In the mysterious and often impenetrable depths of the ocean, toothed whales such as dolphins have developed an extraordinary biological innovation: echolocation. This complex biosonar system allows them to navigate, hunt, and understand their environment by emitting high-frequency clicking sounds and interpreting the returning echoes that bounce off objects. While echolocation has fascinated scientists for decades, the precise neural mechanisms that underlie this remarkable ability have remained elusive. A groundbreaking study published in PLOS ONE on June 6, 2025, led by researchers from the Woods Hole Oceanographic Institution (WHOI), New College of Florida, UC Berkeley, and Oxford University, has now begun to crack open the black box of cetacean auditory processing by comparing the brain networks of echolocating dolphins with a closely related non-echolocating whale species called the sei whale.
The team’s approach was revolutionary. Leveraging advanced diffusion magnetic resonance imaging (MRI) techniques—refined through years of technical innovation by Oxford’s Karla Miller and Berkeley’s Ben Inglis—the researchers mapped neural pathways in the excised brains of deceased, stranded cetaceans. Diffusion MRI allows scientists to visualize the architecture of white matter tracts by tracking water molecule movements along neural fibers. Although widely used in human neuroimaging, applying diffusion imaging to large cetacean brains had been fraught with challenges such as low signal-to-noise ratio, immense brain size, and post-mortem tissue degradation. Overcoming these barriers opened new vistas for comparative neurobiology of marine mammals, enabling a first-ever network-level investigation into how auditory information flows within echolocating versus non-echolocating whales.
Central to the study was the focus on the inferior colliculus, a pivotal midbrain structure present in all mammals and notably enlarged in dolphins. This bilateral auditory relay nucleus serves as a neural "chokepoint," funneling and integrating auditory signals from the ear before projecting them onward to higher brain regions. The researchers mapped the pathways leading from the inferior colliculus to multiple targets in the brain, carefully comparing connectivity patterns in the dolphin and sei whale brains. Surprisingly, despite dolphins’ reliance on high-frequency echolocation clicks—which generate a wealth of auditory data—the strength of ascending cortical projections from the inferior colliculus was not markedly greater in dolphins compared to the sei whale. Rather, the defining difference emerged in the descending projections that link the inferior colliculi to the cerebellum.
Historically recognized for its role in balance and motor coordination, the cerebellum has increasingly been appreciated as a hub where sensory information and motor planning converge to form rapid, predictive control of behavior. The heightened connectivity between the inferior colliculus and cerebellum in dolphins suggests a sophisticated neural mechanism supporting the fine-tuned control needed to execute and target echolocation clicks. Dolphins must precisely aim their sonar "beam" and adjust the timing and intensity of click production as they interrogate their underwater surroundings, akin to the way humans use tactile feedback when groping in darkness. This integration of sensory input with motor output enables dolphins not just to passively receive echoes but to actively shape their auditory exploration in real time.
Sophie Flem, a lead author and member of New College of Florida’s inaugural Marine Mammal Master’s cohort, highlighted the challenge of dolphin neuroanatomy. Unlike well-characterized terrestrial mammals such as primates and rodents, dolphin brains are architecturally unique and largely unmapped with respect to functional neuroanatomy. The study’s innovative pathway tracing thus represents an important first step toward elucidating the neural basis of echolocation, revealing how auditory networks are specialized in aquatic mammals that rely on biosonar.
Technical ingenuity was essential to the study’s success. The sei whale brain examined, nearly three times larger than a human brain, posed formidable obstacles to imaging resolution and data quality. Advances made by Karla Miller and Ben Inglis in pulse sequence design and data acquisition protocols enabled scientists to produce stunningly detailed diffusion images despite these constraints. The resulting high-resolution brain maps revealed intricate neural wiring that was previously hidden, opening new frontiers for marine mammal neuroscience.
Peter Tyack, an emeritus research scholar at WHOI and co-author, contextualized these findings within broader sensorimotor integration. Echolocation is not a simple auditory task but a dynamic interplay between producing acoustic signals and processing their echoes. The cerebellum’s apparent role as a predictive controller may allow dolphins to fine-tune their sonar emissions, sustaining sharp "acoustic touch" as they move through complex environments. This sensorimotor feedback loop transforms echolocation from mere hearing into an active querying of the surroundings, akin to a human’s sense of touch.
Interestingly, the study reinforces evolutionary distinctions within cetaceans. Baleen whales such as the sei whale and toothed whales like dolphins share a common ancestor yet diverged in auditory strategies. Baleen whales depend largely on vocal communication and passive hearing and lack the biosonar capabilities observed in dolphins. This neural divergence, especially in cerebellar connectivity, reflects an evolutionary adaptation of dolphin brain circuitry to the demands of echolocation, illuminating how specific neural structures are remodeled to support novel behaviors.
Senior author Peter Cook, an associate professor of Marine Mammal Science at New College of Florida, emphasized the broader implications of these findings. Comparative neurobiology has long yearned to decode the unique intricacies of cetacean brains, which are vastly different from terrestrial mammals. The fusion of cutting-edge imaging with opportunistic sampling of stranded marine mammals presents an unprecedented opportunity to explore brain evolution, sensory processing, and vocal communication in these enigmatic animals.
The study team plans to expand their investigation by including additional baleen whale brains and delving into other neural circuits, particularly those involved in vocal production. Dolphins possess a distinctive nasal vocal apparatus enabling complex learned vocalizations, and understanding the neural basis of these vocal behaviors promises to shed further light on how cetacean brains have evolved to master acoustic communication.
Ultimately, this research represents a profound leap forward in marine mammal neuroscience, offering the first map of differential auditory pathways in echolocating and non-echolocating whales. It underscores the intricate neural choreography that empowers dolphins to "see" with sound, transforming our understanding of sensory evolution and animal cognition beneath the waves.
Subject of Research: Animals
Article Title: Lateralized cerebellar connectivity differentiates auditory pathways in echolocating and non-echolocating whales
News Publication Date: 6-Jun-2025
Web References: PLOS ONE Article
Image Credits: Credits: IFAW Marine Mammal Rescue
Keywords: Marine mammals
