Natrix in Mexico

140 p., [23] p. of plates (1 folded) : ill. (some col.), maps ; 27 cm.Includes bibliographical references (p. 134-140)."This study is concerned primarily with geographical distribution and variation among the three species of water snakes of the genus Natrix in Mexico, but considerable information on ecology and natural history is also included. Among the three species, Natrix valida occurs in western Mexico, but Natrix erythrogaster and Natrix rhombifera not only inhabit eastern Mexico but also range widely in the United States. In the arid northern portions of Mexico these snakes are confined to the rivers and occasional springs and swamps, and the populations occupying individual drainage systems were formerly completely isolated from those of other systems. Theoretically it is now possible in a few areas, however, for semiaquatic snakes, such as Natrix, to move from one river system to another by following the canals of recently developed irrigation projects. Isolation has been of sufficiently long duration to permit the evolution of two distinct subspecies of Natrix erythrogaster, one each in the Río Nazas and Río Aguanaval. A third race of erythrogaster occurs in northeastern Mexico. Natrix rhombifera, which also has three subspecies in Mexico, ranges widely through the lowlands of eastern Mexico. One race occurs in the northeastern states, a second has its center of abundance in the vast and complex drainage system that discharges into the Gulf of Mexico through the Río Pánuco at Tampico, and the third is widespread through the swamplands of southern Veracruz and Tabasco. The distribution of the collecting localities, when plotted on a map of the drainage systems, indicates that stream piracy probably has occurred in Nuevo León and that the Río San Juan has decapitated the headwaters of the Río San Lorenzo of the Río Conchos-San Fernando system, with consequent transfer of the aquatic and semiaquatic faunas to the drainage of the Rio Grande. The break between the two southern races of rhombifera occurs in the region where the Sierra Chiconquiaco and associated highlands extend eastward to the Gulf of Mexico, south of Nautla, Veracruz, and effectively separate the Gulf coastal plain into northern and southern sectors. On the west coast there is a similar, but less massive disruption of the coastal plain immediately south of San Blas, Nayarit. Three races of Natrix come together in this area. One occurs northward as far as the Río Yaqui in Sonora; a second ranges southward along the tenuous and frequently interrupted coastal plain at least to the vicinity of Acapulco, Guerrero; and the third occupies an outlier of the Mexican altiplano in the vicinity of Tepic, Nayarit. A fourth race of valida is confined to the Cape Region of Baja California. The present distribution of the genus Natrix in Mexico suggests that these reptiles were much more widely distributed during one or more pluvial periods of late Pleistocene or Recent time and that increasing aridity in western North America has resulted in the fragmentation of their ranges and the establishment of numerous isolated populations, especially in rivers in the northern part of Mexico. Earlier pluvial periods may also have permitted Natrix valida to range northward around the head of the Gulf of California and from there southward through Baja California, which would explain the presence of the isolated subspecies that has survived in the Cape Region. The various taxa are discussed in detail in the present paper, with emphasis on variation in scutellation and color patterns. Distributions are plotted on a series of six maps. Ten plates, two in color, illustrate all the various subspecies from life; 12 additional plates illustrate habitats in which these reptiles have been collected. Field work in Mexico was conducted in each of 10 different years, and the total accumulated time, including work on parallel studies on garter snakes of the genus Thamnophis, amounted to almost exactly a full year. During these investigations every state and territory in Mexico was visited at least once, as well as the Federal District"--P. 132