Zoology / Review Essay

Vol. 5, NO. 3 / September 2020

Whales, dolphins, and porpoises have become a fixture of contemporary popular culture. In addition to their star turns at aquarium shows around the world, cetaceans have been immortalized as stars of film and television, and celebrated in documentaries.1 Cetacean-themed tourism has also flourished, with whale- and dolphin-watching tours estimated to be generating more than $2 billion a year in revenue.2 Although perhaps long overdue, the present fascination with these creatures is still relatively new. The International Whaling Commission’s worldwide moratorium on commercial whaling took effect only in 1986.3 Since then, cetaceans have undergone a remarkable transfiguration in the popular imagination. Once hunted to near extinction, these creatures are now thought a natural wonder.

Research has revealed a range of social and prosocial behaviors among cetaceans, including caregiving, tool use, and the teaching and passing on of behaviors across generations.4 Cetaceans rank near the top of any list measuring neuroanatomical sophistication, next to great apes, humans, and elephants.5 The highly social nature of these mammals has led researchers to investigate a potential link between sociality and brain complexity.6 First proposed in the late 1980s,7 the social brain hypothesis posits that the demands of complex, information-rich social interactions will tend to create evolutionary pressure for the development of larger, more complex brains.8 Originally advanced as a model for primate brain evolution, the social brain hypothesis has since been expanded to include many other orders, with cetaceans the most recent addition.9 Though separated from humans by tens of millions of years of independent evolution, whales and dolphins represent, in some ways, our closest living parallel.

A Birth of Scientific Interest

The ancient Greeks were astute observers of cetacean behavior. Aristotle reported that dolphins spontaneously imitated the sounds of human speech and would work cooperatively with fishermen.10 Both of these behaviors have been confirmed by recent research.11 Aristotle also noted that cetacean young fed on breast milk and should therefore be classified as mammals.12

The prescience of Aristotle’s observations were in sharp contrast to the prevailing views of the early nineteenth century, when cetaceans were seen primarily as a valuable source of oil.13 In New York in 1818, a major court battle took place in which opposing sides debated whether cetaceans were mammals or fish.14 A whale-oil merchant, Samuel Judd, had contested the right of the authorities to collect fish-oil taxes on his product, since, he argued, whales were not fish. Despite the ample scientific evidence presented to the contrary, the jury ultimately decided whales were fish.15 Admittedly, Judd was likely more interested in preserving his profits than pursuing scientific truth, but that does not change the fact that he was right.

Cetaceans spend nearly all of their time underwater, often in the open ocean, which makes intimate, long-term studies extraordinarily expensive, difficult, and time-consuming. It was not until the 1960s that serious scientific studies first began appearing, sparked by the early work of John Cunningham Lilly.16 Trained as a medical doctor at the University of Pennsylvania, Lilly went on to conduct psychiatric research into psychedelics and sensory deprivation at the National Institute of Mental Health before becoming fascinated by cetaceans.17 Lilly used a combination of personal and government funds, most of the latter granted by NASA, to set up a dolphin research facility in the Virgin Islands. Lilly believed, and apparently convinced NASA, that a project to communicate with dolphins would provide an ideal proving ground to prepare humans for imminent contact with extraterrestrial intelligence.18 In the course of his work, Lilly had become convinced that dolphins possessed a discernible and comprehensible language.19 He published a series of papers in Science, all of which were focused on the vocalization and communication abilities of the bottlenose dolphin. Lilly’s was the first modern research to note a tendency among dolphins to communicate with each other using complex vocalizations and to identify their distress calls. Lilly also confirmed Aristotle’s observation that dolphins could mimic the sounds of human speech.20

Despite the promise of Lilly’s early publications, deep communication with the dolphins proved elusive, and his grandiose claims concerning “humans of the sea” began to look foolish. Some of Lilly’s dolphins died of infectious disease and other complications—a major setback to research, a financial loss, and a public embarrassment.21 It did not help matters when it became known that Lilly was dosing the dolphins, and himself, with LSD in an effort to establish psychic communication where traditional means had failed.22

Researchers have since felt the need to distance themselves from these unorthodox methods.23 Lilly may not have turned out to be the kind of pioneering figure that later scientists and activists would prefer to be associated with, but he was perhaps precisely the kind who was needed at that time. Irreverent, erudite, and independently wealthy, Lilly took a species that was thought of, if at all, as a source of oil and helped recast them in the public eye as icons of animal intelligence.

