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Brain size was previously considered a major indicator of the intelligence of an animal. However, many other factors also affect intelligence. Recent discoveries concerning bird intelligence have called into question the usefulness of brain size as an indicator. Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks.[unreliable source?] Allometric analysis indicates that mammalian brain size scales at approximately the ⅔ or ¾ exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalization quotient (EQ) that can be used as another indication of the animal's intelligence.
- Sperm whales (Physeter macrocephalus) have the largest known brain mass of any extant animal, averaging 7.8 kg in mature males.
- Killer Whales (Orcinus orca ) have the second largest known brain mass of any extant animal. (5.4-6.8 kg) 
- Bottlenose dolphins (Tursiops truncatus) have an absolute brain mass of 1,500–1,700 grams. This is slightly greater than that of humans (1,300–1,400 grams) and about four times that of chimpanzees (400 grams).
- The brain to body mass ratio (as distinct from encephalization quotient) in some members of the odontocete superfamily Delphinoidea (dolphins, porpoises, belugas, and narwhals) is greater than modern humans, and greater than all other mammals (there is debate whether that of the treeshrew might be second in place of humans). In some dolphins, it is less than half that of humans: 0.9% versus 2.1%. This comparison seems more favorable if the large amount of blubber (15-20% of mass) that dolphins require for insulation is omitted.
- The encephalization quotient varies widely between species. The La Plata dolphin has an EQ of approximately 1.67; the Ganges River dolphin of 1.55; the orca of 2.57; the bottlenose dolphin of 4.14; and the tucuxi dolphin of 4.56; In comparison to other animals, elephants have an EQ ranging from 1.13 to 2.36;:151 chimpanzees of approximately 2.49; dogs of 1.17; cats of 1.00; and mice of 0.50.
- The majority of mammals are born with a brain close to 90% of the adult weight. Humans are born with 28% of the adult weight, chimpanzees with 54%, bottlenose dolphins with 42.5%, and elephants with 35%.
The discovery of spindle cells (neurons without extensive branching, known also as "von Economo neurons", or VENs) in the brains of the humpback whale, fin whale, sperm whale, killer whale, bottlenose dolphins, Risso's dolphins, and beluga whales is another unique discovery. Humans, the great apes, and elephants, species all well known for their high intelligence, are the only others known to have spindle cells.(p242) Spindle neurons appear to play a central role in the development of intelligent behavior. Such a discovery may suggest a convergent evolution of these species.
Elephant brains also show a similar complexity to dolphin brains, and are also more convoluted than that of humans, and with a cortex "thicker than that of cetaceans". It is generally agreed that the growth of the neocortex, both absolutely and relative to the rest of the brain, during human evolution, has been responsible for the evolution of human intelligence, however defined. While a complex neocortex usually indicates high intelligence, there are exceptions to this. For example, the echidna has a highly developed brain, yet is not widely considered to be very intelligent.
In 2015, it was shown for the first time that a species of dolphin, the long-finned pilot whale, has more neocortical neurons than any mammal studied to date including humans. All sleeping mammals, including dolphins, experience a stage known as REM sleep. Unlike terrestrial mammals, dolphin brains contain a paralimbic lobe, which may possibly be used for sensory processing. The dolphin is a voluntary breather, even during sleep, with the result that veterinary anaesthesia of dolphins is impossible, as it would result in asphyxiation.  Ridgway reports that EEGs show alternating hemispheric asymmetry in slow waves during sleep, with occasional sleep-like waves from both hemispheres. This result has been interpreted to mean that dolphins sleep only one hemisphere of their brain at a time, possibly to control their voluntary respiration system or to be vigilant for predators. This is also given as explanation for the large size of their brains.
