Relative size underlies alternative morph development in a salamander
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BEHAVIORAL ECOLOGY –ORIGINAL RESEARCH
Relative size underlies alternative morph development in a salamander Michael P. Moore1,2,5 · Joseph H. K. Pechmann3,4 · Howard H. Whiteman1,4 Received: 20 September 2019 / Accepted: 23 July 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Size thresholds commonly underlie the induction of alternative morphological states. However, the respective importance of absolute and relative size to such thresholds remains uncertain. If absolute size governs expression, morph frequency should differ among environments that influence absolute sizes (e.g. resources, competition), and individuals of the same morph should have similar average sizes across environments. If relative size determines expression, the frequency of each morph may not differ among environments, but morphs within each environment should differ in size relative to one another. We tested these predictions in a salamander (Ambystoma talpoideum) that develops into either a terrestrial metamorph or an aquatic paedomorph. To generate size variation within and among environments, we reared individuals in mesocosm ponds across three conspecific densities. We found that morph frequency did not differ among density treatments, and the morphs were not similarly sized within each density treatment. Instead, within each environment, relatively larger individuals became metamorphs and relatively smaller individuals became paedomorphs. Relative size therefore determined morph development, highlighting the importance of an individual’s social context to size-dependent morph induction. Keywords Facultative paedomorphosis · Intraspecific competition · Life-history variation · Polyphenism · Resource polymorphism
Introduction
Communicated by Jean-François Le Galliard. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00442-020-04723-8) contains supplementary material, which is available to authorized users. * Michael P. Moore [email protected] 1
Watershed Studies Institute and Department of Biological Sciences, Murray State University, Murray, KY 42071, USA
2
Department of Biology, Case Western Reserve University, Cleveland, OH 44106, USA
3
Department of Biology, Western Carolina University, Cullowhee, NC 28723, USA
4
Savannah River Ecology Laboratory, University of Georgia, Aiken, SC 29802, USA
5
Living Earth Collaborative, Washington University, St. Louis, MO 63105, USA
Discrete, alternative phenotypes are taxonomically widespread and often enable individuals to optimize fitness through differential resource use (Smith and Skúlason 1996). The induction of these morphs commonly depends on a suite of external and internal factors, whereby even genetically identical individuals can express alternative phenotypes based on differing developmental environments (Nijhout 2003; West-Eberhard 2003). Such “polyphenisms” broaden the range of environmental and social circumstances to which a single genotype can produce well-adapted phenotypes
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