The Social Brain Hypothesis

Although Lilly’s work on the communicative, linguistic, and social abilities of cetaceans anticipated some aspects of the social brain hypothesis, its ideas are typically traced back to the Machiavellian intelligence hypothesis, proposed by Richard Byrne and Andrew Whiten in 1988.24 Traditional ecological accounts of primate and human brain evolution emphasized foraging and hunting techniques, size and complexity of home range, and tool use.25 The work of Byrne and Whiten emphasized that the most complex and challenging part of a primate’s environment was, in fact, other primates. In densely social species, social standing could easily mean the difference between life and death, or, at least, between sex and celibacy, which in evolutionary terms amounts to the same thing. Presumably, the imperative to assist allies and outwit opponents produces a psychological and neurological arms race. An expansion of cognitive architecture would be required to undertake increasingly complex social behavior and keep track of increasingly elaborate social hierarchies and alliances. Robin Dunbar presented evidence for just such a relationship in 1992, claiming a correlation between the size of the neocortex and group size in 38 genera of primates.26 Dunbar also found no relationship between neocortex size and ecological factors, such as the size of the home range that a genus needed to explore and map or the degree to which complex extraction strategies were employed to exploit food resources. Dunbar’s paper provided the first credible quantitative evidence in favor of social over ecological theories of primate brain evolution.27

It did not take long for scientists to notice similar trends among whales and dolphins. In a 1996 study of odontocetes, Lori Marino published evidence of a correlation between maximum group, or pod, size and brain size relative to body size.28 She observed this trend across 16 species—by no means a majority of the nearly 100 living cetacean species. The correlation was admittedly speculative. In 2001, Luke Rendell and Hal Whitehead provided the first truly comprehensive literature review showing that, in addition to their ecological and genetic conditioning, cetaceans also learn socially. Subgroups within a species display distinct repertoires of behavior that are passed between individuals and generations. Among orcas, different subgroups demonstrate highly distinctive patterns of behavior, diet, and vocalizations, which remain stable over long periods, even when habitats are shared with other subgroups.29 Humpback whales have been studied long enough for scientists to observe novel hunting and feeding strategies emerge in just one or a few individuals and then spread like wildfire through the larger population, through mimicry or perhaps direct teaching.30 Rendell and Whitehead surveyed a wide range of cetacean behavior that was unlikely to be genetic or ecological, and thus more likely sociocultural.31 The article prompted dozens of replies, ranging from the supportive, to the critical, to the openly hostile and dismissive.

Not everyone has agreed with a social brain hypothesis for cetaceans. The most notable dissent has come from the neuroscientist Paul Manger. In 2006, he published a 24,000-word manifesto arguing against the notion that high intelligence and brain size were linked in cetaceans.32 Manger suggested instead that water temperature was the primary driver of cetacean encephalization. He argued that about 35 million years ago cooling ocean temperatures induced anatomical changes in cetaceans, including larger brains rich in myelin. The exceptionally high amount of myelin evolved to counteract heat loss, keeping cetaceans warm as ocean temperatures dropped.33 Manger’s theory had obvious difficulties. Not least was that it overlooked the brain’s role in perceiving, thinking, and planning and controlling actions, as well as myelin’s role in facilitating those activities.34 Two years after Manger’s paper was published, 17 cetacean specialists came together and examined the thermogenesis hypothesis.35 Manger had provided incorrect data about the temperature range of waters inhabited by several cetacean species and had ignored readily available data for several others.36 Rerunning Manger’s regression analyses using accurate data showed no significant relationship between inhabited temperature range and relative brain size, undermining the central pillar of the thermogenesis hypothesis.37

Even as Manger was debunked, little had been done to explore the social brain hypothesis in cetaceans in a rigorous, quantitative manner. A simple correlation between relative brain size and group size was not sufficient. After sifting through the whole of the documentation on social behavior engaged in by each cetacean species, Michael Muthukrishna, Susanne Shultz, and I found compelling correlations.38 The dolphin family tends to have the largest relative brain size, the broadest social repertoires, and the tightest-knit social bonds. Filter-feeding baleen whales showed the opposite pattern, with little by way of social behavior and small brains relative to their large bodies.39

