What can the evolution of the turtle’s inner ear tell us about hunting ability – and agility?
- Analysis of 163 species finds the turtle’s proportionally big inner ear may have evolved to better stabilise its eyes for hunting in water
- Home to key phases of turtle evolution, China is ‘at the forefront of palaeontological headlines for turtles’ as it is for bird evolution, says author
Despite its famed slowness, the turtle has proportionally big inner ears which may have evolved to better stabilise its eyes for hunting in water, the researchers said.
They analysed 163 specimens, including extinct and living species such as softshell turtles, terrapins and loggerhead sea turtles and cite a Chinese fossil specimen as among the first to highlight the distinctive characteristic.
The inner ear, or bony labyrinth, is a tiny structure in the head that senses movement and orientation. The shape and size of the “organ of balance” had been used to infer agility in extinct animals.
But agility might not be the one-size-fits-all purpose for inner ear size across all groups of vertebrates, said lead author Serjoscha Evers, a research associate at the University of Fribourg in Switzerland.
“Turtles are quite slow, but nevertheless have inner ear sizes much larger than those of mammals and as large as those of birds. This indicates that there may be different explanations as to why large inner ear sizes evolve,” the palaeontologist said.
“We hypothesise that large inner ears in turtles facilitate better eye stabilisation, which could be a prerequisite for aquatic hunting behaviour, which first evolved in those turtles that also have large inner ears.”
The team of researchers from institutes in Brazil, Britain, China, Germany, South Africa and Switzerland published their findings in the peer-reviewed journal Nature Communications on Tuesday.
“This result contrasts with the hypothesis that large labyrinth sizes in tetrapods are related to high agility, or turning performance, in birds and some mammals,” they wrote.
“The relative size of the labyrinth in turtles exceeds that seen in mammals and rivals that of many birds, but turtles are conspicuously less agile than those groups, and even some other reptiles.”
Their results also showed that diving turtles did not have small inner ears, unlike other deep-diving animals such as whales, Evers said.
“Further research into diving adaptations in animals like turtles may help humans to mitigate diving-related problems – but this is a long way from our core research and requires experts from medical fields,” he said.
He also said that global collaboration was key to carrying out a comprehensive study of turtles, which had lived in many parts of the world in the past.
“Specimens of living and fossil turtles are spread across the globe in multiple museums,” he said.
“China is one of the nations with a particularly rich fossil record. The xinjiangchelyid turtles, for instance, are among the first fossil turtles to have large inner ears and aquatic habits,” he said, referring to a group of extinct shelled turtles found in places including the western region of Xinjiang and the southwestern province of Sichuan.
“Some of the key phases of turtle evolution may have happened in what today is China, and thus China will remain at the forefront of palaeontological headlines for turtles just as much as it already is for bird evolution, even if it requires specimens from all over the globe to better understand turtle evolution.”