Dolphin brain stem transmission time is faster than that normally found in humans, and is approximately equivalent to the speed found in rats. As echo-location is the dolphin's primary means of sensing its environment – analogous to eyes in primates – and since sound travels four and a half times faster in water than in air, scientists[who?] speculate that the faster brain stem transmission time, and perhaps the paralimbic lobe as well, assist quicker processing of sound. The dolphin's greater dependence on sound processing is evident in the structure of its brain: its neural area devoted to visual imaging is only about one-tenth that of the human brain, while the area devoted to acoustical imaging is about 10 times that of the human brain. Sensory experiments suggest a great degree of cross-modal integration in the processing of shapes between echolocative and visual areas of the brain. Unlike the case of the human brain, the cetacean optic chiasm is completely crossed, and there is behavioral evidence for hemispheric dominance for vision.
Some research shows that dolphins, among other animals, understand concepts such as numerical continuity (but not necessarily counting). A recent study found that dolphins may be able to discriminate between numbers, but that this ability "may involve mimicry. . . as dolphins are unsurpassed in imitative abilities among nonhuman animals".
Several researchers observing animals' ability to learn set formation tend to rank dolphins at about the level of elephants in "intelligence" and show that dolphins do not have any unusual talent with problem solving compared with the other animals classed with very great intelligence. Macphail in his "Brain and intelligence in vertebrates" compared data from studies regarding learning "set formation" of animals. The results show that dolphins are skilled at performing this sort of standardized testing but not as adept as other animals in the study.
Dolphin group sizes vary quite dramatically. River dolphins usually congregate in fairly small groups from 6 to 12 in number or, in some species, singly or in pairs. The individuals in these small groups know and recognize one another. Other species such as the oceanic Pantropical Spotted Dolphin, Common Dolphin and Spinner Dolphin travel in large groups of hundreds of individuals. It is unknown whether every member of the group is acquainted with every other. However, there is no doubt that such large packs can act as a single cohesive unit – observations show that if an unexpected disturbance, such as a shark approach, occurs from the flank or from beneath the group, the group moves in near-unison to avoid the threat. This means that the dolphins must be aware not only of their near neighbors but also of other individuals nearby – in a similar manner to which humans perform "Audience waves". This is achieved by sight, and possibly also echolocation. One speculative hypothesis proposed by Jerison (1986) is that members of a pod of dolphins are able to share echolocation results with each other to create a better understanding of their surroundings.
Resident orcas living in British Columbia, Canada, and Washington, United States live in extremely stable family groups. The basis of this social structure is the matriline, consisting of a mother and her offspring, who travel with her for life. Male orcas never leave their mothers' pods, while female offspring may branch off to form their own matriline if they have many offspring of their own. Males have a particularly strong bond with their mother, and travel with them their entire lives, which can exceed 50 years. It is interesting behavior, as it may seem that there would be no benefit from this except perhaps in hunting techniques, although they could join other groups to hunt. There are two interesting examples of this familial bond in males. Two male sons, identified as A38 and A39, constantly accompany their mother A30, despite the fact that she does not need protection and they can all hunt by themselves, and rarely leave her side. Researchers have noted that if one son wanders away, one always remains with the mother. Another example are the brothers A32, A37 and A46, whose mother (A36) died. Instead of the family disbanding, the three brothers remain constantly together.
Relationships in the orca population can be discovered through their vocalizations. Matrilines who share a common ancestor from only a few generations back share mostly the same dialect, comprising a pod. Pods who share some calls indicate a common ancestor from many generations back, and make up a clan. Interestingly, the orcas use these dialects to avoid in-breeding. They mate outside the clan, which is determined by the different vocalizations. There is evidence that other species of dolphins may also have dialects.