The most significant finding was that both social repertoire and brain size are largest in the species that associate in midsized groups, and smaller in solitary species and those that associate in large but relatively anonymous mega-pods. Contrary to the earlier and simpler version of the social brain hypothesis, which predicted the largest brains in the largest groups,40 our dataset suggested that it is not simply large groups that are the key factor in brain size; rather, the depth and diversity of social interactions counts most.41

Captivity and Its Consequences

Even as the slaughter of cetaceans in whaling has subsided, new moral dilemmas have arisen, somewhat paradoxically, as the direct result of a newfound appreciation for the complex behavior of dolphins and whales. It is estimated that more than 2,000 cetaceans are in captivity, and that more than 5,000 have died in captivity since the confinement of cetaceans began in the 1950s.42 Captive cetaceans demonstrate a wide range of sociopathic and psychopathic behavior rarely if ever witnessed in the wild.43 These include abnormal repetitive behavior, self-mutilation, attempted suicide, and murder—killing trainers and aquarium visitors who have entered their tanks. Such outcomes are similar to the sociopathic behavior induced by overcrowded prisons and solitary confinement among our own species. To mitigate these pathological behaviors, captive cetaceans are administered a range of antidepressants, antianxiety drugs, antibiotics for stress-induced stomach ulcers, and hormone therapy to control sexual urges.44 It should come as no surprise that the average lifespan of cetaceans in captivity is considerably lower than in the wild.45

Most research observations have been made in captivity and from the water’s surface, but in the wild cetaceans spend upwards of 95% of their time underwater. What goes on below the surface has remained largely mysterious. It was only a few years ago that a wildlife photographer was able to capture the first footage of a sperm whale giving birth in the wild. In the video, several other sperm whales can be seen attending to the mother and then ushering the newborn calf to the surface for its first breath.46 Death is also accompanied by distinctive sociocultural behaviors. Cetaceans display a wide range of grieving behavior following the death of offspring, relatives, and even unrelated members of the same species. Giovanni Bearzi et al. recently found that the odontocetes, and especially dolphins, were far more likely to grieve than baleen whales.47 Bottlenose dolphins have been known to carry around dead newborns and attend to dead calves.48 As to be expected, dolphins are also the cetaceans with the largest relative brain mass, the largest social repertoire, the tightest-knit social groups,49 and the most sonically complex communication.50

More about cetaceans’ behavior beneath the waves remains to be discovered. What is certain now is that the study of cetaceans in captivity has confirmed that these animals display cognitive skills surpassing any other animal.51 In one study, two dolphins were able to correctly follow symbolic instructions in a screen display.52 The dolphins were able to perform the instructions almost perfectly after one exposure, and even as the instructions became more and more abstract: ultimately, the trainers disappeared from the video and were replaced with two white circles representing the key movements of the gestural commands. Such a feat had never been accomplished by another animal, not even the chimpanzee.53 Dolphins, despite having no hands or fingers of their own, also easily understand the symbolic significance of a human’s pointing gesture without any explicit training, another feat unknown in primates and demonstrated elsewhere only in dogs, which have a long history of domestication.54 Careful study of cetaceans in their natural habitats could reveal a host of other social and cognitive abilities so far unsuspected.


