Mammalian Olfactory Systems

The sense of smell (olfaction) is mediated by two systems in most mammals - the main olfactory system and the vomeronasal system. The vomeronasal system is particularly interesting because it has been lost in some bats, whales and dolphins, and some primates. Loss of this system is often attributed to a trade-off with other sensory systems, such as the visual system in primates. For my dissertation I compared the vomeronasal organ (the primary organ for the vomeronasal system) across primates, and identified a bony-correlate called the vomeronasal groove. The vomeronasal organ is well-developed in lemurs and lorises, but is reduced in size in tarsiers and New World monkeys, and absent in adult apes and Old World monkeys. The size of the vomeronasal organ may predict sensitivity of the vomeronasal system, and appears to be related to mating behavior and changes in the visual system.  

Schematic diagram of the olfactory systems of a mouse lemur. The main olfactory system is denoted by orange, and the vomeronasal system is denoted by purple. The main olfactory epithelium (MOE) is the peripheral sensory organ of the main olfactory system, while the vomeronasal organ (VNO) is the peripheral sensory organ of the vomeronasal system. The olfactory bulb (OB) is the primary brain center where main olfactory information is processed, while the accessory olfactory bulb (AOB) is the primary brain center where vomeronasal olfactory information is processed.

Schematic diagram of the olfactory systems of a mouse lemur. The main olfactory system is denoted by orange, and the vomeronasal system is denoted by purple. The main olfactory epithelium (MOE) is the peripheral sensory organ of the main olfactory system, while the vomeronasal organ (VNO) is the peripheral sensory organ of the vomeronasal system. The olfactory bulb (OB) is the primary brain center where main olfactory information is processed, while the accessory olfactory bulb (AOB) is the primary brain center where vomeronasal olfactory information is processed.

Genomics

Each olfactory system is encoded by distinct receptor genes. The receptor proteins of the main olfactory system are encoded by olfactory receptor (OR) genes, and the receptor proteins of the vomeronasal system are encoded by vomeronasal receptor (VR) genes. I am interested in exploring variation in the number of functional OR and VR genes in primates and other mammals in order to understand the role smell has played in their evolution.

Paleontology

Loss of a functional vomeronasal system in apes and Old World monkeys, primates with excellent visual adaptations, is fascinating and implies a trade-off between vision and this particular olfactory system. In order to understand this supposed trade-off from an evolutionary framework, fossils should be studied. I am interested in looking variation of the vomeronasal groove (the bony correlate of the vomeronasal organ) when it is preserved in primate fossils to see if this trade-off has been documented in the fossil record.

Figure adapted from Garrett et al. 2013. Anatomical Record. A shows a 3-D rendered skull of a loris with the yellow arrow pointing to the vomeronasal groove. B, C, and D are serial levels of the soft tissue vomeronasal organ, also from a loris. The vomeronasal groove is occasionally preserved in fossils, and its presence/absence and size can inform how developed the vomeronasal system was in extinct primates.

Figure adapted from Garrett et al. 2013. Anatomical Record. A shows a 3-D rendered skull of a loris with the yellow arrow pointing to the vomeronasal groove. B, C, and D are serial levels of the soft tissue vomeronasal organ, also from a loris. The vomeronasal groove is occasionally preserved in fossils, and its presence/absence and size can inform how developed the vomeronasal system was in extinct primates.