Variation in cranial morphology and ecological niche of big-eared tree rats, Ototylomys phyllotis and O. chiapensis

Ototylomys Merriam, 1901 is a genus of tree rats in which significant differences in body and skull size have been reported at inter- and intraspecific levels. In Mexico, this genus inhabits several biogeographic and physiographic provinces with different environmental characteristics. However, their ecological niches have not been evaluated. Ototylomys phyllotis Merriam, 1901 is widespread in the south-southeast of the country and its phylogeographic structure corresponds to the Mexican biogeographic provinces it inhabits, indicating divergence between the populations of Los Altos de Chiapas and the Península de Yucatán. Ototylomys chiapensis Porter et al. 2017 is restricted to Chiapas and is found in the La Pera Ecological Reserve. This study assessed variation in skull morphology and the niche ecological of the 2 species and populations of O. phyllotis distributed throughout the biogeographic and physiographic provinces of Mexico, based on geometric morphometric methods and ecological niche modeling testing niche similarity and overlap in their potential distributions. Significant variation in skull shape was found between O. chiapensis and O. phyllotis—as well as divergence between their ecological niches—suggesting that the species have undergone distinct morphological and ecological adaptations, which may have played a key role in their evolutionary divergence. For populations of O. phyllotis, the results were consistent when considering biogeographic or physiographic provinces. In addition to differences in skull size, variation was found in cranial shapes. The greatest morphological distances were observed between populations from Los Altos de Chiapas and the rest of the provinces in Chiapas, while the smallest distances were found between populations from the Península de Yucatán and the Sierra Lacandona provinces. These findings revealed 3 morphological groups that showed no niche similarity and minimal overlap in their potential distributions: Península de Yucatán and Sierra Lacandona (PY-SL); Sierras del Norte de Chiapas (SNCh); and Altos de Chiapas (ACh). These morphological groups partially correspond to their phylogeographic structure. We believe that morphological and environmental variation of O. phyllotis are largely governed by factors related to geological and evolutionary history, including geographic isolation, geological history, and climatic history, resulting in the appearance of Pleistocene refugia and the dispersal history of the species.