Vomeronasal organ in bats and primates: Extremes of structural variability and its phylogenetic implications

The mere appearance of a tubular, epithelially-covered, bilateral structure, no matter how minuscule, on the anteroventral nasal septum of tetrapods, is generally called the vomeronasal organ (of Jacobson). However, considering the functionality of this chemosensory structure, the presence of a non-cilated (microvillar) neuroepithelium (and not just any odd type of epithelium) encased in a variously shaped vomeronasal cartilage, along with vomeronasal nerve bundles and above all an accessory olfactory bulb connected to the limbic vomeronasal amygdala, are the absolute essential neurostructural characteristics and anatomic requirement for a functional VNO and the accessory olfactory system in any tetrapod. The distribution of the vomeronasal organ is reported here in two mammalian orders: Chiroptera and Primates. An impressive data pool on the vomeronasal organ of bats is now available, pointing to the fact that at this time bats may be the only group in which this organ system is extremely variable, ranging from total absence (even in the embryo) to spectacular development with numerous intervening stages in different chiropteran species. Of the eighteen bat families, only one family of New World leaf-nosed bats, family Phyllostomidae, exhibits functional vomeronasal organs. The vespertilionid bat Miniopterus, and the mormoopid bat Pteronotus, present exceptions to this rule. Among Primates, very few species have been rigorously studied. As a result, developmental variability of the vomeronasal organ is almost unknown; either the vomeronasal organ is well developed (such as in New World monkeys) or absent (as in Old World monkeys and great apes) in the adult. The concept whether adult humans or embryonic and fetal forms are endowed with this so-called sixth sense, is a controversial one and is under intense study in our laboratory and by others. The general phylogenetic implications based on our cladistic analysis of bats are that the vomeronasal organ complex has evolved several times. Among the prosimians and platyrrhine primates, the organ is well developed, although to a varying degree. Among catarrhine primates, its loss has occurred only once, as it is generally absent in the adult forms.