Morphological and Cranial Variation and Reproductive Data of Invasive Rodents in Southeastern Mexico

This study reports the first comprehensive analysis of body variation and cranial morphology (using traditional morphometrics) in Rattus norvegicus and R. rattus in San Cristobal de Las Casas (SCLC), in the Mexican state of Chiapas in southeastern Mexico. Our results provide a valuable tool for identifying these species in the region and shed light on their variability. A total of 125 specimens were collected, 78 R. norvegicus and 47 R. rattus, with sex ratios of 1Female:0.75Male and 1Female:0.51Male, respectively. Of the collected individuals, 47% of R. norvegicus and 44% of R. rattus were in the reproductive stage. Notably, 60% of R. norvegicus and 42% of R. rattus weighed < 100 g. Reproductive R. norvegicus averaged 131 g in body mass and 175.70 mm in body length. We found sexual dimorphism in R. norvegicus: males exhibited significantly larger ears (p = 0.044). Reproductive R. rattus specimens had an average body mass of 103.64 g and a body length of 166.21 mm. Cranial morphology differed substantially between species: R. norvegicus showed a robust skull with wider interorbital constriction and foramen magnum, whereas R. rattus showed a reduced rostral depth and a shorter mandibular ramus. SCLC R. norvegicus specimens were smaller by approximately 400 g (body mass) and 249.1 mm (mean length) than specimens from Baltimore, US (the largest recorded American population). Conversely, R. rattus specimens from SCLC (mean weight = 103.64 g; mean length = 166.21 mm) were larger than Yucatan populations (~70 g body mass).