Toothed whales, the suborder Odontoceti, make up the majority of cetacean species alive today and include dolphins, porpoises, sperm whales, beaked whales, and narwhals. What sets them apart from their baleen whale relatives goes far beyond the presence of teeth: toothed whales are the only marine mammals that echolocate, using high-frequency sound pulses to map their surroundings and find prey in dark or murky water. That single ability has shaped nearly every aspect of their anatomy, from lopsided skulls to fat-filled foreheads, and it exists alongside some of the most complex social lives found in the animal kingdom.
How a Lopsided Skull Tells an Evolutionary Story
One of the most striking features of a toothed whale skull is that it is not symmetrical. The bones of the face and nasal region are shifted to one side, sometimes dramatically. This asymmetry is directly tied to echolocation: the nasal passages on the right side became specialized for producing the high-frequency clicks used in sonar, while the left side took on a different acoustic role. The connection between cranial asymmetry and echolocation is one of the defining traits of the entire suborder.1PubMed Central. The ontogeny of asymmetry in echolocating whales
This didn’t happen overnight. The earliest whale ancestors, the archaeocetes that lived roughly 40 to 50 million years ago, already had some skull asymmetry, but it had nothing to do with sonar. Their skulls showed curvature and torsion linked to directional hearing in water, which helped them locate sound-producing fish. True ultrasonic echolocation did not appear until the Oligocene, roughly 30 million years ago, when the odontocete lineage branched off and the nasal region began to reshape itself into the complex sound-production apparatus we see today.2PubMed Central. Cranial asymmetry in Eocene archaeocete whales and the evolution of directional hearing in water Early ancestors of living whales had little cranial asymmetry and were likely unable to echolocate. The nasofacial asymmetry that defines modern toothed whales became a significant feature in the Early Oligocene and has reached its highest levels in living species.3PubMed Central. Wonky whales: the evolution of cranial asymmetry in cetaceans
Making Sound Without a Larynx (at Least Not in the Usual Way)
Unlike most mammals, toothed whales do not produce their echolocation clicks using the larynx in the throat. Instead, all the action happens in the nasal passages just below the blowhole. Air is pushed through structures called phonic lips, which vibrate to create sound. The surrounding air sacs capture and recycle air so the whale can keep clicking without exhaling a stream of bubbles, and a dense connective tissue structure behind the phonic lips acts as a reflector, bouncing sound forward.4Handbook of Behavioral Neuroscience. Anatomy of Underwater Sound Production With a Focus on Ultrasonic Vocalization in Toothed Whales Including Dolphins and Porpoises Recent research has shown that this nasal sound-production system is functionally analogous to the way vocal cords work in the larynx, and that tissue vibrations in different “registers” produce distinct types of signals: one register for echolocation clicks, another for communication calls.5PubMed. Toothed whales use distinct vocal registers for echolocation and communication
In dolphins, the echolocation clicks are produced almost exclusively by the right pair of phonic lips, while whistles used for social communication come from the left pair. This functional division of labor mirrors the skull asymmetry itself. A study on bottlenose dolphins and false killer whales found that the entire dynamic range of click outputs, from faint search clicks to powerful buzzes, can be achieved by modulating air pressure and lip tension on the right side alone, without needing both phonic lip pairs to fire at once.6PubMed. Nasal sound production in echolocating delphinids (Tursiops truncatus and Pseudorca crassidens) is dynamic, but unilateral: clicking on the right side and whistling on the left side
The Melon and the Lower Jaw as Acoustic Hardware
Once a click is generated, it needs to be focused into a directional beam. That job belongs to the melon, the rounded, fatty structure on the forehead of every toothed whale. The melon is not uniform inside; it has a complex internal topography of lipids with different densities. These density gradients bend sound waves the way a glass lens bends light, focusing the outgoing clicks into a narrow beam that the whale can aim at targets.7Marine Mammal Science. Morphology of the odontocete melon and its implications for acoustic function In the North Atlantic pilot whale, researchers measured a minimum sound velocity of about 1,340 meters per second in the melon’s core, a region rich in unusual branched-chain fatty acids. The surrounding tissue transmits sound faster, so the speed gradient causes refraction, bending the click energy forward and outward.8PubMed. Variations in lipid composition and sound velocity in melon from the North Atlantic Pilot whale, globicephala melaena melaena
Receiving the returning echoes is equally specialized. Toothed whales have essentially lost functional external ear canals. Instead, sound enters through the lower jaw, which contains a fat-filled channel running from the chin region back toward the ear bones. Small openings called mental foramina on the lower jaw act as the new “outer ear,” conducting sound across the full frequency range of the whale’s hearing and funneling it through the mandibular fat pad to the middle ear.9PubMed. Anatomy and physics of the exceptional sensitivity of dolphin hearing (Odontoceti: Cetacea)10St. Petersburg Polytechnical University Journal: Physics and Mathematics. The effect of acoustic shielding of the region of a dolphin’s mental foramina on its hearing sensitivity The thin tympanic bone plate that receives these vibrations replaces the eardrum of land mammals and contributes to an impedance match that gives dolphins remarkably sensitive hearing underwater.
