Chemical aposematism
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CHEMOECOLOGY
COMMENTARY
Chemical aposematism Paul J. Weldon
Received: 8 July 2013 / Accepted: 29 July 2013 / Published online: 22 August 2013 Springer Basel 2013
Abstract Discussions of aposematism traditionally have focused on the visual displays of prey that denote unpalatability or toxicity to predators. However, the construct of aposematism accommodates a spectrum of unprofitable traits signaled through various sensory modalities, including contact and distance chemoreception. Aposematism, involving learned aversions by signal receivers or selection for their unlearned avoidances, arises in predator–prey or other interspecific interactions where a mutually beneficial avoidance of signal emitters by signal receivers exists. Aposematism evolves by selection against signal receivers, e.g., predators, imposed by signal emitters, e.g., unprofitable prey, and vice versa, where both nondiscriminating signal receivers and unrecognized signal emitters are imperiled. Chemical aposematism entails concurrent reciprocal selection where signal emitters select for chemosensory avoidance responses in signal receivers, and where signal receivers select for the emission of identifiable (distinctive) chemicals in signal emitters. Keywords
Aposematism Chemical defense
Charles Darwin and Alfred Russel Wallace, the celebrated co-proponents of natural selection as a mechanism of evolution, proffered different explanations for colorful ostentation among animals. Darwin (1874) posited that bright plumage and other conspicuous traits arise through Handling Editor: Michael Heethoff. P. J. Weldon (&) Smithsonian Conservation Biology Institute, National Zoological Park, 1500 Remount Road, Front Royal, VA 22630, USA e-mail: [email protected]
sexual selection, and are displayed chiefly by males ‘‘to charm the female’’. However, he was puzzled by the gaudy markings of some caterpillars, which, given their life stage, could not plausibly be adorned for mate recognition (Wallace 1867). Wallace (1867) suggested that these patterns in caterpillars evolved as signals to potential predators denoting unpalatability: ‘‘… distaste alone would be insufficient to protect a larva unless there were some outward sign to indicate to its would-be destroyer that his contemplated prey would prove a disgusting morsel, and so deter him from attack’’. Poulton (1890) coined the term aposematism (Gr a´po´, away; rnˇla´, sign) for the phenomenon postulated by Wallace, defining it as ‘‘an appearance which warns off enemies because it denotes something unpleasant or dangerous’’. As Poulton’s definition of aposematism licenses, the range of known or suspected features supporting the aposematic status of signalers has been expanded beyond prey unpalatability to embrace a spectrum of unprofitable traits, including toxicity, pugnacity, and mechanical inaccessibility. In addition, some authors have justly amended Poulton’s definition to accommodate aposematism that is manifest by means other than through appearance, i.e., where signals are perceived through non-visua
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