Book Description

"The genus Microryzomys contains certain small-bodied, long-tailed oryzomyine rodents that are endemic to the northern and central Andes, their distribution describing an arc from northern Venezuela, through Colombia, Ecuador and Peru, to central Bolivia. Based on the examination of over 900 museum specimens representing 105 principal collecting localities, two species, without subspecific divisions, are recognized: M. minutus (dryas Thomas, d. humilior Thomas, fulvirostris Allen, and aurillus Thomas in synonomy) and M. altissimus (a. hylaeus Hershkovitz and a. chotanus Hershkovitz in synonomy). Microryzomys minutus can be distinguished from its congener altissimus by a combination of size and proportional differences, which were underscored in principal component analysis of 16 craniodental and 3 external variables using representative population samples of each species. These results, together with 14 locality records of sympatry (or parapatry) in the mountains of Colombia, Ecuador and Peru, support an interpretation of only two biological species. Populations of Microryzomys altissimus are more restricted geographically, altitudinally, and perhaps ecologically than are those of M. minutus. The geographic limits of M. minutus define the distributional boundaries of the genus, whereas, M. altissimus occurs only in the Cordillera Central of Colombia, throughout the Andes of Ecuador, to the highlands of central Peru. Locality records document that M. altissimus occupies a higher altitudinal zone (mostly from 2500 to 4000 m) compared to M. minutus (mostly from 1500 to 3500 m); most instances of sympatry occur within the 2500 to 3500 m interval. Information gleaned from specimen tags and fieldnotes suggests that the two species inhabit different morphoclimatic vegetation zones consistent with the altitudinal belts they occupy. Microryzomys minutus dwells in a variety of wet, cool Andean forests ranging from lower and upper montane rain forest to subalpine rain forest; at its upper altitudinal limits, it may frequent various habitats found at the ecotone between upper Andean forests and páramo. Microryzomys altissimus is more regularly associated with páramo environments, the forest-páramo ecotone, and subalpine rain forest. Both species range on either side of the Huancabamba Depression in northern Peru without evidence of appreciable differentiation. Neither species penetrates the Altiplano biome in southern Peru and Bolivia; the southernmost distribution of M. minutus adheres to wet forest along Amazon-facing slopes east of the Altiplano. Examination of external, cranial, dental and gastric morphology revealed a suite of qualitative traits, apparently unrelated to overall size, by which species of Microryzomys can be distinguished from those of Oligoryzomys and from Oryzomys palustris, the type species of Oryzomys. The concordance of these discrete character states contradicts the notion that Microryzomys and Oligoryzomys are artificial constructs used to segregate the smaller species within a speciose genus highly varied in body size. Instead, such character associations support the monophyly of species assigned to Microryzomys and to Oligoryzomys relative to other groups of oryzomyine rodents. Emended diagnoses, framed at the generic level, are provided for these two taxa. Evaluation of 20 qualitative characters reveals that Oryzomys palustris is highly differentiated from both Microryzomys and Oligoryzomys, which share many traits thought to be plesiomorphic. Certain derived character states suggest that Microryzomys and Oligoryzomys are sister taxa, but the evidence for a close relationship is weak, and this possibility must await further corroboration and the study of a broader array of oryzomyines"--P. 3.