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Proximate and Functional Causes of Polyphenism in an Anuran Tadpole

D. W. Pfennig
Functional Ecology
Vol. 6, No. 2 (1992), pp. 167-174
DOI: 10.2307/2389751
Stable URL: http://www.jstor.org/stable/2389751
Page Count: 8
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Proximate and Functional Causes of Polyphenism in an Anuran Tadpole
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Abstract

Few systems have been explored in sufficient detail to link the proximate causes of polyphenism to the ecological factors that favour discontinuous, environmentally induced variation. I examined the developmental and physiological bases underlying polyphenism in southern spadefoot toad tadpoles (Scaphiopus multiplicatus). Scaphiopus often occurs as two discrete trophic morphs: carnivores and omnivores. Carnivores develop from omnivores if the latter are fed macroscopic prey (anostracan shrimp and tadpoles). I found that tadpoles can change to the alternative morphology if their diet was switched. Static allometric analyses indicated that morph determination results from acceleration (or retardation) of certain anatomical features. I hypothesized that morph determination is triggered by the presence of a potent accelerator of amphibian development occurring in the carnivore's prey: thyroid hormone (or its constituents). Omnivores exposed exogenously to thyroxine assumed the carnivore morphology within days. This endocrine response enables tadpoles to develop into the morph with the greater fitness since the rapidly developing carnivores are favoured in highly ephemeral ponds, which contain more shrimp. Carnivores also are more efficient predators of shrimp. This study thus links the underlying physiological causes of a polyphenism to the ecological factors that likely favour the occurrence of alternative morphs. More generally, this study illustrates how an endocrine signal may allow an individual to assess and continuously adapt to a changing environment.

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