Variation in Rana Pipiens Schreber in Texas

Abstract

Leopard frogs (Rana pipiens Schreber) from Texas were examined morphologically and used in intraspecific hybridization tests to describe the variation of this anuran in the state. The morphological analysis suggests that there are three morpho-units separated by lines running along the western boundary of the Texan province and the southern boundary of the Kansan province. Hybridization tests indicate that Central x East Texas hybrid embryos are recognizable from their controls only when reared under conditions of salinity stress. This work was initiated to determine whether the leopard frog (Rana pipiens Schreber) has developed a demonstrable morphophysiological gradation of characters along a habitat gradient which extends from Central through East Texas. Specimens from outside the study area were also examined to elucidate statewide trends in morphological variation. This study is based on the examination of 625 adult (i.e., over 45 mm snout-vent length) and 215 juvenile leopard frogs, and upon the results of 76 artificial hybridization experiments. The study was undertaken for several reasons. First, cursory unpublished experiments conducted at the University of Texas had indicated that offspring from crosses between Central and East Texas leopard frogs were not as viable as their controls. Second, the morphology of the adults varies over the state, but this variation has never been adequately described using a large sample of specimens. Third, although numerous crossings experiments with Texas R. pipiens have been conducted (Moore, 1946; 1949), the specimens used have all been collected in Ward County. Genetic variation within the state of Texas as revealed by crossing experiments is therefore an untouched field. Finally, although Moore and others (Moore, 1939, et. seq.; Porter, 1941; Ruibal, 1955, 1957; Volpe, 1957) have attempted to establish the major trends in variation of certain physiological characters of R. pipiens embryos, no one has made a concentrated study of specimens from a relatively small geographic area. I am indebted to Dr. W. F. Blair for suggesting this problem and for advice during its progress. My wife, Martha Kay, assisted in the field collecting and in preparation of the figures.