Scientific consensus: How theories of animal intelligence have changed over time

An overview of leading proponents of animal intelligence and their views

Scala Naturae
David Hume
Wolfgang Köhler

Aristotle on animals

Humans have studied animal intelligence for thousands of years. In the 4th century BCE, Aristotle wrote his famous *History of Animals*, which explored the subject in detail.

Aristotle believed that animals possessed a form of basic intelligence. He gave the nest-building abilities of certain birds as an example of “pre-eminent intelligence.”

But this intelligence differed from human intelligence in terms of quantity; humans had more, while animals had less. He called this the Scala Naturae, or ‘Ladder of Being’ – a natural hierarchy with humans at the top, and all other species underneath.

Descartes on animals

Two thousand years after Aristotle, René Descartes tackled the subject of animal intelligence. He rejected the idea of a ladder of intelligence, deciding instead that humans were alone in their capacity for intelligent thought.

He believed that humans were made of two pieces: the body and the mind. The body performed automatic processes, like a biological machine, while the mind provided consciousness, intelligence, and thought. This is known as mind–body dualism, or Cartesian dualism.

Descartes thought that animals lacked the ‘mind’ half of the dualism. Instead, they were simply biological machines, which he referred to as automata.

Hume on animals

About a hundred years after Descartes, in the middle of the 18th century, Scottish philosopher David Hume argued against the Cartesian theory that animals were mindless automata.

In his famous work, *A Treatise on Human Nature*, he wrote that “no truth appears to me more evident, than that beasts are endowed with thought and reason as well as men.”

He based this theory on the fact that animals often behave like humans; they run from fires, form bonds with masters, avoid strangers. If their external behaviors are similar, their internal processes were probably similar too.

Morgan's Canon

When Hume and Descartes explored the subject of animal intelligence, their theories were based on anecdotal observation. In the 19th century, British psychologist Conwy Lloyd Morgan warned that this approach was dangerous.

He said that people observing the behavior of an animal had a tendency to anthropomorphize. When a dog started whining, people thought it was missing its owner, in a display of human-like intelligence. In reality, the dog might just have been hungry, and instinctively whining for food.

He argued that, when observing animals, we must always assume a simpler explanation (instinctive behavior) before jumping to big conclusions (intelligent behavior). This idea is known as Morgan’s Canon, and it has influenced studies of animal intelligence ever since.

Edward Thorndike

At the end of the 19th century, Edward Thorndike became one of the first scientists to study animal intelligence in a laboratory setting. He pioneered a school of thought which later became known as behaviorism.

Thorndike wanted to know if an animal could intelligently learn the solution to a problem. He put a cat in a box, then showed the cat how a lever would open the door. If the cat was intelligent, it would learn from the demonstration.

The cat did not learn from the demonstration. It only learned through trial-and-error: it stood on the lever by accident, saw the door open, and exited the box. Thorndike decided this type of learning was random, and not a sign of intelligence.

Operant conditioning

After his experiments on cats, Edward Thorndike established what he called the law of effect: when an animal’s behavior leads to a positive outcome, like the door to a puzzle-box clicking open, they will repeat that behavior in the future.

In the early 20th century, Thorndike’s law of effect was built upon by scientists like B.F. Skinner and Ivan Pavlov. It developed into the theory of operant conditioning.

This theory argued that animal behavior is constantly shaped by trial-and-error. If a random behavior leads to a reward, the animal will repeat it in the future. If a random behavior leads to a punishment, the animal will avoid it next time.

Operant conditioning suggests that animals are simple and machine-like, and not capable of intelligent thought.

Wolfgang Köhler

Operant conditioning was a dominant theory in the 20th century, but it was not the only theory of animal intelligence. It was challenged by a German psychologist, Wolfgang Köhler, who wrote a famous book called *The Mentality of Apes*.

He observed that, when a chimp was confronted by a novel problem, it would stop, and think, before coming up with a solution. Often, this solution would work first time. For example, they might stack some boxes in order to reach a banana, without trying other behaviors first.

Köhler described his chimps as “unwaveringly purposeful,” and argued that they were solving problems through intelligent insight, as opposed to trial-and-error conditioning.

Evolutionary approach

The debate between conditioning and insight is still ongoing, but most modern scientists believe in a compromise. While animals (and humans) can learn through conditioning, they are also capable of intelligent, insightful thought.

The next question to ask was whether this rule applied to every animal species. In the 1980s, some scientists believed in a null hypothesis: that every species, apart from humans, had exactly the same intelligence.

But in 1987, American psychologist Alan Kamil wrote a pioneering article: *A synthetic approach to the study of animal intelligence*. He argued that every species had a unique intelligence, just as it had a unique anatomy.

It came down to evolutionary pressures. Animals evolved a type of intelligence which best suited their specific needs.

Modern studies

In the 21st century, studies into animal minds are becoming more and more advanced. Many of these studies focus on the ways that different animals have evolved their own types of intelligence.

Laboratory experiments are often used to study smaller animals. They are placed in mazes, given memory tests, and asked to respond to images. Meanwhile, field studies observe how animals behave in their natural habitats, especially larger species, like whales.

Functional MRI scans can be used to study neural activity, but only in certain species. Animals must be trained, or restrained, before entering the scanner. A crocodile brain was successfully scanned in 2018, but only after the animal was sedated, and its mouth taped firmly shut.

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