In mammals, social chemical cues are primarily detected in the vomeronasal system. While vomerolfaction is well conserved due to its role in fitness-related behaviors, several mammalian groups have lost function, including Old World primates, some aquatic mammals, and most bats. These losses and gains might relate to ecological variation, such as circadian rhythm, social system, or habitat specialization, but it is unclear if these patterns are related to ecology, phylogeny, or both. We used vomeronasal genetic machinery and morphology to model the evolutionary dynamics of loss and gain vomerolfaction. Vomerolfactory cues are detected by V1R receptors in the vomeronasal organ (VNO), including proteins encoded V1R genes. Both V1Rs and VNO size and shape are highly variable among mammals. We obtained the V1R profiles from nearly every order of mammal, and estimated the birth and death rates of gene duplication and loss. We also quantified VNO morphology from iodine-stained soft tissue µCT-scans. Our results reveal the evolutionary history of the mammalian vomerolfaction was complex, with no clear connection to ecological explanations of loss or gain. Many mammalian orders have experienced decreased birth rates of V1R genes, relative to other mammals. However, many lineages with low V1R birth rates still possess a well-developed VNO and retain intact orthologous receptors with distantly related species, suggesting strong purifying selection in light of low diversification. Our study highlights the importance of incorporating birth-death models and phylogenetic comparative methods to understand the evolutionary history of complex traits, such as a sensory system.