Series: Bulletin of the American Museum of Natural History (AMNH Bulletins) Volume: 210
Members of the Neotropical muroid rodent genus Zygodontomys are easily studied in the field and laboratory, providing opportunities for innovative research in many biological disciplines. A major impediment, however, is the confused systematics of the genus: no adequate diagnosis of Zygodontomys is available to permit unambiguous identifications, phylogenetic relationships to other muroids are unknown, and the species-level classification is in disarray. This monograph provides a systematic basis for future research with Zygodontomys, summarizes what is known concerning diverse ecological and biogeographic topics, and suggests where new investigations are most likely to yield important results. Zygodontomys can be distinguished from other Neotropical muroids by a unique combination of morphological attributes including external proportions, mammae number, qualitative details of cranial architecture, molar occlusal morphology, molar root numbers, and characters of the viscera. Morphological comparisons among Zygodontomys and putatively related species in the genera Bolomys, Calomys, and Pseudoryzomys afford few characters suitable for phylogenetic analysis, the results of which are inconclusive by the criterion of unweighted parsimony. A broader taxonomic survey of one character, presence or absence of the gall bladder, however, reveals that the presumptive apomorphy (absence) occurs in all oryzomyines (sensu stricto) together with Zygodontomys, Pseudoryzomys, Holochilus, and a few other taxa; some instances of homoplasy are obvious, but the implication of recent ancestry among the above-named genera and oryzomyines (s.s.) merits the attention of future investigators. Variation in quantitative and qualitative morphological characters among 2623 specimens of Zygodontomys is interpreted to reflect the existence of two species. Z. brunneus inhabits the intermontane valleys of the upper Río Magdalena, the upper Río Cauca, the upper Río Dagua, and the upper Río Patía in Colombia. In the upper Río Magdalena valley, Z. brunneus occurs sympatrically with Z. brevicauda, but elsewhere the two species are allopatric. The range of Z. brevicauda extends from the Pacific littoral of eastern Costa Rica through Panama, Colombia, Venezuela, and the Guianas, to northern Brazil. Qualitative character variation in Z. brevicauda reveals geographic patterns of population divergence that serve as the basis for three subspecies: Z. b. brevicauda, Z. b. cherriei, and Z. b. microtinus. Insular populations of Z. b. cherriei in Panama and of Z. b. brevicauda on Trinidad and Tobago average larger in craniodental dimensions than adjacent mainland populations but are not differentiated from them in qualitative characters. Species of Zygodontomys inhabit open savannas, savanna woodlands, thornscrub, shrublands, pastures, agricultural regions, and other types of natural or anthropogenic nonforest habitats on the Central and South American mainland. On some continental-shelf islands, however, Z. brevicauda is known to occur in closed-canopy forests. Most collection records are from elevations below 100 m, but there are numerous well-documented collections from higher altitudes, up to about 1300 m. Z. brevicauda is nocturnal, strictly terrestrial, and apparently omnivorous; it is numerically abundant in most suitable habitats within its ecogeographic range. Despite the dramatic seasonality of rainfall in some regions, populations of Z. brevicauda reproduce continuously throughout the year. Fifty-five species of arthropod ectoparasites have been collected from Z. brevicauda in Panama and Venezuela. Zygodontomys is part of a nonforest vertebrate fauna with a disjunct distribution in northern South America. Other mammals that belong to this fauna include the opossum Lutreolina crassicaudata, the armadillo Dasypus sabanicola, several muroid rodents (Calomys hummelincki, Sigmodon alstoni, and S. hispidus), the cavy Cavia aperea, and the rabbit Sylvilagus floridanus. The occurrence of these and other flightless nonforest vertebrate species in isolated enclaves of savanna and other types of open vegetation surrounded by forests is most parsimoniously explained by vicariance. Independent evidence of paleoclimates suggests that nonforest vegetation in northern South America was more extensive during the last glacial maximum than at present, and the disjunct distributions of some modern nonforest organisms are presumably the consequence of postglacial expansions of rain forests. On this assumption, an evolutionary scenario is proposed to account for the geographic variants of Z. brevicauda. Some of the South American zoogeographic literature is compromised by an uncritical reliance on antiquated subspecies taxonomies, and more revisionary systematic studies will be required to serve as the basis for meaningful historical analyses of the nonforest vertebrate fauna.
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