One of the main evolutionary forces that has led to high brain power is thought to be the demands of social life. Animals such as corvids and primates, including humans, may be intelligent because they need to keep track of relationships with others in their group, and also the relationships between others. This is important for winning in conflicts with group members. The evidence for the social intelligence hypothesis comes from comparing the brain size across closely-related species, but some studies have failed to find the expected patterns, instead suggesting diet complexity or environmental variation are more important.
The prediction should also apply to animals living in different group sizes within species. The Australian magpie (Cracticus tibicen dorsalis) is an intelligent bird that lives in family groups from 3 to 12 individuals. We tested 56 wild birds from 4 groups with four tests of learning and memory. Performance on all four tests were correlated with each other and with group size, indicating that individuals in living in more complex societies were generally more intelligent. By testing young birds 3 times, we showed that this variation develops over time rather than being genetic. By measuring the number of offspring females had, we found that intelligence affects fitness.
Our results show support for the social intelligence hypothesis within a species. They suggest that this intelligence develops after birth in response to social complexity. This means that the observed differences between species might be due to nurture rather than nature (development not genetics). The fact that intelligence leads to having more offspring means that there is a chance for evolution to act to make some species more intelligent.