nature of the relationship between altered ear bone shape and the evolution of exaggerated ear lengths, the authors write that the study provides support for a classic Darwinian hypothesis concerning domestication and natural selection, which has been previously lacking.
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