The Development of an Asymmetric Accessory Olfactory Bulb in Octodon degus and the Influence of Social Behavior in Its Establishment

The vomeronasal system (VNS) has been extensively associated with the regulation of sexual and social behaviors in mammals. In rodents, this system comprises two types of vomeronasal neurons sending parallel projections from the vomeronasal organ (VNO) to the anterior and posterior portions of the accessory olfactory bulb (aAOB and pAOB, respectively). Caviomorph rodents develop either a larger aAOB bias or a larger pAOB bias, which has been associated with ecological traits related to the pheromonal communication displayed by these species. However, it is still unknown how and when such AOB asymmetries emerge and whether experience may influence their establishment. To explore these factors, we studied the development of the VNS in Octodon degus , a caviomorph that exhibits an asymmetric AOB, in which the aAOB is larger and has more glomeruli than the pAOB. We found that both the VNO and the AOB are present at birth, but exhibit substantial immature traits. Both VNS structures develop postnatally, showing mature patterns by the end of the first postnatal month. By the second postnatal week, the aAOB becomes larger in size than the pAOB; however, in the glomerular layer, such a bias appears later, by the end of the first postnatal month. The AOB asymmetry increases in extent until adulthood, as in this period the pAOB shows a slower growth rate than the aAOB. Additionally, we found that animals raised in restricted social contexts displayed a reduced AOB asymmetry in comparison to wild degus. Overall, we suggest that social experiences contribute to the development of the AOB asymmetry in degus.