The Sperm Whale’s Giant Nose
Sperm whales have taken the basic toothed-whale sound-production plan and scaled it to extremes. Their massive head, which can make up a third of total body length, contains the spermaceti organ, a structure filled with waxy oil, flanked by air sacs at each end. The “junk,” a compartmentalized fatty structure below the spermaceti organ, sits between the sound source and the water. When the phonic lips at the front of the head fire, the resulting pulse bounces back and forth between the air sacs at each end of the spermaceti organ, producing the distinctive multi-pulse click that sperm whales are known for. Post-mortem experiments confirmed this reflection pattern: when researchers injected frequency-modulated sound pulses at the front of the head, they recorded a decaying series of reflected pulses spaced by the travel time through the organ, matching the structure of natural sperm whale clicks.11PubMed. Sound transmission in the nose of the sperm whale Physeter catodon. A post mortem study The junk readily transmitted sound and is likely in acoustic contact with the spermaceti organ, meaning the entire forehead functions as one integrated acoustic system.
Porpoises and the Stealth Click
Not all toothed whales use the same kind of sonar signal. Porpoises produce narrow-band, high-frequency clicks centered well above 100 kHz, far above the hearing range of most potential predators like killer whales. Harbour porpoises and Dall’s porpoises produce clicks with an average directivity index of about 25 dB, meaning the beam is extremely focused. Where these two species overlap geographically, a small but consistent frequency difference appears: Dall’s porpoises in Canadian waters click at around 137 kHz on average, while harbour porpoises in the same area click at about 141 kHz. Interestingly, harbour porpoises in Danish waters, far from any Dall’s porpoises, click at roughly 136 kHz, more similar to Dall’s porpoises than to their own Canadian relatives.12PubMed. Clicking in a killer whale habitat: narrow-band, high-frequency biosonar clicks of harbour porpoise (Phocoena phocoena) and Dall’s porpoise (Phocoenoides dalli) This frequency shift in sympatric populations may serve as a form of acoustic character displacement, helping the two species distinguish each other’s signals.