  1. Cetaceans star in television series and films such as Flipper (1964–67), Free Willy (1993), and Blackfish (2013). 
  2. Simon O’Connor et al., Whale Watching Worldwide: Tourism Numbers, Expenditures and Expanding Economic Benefits (Yarmouth, MA: International Fund for Animal Welfare, 2009). 
  3. Although American whaling peaked in the mid-1800s, other latecomer nations kept the pressure up, and one species after another was progressively hounded to near-extinction well into the mid-twentieth century. A few nations (notably Norway and Japan) still stubbornly continue whaling, ignoring international agreements and mounting public outrage. D. Graham Burnett, The Sounding of the Whale: Science and Cetaceans in the Twentieth Century (Chicago: University of Chicago Press, 2012). Andrew Brierley, “No Case for Japan to Kill Minke Whales,” Nature 520 (2015): 157, doi:10.1038/520157c. Andrew Brierley and Phillip Clapham, “Japan’s Whaling Is Unscientific,” Nature 529 (2016): 283, doi:10.1038/529283a. 
  4. Luke Rendell and Hal Whitehead, “Culture in Whales and Dolphins,” Behavioral and Brain Sciences 24, no. 2 (2001): 309–24, doi:10.1017/s0140525x0100396x. Rachel Smolker et al., “Sponge Carrying by Dolphins (Delphinidae, Tursiops sp.): A Foraging Specialization Involving Tool Use?Ethology 103, no. 6 (1997): 454–65, doi:10.1111/j.1439-0310.1997.tb00160.x. Katharine Payne and Roger Payne, “Large Scale Changes over 19 Years in Songs of Humpback Whales in Bermuda,” Zeitschrift für Tierpsychologie 68, no. 2 (1985): 89–114, doi:10.1111/j.1439-0310.1985.tb00118.x. Roger Payne and Scott McVay, “Songs of Humpback Whales,” Science 173 (1971): 585–97, doi:/10.1126/science.173.3997.585. Hal Whitehead and Luke Rendell, The Cultural Lives of Whales and Dolphins (Chicago & London: University of Chicago Press, 2014). 
  5. Patrick Hof, Rebecca Chanis, and Lori Marino, “Cortical Complexity in Cetacean Brains,” The Anatomical Record Part A: Discoveries in Molecular Cellular and Evolutionary Biology 287 (2005): 1,142–52, doi:10.1002/ar.a.20258. Richard Connor, “Dolphin Social Intelligence: Complex Alliance Relationships in Bottlenose Dolphins and a Consideration of Selective Environments for Extreme Brain Size Evolution in Mammals,” Philosophical Transactions of the Royal Society of London B: Biological Sciences 362, no. 1,480 (2007): 587–602, doi:10.1098/rstb.2006.1997. Lori Marino, “Comparison of Encephalization between Odontocete Cetaceans and Anthropoid Primates,” Brain and Behavioral Sciences 51 (1998): 230–38, doi:10.1159/000006540.

