A craniomandibular appraisal of size and shape variation in the genus Holochilus (Cricetidae, Sigmodontinae) by means of geometric morphometrics

Marsh rats of the genus Holochilus are medium to large-sized amphibious sigmodontines, widely distributed across the wetlands and rivers of lowland regions of South America. Alpha-taxonomy in the genus has undergone numerous revisions over recent decades, incorporating various data types and methodological approaches, including morphology and genetics. We applied the first geometric morphometric data to analyze cranial and mandibular size, shape, and their interaction and compared results with the current taxonomic classification of Holochilus. The species studied included H. brasiliensis, H. chacarius, H. nanus, H. oxe, H. sciureus, H. venezuelae, and H. vulpinus, involving examination of 1,190 adult specimens from about 150 different localities. Results indicate that shape is influenced significantly by size (centroid size). Holochilus vulpinus exhibits a larger size and a distinct allometric slope compared to the other species studied, and also performed a higher classification accuracy in the linear discriminant analysis. All species demonstrate overall conserved morphology in terms of shape and nonsignificant variation in terms of sexual dimorphism. This study reveals new insights into Holochilus morphology, confirming the distinctiveness of H. vulpinus due to its larger size and supporting allometry, validating its species status.