Comparison of brain structure volumes in Insectivora and Primates. II. Accessory olfactory bulb (AOB).

A total of 134 individuals from 75 species were investigated with regard to existence and size of the accessory olfactory bulb (AOB). Based on volume measurements the AOB and its three measured components do not show any uniform trend in size changes from lower Insectivora through higher Primates. The investigated structures are in most prosimians clearly larger than in low Insectivora, in most New World simians clearly smaller, and in Old World simians absent. Even within narrowly related groups there may exist strong differences in size and structural differentiation. In AOBs of large relative size, the inner granular layer (layer 6) is relatively larger than in AOBs of small relative size. The variability in size is especially large in those individuals and species with a relatively small AOB. No differences were found between the AOBs (1) of the two sides within the same individual (brain) and (2) between the two sexes within the same species. There is no interdependency in size between MOB (main olfactory bulb) and AOB. The relative size of the AOB is discussed in relation to various ecological and behavioural considerations. No clear relations could be found between the size of the AOB and dietary specialization. A comparison of the relative size of the AOB with activity cycle, sexual behaviour and the use of pheromones in various species of Primates supports the suggestion that the vomeronasal system may be involved in sexual and/or social interaction.