Can social behaviour drive accessory olfactory bulb asymmetries? Sister species of caviomorph rodents as a case in point

Abstract

In mammals, the accessory olfactory or vomeronasal system exhibits a wide variety of anatomical arrangements. In caviomorph rodents, the accessory olfactory bulb (AOB) exhibits a dichotomic conformation, in which two subdomains, the anterior (aAOB) and the posterior (pAOB), can be readily distinguished. Interestingly, different species of this group exhibit bias of different sign between the AOB subdomains (aAOB larger than pAOB or vice versa). Such species‐specific biases have been related with contrasting differences in the habitat of the different species (e.g. arid vs. humid environments). Aiming to deepen these observations, we performed a morphometric comparison of the AOB subdomains between two sister species of octodontid rodents, Octodon lunatus and Octodon degus. These species are interesting for comparative purposes, as they inhabit similar landscapes but exhibit contrasting social habits. Previous reports have shown that O. degus, a highly social species, exhibits a greatly asymmetric AOB, in which the aAOB has twice the size of the pAOB and features more and larger glomeruli in its glomerular layer (GL). We found that the same as in O. degus, the far less social O. lunatus also exhibits a bias, albeit less pronounced, to a larger aAOB. In both species, this bias was also evident for the mitral/tufted cells number. But unlike in O. degus, in O. lunatus this bias was not present at the GL. In comparison with O. degus, in O. lunatus the aAOB GL was significantly reduced in volume, while the pAOB GL displayed a similar volume. We conclude that these sister species exhibit a very sharp difference in the anatomical conformation of the AOB, namely, the relative size of the GL of the aAOB subdomain, which is larger in O. degus than in O. lunatus. We discuss these results in the context of the differences in the lifestyle of these species, highlighting the differences in social behaviour as a possible factor driving to distinct AOB morphometries.

The vomeronasal system of the newborn capybara: a morphological and immunohistochemical study

“…Although the small number of available individuals available prevented the realization of a rigorous statistical study, all of our newborn specimens showed a caudal portion that was larger than the rostral portion, which demonstrated that starting at the perinatal stage, the capybara shows the bias toward a more prominent caudal AOB that was previously described in adult individuals by Suarez et al 23,41 These results, combined with the recent observations regarding the morphometrical parameters of the AOB, in two closely related degus species with contrasting social habits 20 , invite the hypothesis that some structural features of the AOB reflect the species lifestyle and arise during an early stage of the ontogeny.…”

“…Hystricognathi has become a model group for the study of the anatomical diversity of the VNS, as shown by the studies in chinchillas 16 , 17 , guinea pigs 18 , degus 19 , 20 , and mole rats 21 , 22 . Suárez et al 23 paid special attention to the organisation of the first integrative centre of the VNS, the accessory olfactory bulb (AOB) in capybaras Hydrochoerus hydrochaeris , particularly to the morphometry of the anteroposterior zonation, which is determined by the expression of the G proteins.…”

Pheromone Sensing in Mammals: A Review of the Vomeronasal System

“…This characteristic was previously described in adult individuals [442]. These results, combined with recent observations on the morphometric parameters of the AOB in two degu species with contrasting social habits, suggest that some AOB structural features are tied to the species lifestyle and arise during an early ontogeny stage [443].…”

The vomeronasal system in semiaquatic beavers

“…In beavers, areas of the rostral and caudal parts of the AOB, respectively, labeled with anti‐G αi2 (V1R subdomains) and anti‐G αo (V2R subdomains) were similar, like those in laboratory myomorph mice (Jia & Halpern, 1996) and rats (Shinohara et al, 1992). In contrast, the terrestrial degus ( Octodon degus ) have a larger rostral AOB than the caudal AOB (large V1R subdomain) (Fernández‐Aburto et al, 2020; Suárez & Mpodozis, 2009) and the semiaquatic capybaras have the larger caudal AOB than the rostral AOB (large V2R subdomain) (Suárez et al, 2011), probably because this asymmetry of AOB subdomains may reflect ecological traits of animals (Suárez & Mpodozis, 2009). Although we could not establish why beavers had a similar size of AOB subdomains, despite being semiaquatic rodents like capybaras, asymmetric AOB subdomains might be specific to a hystricomorph rodent phylogeny.…”