Adaptive Color Polymorphism in Cricket Frogs

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[Geographic variation of the vertebralstripe color polymorphism was studied in 7155 live frogs from 92 localities across the ranges of the two sibling species Acris crepitans and Acris gryllus in eastern and central North America. The four phenotypes, gray, red, green and red-green occur in nature in the following order of frequency: gray ≥ red > green, the red-green being extremely rare. The color polymorphism is probably stable and appears in both species but in different proportions probably due to macro- and micro-habitat differences between the species. It varies spatially and temporally suggesting its maintenance by balancing natural selection. All gryllus populations and central ones of crepitans, inhabiting apparently more varied environments, are trimorphic and highly heterozygous. Distributional pattern of the three color morphs in those populations is mosaic and is correlated with local substrate color (r = .286, P < .001). The red and green alleles disappear peripherally toward the west and north, where gray monomorphism prevails. Regional morph clines are detected, strongly correlated with rainfall, and to a lesser degree with temperature. The frequency of the gray morph increases while that of the red and green morph decreases along increasingly arid habitats. Laboratory desiccation experiments suggested that extralarge gray morphs lost significantly less water than red ones of comparable size. The gray morph proved more resistant than the red one in three cases of incidental laboratory infections. The above evidence suggests that the color polymorphism is maintained by several selective ecophysiological factors. These involve at least (a) visual selective predation, (b) climatic selection, and (c) possible 'immune' selection. The gray morph, combining general concealment and physiological hardiness predominates (.758), while the green and red morphs, though having selective advantage on green and red substrates, are apparently less hardy under extreme ecological conditions. The ecophysiological superiority of the gray morph enables it to colonize marginal environments and thereby extend the Acris range into grasslands and forests, arid and cold frontiers.]
Original languageEnglish
Pages (from-to)353-367
Number of pages15
Issue number3
StatePublished - Sep 1973


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