    Comparative neurology across species, however, remains a relatively crude business. Susan Healy and Candy Rowe, “A Critique of Comparative Studies of Brain Size,” Proceedings of the Royal Society of London B: Biological Sciences 274 (2007): 453–64, doi:10.1098/rspb.2006.3748. 
  6. Lori Marino et al., “Cetaceans Have Complex Brains for Complex Cognition,” PLoS Biology 5, no. 5 (2007): e139, doi:10.1371/journal.pbio.0050139.g001. 
  7. Machiavellian Intelligence: Social Expertise and the Evolution of Intellect in Monkeys, Apes, and Humans, ed. Richard Byrne and Andrew Whiten (New York: Clarendon Press & Oxford University Press, 1988). Andrew Whiten and Richard Byrne, “Tactical Deception in Primates,” Behavioral and Brain Sciences 11, no. 2 (1988): 233–44, doi:10.1017/s0140525x00049682. Robin Dunbar, “The Social Brain Hypothesis,” Evolutionary Anthropology 6, no. 5 (1998): 178–90, doi:10.1002/(SICI)1520-6505(1998)6:5<3C178::AID-EVAN5>3E3.0.CO;2-8. Robin Dunbar, “Neocortex Size as a Constraint on Group Size in Primates,” Journal of Human Evolution 20, no. 6 (1992): 469–93, doi:10.1016/0047-2484(92)90081-j. 
  8. Dunbar, “The Social Brain Hypothesis.” Robin Dunbar and Susanne Shultz, “Evolution in the Social Brain,” Science 317 (2007): 1,344–47, doi:10.1126/science.1145463. 
  9. Susanne Shultz and Robin Dunbar, “Encephalization Is Not a Universal Macroevolutionary Phenomenon in Mammals but Is Associated with Sociality,” Proceedings of the National Academy of Sciences USA 107 (2010): 21,582–86, doi:10.1073/pnas.1005246107. Javier Pérez-Barbería, Susanne Shultz, and Robin Dunbar, “Evidence for Coevolution of Sociality and Relative Brain Size in Three Orders of Mammals,” Evolution 61, no. 12 (2007): 2,811–21, doi:10.1111/j.1558-5646.2007.00229.x. Kieran Fox, Michael Muthukrishna, and Susanne Shultz, “The Social and Cultural Roots of Whale and Dolphin Brains,” Nature Ecology & Evolution 1 (2017): 1,699–1,705, doi:10.1038/s41559-017-0336-y. 
  10. Aristotle: History of Animals: Books VII–X, ed. and trans. David Balme (Cambridge & London: Harvard University Press, 1991). 
  11. Sam Ridgway et al., “Spontaneous Human Speech Mimicry by a Cetacean,” Current Biology 22, no. 20 (2012): PR860–61, doi:10.1016/j.cub.2012.08.044. Karen Pryor et al., “A Dolphin­–Human Fishing Cooperative in Brazil,” Marine Mammal Science 6 (1990): 77–82, doi:10.1111/j.1748-7692.1990.tb00228.x. 
  12. Balme, Aristotle: History of Animals
  13. Walter Tower, A History of the American Whale Fishery (Philadelphia: University of Pennsylvania, 1907). 
  14. D. Graham Burnett, Trying Leviathan (Princeton, NJ: Princeton University Press, 2010). 
  15. Burnett, Trying Leviathan
  16. Burnett, The Sounding of the Whale
  17. John Lilly, The Scientist: A Metaphysical Autobiography (Berkeley: Ronin Publishing, 1996). 
  18. John Lilly, Man and Dolphin: Adventures of a New Scientific Frontier (Garden City, NY: Doubleday, 1961), 145–46. John Lilly, Communication between Man and Dolphin: The Possibilities of Talking with Other Species (New York: Crown Publishers, 1978), 125–26. 
  19. John Lilly, Lilly on Dolphins: Humans of the Sea (Garden City, NY: Anchor Press, 1975). 
  20. John Lilly and Alice Miller, “Sounds Emitted by the Bottlenose Dolphin: The Audible Emissions of Captive Dolphins under Water or in Air Are Remarkably Complex and Varied,” Science 133 (1961): 1,689–93, doi:10.1126/science.133.3465.1689. John Lilly and Alice Miller, “Vocal Exchanges between Dolphins: Bottlenose Dolphins ‘Talk’ to Each Other with Whistles, Clicks, and a Variety of Other Noises,” Science 134 (1961): 1,873–76. John Lilly, “Distress Call of the Bottlenose Dolphin: Stimuli and Evoked Behavioral Responses,” Science 139 (1963): 116–18, doi:10.1126/science.139.3550.116. John Lilly, “Vocal Mimicry in Tursiops: Ability to Match Numbers and Durations of Human Vocal Bursts,” Science 147 (1965): 300–01, doi:10.1126/science.147.3655.300. 
  21. Lilly, Lilly on Dolphins. To make matters worse, revelations of sexual relations between a male dolphin and the primary linguistic and behavioral trainer, Margaret Howe, raised eyebrows—and hackles. Christopher Riley, “The Dolphin Who Loved Me: The NASA-Funded Project that Went Wrong,” The Guardian, June 8, 2014. 
  22. Riley, “The Dolphin Who Loved Me.” 
  23. Burnett, The Sounding of the Whale. Cetacean Societies: Field Studies of Dolphins and Whales, ed. Janet Mann et al. (Chicago & London: University of Chicago Press, 2000). 
  24. Machiavellian Intelligence, ed. Byrne and Whiten. Machiavellian Intelligence II: Extensions and Evaluations, ed. Richard W. Byrne and Andrew Whiten (New York: Cambridge University Press, 1997).

