Aristotle’s ladder of nature is something you probably aren’t too familiar with. The man himself was one of the most influential of the early philosophers  (384 – 322 BC). Much of his work centred on animal and plant life and he was the first person to group animals into orders (Historia Animalium) as well as the first to use a binomial system of classification (meaning that each animal had two names) using the type of animal and the species. His taxonomy is still in use today.

Aristotle’s main work on this classification of animals was the Scala Naturae or Ladder of Nature, which was not evolutionary in its structure as it placed contemporary species on the rungs of the model. However, it can be interpreted as having a loose basis on the evolutionary model as the steps up the ladder reflect the bigger picture of human evolution.

The one of the original Ladders of Nature included God, angels, humanity, animals, plants and minerals, so Aristotle’s work was an exploration of the chain within the humanity/animals/plants link in this wider concept.

The rungs on Aristotle’s Ladder of Nature (in order) are as follows:

Humans
Mammals
Birds
Reptiles and amphibians
Whales and porpoises
Fish
Squid and octopi
Lobsters, crabs etc.
Snails, clams etc.
Insects, spiders etc.
Jellyfishes, sponges etc.
Higher plants
Lower plants
Inanimate matter.

So – What Does The Ladder Actually Teach Us?

We can see that none of the adjacent groups have a direct evolutionary relationship to each other, but that from the bottom to the top rung we can trace the basic steps of evolution. This model does not reflect the branches of evolution like Darwin’s model, which is more familiar in modern science.

The main criteria for placement on the Ladder of Nature is the ability to move and sense, hence plants and inanimate matter coming at the bottom and animals at the top. The animals were graded by their reproductive method and whether they ‘possessed blood’ (Aristotle viewed invertebrates as having no blood). The overall effect of this classification is that higher organisms are defined as having vitality and movement, while lower organisms are bloodless and motionless.

Aristotle’s higher organisms are considered intelligent, able to use tools and display emotion. The lower organisms are considered bereft of emotion and intelligence (at least as far as human understanding of animal emotion goes) and less able to interact with their environment to achieve results (such as finding food).

Scientific studies of animal behaviour and intelligence have used Aristotle’s linear classification of organisms as the basis for research into memory, emotion and problem solving in a variety of animals. Often, the results of these research programmes show that animals are more intelligent than previously thought.

Chimpanzees, Intelligence and Ladders

Chimpanzees are quite close to humans on the evolutionary ladder, so it’s no surprise that many experiments have been designed with these animals in mind. In 2007, a chimpanzee at a Japanese university was tested against a group of students on memory recall. Ayumu, the chimpanzee, was trained on a touch screen that displayed a series of numbers from 1 to 9 for a fraction of a second. The numbers were then replaced with squares and Ayumu could touch the squares in ascending order according to where the numbers had been displayed. Ayumu beat all the students by a wide margin, and even beat Ben Pridmore, a memory champion in 2008. According to this research, chimpanzees have a better memory than humans.

New understanding of how to conduct animal intelligence experiments has led to better outcomes for animals in this kind of research. Instead of designing experiments with human subjects in mind, scientists are trying to design the experiments around the traits and physiology of the animals studied. For example, studies on chimpanzees designed to see whether they could recognise faces, were conducted with pictures of humans. Lisa Parr then tested chimpanzees using pictures of their own species. The apes were trained to find similarities between faces, and then shown a picture of an unknown female chimpanzee. They were then shown two more pictures, one of which was the child of the chimpanzee in the first picture (again, the test subjects did not know any of the chimpanzees in the pictures). They picked the offspring of the first picture almost every time, based on the family resemblance between them. Could a human detect the family resemblance between two monkeys? Probably, but not as well as we can detect the familial similarities between two humans. This change in the method of the experiment showed that chimpanzees have a similar ability to humans in recognising facial similarities, but until the test was done with their own species, this was not known.

A perfect example of a primates intelligence was recently demonstrated at a zoo in Hanover, Germany, where 5 chimpanzees built a makeshift ladder and escaped their compound. 4 of the chimpanzees returned of their own volition, presumably using the same ladder (constructed from tree branches) to climb back down!

Another popular test for animal intelligence is problem solving to obtain food. Researchers tested primates with food placed in out of reach branches, and a long stick. Primates were able to connect the use of the stick with the action of shaking the food down from the tree, displaying the ability to recognise and use tools to help them get food. Elephants were tested with the same apparatus, as researchers assumed that the elephants would use their trunks to pick up the stick and shake the food down from the tree. Researchers made this assumption on the basis that in the wild, elephants have been observed picking up sticks with their trunk to scratch themselves. However, using their trunk to pick up a stick to get food makes no sense to an elephant. Their primary sense for detecting food is smell, and since their nose is their trunk, they need this to be able to find the food. As soon as the elephant picks up the stick, the nose is blocked and it can’t find the food as easily. Researchers had not thought of the suitability of the tool for the animal, and concluded that the elephant was less intelligent than the primates.

When the experiment was repeated with different tools, the elephants showed they do possess the problem solving skills they had previously been thought to lack, as well as a more complex understanding of their surroundings. A young male elephant was presented with food hung out of reach, several sticks and a large box, strong enough to take his weight. The elephant ignored the sticks, but examined the box. He kicked the box underneath the food, stood on the box and was able to reach the food with his trunk. Researchers were amazed, and repeated the experiment putting the box in another corner of the yard, well away from the food. Again, the elephant sought out the box, even from far away, and moved it to where he needed to use it to get the food. This demonstrates complex problem solving, as when the solution is far away from the problem, the animal needs to remember the solution, leave the problem to find it, and bring the solution back to the problem. Not many species will do this, and the ones that do are considered the most intelligent.

Elephants have also been observed grieving and consoling one another after the death of a herd member, demonstrating (albeit anecdotally) that elephants are capable of compassion and complex emotions. Similarly, dolphins have been observed displaying emotion and problem solving behaviour, perhaps most surprisingly recently on the news, with the case of the dolphin trapped by fishing nets appearing to ask a diver for help. The diver in question was underwater with a crew filming for a television programme when a dolphin swam very close to him, seeming to display a flipper very prominently. The diver inspected the flipper, curious as to what the dolphin was up to. When he realised there was fishing line caught around the flipper he used a diving knife to cut the animal free. All the way through this the dolphin stayed completely still and by the diver’s side, as if the dolphin had recognised the diver as a potential solution to a problem that the dolphin could not solve alone. This display of searching for assistance from another species displays a great complexity of thought from the dolphin. To actively seek assistance from another being, especially a stranger, and trust that person to help shows trust and empathy, as well as the understanding that the dolphin, although he could not free himself, could be freed by someone else.

These impressive displays of animal intelligence seem to make the rungs on Aristotle’s Ladder of Nature open to movement and interpretation; if a dolphin can display intelligence, understanding and emotion like in the case above, what is to say that the species should come a whole four steps below humans? This new, less anthropocentric way of testing and viewing animal intelligence, is opening our minds to a new way of looking at the understanding of species that, until recently, had always been viewed as inferior to humans. Aristotle’s Ladder of Nature is due to have a couple of rungs replaced.