Suction Feeding and Tooth Loss
The name “toothed whale” is slightly misleading, because several lineages within Odontoceti have drastically reduced their teeth or lost functional dentition altogether. Sperm whales have teeth only on the lower jaw, and most beaked whales retain just one or two pairs of teeth, often erupted only in males. These species have converged on suction feeding, using rapid expansion of the throat to create a vacuum that pulls prey into the mouth. A recent morphological analysis found that reduced dentition, non-occluding jaws, a specialized palate, and weak jaw-closing muscles were the strongest predictors of suction feeding across multiple independent lineages. Skull shape itself was a poor predictor, meaning these lineages arrived at similar feeding strategies without their skulls looking alike.13PubMed Central. Toothed whales independently evolve specialized suction feeding without converging on skull shape
Matrilines, Grandmothers, and Why Old Females Lead
Toothed whale societies range from loose aggregations to some of the most tightly bonded family groups known in mammals. Killer whales are the best-studied example. Southern resident killer whales off the Pacific coast of North America live in pods that are organized around matrilines: a mother, her offspring, and her daughters’ offspring, all staying together for life. Analysis of a 29-year photo-identification database confirmed that whales within the same pod and matriline cluster together far more often than chance would predict.14Animal Behaviour. The social dynamics of southern resident killer whales and conservation implications for this endangered population
One of the most remarkable features of killer whale societies is that females live for decades after they stop reproducing. This extended post-reproductive lifespan exists in only a handful of mammal species, and it has puzzled biologists for years. The leading explanation is the grandmother effect: by ceasing reproduction, older females avoid competing with their own daughters for resources and instead invest in the survival of their grandoffspring. A study of resident killer whales showed that post-reproductive grandmothers provide survival benefits to grandoffspring above and beyond what reproductive grandmothers provide, and that these benefits are most pronounced during years of low salmon abundance.15PubMed Central. Postreproductive killer whale grandmothers improve the survival of their grandoffspring
These grandmothers don’t just help by being present. They actively lead. Post-reproductive females are more likely than younger adult females to be at the front of group movements, and their leadership role intensifies when salmon stocks are scarce. In years of low food availability, the oldest females guide the group to foraging areas, presumably drawing on ecological knowledge accumulated over decades.16Current Biology. Ecological Knowledge, Leadership, and the Evolution of Menopause in Killer Whales
Dialects and Pod-Specific Whistles
Social identity in toothed whales is reinforced by sound. Resident killer whales off British Columbia form four acoustically distinct clans, each with its own dialect of pulsed calls.17Animal Behaviour. Stability and group specificity of stereotyped whistles in resident killer whales, Orcinus orca, off British Columbia On top of these calls, killer whales also produce tonal whistles that function in close-range communication. Among southern resident killer whales, researchers identified six stereotyped whistle types, and no single pod produced all of them. K-Pod and L-Pod had a higher proportion of stereotyped whistles (roughly 75% and 57% of recorded whistles, respectively), while J-Pod’s whistle repertoire was more variable, with about 62% of its whistles falling outside stereotyped categories. One whistle type appeared only in recordings where K-Pod was present, suggesting it may be exclusive to that group.18PubMed Central. Stereotyped whistles in southern resident killer whales
These acoustic signatures are learned socially and maintained across generations, functioning much like cultural traditions. Calves grow up hearing their pod’s calls and whistles and gradually adopt them. When pods with different dialects encounter each other, their distinct repertoires may serve as badges of group membership and relatedness.
Tool Use and Cultural Transmission
Bottlenose dolphins in Shark Bay, Australia, provide one of the clearest examples of animal culture outside humans and great apes. Some dolphins there carry marine sponges on their rostrums while foraging along the sandy seafloor, apparently using them to protect their snouts as they probe for buried prey. Mitochondrial DNA analysis showed that this “sponging” behavior is transmitted almost exclusively from mother to female offspring, following a single matriline.19PubMed Central. Cultural transmission of tool use in bottlenose dolphins A later study using network-based diffusion analysis confirmed that sponging spreads vertically from mother to primarily female offspring, rather than being independently invented or picked up from unrelated individuals in the population.20PubMed Central. Multi-network-based diffusion analysis reveals vertical cultural transmission of sponge tool use within dolphin matrilines This makes it one of the best-documented cases of socially transmitted tool use in a non-primate.
