Dolphins top chimps as our closest intellectual relations

BIOLOGISTS have long thought that the chimpanzee was the animal closest in intelligence to the human, but recent work assigns that honour to the dolphin.

Some scientists are so impressed by the mental capacity of the dolphin that they believe dolphins should be categorised and treated as “non-human persons”. A leading US researcher in this field, Lori Marino, Emory University, Altanta, Georgia, has compared the brains of dolphins and primates.

The dolphin is an aquatic mammal closely related to whales and porpoises. About 32 species of dolphin exist – the bottle-nosed dolphin has been studied the most. Dolphins emit clicking sounds that they use for echolocation to help them move among their companions and to hunt prey fish. They also squeal to communicate alarm, sexual excitement and, probably, different emotional states. They are clearly intelligent and friendly, and easily trained in captivity. Morani uses magnetic resonance imagery to scan dolphin brains and she concludes that their brain neuroanatomy suggests “psychological continuity between humans and dolphins and has profound implications for the ethics of human-dolphin interactions”.

The cortex of the human brain is the part principally associated with intelligence. The human cortex is deeply convoluted, a device to increase its surface area/capacity and to facilitate brain cells to interconnect with each other. Marino found that the brain cortex of dolphins had the same convoluted structure as humans, although they evolved along different neuro-anatomical trajectories.

When it comes to correlation with intelligence, brain size relative to body size is more significant than absolute brain size. Thus, the chimpanzee brain weighs 0.34kg, the human brain 0.9kg-1.8kg, and the sperm whale brain over 8.6kg, but the ratio of brain to body weight is greater in the human than in most other animals. Marino and colleagues found the brain cortex of bottlenose dolphins so large that “the anatomical ratios that assess cognitive capacity place it second only to the human brain”.

The following list expresses brain weight as a percentage of body weight for a number of species – the percentage weight is the figure in brackets after each species: human (2.1), dolphin (0.94), elephant (0.15), cow (0.1), killer whale (0.094), pilot whale (0.076). But this neat correlation with intelligence is disrupted by mouse (3.2) and small birds (8).

Such neuroanatomical studies harmonise with many behavioural studies of dolphins that indicate a significant degree of intelligence. For example, bottle-nosed dolphins recognise themselves in a mirror and can use the mirror to look at various parts of their bodies. Dolphins can learn simple symbol-based language. Wild dolphins live in groups that have complex social structures and emotional sophistication. Hunting dolphins co-operate with great precision when hunting shoals of fish.

Leake reports how social interaction between dolphins was illustrated recently when an injured wild dolphin was placed in a dolphinarium to recuperate, taught the trick of tail-walking and then released to the wild again. Scientists continued to observe this dolphin group, watching with amazement to see the trick of tail-walking spreading through the group.

Many researchers who study dolphins now feel that because of their high intelligence, and other evidence of their inner lives, it is morally repugnant to mistreat them. I would certainly agree that it is wrong to kill such animals, or indeed whales or porpoises, for food. I also think that it is wrong to keep them in amusement parks, unless they are inclined to stay there voluntarily. I don’t agree that dolphins should be categorised as “non-human persons” as suggested by some researchers.

The category of “person” has always been used synonymously with “human being”. If we start calling animals persons this would cause all sorts of mix-ups. The use of the term person for the human being classifies the human as different from the animal. If we categorise some animals as persons we will begin to think of humans as animals also. I am generally uneasy at the notion of elevating one animal over all others and doing so using the criterion of intelligence. We have an obligation to respect all animals and not just those who are most like us in our most distinctive capacity. And we must also remember that the experience of strong human preferment for the most intelligent and able among ourselves led to a sorry chapter in our history. Also, assigning such high value to animal intelligence could cause other problems. I understand that pigs are more intelligent than dogs, yet we eat pigs; but, on this side of the world, are shocked at the idea of eating dogs.

William Reville is associate professor of biochemistry and public awareness of science officer at UCC – understandingscience.ucc.ie