Mechanisms for dominance linearity as logic in social structure were investigated by (1) quantitative neuroanatomical comparison of pallial nucleic volumes between jungle crows and zebra finches, and by (2) examining the function of allopreening in group-housing crows. In experiment-1, proportions of the pallial nuclei to the entire telencephalon were compared between jungle crows and zebra finches. Ratios of those nuclei to basal ganglion volume were also compared between the two species. We found that proportions of pallial nuclei were relatively similar in the two species but major enlargement of crow brain in comparison to finch brain emerged in dorsal 'association' pallium, including mesopallium and nidopallium, based on the ratio index. In experiment-2, allopreening and agonistic interactions were recoded in a form of matrices and analyzed for examining the potential relationship between the two social interactions. We found no reciprocity between preeners and preenees but significant relationships between preeners and dominant individuals (i.e., preenees and subordinates). This result suggest the function of allopreening as dominance status signal which may contribute to maintain dominance linearity.

Part 1: Brain and Evolution