Horses can count, according to a new study that suggests they are more intelligent than previously thought.
Researchers found that, when offered a choice, they consistently choose buckets containing higher numbers of apples.
Babies aged from 10-months-old have been shown to have an innate tendency to opt for containers holding larger numbers of food items, as have many non-human primates such as rhesus macaques and lemurs.
Dr Claudia Uller, of the University of Essex, was inspired to investigate whether horses could count by the story of Clever Hans, a horse that caused a sensation 100 years ago with his apparent abilities to simple arithmetic and keep track of the calendar.
In public performances in Germany he is said to have communicated the answers to questions by tapping his foot.
However psychologist Oskar Pfungst carried out an investigation and reported in 1911 that Clever Hans was not performing arithmetic, but had learnt to obtain the required answers by interpreting the reactions of his maths teacher owner and other observers.
Dr Uller, speaking at the British Psychological Society conference in Dublin yesterday said: "Nobody has been able to show any mathematical abilities in horses since then.
"However our results suggest that horses too, and not only primates, are able to spontaneously discriminate between two small numbers.
"It shows horses are more intelligent than we thought. This may be another piece in the jig saw explaining the evolutionary origins of our ability to count."
Dr Uller and colleague Jennifer Lewis carried out a series of experiments involving riding school and privately owned horses stabled near Colchester, Essex.
In one task, 11 of 13 horses consistently selected buckets containing three plastic apples over another containing two when offered a choice. Fake fruit was used to ensure no difference in smell.
Researchers then showed 12 different horses a box holding either two identical small apples or another containing one large apple with double the surface area. Again, all but two selected the greater number of apples.
In a study published in February, Italian researchers found certain species of fish can count up to four.