A new species of big-eared climbing rat, genus (Cricetidae: Tylomyinae), from Chiapas, Mexico.

An allopatric population of big-eared climbing rats () from the Northern Highlands of Chiapas, Mexico, is described as a new species. The new taxon is part of a unique montane rainforest community that includes several other endemic species in the limited geographic range between the Río Grijalva and the Central Depression of Chiapas. Several cranial, external, and molecular characters distinguish this new species of big-eared climbing rat from its more widely distributed congener, . We performed principal component and discriminate function analyses of cranial measurements, and found that specimens of the new species consistently could be distinguished from other with strong statistical support. Compared with exemplars of from elsewhere in their range, the new species possesses a karyotype that differs by 3 additional biarmed chromosome pairs, is fixed or nearly fixed for distinct electromorphs at 12 allozyme loci, and the mean genetic distance exceeds 14%, based on comparisons of the mitochondrial cytochrome gene between the new species of and representatives of . The restricted distribution in montane karst rainforest suggests that the species and its habitat may be a matter of conservation concern.