In bottlenose dolphin studies by Wells in Sarasota, Florida, and Smolker in Shark Bay, Australia, females of a community are all linked either directly or through a mutual association in an overall social structure known as fission-fusion. Groups of the strongest association are known as "bands", and their composition can remain stable over years. There is some genetic evidence that band members may be related, but these bands are not necessarily limited to a single matrilineal line. There is no evidence that bands compete with each other. In the same research areas, as well as in Moray Firth, Scotland, males form strong associations of two to three individuals, with a coefficient of association between 70 and 100. These groups of males are known as "alliances", and members often display synchronous behaviors such as respiration, jumping, and breaching. Alliance composition is stable on the order of tens of years, and may provide a benefit for the acquisition of females for mating. The complex social strategies of marine mammals such as bottlenose dolphins, "provide interesting parallels" with the social strategies of elephants and chimpanzees.(p519)
Dolphins are known to engage in complex play behavior, which includes such things as producing stable underwater toroidal air-core vortex rings or "bubble rings". There are two main methods of bubble ring production: rapid puffing of a burst of air into the water and allowing it to rise to the surface, forming a ring; or swimming repeatedly in a circle and then stopping to inject air into the helical vortex currents thus formed. The dolphin will often then examine its creation visually and with sonar. They also appear to enjoy biting the vortex-rings they've created, so that they burst into many separate normal bubbles and then rise quickly to the surface. Certain whales are also known to produce bubble rings, or even bubble-nets for the purpose of foraging. Many dolphin species are also known for playing by riding in waves, whether natural waves near the shoreline in a method akin to human "body-surfing", or within the waves induced by the bow of a moving boat in a behavior known as bow riding.
There have been instances in captivity of various species of dolphin and porpoise helping and interacting across species, including helping beached whales. Also they have been known to live alongside Resident (fish eating) Orca Whales for limited amounts of time. Dolphins have also been known to aid human swimmers in need, and in at least one instance a distressed dolphin approached human divers seeking assistance.
Aside from having exhibited the ability to learn complex tricks, dolphins have also demonstrated the ability to produce creative responses. This was studied by Karen Pryor during the mid-1960s at Sea Life Park in Hawaii, and was published as The Creative Porpoise: Training for Novel Behavior in 1969. The two test subjects were two rough-toothed dolphins (Steno bredanensis), named Malia (a regular show performer at Sea Life Park) and Hou (a research subject at adjacent Oceanic Institute). The experiment tested when and whether the dolphins would identify that they were being rewarded (with fish) for originality in behavior and was very successful. However, since only two dolphins were involved in the experiment, the study is difficult to generalize.
Starting with the dolphin named Malia, the method of the experiment was to choose a particular behavior exhibited by her each day and reward each display of that behavior throughout the day's session. At the start of each new day Malia would present the prior day's behavior, but only when a new behavior was exhibited was a reward given. All behaviors exhibited were, at least for a time, known behaviors of dolphins. After approximately two weeks Malia apparently exhausted "normal" behaviors and began to repeat performances. This was not rewarded.
According to Pryor, the dolphin became almost despondent. However, at the sixteenth session without novel behavior, the researchers were presented with a flip they had never seen before. This was reinforced. As related by Pryor, after the new display: "instead of offering that again she offered a tail swipe we'd never seen; we reinforced that. She began offering us all kinds of behavior that we hadn't seen in such a mad flurry that finally we could hardly choose what to throw fish at".
The second test subject, Hou, took thirty-three sessions to reach the same stage. On each occasion the experiment was stopped when the variability of dolphin behavior became too complex to make further positive reinforcement meaningful.
The same experiment was repeated with humans, and it took the volunteers about the same length of time to figure out what was being asked of them. After an initial period of frustration or anger, the humans realised they were being rewarded for novel behavior. In dolphins this realisation produced excitement and more and more novel behaviors – in humans it mostly just produced relief.
Captive orcas have often displayed interesting responses when they get 'bored' with activities. For instance, when Dr. Paul Spong worked with the orca Skana, he researched her visual skills. However, after performing favorably in the 72 trials per day, Skana suddenly began consistently getting every answer wrong. Dr Spong concluded that a few fish were not enough motivation. He began playing music, which seemed to provide Skana with much more motivation.
At the Institute for Marine Mammal Studies in Mississippi, it has also been observed that the resident dolphins seem to show an awareness of the future. The dolphins are trained to keep their own tank clean by retrieving rubbish and bringing it to a keeper, to be rewarded with a fish. However, one dolphin, named Kelly, has apparently learned a way to get more fish, by hoarding the rubbish under a rock at the bottom of the pool and bringing it up one small piece at a time.
