Comparative Analysis of the Olfactory Bulbs of the Nile Crocodile (Crocodylus niloticus), Rock Monitor Lizard (Varanus albigularis) and Nile Monitor Lizard (Varanus niloticus)

Abstract

Introduction/Background: Reptilian brains have been poorly studied historically, which lead to many scientific misconceptions. Over the last few decades more research has been dedicated to these taxa demystifying some of these earlier misconceptions. One area of interest that requires clarity relates to the reptilian olfactory system and its connection to the vomeronasal system (VNS). To understand these systems better we require a comprehensive anatomical account of the structures involved. By understanding the structure more accurately, inferences can be made about the function and presence of the associated underlying systems. Crocodilians are suggested to lack a vomeronasal system, but contradictory findings have indicated the presence of a putative accessory olfactory bulb which forms part of the vomeronasal system. No study provides a complete anatomical account of the olfactory bulb to confirm or negate these findings for crocodilians. Squamates present with a well- established vomeronasal system and is the ideal comparator to identify the structural components of the olfactory bulbs that are involved in the VNS. Most studies defining the anatomy of the olfactory bulbs are confined to snakes with few studies focused on lizards. Aim: The aim of the current study was to identify and describe the chemoarchitecture of the olfactory bulbs in one of the Squamate groups where the vomeronasal/accessory olfactory system is apparently well-established, monitor lizards (Varanus albigularis and Varanus niloticus) and in a Crocodilian group where the VNS is suspected to be absent, Nile Crocodiles (Crocodylus niloticus). Methods: The olfactory bulbs of three reptile species (Nile Crocodile, Rock Monitor and Nile Monitor lizard) were sectioned and stained for Nissl and immunohistochemically stained for Calbindin, Calretinin, Parvalbumin, NMDA receptor 2B and Ga0 protein. The resultant sections were viewed under a low power stereomicroscope and the relevant cytoarchitectural boundaries were traced using a camera lucida. Immuno-labelled sections were viewed under a low power dissecting microscope and the relative cellular density profiles were determined by observation and stereology. 4 Results: The Monitor Lizards presented both the main olfactory bulb (MOB) and accessory olfactory bulb (AOB). The Nile Crocodile presented a MOB and a unique region we believe to be a putative accessory olfactory bulb. Discussion/Conclusion: This unique region lacks the classical features of the AOB described for squamates. However, it is independent and separated from the MOB with clear cell layers and cellular boundaries. This structure may be an evolutionary remnant of an AOB that has remained in the olfactory bulb of the Nile Crocodile. Functional and behavioural studies would need to be performed to determine the role of this region in olfaction and its possible association with VNS.