Vocal sac condition of 460 frogs was examined by dissection for five western North American Rana (Rana boylii group sensu Case, 1978): R. aurora (N = 280), R. boylii (N = 24), R. cascadae (N = 113), R. muscosa (N = 22) and R. pretiosa (N = 21). R. boylii consistently exhibits small, paired subgular sacs, whereas R. muscosa and R. pretiosa lack vocal sacs. R. cascadae typically lacks vocal sacs, whereas R. aurora may have no vocal sacs, a single asymmetric vocal sac, paired sac rudiments or paired vocal sacs. R. aurora exhibits a striking step-cline in vocal sac condition: northern populations lack vocal sacs, southern populations have small, paired subgular sacs and frogs with intermediate vocal sac conditions occur mostly in the 480 km between these northern and southern population assemblages. The two most common vocal sac variants within R. aurora (an absence of vocal sacs and paired subgular sacs) are congruent with available data that suggest trenchant morphological and behavioral differences between the two previously defined subspecies, R. a. aurora and R. a. draytonii. R. aurora, as currently defined, may actually be two species in secondary contact. Reduced vocal sac apparatus among members of the R. boylii group is correlated with their weak vocalizations.
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文章 : 英语