Use of tools
As of 1984[update], scientists have observed wild bottlenose dolphins in Shark Bay, Western Australia using a basic tool. When searching for food on the sea floor, many of these dolphins were seen tearing off pieces of sponge and wrapping them around their rostra, presumably to prevent abrasions and facilitate digging.
Whale song is the sounds made by whales and which is used for different kinds of communication.
Dolphins emit two distinct kinds of acoustic signals, which are called whistles and clicks:
- Clicks – quick broadband burst pulses – are used for echolocation, although some lower-frequency broadband vocalizations may serve a non-echolocative purpose such as communication; for example, the pulsed calls of Orcas. Pulses in a click train are emitted at intervals of ≈35–50 milliseconds, and in general these inter-click intervals are slightly greater than the round-trip time of sound to the target.
- Whistles – narrow-band frequency modulated (FM) signals – are used for communicative purposes, such as contact calls, the pod-specific dialects of resident Orcas, or the signature whistle of bottlenose dolphins.
There is strong evidence that some specific whistles, called signature whistles, are used by dolphins to identify and/or call each other; dolphins have been observed emitting both other specimens' signature whistles, and their own. A unique signature whistle develops quite early in a dolphin's life, and it appears to be created in an imitation of the signature whistle of the dolphin's mother. Imitation of the signature whistle seems to occur only among the mother and its young, and among befriended adult males.
Xitco reported the ability of dolphins to eavesdrop passively on the active echolocative inspection of an object by another dolphin. Herman calls this effect the "acoustic flashlight" hypothesis, and may be related to findings by both Herman and Xitco on the comprehension of variations on the pointing gesture, including human pointing, dolphin postural pointing, and human gaze, in the sense of a redirection of another individual's attention, an ability which may require theory of mind
The environment where dolphins live makes experiments much more expensive and complicated than for many other species; additionally, the fact that cetaceans can emit and hear sounds (which are believed to be their main means of communication) in a range of frequencies much wider than that of humans means that sophisticated equipment, which was scarcely available in the past, is needed to record and analyse them. For example, clicks can contain significant energy in frequencies greater than 110 kHz (for comparison, it is unusual for a human to be able to hear sounds above 20 kHz), requiring that equipment have a sampling rates of at least 220 kHz; MHz-capable hardware is often used.
In addition to the acoustic communication channel, the visual modality is also significant. The contrasting pigmentation of the body may be used, for example with "flashes" of the hypopigmented ventral area of some species, as can the production of bubble streams during signature whistling. Also, much of the synchronous and cooperative behaviors, as described in the Behavior section of this entry, as well as cooperative foraging methods, likely are managed at least partly by visual means.
While there is little evidence for dolphin language, experiments have shown that they can learn human sign language and can use whistles for 2-way human-animal communication. Phoenix and Akeakamai, bottlenose dolphins, understood individual words and basic sentences like "touch the frisbee with your tail and then jump over it" (Herman, Richards, & Wolz 1984). Phoenix learned whistles, and Akeakamai learned sign language. Both dolphins understood the significance of the ordering of tasks in a sentence.
A study conducted by the University of Chicago showed that bottlenose dolphins can remember whistles of other dolphins they'd lived with after 20 years of separation. Each dolphin has a unique whistle that functions like a name, allowing the marine mammals to keep close social bonds. The new research shows that dolphins have the longest memory yet known in any species other than people.
Self-awareness is seen, by some, to be a sign of highly developed, abstract thinking. Self-awareness, though not well-defined scientifically, is believed to be the precursor to more advanced processes like meta-cognitive reasoning (thinking about thinking) that are typical of humans. Scientific research in this field has suggested that bottlenose dolphins, alongside elephants and great apes, possess self-awareness.
A recent study conducted by the University of Chicago shows that bottlenose dolphins can remember whistles of other dolphins they'd lived with after 20 years of separation. Each dolphin has a unique whistle that functions like a name, allowing the marine mammals to keep close social bonds. This also shows that Dolphins see themselves and other Dolphins as distinct individuals. The new research shows that dolphins have the longest memory yet known in any species other than people.
