The evidence that the vomeronasal organ (Jacobson's organ) is responsible for pheromone detection is at present confusing. The organ is sexually dimorphic, found in virtually all animals, but apparently absent in birds, crocodiles, cetaceans, bats and some primates ¹¹⁸. Teleosts (bony fish) have no anatomically distinct VNO, but the medial portion of the olfactory tract has pheromone receptors. Surgical removal of the VNO reduces investigatory and sexual behaviour, but this can be restored by luteinising hormone releasing hormone ¹¹⁹ and (in rodents) has less profound effects on animals that have prior sexual experience.

There is growing evidence that the signal transduction pathways in the vomeronasal organ are different to those in the olfactory epithelium. The OBPs ¹²⁰, G-proteins ¹²¹ and adenylate cyclase expressed by the vomeronasal gland are different to those of the olfactory epithelium, although they both express the oCNC2 ion channel ^{122 123}. The turtle vomeronasal gland has cAMP and IP3 gated ion channels, but the cAMP pathway is not involved in transduction of common odorants ¹²⁴. It is an appealing idea that the vomeronasal organ may exclusively or primarily use the IP3 pathway to transduce odorant detection ¹²⁵, and that the receptors used may be homologous to those used by insects to detect pheromones. Homology between these receptors and signalling pathways might go some way towards explaining the extraordinary coincidences in pheromone structures used by organisms discussed above.