Research Report Sexual dimorphism in the vomeronasal system of the rabbit Santiago Segovia a , Alicia Garcia-Falgueras a , Beatriz Carrillo a , Paloma Collado a , Helena Pinos a , Carmen Perez-Laso a , Concepcion Vinader-Caerols b , Carlos Beyer c , Antonio Guillamon a, ⁎ a Departamento de Psicobiología, UNED, c/ Juan del Rosal, 10; 28040 Madrid, Spain b Área de Psicobiología, Facultad de Psicología, Universitat de Valencia, Av. De Blasco Ibáñez, 21; 46010 Valencia, Spain c Centro de Investigación en Reproducción Animal, Universidad Autónoma de Tlaxcala, Tlaxcala, México ARTICLE INFO ABSTRACT Article history: Accepted 1 May 2006 Available online 27 June 2006 Studies have shown that the vomeronasal system (VNS), an olfactory neural network that participates in the control of reproductive physiology and behavior, is sexually dimorphic in the rat. These works have also shown two main characteristics of brain sexual dimorphism: (a) dimorphism appears in neural networks related to reproduction and (b) it can present two morphological patterns: one in which males present greater morphological measures than females (male > female) and another in which the opposite is true (female > male). The present work extends the hypothesis to the rabbit, as a representative species of Lagomorpha. In addition, the locus coeruleus (LC), which is known to send rich noradrenergic projections to VNS structures, was also studied. Sex differences were found in: (a) the number of mitral, and dark and light granule cells (female > male) of the accessory olfactory bulb (AOB); (b) the medial amygdala (Me) and its dorsal (Med) and ventral (Mev) subdivisions, males showing greater values than females in volume and number of neurons, while in the posteromedial cortical amygdala (PMCo or C 3 ), females show greater density of neurons than males and (c) the posteromedial division of the bed nucleus of the stria terminalis (BSTMP) in which males have more neurons than females. No sex differences were seen in the bed nucleus of the accessory olfactory tract (BAOT) and the LC. These results evidence that, as it was observed in rodents, sex differences are also seen in the VNS of Lagomorpha and that these sex differences present the two morphological patterns seen in Rodentia. Differences between orders are discussed with respect to the species-specific physiological and behavioral peculiarities. © 2006 Elsevier B.V. All rights reserved. Keywords: Rabbit Lagomorph Rat Sex difference Sexual dimorphism Vomeronasal system 1. Introduction The dual olfactory system hypothesis, which proposes the existence of two separate olfactory pathways: the main and vomeronasal pathways (Scalia and Winans, 1975, 1976; Winans and Scalia, 1970), has generated a huge amount of research and has helped us to understand important aspects of vertebrate reproductive behaviors. Starting with Powers and Winans (1975) who demonstrated that the vomeronasal organ (VNO) participates in the control of copulatory behavior of male hamsters, many studies have appeared in the literature suggesting that the VNS mediates the action of pheromones implicated in the expression of masculine and feminine behavior, maternal behavior and physiological primer BRAIN RESEARCH 1102 (2006) 52 – 62 ⁎ Corresponding author. E-mail address: aguillamon@psi.uned.es (A. Guillamon). 0006-8993/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2006.05.017 available at www.sciencedirect.com www.elsevier.com/locate/brainres