Monday, March 21, 2011

Brighter Coloration Does Not Always Mean Greater Toxicity


A red morph of the Granulated
Dart Frog, Oophaga granulifera
Photo credit: Patrick Gijsbers.
Hugh Cott, noted the relationship between warning coloration and toxins in 1940 when he wrote, "Perhaps the most specialized and effective of all methods of defense is the use of poisons. Toxic properties have been developed in a wide range of animal life, and are frequently associated with warning coloration." A new study now suggests that at least one brightly colored morph of a dendrobatid frog may be less toxic than morphs with more subdued coloration. Ian Wang (2011) notes that the prevailing theory, following Cott, suggests that aposematic coloration evolves with toxicity so that increased toxicity will accompany greater conspicuousness. Dart frogs in the Dendrobates (=Oophaga) histrionicus group (D. histrionicus, D. pumilio, and D. granuliferus) have populations with unique color morphs spanning the visual spectrum, and while some of the morphs would seem likely to to be aposematic others are cryptic to the human eye. How these polymorphisms are mainatined is porrly understood. Wang measured spectral reflectance, toxicity, and did a phylogenetic reconstruction on nine populations of  O. granuliferus (Taylor, 1958) on the Pacific coast of Costa Rica. and found  that the less conspicuous color morphs are actually significantly more toxic than the brightest, most conspicuous phenotypes and that the more toxic, less-conspicuous form evolved from a less toxic, more conspicuous ancestor. Through gas chromatography—mass spectrometry analysis of toxin profiles, Wang traced the increase in toxicity in the less-conspicuous populations to an acquisition of specific alkaloids, some of which are convulsants. Wang's results challenge the idea that increased conspicuousness always evolves with increased toxicity and support the idea that once aposematism has been established in a species, phenotypic variation may evolve from brightness and toxicity becoming decoupled.

Literature
Cott, H. B. 1940. Adaptive coloration in Animals. London: Methuen & Co. Ltd. (Quote from page 253.)


Wang, I. J. 2011. Inversely related aposematic traits: reduced conspicuousness evloves with increased toxicity in a polymorphic poison-dart frog. Evolution, doi: 10.1111/j.1558-5646.2011.01257.x

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