Although dolphins have a large and complex brain, it is very different from their terrestrial relatives. For more than 35 million years, dolphin ancestors were evolving a brain under natural selection shaped by the need to thrive in an underwater environment. The result is a specialized brain capable of processing huge amounts of sensory input, communicating intricate information, and organizing a vast array of social protocols.
The dolphin brain is slightly larger than our own. For many mammals, the brain size is generally proportional to body size. Measuring the brain size to body size results in what is called an encephalization quotient, or EQ. An adult human has an EQ of 7.44, a chimpanzee is 2.49. A bottlenose dolphin is in-between at 5.31 – that is almost four-and one-half times larger than the EQ of an average terrestrial mammal of the same size.
One feature of the dolphin brain that sets it apart from many other mammals is the large and extensively folded cerebral cortex. This surface area is the part of the brain that controls complex functions such as correlation, association, learning, and conscious control of bodily functions. Toothed whales actually have more folding, (called fissurization), than humans and all other animals.
However, the dolphin cortex is quite a bit thinner than our own. Some anatomists even consider the microscopic structure of the dolphin cortex to be somewhat “primitive.” All in all, we still know very little about the functional organization of this brain.
The dolphin brain has obviously evolved the ability to process the massive amount of acoustical input associated with echolocation. In fact, many scientists agree that the animal’s necessity for precision and speed in processing sound, is likely to be a primary basis for their large brain.
Nearly all land mammals have auditory nerves leading from their inner ears to the temporal lobe of the brain. The same is true for dolphins. However, new research in 2015 showed that dolphins also have neural pathways to their primary visual region of the brain as well. This is strong evidence to suggest that dolphins may indeed be “seeing” with sound.
Another particularly interesting aspect of the dolphin brain is the animal’s ability to sleep with one-half of the brain, while the other half remains awake. This is an obvious advantage for an animal that must continuously surface to breathe, and stay alert to avoid danger.
Based on electroencephalograms, (EEG), from the right and left hemispheres of bottlenose dolphins, one group of researchers found that the animals spent an average of 33.4% of a 24 hour day with one or the other hemisphere asleep.
This information coincides with the finding that the corpus collosum, a bundle of nerve fibers linking the two cerebral hemispheres, is relatively small in the dolphin. This means each hemisphere may operate somewhat independently, with lesser hemispheric coordination. This helps to explain the dolphin’s ability to operate the eyes separately. In fact, during sleep, one eye is usually closed while the other remains open.
After analyzing and scrutinizing the dolphin brain, we are still faced with the ultimate question of its capacity. The question of intelligence remains. Perhaps one of the best ways to evaluate the mental capabilities of the dolphin is through controlled clinical studies of memory and problem solving skills.
Between 1980 and 2004, work at the Kewalo Basin Marine Mammal Laboratory of the University of Hawaii shed a great deal of light on the dolphin’s cognitive capabilities. Under the direction of Dr. Louis Herman, it was learned that the bottlenose dolphin can remember things heard and seen equally well, can understand a simple artificial language, and report on the presence or absence of named objects.
In one of their studies, a dolphin was trained to listen to a list of electronically generated sounds, and then decide whether or not a succeeding “probe” sound was a member of that list. The dolphin would press a “yes” paddle if she recognized the sound, or a “no” paddle if she did not. The dolphin was over 90% correct for lists of one or two sounds; over 80% correct for lists of three or four sounds; and about 70% correct for lists of six to eight sounds.
As might be expected, she remembered the sounds near the end of the list better than the ones at the beginning. This “recency” effect also occurs in human memory tests.
In a visual memory test, a dolphin was briefly shown a single object, and then asked to pick out that object from several alternatives. The dolphin was over 90% correct even when there were delays up to one minute between the presentation of the sample object and the test.
Another of the lab’s studies involved teaching a dolphin an artificial language based on the gestures of a trainer’s arms and hands -- basically a sign language. Signs referred to objects in the pool, to actions that might occur to the objects, and to modifiers of place or location.
