My son pointed me to work by Wobber et al. which makes me feel much better about my curmudgeonly nature. They compared the behavior of Chimpanzees, who can be quite grumpy, that that of Bonobos, who maintain childlike playfulness throughout their lives. They suggest the chimps' ability to put aside their sociability is one of the reasons they are more intelligent and civilized than their genetically similar great ape cousins. Perhaps being aggressive, intolerant and short-tempered could be a sign of a more advanced nature! Here is their abstract:
Phenotypic changes between species can occur when evolution shapes development. Here, we tested whether differences in the social behavior and cognition of bonobos and chimpanzees derive from shifts in their ontogeny, looking at behaviors pertaining to feeding competition in particular. We found that as chimpanzees (n = 30) reached adulthood, they became increasingly intolerant of sharing food, whereas adult bonobos (n = 24) maintained high, juvenile levels of food-related tolerance. We also investigated the ontogeny of inhibition during tasks that simulated feeding competition. In two different tests, we found that bonobos (n = 30) exhibited developmental delays relative to chimpanzees (n = 29) in the acquisition of social inhibition, with these differences resulting in less skill among adult bonobos. The results suggest that these social and cognitive differences between two closely related species result from evolutionary changes in brain development.