How often is a job so easy that even a monkey could do it? That may not be such a bad thing after all. In addition to all the other stuff we share with our closest relatives, add mathematical ability to the list. Rhesus monkeys have been found to be able to learn simple addition, and their rare errors may tell us something about how we estimate quantities ourselves.
Many animals have a sense of number. Chimpanzees have been known to use the number of taps on a tree to communicate over distances, and lions have been shown to be able to "count" roars in a foreign Pride to determine if they are outnumbered.
Adding and subtracting numbers in symbolic form is a different beast however. Professor Margaret Livingstone of the Harvard Medical School taught rhesus monkeys the meaning of numbers from zero to 25 using the symbols for 0-9 and 16 letters of the alphabet. The teaching was done using a Pavlovian method of associating larger amounts of reward with the larger symbol. The monkeys were then given a choice of two symbols and given a number of drops of the reward equal to the side they chose.
Once this had been learned successfully the monkeys were given two symbols and had to compare them with a single one. At first the monkeys were inclined to choose the side with the single number, if it was larger than either numbers on the other side on their own. With time they got better, realizing that two smaller numbers combined could be better than one larger one. The study found that the animals were successful 90% of the time. Interestingly, however, the monkeys still placed more weight on the larger number than the smaller one – that is they were more likely to pick a side with a 2 and a 9, when compared with a 10, than a 4 and 7, even though both add to 11.
Past tests of animal intelligence have often run into trouble because certain animals reached the same conclusion through a different method from what was expected. The team worried that through prolonged practice the monkeys might have memorized all possible pairings, demonstrating an extraordinary memory rather than any capacity for calculation. So Livingstone and her colleagues gave their subjects a new set of characters and taught them what each meant. Without further prompting the monkeys started using math to work out which combinations were larger.
A further intriguing insight into the monkey mind came from the observations that when the monkeys did get it wrong it was usually when the totals were close together; 6+7 was hard when compared to 12, but easy when compared to 9. Rather than having a precise calculation they may be using estimates, which may offer insight into the way humans do the same calculations.
This likely just goes to show you that the Barenaked Ladies were on to something with their song "Another Postcard"