For example, with the pool full of various objects, the dolphin would be given the sentence “basket, pipe, in.” She would then swim to the pipe, carry it to a basket, and place the pipe inside. Some instructions contained as many as five words. The animal consistently demonstrated an understanding not only of the individual signs, but how the order of the signs affected the meaning of the sentence.
In yet another study, named objects would be placed in the pool. Through the trainer’s gestures, the dolphin would be asked if an object was present. The dolphin would then report by pressing a “yes” or a “no” paddle. Since the dolphin often responded immediately without conducting an active search, she seemed capable of storing the concept of an object not immediately present.
Although bottlenose dolphins have proven to understand the rules associated with the simple artificial languages implemented in clinical studies, there is yet no evidence that they use anything resembling “language” in nature. This is not to suggest that they do not communicate. Their means of information exchange may indeed be quite complex -- utilizing vocalizations, body posturing, and perhaps, their echolocation abilities.
Dolphins produce three basic types of sound: clicks, whistles, and burst-pulse sounds. While the clicks are used in echolocation, the other sounds seem to be involved in certain forms of limited communication.
Burst-pulse sounds seem to be related to a dolphin’s emotional state. For example, those sounds often associated with aggression have a “squawking” or “barking” quality. Dolphins might produce “squeaking” tones when engaging in playful activities. “Chirping” sounds can sometimes be heard during sexual interaction. It should be noted that these sound descriptions are based on the way we hear them. Dolphins may here them quite differently. While these vocalizations may sound like pure tones to our ears, some are actually trains of short pulses strung very close together.
The pure-tone whistles of the bottlenose dolphin have received a great deal of attention from researchers. We know that each individual dolphin develops a distinct whistle pattern, or “signature whistle” that remains unchanged for life.
These signature whistles are likely to establish or maintain vocal contact between dolphins. Various studies have shown dolphins will whistle when separated from specific group members. This is particularly so for mothers and calves. Dolphins often respond to whistles by either whistling themselves or by approaching the whistler.
Other evidence indicates that one dolphin is capable of imitating the signature whistle of another. Computer analysis reveals such mimicry to be precise enough to produce nearly identical spectrograms. It is quite possible that dolphins mimic signature whistles in order to refer to each other by name.
The production of signature whistles may not always trigger an obvious response. A group of dolphins will often whistle simultaneously. Assuming each dolphin within a group has learned the whistle (and thus the identity) of the others, then repetition of these whistles may help individuals keep track of one another when out of visual range.
Another form of vocal communication may be the dolphin’s echolocation capabilities. Research has shown that one dolphin, positioned near an echolocating companion, can gain information about a target by “listening in” on the returning echoes. Therefore, a dolphin may be able to “point” out a target with an echolocation beam.
There is evidence that young dolphins must learn how to use their echolocation skills. A dolphin calf may depend upon the mother to project her own sounds on a target, in order to discover which frequencies work best for discerning certain objects or species of fish.
Body posturing or “body language” is an obvious way in which dolphins exchange a wealth of information. Physical contact itself is one important way that dolphins communicate fellowship. Dolphins are tactile animals and are often observed touching and caressing each other with their pectoral fins, rostrum, and flukes. Such contact reaffirms friendly relations among individuals and groups.
Synchronous movements are another way in which dolphins reveal solidarity. Dolphins surfacing together, turning simultaneously, swimming in sync, and even jumping together display their close association. This type of behavior is especially common during courtship displays.
Aggression is often communicated by an open mouth, exaggerated movements, jaw clapping, and inverted swimming. Aggressive posturing is frequently a prelude to biting, butting, or tail-slapping (called “fluking”). The number of scratches from other dolphins’ teeth, called rake marks, are a tell-tale sign of an individual’s fighting skill.
Dolphins will sometimes produce sounds by slapping the surface of the water with the pectoral fins or flukes. They will also jump into the air and land on one side for a louder effect. Such slapping and breaching may communicate an animal’s location, frame of mind, or even be used as an alarm to warn against danger.
The ability to communicate practical information, as well as abstract emotion, is vital in order for an individual dolphin to function successfully in both the natural environment, and a complex social atmosphere.