In a ground-breaking first, researchers have measured brain activity in live crocodiles to understand how brains evolved to process sights and sounds. As crocodiles are a recent ancestor of birds and, more distantly, mammals, seeing how far back in evolutionary time brain circuits originate in animals provides a clue for when brains first started operating as they do in vertebrates. Although many animals share brain molecules and cell types, the real test of brain conservation is how similarly they function across species. For example, despite mammal and bird brains being hugely different in appearance, both are similarly wired, resulting in them perceiving their environment in analogous ways, namely in a hierarchical fashion.
Hierarchical processing starts with simple sensory information (e.g. tones, noises) being initially perceived in ‘earlier’ brain regions, whereas complex sounds (e.g. music, language) solely activate downstream, ‘higher-order’ brain regions. Hierarchical processing is present in birds and mammals, but until recently it was unclear at which point the brain pathways were gained. Enter the crocodile, stage left. Crocodiles are the closest relatives to birds and haven't shared a common ancestor with mammals for nearly 300 million years. If crocodiles, birds and mammals share similar brain regions important for processing sights and sounds, it would suggest that these brain circuits were established early on in evolutionary time and were passed down to all subsequent species. In an extraordinary new paper in the journal Proceedings of the Royal Society B, Mehdi Behroozi from Ruhr University Bochum, Germany, and Brendon Billings from the University of the Witwatersrand, South Africa, recorded brain activity from mildly sedated crocodiles while the animals listened to tunes and watched a laser-light show, and the results hint that these brain regions may have emerged before birds and mammals went their separate evolutionary ways.
After ironing out a number of technical kinks, the international team of researchers led by Felix Ströckens carefully placed juvenile Nile crocodiles into a brain scanner in order to measure brain activity. Once the crocs were relaxed, the researchers presented a brief flash of red or green lights twinkling at different speeds while Behroozi and Billings watched how their brain responded. Overall, the lights excited two suspected visual areas, including one ‘higher-order’ region that responded more to red lights, suggesting subtle selectivity. These results are the first to show visually triggered brain areas in crocodiles; however, there was a lack of clear evidence for hierarchal processing as observed in other animals or senses, such as hearing.
In order to expand the diversity of senses tested, the team then broadcasted several sounds to the crocodiles in a separate part of the study. The crocs listened to two different simple sounds (random chord noises at around 1000 or 3000 Hz), as well as one complex sound: a snippet from Johann Sebastian Bach's Brandenburg Concerto No. 4. Although ‘lower’ auditory brain areas responded to all three of the sounds, a ‘higher-order’ brain area responded solely to the symphony in a similar region found in birds and mammals that is similarly selective for complex sounds such as music and birdsong.
Taken together, these findings elevate crocodiles’ status from fear-inducing, cold-blooded, modern-day dinosaurs to like-minded, informative and capable animals that could teach us about brain function and evolution through non-invasive methods. Crocodiles may have split from the mammalian evolutionary tree nearly 300 million years ago, but their brains appear to be more in sync with those of birds, and possibly humans, than previously thought.