    Similar notions had been advanced earlier by numerous other researchers. Nicholas Humphrey, “The Social Function of Intellect,” in Growing Points in Ethology, ed. Patrick Bateson and Robert Hinde (Cambridge, UK: Cambridge University Press, 1976), 303–17. Alison Jolly, “Lemur Social Behavior and Primate Intelligence,” Science 153 (1966): 501–06, doi:10.1126/science.153.3735.501. Hans Kummer, “Social Knowledge in Free-Ranging Primates,” in Animal Mind—Human Mind, ed. Donald Griffin (Berlin, Heidelberg, & New York: Springer-Verlag, 1982), 113–30. 
  25. Tim Clutton-Brock and Paul Harvey, “Primates, Brains and Ecology,” Journal of Zoology 190, no. 3 (1980): 309–23, doi:10.1111/j.1469-7998.1980.tb01430.x. Paul Harvey and John Krebs, “Comparing Brains,” Science 249 (1990): 140–46, doi:10.1126/science.2196673. Katharine Milton, “Foraging Behaviour and the Evolution of Primate Intelligence,” in Machiavellian Intelligence, 285–305. Kathleen Gibson, “Cognition, Brain Size and the Extraction of Embedded Food Resources,” in Primate Ontogeny, Cognition and Social Behaviour, ed. Phyllis Lee and James Else (Cambridge, UK: Cambridge University Press, 1986), 93–104. 
  26. Dunbar, “Neocortex Size as a Constraint.” 
  27. Although far from comprehensive, Dunbar’s paper was nonetheless the catalyst for much of the research and discussion that followed. Alex DeCasien, Scott Williams, and James Higham, “Primate Brain Size Is Predicted by Diet but Not Sociality,” Nature Ecology & Evolution 1, no. 5 (2017), doi:10.1038/s41559-017-0112. Fox, Muthukrishna, and Shultz, “Social and Cultural Roots.” Marino et al., “Cetaceans Have Complex Brains for Complex Cognition.” Connor, “Dolphin Social Intelligence.” 
  28. Lori Marino, “What Can Dolphins Tell Us about Primate Evolution?Evolutionary Anthropology 5, no. 3 (1996): 81–86, doi:10.1002/(sici)1520-6505(1996)5:3<3C81::aid-evan3>3E3.0.co;2-z. 
  29. Rendell and Whitehead, “Culture in Whales and Dolphins.” 
  30. Jenny Allen et al., “Network-Based Diffusion Analysis Reveals Cultural Transmission of Lobtail Feeding in Humpback Whales,” Science 340 (2013): 485–88, doi:10.1126/science.1231976. Mason Weinrich, Mark Schilling, and Cynthia Belt, “Evidence for Acquisition of a Novel Feeding Behaviour: Lobtail Feeding in Humpback Whales, Megaptera novaeangliae,” Animal Behaviour 44, no. 6 (1992): 1,059–72, doi:10.1016/s0003-3472(05)80318-5. 
  31. Rendell and Whitehead, “Culture in Whales and Dolphins.” 
  32. Paul Manger, “An Examination of Cetacean Brain Structure with a Novel Hypothesis Correlating Thermogenesis to the Evolution of a Big Brain,” Biological Reviews of the Cambridge Philosophical Society 81, no. 2 (2006): 293–338, doi:10.1017/s1464793106007019. 
  33. Manger was by no means alone in envisioning the brain as thermostat. Aristotle’s theory, later demolished by Galen, that the brain served as a radiator to disperse heat is well-known. A modern anthropologist has proposed a similar theory. Nonetheless, any such theory encounters immense difficulties. See Olga Longo, “Hot Heads and Cold Brains: Aristotle, Galen and the ‘Radiator Theory,’” Physis: Rivista internazionale di storia della scienza 33 (1996): 259–66. Dean Falk, “Brain Evolution in Homo: The ‘Radiator’ Theory,” Behavioral and Brain Sciences 13 (1990): 333–44, doi:10.1017/s0140525x00078973. 
  34. Jim Salzer and Bernard Zalc, “Myelination,” Current Biology 26, no. 20 (2016): PR971–75, doi:10.1016/j.cub.2016.07.074. Daniel K. Hartline, “What Is Myelin?Neuron Glia Biology 4, no. 2 (2008): 153–63, doi:10.1017/s1740925x09990263.

