Are cetaceans really intelligent?

Is there such a thing as non-human intelligence? The question, long set aside as metaphysical, has become a scientific one over the past half-century. Several species, chimpanzees, elephants, corvids, parrots, have in turn overturned the idea that complex cognitive abilities are uniquely human. But perhaps none has unsettled the scientific community as much as the cetaceans. Their brains are enormous, their social behaviour complex, their communication structured, their culture stable and transmissible. Several of them even pass a test long believed to be exclusive: mirror self-recognition. This page attempts to take stock of what science allows us to state today without romantic projection.

The brain: an impressive substrate

The brain of a common bottlenose dolphin weighs about 1.6 kilograms, more than that of a human (1.4 kg). That of a killer whale reaches 5.4 kilograms. That of a sperm whale averages 7.8 kilograms, up to 9.2 kg in the largest males. It is the largest brain ever documented in the animal kingdom, across all taxa, with the possible exception of some large sauropods (though their brains were tiny relative to their bodies).

The ratio between brain size and body size, known as the encephalisation quotient (EQ), is often regarded as a better indicator of cognitive ability than absolute size. Humans have an EQ of about 7. The common bottlenose dolphin has an EQ of 4 to 5, the highest after humans. The killer whale, being more massive, reaches an EQ of about 2.5, equivalent to that of the chimpanzee. The sperm whale, whose body is colossal, sees its EQ fall to around 0.6, even though its absolute brain is the largest known. None of these figures is enough to settle the question of intelligence, but they do attest to a massive energetic investment in the central nervous system.

More interesting than size: structure. The cerebral cortex of cetaceans is extremely folded, more so than the human cortex, although on average it is thinner. It contains large numbers of von Economo cells, a distinctive type of neuron first identified in humans and associated with complex social processing, with empathy and with self-awareness. Patrick Hof, a neuroanatomist at the Mount Sinai School of Medicine in New York, published a study in 2007 (Anatomical Record) showing that humpback whales have von Economo cells in quantities proportionally comparable to those of humans.

The mirror test

Devised by the psychologist Gordon Gallup in 1970, the mirror test consists in secretly placing a coloured mark on an animal’s body (in an area it cannot see directly), then presenting it with a mirror. If the animal touches the mark on its own body after seeing itself in the mirror, this suggests that it recognises itself as a distinct individual in the reflection, a form of self-awareness.

Very few animals pass this test. Humans from 18 to 24 months. The great apes (chimpanzees, orangutans, bonobos, gorillas). Elephants. Magpies. And among cetaceans: common bottlenose dolphins and killer whales. Diana Reiss and Lori Marino published the demonstration for the common bottlenose dolphin in 2001 in PNAS: the marked animals examine their own body in the mirror, inspect the mark, check their other marks. It is a robust demonstration, and it has been repeated since. For killer whales, the experiments are harder to set up, but convergent results exist.

Tool use in the Shark Bay dolphins

At Shark Bay, on the west coast of Australia, part of the common bottlenose dolphin population uses marine sponges to protect the rostrum while probing the sandy bottom to flush out fish. The technique was identified in the 1990s by Janet Mann and her team. The sponges, fitted over the rostrum, protect the skin against spines and sharp edges on the seabed. The practice is passed on exclusively from mother to daughter: a rare case of cultural transmission confined to one sex.

Tool use is not uniquely human, it is widespread among corvids, sea otters and chimpanzees, but the Shark Bay dolphins are one of the few documented examples in a marine mammal, and the strictly matrilineal transmission makes it a textbook case for the study of animal culture.

Grief in J35

In July 2018, the killer whale J35, known as Tahlequah, of the Southern Resident community, lost her calf a few hours after its birth. Instead of leaving it behind, as might have been expected, she carried the small body on her head or her rostrum for 17 days and more than 1,600 kilometres. When she stopped to breathe, she let the body float briefly before taking it up again. The other members of the pod swam in formation around her, slowing their pace to keep with her. This behaviour, which travelled round the world, is an extremely well documented example of what ethologists now call prolonged grief in a cetacean.

Tahlequah is not an isolated case. The carrying of dead calves has been observed in humpback whales, sperm whales and dolphins. Several studies have drawn these observations together systematically (Bearzi et al., 2018). There is still hesitation about using the word “grief” without quotation marks, since it implies an emotional charge that cannot be measured directly. But the behavioural, energetic and social investment involved in these behaviours suggests that they cannot be reduced to a simple reflex.

Cultures and shared knowledge

Perhaps the most striking aspect of cetacean intelligence is the cultural spread of complex behaviours. Bubble net feeding in humpback whales (see our dedicated page), the dialectal codas of sperm whale clans, the hunting techniques specific to killer whale ecotypes, the use of sponges by the Shark Bay dolphins: all of them behaviours that are not genetically programmed, that vary between populations, and that are passed on by social learning across generations.

The reference work on this subject is The Cultural Lives of Whales and Dolphins (2015) by Hal Whitehead and Luke Rendell, at Dalhousie University. It argues that cetaceans have cultures comparable in complexity to those of humans, but structured differently, oriented towards acoustics and the social dimension rather than towards material tools.

A different intelligence, not an inferior one

Comparing the intelligence of cetaceans with that of humans in terms of “more” or “less” makes little sense. Their cognition is oriented differently. Highly developed in acoustic processing, in spatial and social memory, in group coordination. Far less developed in the manipulation of objects (they have no prehensile hands). Probably more oriented towards the collective immediate present than towards the abstract projections in time that we make.

As Whitehead writes: “cetaceans have had fifty million years to develop an intelligence suited to their world; we have had far less. Comparing our solutions only makes sense within their respective contexts.”

Sources

• Marino, L. (2002). “Convergence of complex cognitive abilities in cetaceans and primates”. Brain, Behavior and Evolution, 59.
• Reiss, D. & Marino, L. (2001). “Mirror self-recognition in the bottlenose dolphin: a case of cognitive convergence”. PNAS, 98.
• Hof, P.R. & Van der Gucht, E. (2007). “Structure of the cerebral cortex of the humpback whale, Megaptera novaeangliae“. Anatomical Record, 290.
• Whitehead, H. & Rendell, L. (2015). The Cultural Lives of Whales and Dolphins. University of Chicago Press.
• Mann, J. et al. (2008). “Why do dolphins carry sponges?”. PLoS ONE, 3.
• Bearzi, G. et al. (2018). “Whales and dolphins in love and grief”. Zoology.

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