Comparative morphology of the rhinarium and upper lip in sigmodontine rodents: Refined nomenclature, intertribal variation in a phylogenetic framework, and functional inferences

Abstract

Rodents have received substantial attention in the study of olfaction. However, therhinarium, the naked part of the nose, which plays an important role in chemical, tactile,and thermal perception, has been relatively overlooked. This study presents a comprehensive analysis of the rhinarium morphology and spatially associated structures(i.e., upper lip, and philtrum) in sigmodontines, a diverse group within the Cricetidaerodents. The research covers 483 specimens representing 145 species, accounting for74% of genera in the clade, including all 13 recognized tribes, three incertae sedis genera,and the murid representatives Mus musculus and Rattus norvegicus. The inconsistent useof terminology in describing rhinarium traits across the literature poses a challenge forcomparative analyzes. To address this issue, a standardized terminology was proposed tocharacterize the rhinarium. A paired complex protuberance typically with epidermal ridges(i.e., rhinoglyphics), termed here the tubercle of Hill, was identified as a distinctive featurein muroid rhinaria. Comparative assessments among tribes revealed unique sets of features defining each major clade, encompassing variations in hairiness, dorsum nasi complexity, size and positioning of the tubercle of Hill, and other key attributes. Two primaryrhinarium configurations were discerned: one shared by Oryzomyalia and Sigmodontiniand another specific to Ichthyomyini. The former groups display a ventrally positionedrhinarium prominently featuring the tubercle of Hill and sculptured areola circularis. Incontrast, Ichthyomyini exhibit a frontally directed rhinarium characterized by an enlargeddorsum nasi fused to the tubercle of Hill, resulting in a distinctive “cherry” appearance.Convergent rhinarium structures observed in fossorial species, characterized by welldeveloped plica alaris and hair fringes, are presumed to mitigate potential damage duringdigging. Conversely, semiaquatic carnivorous sigmodontines showcase an integratedapical structure in their rhinarium, facilitating enhanced somatosensory capabilities crucialfor predation activities during diving expeditions.