    In addition, cetaceans already have a thick layer of blubber that serves as a highly effective conserver of body heat in cold ocean waters. It is difficult to see how the approximately 8-kilogram brain of the sperm whale, no matter how efficient a producer of heat, could possibly have any meaningful impact on the temperature of an approximately 50,000-kilogram body. 
  35. Lori Marino et al., “A Claim in Search of Evidence: Reply to Manger’s Thermogenesis Hypothesis of Cetacean Brain Structure,” Biological Reviews of the Cambridge Philosophical Society 83, no. 4 (2008): 417–40, doi:10.1111/j.1469-185x.2008.00049.x. 
  36. Marino et al., “A Claim in Search of Evidence,” 421. 
  37. Others have tried to replicate Manger’s finding—using larger, more accurate datasets, as well as more sophisticated analyses—and failed. Calo Maximino, “A Quantitative Test of the Thermogenesis Hypothesis of Cetacean Brain Evolution, Using Phylogenetic Comparative Methods,” Marine and Freshwater Behaviour and Physiology 42 (2009): 1–17, doi:10.1080/10236240902761656. Manger has a new paper under review on this topic: Paul Manger et al., “Amplification of Potential Thermogenic Mechanisms in Cetacean Brains.” 
  38. We were inspired by investigations in birds and primates that had amassed detailed datasets on social and innovative behaviors: Louis Lefebvre, Simon Reader, and Daniel Sol, “Brains, Innovations and Evolution in Birds and Primates,” Brain, Behavior and Evolution 63 (2004): 233–46, doi:10.1159/000076784. Ana Navarrete et al., “The Coevolution of Innovation and Technical Intelligence in Primates,” Philosophical Transactions of the Royal Society B Biological Sciences 371, no. 1,690 (2016), doi:10.1098/rstb.2015.0186. Simon Reader and Kevin Laland, “Social Intelligence, Innovation, and Enhanced Brain Size in Primates,” Proceedings of the National Academy of Sciences 99, no. 7 (2002): 4,436–41, doi:/10.1073/pnas.062041299. 
  39. Fox, Muthukrishna, and Shultz, “Social and Cultural Roots.” 
  40. Dunbar, “Neocortex Size as a Constraint.” 
  41. Fox, Muthukrishna, and Shultz, “Social and Cultural Roots.” 
  42. CFAF’s Work for Whales and Dolphins in Captivity,” Change for Animals Foundation. 
  43. Lori Marino and Toni Frohoff, “Towards a New Paradigm of Non-Captive Research on Cetacean Cognition,” PloS One 6, no. 9 (2011): e24121, doi:10.1371/journal.pone.0024121. 
  44. Marino and Frohoff, “Towards a New Paradigm.” John Hargrove and Howard Chua-Eoan, Beneath the Surface: Killer Whales, SeaWorld, and the Truth beyond Blackfish (New York: St. Martin’s Press, 2015). 
  45. Marino and Frohoff, “Towards a New Paradigm.” Vanessa Williams, Captive Orcas: Dying to Entertain You! (Chippenham, UK: Whale and Dolphin Conservation Society, 2001). Robert Small and Douglas Demaster, “Survival of Five Species of Captive Marine Mammals,” Marine Mammal Science 11, no. 2 (1995): 209–26, doi:10.1111/j.1748-7692.1995.tb00519.x. 
  46. StoryTrender, Remarkable Footage of Sperm Whale Giving Birth, Youtube.com, December 4, 2014. 
  47. They examined reports of 78 records involving 20 cetacean species. Giovanni Bearzi et al., “Whale and Dolphin Behavioural Responses to Dead Conspecifics,” Zoology 128 (2018): 1–15, doi:10.1016/j.zool.2018.05.003. 
  48. These calves are usually assumed to be their own. Bearzi et al., “Whale and Dolphin Behavioural Responses.” 
  49. Fox, Muthukrishna, and Shultz, “Social and Cultural Roots.” 
  50. Laura May-Collado, Ingi Agnarsson, and Douglas Wartzok, “Phylogenetic Review of Tonal Sound Production in Whales in Relation to Sociality,” BMC Evolutionary Biology 7, no. 136 (2007), doi:10.1186/1471-2148-7-136. 
  51. Louis Herman, “What Laboratory Research Has Told Us about Dolphin Cognition,” International Journal of Comparative Psychology 23, no. 3 (2010): 310–30. 
  52. Louis Herman, Palmer Morrel-Samuels, and Adam Pack, “Bottlenosed Dolphin and Human Recognition of Veridical and Degraded Video Displays of an Artificial Gestural Language,” Journal of Experimental Psychology: General 119, no. 2 (1990): 215–30, doi:10.1037/0096-3445.119.2.215. 
  53. E. Sue Savage-Rumbaugh, Ape Language: From Conditioned Response to Symbol (New York: Columbia University Press, 1986). 
  54. Herman, “What Laboratory Research Has Told Us.” 

Kieran Fox is a neuroscientist working in the Department of Neurology and Neurological Sciences at Stanford University.

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