Kari Allen and Rich Kay report in the Proceedings of the Royal Society B on a study of the relationship between dietary quality and relative brain size in platyrrhine primates (New World monkeys). This study is relevant to human evolution because of a big idea in paleoanthropology known as the expensive tissue hypothesis (ETH, Aiello & Wheeler, 1995). The ETH predicts that dietary quality and relative brain size should be correlated. As the picture at the top of this post shows, the idea is that low quality diets (e.g. leaves) are a constraint on primates, which have to devote lots of energy to make big guts capable of digesting these difficult foods. With so much energy invested in making a big gut, there isn't enough energy left over to make lots of another metabolically expensive tissue...brains. The story as relates to humans is that humans had this constraint lifted at some point in their evolution, perhaps related to an increase in easy-to digest meat acquired with the help of tools. Humans then devoted their excess energy -- no longer needed to make a big gut -- to make a big brain.
However, Allen and Kay find no relationship between a measure of dietary quality and relative brain size in New World monkeys. In particular Brachyteles has a very low dietary quality and a relatively large brain, going against predictions. This doesn't necessarily mean that the ETH isn't valid for humans, but it is certainly worth thinking about why the underlying relationship doesn't appear to hold for platyrrhines.