Crocodilians – the group with includes crocodiles and alligators – have lived on earth for about 100 million years and are the dinosaurs’ closest living relatives.
To determine where a sound is coming from, animals unconsciously recognise the tiny difference in time it takes for a sound to reach each ear – a cue known as interaural time difference.
How the cue is interpreted once the signals reach the brain depends on the kind of animal doing the hearing.
Studies of birds have previously shown they have exceptionally well-developed neural maps – brain passageways which chart the locations of sounds. It is also known that mammals use a different system, but little was known about how alligators pinpointed sounds.
Previous studies examining animals’ hearing capabilities have often focused on skull-shapes and size, but scientists Catherine Carr, a professor of biology at the University of Maryland, and her colleague Lutz Kettler from the Technische Universität München, focused on evolutionary relationships between groups of species.
Birds have very small head sizes compared with alligators, but the two groups share a common ancestor – the archosaur – which predates dinosaurs.
Archosaurs began to emerge around 246 million years ago and split into two lineages – one that led to alligators and one that led to dinosaurs.
Although most dinosaurs died out during the mass extinction event 66 million years ago, some survived to evolve into modern birds.
Professor Carr and Dr Kettler’s findings indicate the hearing strategy birds and alligators share may have less to do with head size and more to do with common ancestry.
“Our research strongly suggests that this particular hearing strategy first evolved in their common ancestor,” Professor Carr said.
“The other option, that they independently evolved the same complex strategy, seems very unlikely.”
To study how alligators identify where sound comes from, the researchers anesthetised 40 American alligators with ketamine injections and fitted them with earphones. They played tones for the reptiles and measured the response of a structure in their brain stems called the nucleus laminaris.
This structure is the core area of auditory signal processing.
The results showed that alligators create neural maps very similar to those previously observed in barn owls and chickens. The same maps have not been recorded in the equivalent structure in mammal brains.
“We know so little about dinosaurs,” Professor Carr said. “Comparative studies such as this one, which identify common traits extending back through evolutionary time add to our understanding of their biology.”
The study is published in the Journal of Neuroscience.
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