Chiropteran vomeronasal complex and the interfamilial relationships of bats

Within the extant orders of living mammals, the distribution of the vomeronasal organ (VNO) and associated structures is very stable, being universally present in the vast majority or universally absent in cetaceans and sirenians. Chiroptera is the most noteworthy exception, with variation in the absence or presence of the vomeronasal complex occurring even at the species level in some instances. The VNO and/or its component structures, such as the accessory olfactory bulb, were studied in serially sectioned snouts and brains from 114 genera and 292 species representing all extant chiropteran families except Myzopodidae and Antrozoidae. Taxa were scored for the following characters: (1) degree of formation of the vomeronasal epithelial tube, (2) shape of the vomeronasal cartilage, (3) occurrence of the nasopalatine duct, and (4) occurrence of the accessory olfactory bulb. To reconstruct the evolutionary history of the bat vomeronasal complex, the distributions of these four characters were mapped, using the computer program MacClade, onto chiropteran phylogenies in the literature derived from other data sets. In all phylogenies, these four characters exhibit a high degree of homoplasy, only part of which is accounted for by several polymorphic taxa. However, perhaps the most remarkable result is that in the most parsimonious solutions the absence of the vomeronasal epithelial tube and accessory olfactory bulb is identified as primitive for Chiroptera, with both structures reevolving numerous times: such a scenario would be unique to bats among mammals. An alternative, though less parsimonious interpretation, which does not require reevolution of this very complex system, is that a well-developed vomeronasal epithelial tube is primitive for Chiroptera, as in nearly all other orders of mammals, but has been reduced or lost in the majority of families. Explication of the peculiar evolutionary history of the vomeronasal system in bats awaits studies on the adult morphology in the more than 630 species not yet examined and, in particular, on ontogeny, which to date is known for only a handful of taxa.A preliminary account of this research was presented at the Tenth International Bat Research Conference and Twenty-Fifth North American Bat Research Symposium held at Boston University, Massachusetts, on 6–11 August 1995.