Dolphin super hearing at least 26 million years old

The earliest toothed whales, which lived 26 million years ago, had super hearing like their modern descendants, a new study has revealed.

This group of mammals, which includes dolphins, killer whales and porpoises, have risen to evolutionary power in part because of their extra-special ability to interpret minute details through their hearing.

Their unique talent for producing high-frequency sounds and listening to echoes to gauge their surroundings has contributed to their ability to navigate, hunt and find food effectively underwater.

But precisely when this skill, known as echolocation, developed among toothed whales, has remained a mystery.

So researchers led by Travis Park at Melbourne’s Monash University scanned the inner ear of a 26-million-year-old fossilised Oligocene Xenorophidae – one of the oldest species of the toothed whale group.

When he first saw CT images of the pre-historic giant’s cochlear (a spiral-shaped cavity within the inner ear), Park says he “was blown away by just how similar this incredibly old toothed whale was to a modern echolocating dolphin”, and suggests the Xenorophidae had similarly robust hearing, when it came to high-frequency sounds, as today’s toothed whales.

Previous studies have shown the species’ ancient ancestors probably had the anatomy needed to produce high-frequency sounds, thanks to their head shape – and hence, could likely echolocate like their modern cousins.

“Until now we have lacked anatomical evidence from [a fossilised] inner ear to test for high-frequency hearing, and verify functional echolocation,” the paper reads.

Because the fossil in question came from the earliest known divergent of the toothed whale group, the researchers say this evolutionary trait may have been present in all sub-species, but more research is needed to confirm this theory.

The research was published this week in Biology Letters.

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