The most widely used test for self-awareness in animals is the mirror test, developed by Gordon Gallup in the 1970s, in which a temporary dye is placed on an animal's body, and the animal is then presented with a mirror.
Some scientists[who?] still disagree with these findings, arguing that the results of these tests are open to human interpretation and susceptible to the Clever Hans effect. This test is much less definitive than when used for primates, because primates can touch the mark or the mirror, while dolphins cannot, making their alleged self-recognition behavior less certain. Critics argue that behaviors that are said to identify self-awareness resemble existing social behaviors, and so researchers could be misinterpreting social responses to another dolphin. The researchers counter-argue that the behaviors shown to evidence self-awareness are very different from normal responses to another dolphin, including paying significantly more attention to another dolphin than towards their mirror image. Whereas apes can merely touch the mark on themselves with their fingers, dolphins show less definitive behavior of self-awareness, twisting and turning themselves to observe the mark.
In 1995, Marten and Psarakos used television to test dolphin self-awareness. They showed dolphins real-time footage of themselves, recorded footage, and another dolphin. They concluded that their evidence suggested self-awareness rather than social behavior. While this particular study has not been repeated since then, dolphins have since passed the mirror test.
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Research of the comparative cognition of the dolphin is one of the primary methods of the investigation of cetacean intelligence.
Examples of cognitive abilities investigated in the dolphin include concept formation, sensory skills, and the use of mental representation of dolphins. Such research has been ongoing since the late 1970s, and includes the specific topics of: acoustic mimicry, behavioral mimicry (inter- and intra-specific), comprehension of novel sequences in an artificial language (including non-finite state grammars as well as novel anomalous sequences), memory, monitoring of self-behaviors (including reporting on these, as well as avoiding or repeating them), reporting on the presence and absence of objects, object categorization, discrimination and matching (identity matching to sample, delayed matching to sample, arbitrary matching to sample, matching across echolocation and vision, reporting that no identity match exists, etc.), synchronous creative behaviors between two animals, comprehension of symbols for various body parts, comprehension of the pointing gesture and gaze (as made by dolphins or humans), problem solving, echolocative eavesdropping, and more. Some researchers include Louis Herman, Mark Xitco, John Gory, Stan Kuczaj, Adam Pack, and many others.
While these are largely laboratory studies, field studies relating to dolphin and cetacean cognition are also relevant to the issue of intelligence, including those proposing tool use, culture, fission-fusion social structure (including tracking alliances and other cooperative behavior), acoustic behavior (bottlenose dolphin signature whistles, sperm whale clicks, orca pod vocalizations), foraging methods (partial beaching, cooperation with human fishermen, herding fish into a ball, etc.). See: Richard Connor, Hal Whitehead, Peter Tyack, Janet Mann, Randall Wells, Kenneth Norris, B. Wursig, John Ford, Louis Herman, Diana Reiss, Lori Marino, Sam Ridgway, Paul Nachtigall, Eduardo Mercado, Denise Herzing, Whitlow Au.
In contrast to the primates, cetaceans are particularly far-removed from humans in evolutionary time. Therefore, cognitive abilities generally cannot be claimed to derive from a common ancestor, whereas such claims are sometimes made by researchers studying primate cognition. Though cetaceans and humans (in common with all mammals) had a common ancestor in the distant past, it was almost certainly of distinctly inferior cognitive abilities compared to its modern descendants. The early divergence of the human – dolphin ancestry line creates a problem in what cognitive tasks to test for because human/dolphin brains have evolved so differently, with completely different cognitive abilities favoring their very different environments. Therefore, an anthropomorphic problem exists with exactly what cognitive abilities to test, how to test them, as well as the validity of the experimental results because of the completely different evolutionary lineage and environment human and cetaceans have. It was for this reason Dr. John C. Lilly proposed that developing a means of communicating with dolphins is necessary to have any future hope of communicating with an extraterrestrial organism of equal-or-greater intelligence to man, which also would have evolved in a different environment and evolutionary lineage.
