Chemical Signals and Vomeronasal System Function in Axolotls (ambystoma Mexicanum)

Most tetrapods, the group of vertebrates that includes amphibians, reptiles, and mammals, possess separate olfactory and vomeronasal systems. Why? What are the functions of these two anatomically distinct chemosensory systems? Although the vomeronasal system is often presumed to be specialized for mediating responses to pheromones and the olfactory system assumed to respond to “general odorants” that are not pheromones, this dichotomy does not hold up for any group of vertebrates studied to date. In mammals, some pheromonal effects are mediated by the olfactory system (Dorries et al., 1997; Cohen-Tannoudji et al., 1989), and the vomeronasal organ responds to some general odorants (Sam et al., 2001). In squamate reptiles the vomeronasal system is critically involved in both foraging and in responding to pheromones (Halpern and Kubie, 1984). In salamanders, the vomeronasal organ has recently been suggested to play a role in foraging (Placyk and Graves, 2002), and both the olfactory and vomeronasal organs respond to pheromones (Toyoda et al., 1999; Park and Propper, 2002). Thus, the vomeronasal system cannot be considered to be a labeled line for pheromone processing (Shepherd, 1985), although certainly many pheromonal effects are mediated by the vomeronasal system (see Halpern and Martínez-Marcos, 2003, for a recent review). An alternative hypothesis suggests that the vomeronasal system mediates responses to large, high-molecular-weight molecules that are not volatile in air, and that the olfactory system responds to molecules that have a sufficiently low molecular weight to become airborne and contact the olfactory epithelium (Halpern and Kubie, 1980; Wysocki et al., 1980). This hypothesis is attractive, as it might explain why the vomeronasal system has been lost repeatedly in aquatic and arboreal tetrapods, including in crocodilians and birds (Parsons, 1959, 1967, 1970), some cetaceans (Lowell and Flanigan, 1980; Oelschläger and Buhl, 1985), manatees (Mackay-Sim et al., 1985), several species of bat (Bhatnagar, 1980), and some primates (Smith et al., 2001, 2002). Further, if mammalian pheromones tend to be high-molecular-weight molecules (Singer,