Animals found to have a 'sixth sense,' changing some evolutionary theories

Two centuries ago, naturalists thought they had reptile hearing figured out. Animals like lizards and snakes, they said, relied mostly on their senses of sight and smell, while their inner ears handled balance and little else.

That tidy picture just got a shake-up – literally. Fresh research finds that a gecko’s balance organ also acts as a sensitive microphone for ground-borne vibrations, giving the animal a stealthy extra sense.

That discovery matters because it rewrites part of the evolutionary story of hearing. It hints that an ancient vibration pathway never vanished when vertebrates crawled onto land; it merely slipped below the scientific radar.

Studying the tokay gecko

The new study zeroed in on the tokay gecko, a hardy lizard famous for its loud bark. Deep inside its skull sits the saccule, a fluid-filled pouch long tagged as a balance sensor.

Researchers gently recorded nerve signals while delivering low-frequency shakes – think deep rumbles between 50 and 200 Hz. The saccule lit up.

Those frequencies fall well below the range the gecko’s eardrum usually detects, showing that the animal runs two parallel sound channels: one for airborne squeaks, one for tremors underfoot.

“The ear, as we know it, hears airborne sound. But this ancient inner pathway, which is typically linked to balance, helps geckos detect vibrations that travel through media such as ground or water,” explained study co-author Catherine Carr, a Distinguished University Professor of Biology at the University of Maryland.

“This pathway exists in amphibians and fish, and now it has been proven to be preserved in lizards as well. Our findings shed light on how the auditory system evolved from what you see in fish to what you see in land animals including humans.”

Carr and her colleagues published their findings in Current Biology, under the title “Auditory pathway for detection of vibration in the tokay gecko.” Leading the bench work was postdoctoral researcher Dawei Han, who first explored the idea during his graduate studies.

How some animals sense vibrations

Electrical traces told only part of the story. The team also mapped the gecko brain and found a relay station called the nucleus vestibularis ovalis.

This pocket receives input solely from the saccule, then forwards the information to higher auditory centers. It behaves like a dedicated vibration highway, separate from the regular hearing roadbed.

Similar brain nuclei turned up in snakes and the ancient New Zealand reptile Sphenodon, hinting at a shared blueprint across the reptile family tree.

“Many snakes and lizards were thought to be ‘mute’ or ‘deaf’ in the sense that they do not vocalize or hear sounds well,” Han explained.

“But it turns out they could potentially be communicating via vibrational signals using this sensory pathway instead, which really changes the way scientists have thought about animal perception overall.”

Desert sand-diving snakes, burrowing skinks, and even turtle hatchlings thumping inside their nests may all trade information by shaking their surroundings rather than shouting through the air.

Evolution of this new animal sense

Fish rely on inner-ear organs to feel pressure waves in water, while amphibians straddle both water and land. The gecko work suggests that early tetrapods carried a vibration sense ashore and kept it alongside eardrum-based hearing.

Over millions of years, some lineages dialed it down, but others – like the tokay – held on. That persistence reminds biologists that evolution often repurposes old tools instead of discarding them.

“Think about when you’re at a live rock concert. It’s so loud that you can feel your whole head and body vibrate in the sound field. You can feel the music, rather than just hearing it,” Carr continued.

“That feeling suggests that the human vestibular system may be stimulated during those loud concerts, meaning our senses of hearing and balance may also be closely linked.”

Her point: our own balance organs might pitch in when sounds get strong enough, blurring the line between feeling and hearing.

Why does any of this matter?

“The implications of this research extend beyond the world of reptiles,” Han said. “As we uncover these hidden mechanisms, we’re also gaining a richer and more nuanced picture of how animals perceive and interact with their environments – and potentially, new insights into our own sensory experiences.”

If the vestibular system partners with the cochlea at extreme volumes, it could influence therapies for balance disorders or tinnitus.

Engineers designing earthquake sensors or underwater microphones might also borrow tricks from the saccule’s elegant hair cells.

Nature never ceases to amaze

Geckos clinging to cave walls are now known to sense the faint shiver of approaching footsteps. Snakes gliding through leaf litter likely feel the scurry of prey. And humans, rocking out in packed arenas, might engage an ancient inner circuit we rarely acknowledge.

The new research opens a wider conversation about how vertebrates keep tabs on their world – through both ears and bones.

By tuning into the quiet language of vibrations, scientists are uncovering a sensory mosaic that stretches back hundreds of millions of years.

The full study was published in the journal Current Biology.

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