- Dolphin Communication and Cognition: Past, Present, and Future, edited by Denise L. Herzing and Christine M. Johnson, 2015, MIT Press
- Animal cognition
- Animal consciousness
- Morgan's Canon
- John C. Lilly – pioneer researcher in human/dolphin communication.
- Louis Herman – scientist in dolphin cognition and sensory abilities
- Animal language
- Human-animal communication
- Vocal learning
- Spindle neuron
- Military dolphin
- U.S. Navy Marine Mammal Program
- So Long, and Thanks for All the Fish – fiction novel which derives its title from the idea of dolphins leaving the Earth.
- Uplift Universe – a series of novels, involving genetically-enhanced ("uplifted") intelligent dolphins
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- Cetacean brain
- Brain facts and figures.
- Neuroanatomy of the Common Dolphin (Delphinus delphis) as Revealed by Magnetic Resonance Imaging (MRI).
- "The Dolphin Brain Atlas" – A collection of stained brain sections and MRI images.
- Bottle-nose dolphin brain from the comparative mammalian brain collection.
- "Dolphin brains" – An AAAS Science Netlinks feature.
- "Brains, Behaviour and Intelligence in Cetaceans (Whales, Dolphins and Porpoises)" A review of scientific literature in the field which was originally written for a conference on "ethical whaling practices" and later published in the British science magazine New Scientist.
- "Whales share human brain cells" article comparing human and cetacean brains.
- Popular media
- "Watching Whales Watching Us" by Charles Siebert. New York Times Magazine, July 8, 2009
- "Deep thinkers" – Article from The Guardian about dolphin intelligence.
- "Prologue to encounters with Whales and Dolphins" – Article mainly on research difficulties with whales and dolphins.
- "Clever Dolphins Use Shells to Catch fish" – Wired, August 25, 2011
- "How smart are killer whales? Orcas have 2nd-biggest brains of all marine mammals – Phys.org, March 8, 2010
- Scientific or academic sources
- "Memory for recent actions in the bottlenose dolphin (Tursiops truncatus): Repetition of arbitrary behaviors using an abstract rule" – Research study examining ability of dolphins to remember, learn and abstract.
- "The interplay between social networks and culture: theoretically and among whales and dolphins" – Cantor, M. and H. Whitehead, 2013. Philosophical Transactions of the Royal Society B 368.
- The Dolphin Institute – Affiliated with Louis Herman's Kewalo Basin Marine Mammal Laboratory. See also their dolphin publications list for cognitive and sensory research.
- "Individually distinctive acoustic features in sperm whale codas" – Antunes, R., T. Schulz, S. Gero, H. Whitehead, J. Gordon and L. Rendell, 2011. Animal Behaviour 81: 723-730.
- Self-awareness research
- "Dolphins deserve same rights as humans, say scientists", BBC News, 21 February 2012
- "Evidence of self-awareness in the bottlenose dolphin" – Academic study of dolphin self-awareness by Marten and Psarakos(1994).
- "Using Self-View Television to Distinguish between Self-Examination and Social Behavior in the Bottlenose Dolphin (Tursiops truncatus)" – Another study by Marten and Psarakos on self-reflection using TV (1995).
- "Mirror self-recognition in the bottlenose dolphin: A case of cognitive convergence" – Academic study on self-awareness by Reiss and Marino (1998) (HTML version).
- "Dolphin Self-Recognition Mirrors Our Own" – Another article concerning Reiss and Marino study.
- "Research suggests dolphins capable of self-recognition" – Article concerning two studies on self recognition in dolphins.
- Other or uncategorized
- "Towards Communication with Dolphins" – Project using computer software to try to understand dolphin communication.
- Leafy Seadragon – Open source software for researching dolphin communication using emission and recognition of underwater whistles.
- "Organisation of Communication System in Tursiops Truncatus Montagu" by Vladimir I. Markov and Vera M. Ostrovskaya.
- "Bubble Ring Play of Bottlenose Dolphins (Tursiops Truncatus): Implications for Cognition" – McCowan B, Marino L, Vance E, Walke L, Reiss D, Journal of Comparative Psychology.