A Brain Built for Social Complexity
The cognitive demands of living in tight-knit, communication-rich societies appear to have left traces in toothed whale brains. Von Economo neurons, a specialized type of brain cell previously thought to be unique to humans and great apes, have been identified in several toothed whale species, including bottlenose dolphins, Risso’s dolphins, and beluga whales. These neurons appear in the anterior cingulate cortex, anterior insular cortex, and frontopolar cortex, brain regions involved in social awareness, emotional processing, and decision-making. Their distribution in cetaceans is comparable to what is found in humans, great apes, and elephants, despite none of these groups being closely related. The implication is that Von Economo neurons evolved independently in multiple lineages facing similar pressures related to complex social and emotional processing.21PubMed. Total number and volume of Von Economo neurons in the cerebral cortex of cetaceans
Managing Nitrogen on Deep Dives
Many toothed whales are extreme divers. Sperm whales routinely descend past 1,000 meters, and beaked whales hold the records for the deepest and longest mammalian dives. For decades, scientists assumed these animals were essentially immune to decompression sickness because of anatomical features that reduce nitrogen absorption: collapsible lungs, stiff rib cages, and behavioral patterns that limit bottom time at extreme depth. That view has shifted. Gas bubble formation and tissue injury consistent with decompression stress have been documented in stranded marine mammals, and gas kinetic models based on actual dive profiles suggest that blood and tissue nitrogen levels can climb surprisingly high. The current thinking is that diving mammals actively manage their nitrogen load rather than simply avoiding it, adjusting their physiology and behavior in response to multiple stressors.22PubMed Central. Deadly diving? Physiological and behavioural management of decompression stress in diving mammals
When Species Lines Blur
Toothed whales occasionally form mixed-species groups and, in some cases, hybridize across species lines. Off the coast of Australia, humpback dolphins and Indo-Pacific bottlenose dolphins regularly form mixed-species groups. About a third of these mixed-species sightings began with the animals socializing together, a rate far higher than the socializing rate in single-species groups of either species. The majority of mixed-species encounters involved only neutral or affiliative behavior, though about a third included aggressive or sexual interactions typically initiated by bottlenose dolphins.23Behavioral Ecology and Sociobiology. Multiple social benefits drive the formation of mixed-species groups of Australian humpback and Indo-Pacific bottlenose dolphins
Hybridization goes further than socializing. In the Greek Seas, genetic analysis of striped dolphins and common dolphins found evidence of introgressive hybridization between the two species. Out of samples from 60 individuals, 15 were identified as potential hybrids, including three animals with intermediate body shapes. These two species belong to different genera, making this a case of intergeneric hybridization occurring naturally in the wild.24PubMed. Evidence of introgressive hybridization between Stenella coeruleoalba and Delphinus delphis in the Greek Seas The phenomenon raises questions about how rigid species boundaries really are among delphinids, particularly in areas where ranges overlap and population sizes may be declining.
Noise Pollution and Hearing Damage
Because toothed whales depend on sound for virtually everything, from finding food to maintaining social bonds, anthropogenic noise in the ocean poses a unique threat. A stranded melon-headed whale tested using auditory evoked potentials showed hearing thresholds 20 to 65 dB above normal across the 10 to 100 kHz frequency range compared to its closest relative, the pygmy killer whale. That level of hearing loss would cripple the animal’s ability to echolocate and communicate effectively. The researchers concluded the damage was probably caused by intense sonar exposure or chronic shipping noise.25PubMed. Auditory evoked potential in stranded melon-headed whales (Peponocephala electra): With severe hearing loss and possibly caused by anthropogenic noise pollution
Fishing gear entanglement is another growing concern, particularly for coastal and estuarine populations. In Western Australia’s Peel-Harvey Estuary, eight entanglements of Indo-Pacific bottlenose dolphins in recreational fishing gear were recorded over a six-year period, three of which were fatal. While the population-level impact was assessed as low in that particular case because most victims were juvenile males, the animal welfare implications are severe, and small, isolated populations with low reproductive rates could be pushed toward decline by even modest rates of entanglement-related mortality.26Marine Pollution Bulletin. Entanglement in recreational fishing gear poses a threat to estuarine and coastal dolphins For animals that depend on echolocation and tight social bonds to survive, losing even a few experienced adults can ripple through a group’s ability to find food and navigate its environment.