Are direct density cues, not resource competition, driving life history trajectories in a polyphenic salamander?

Abstract

Polyphenisms, where multiple, discrete, environmentally-cued phenotypes can arise from a single genotype, are extreme forms of phenotypic plasticity. Cue acquisition and interpretation are vital for matching phenotypes to varying environments, but can be difficult if cues are unreliable indicators or if multiple cues are present simultaneously. Facultative paedomorphosis, where juvenile traits are retained at sexual maturity, is a density-dependent polyphenism exhibited by many salamanders. Favorable conditions such as low larval densities and stable hydroperiod delay metamorphosis and promote a paedomorphic strategy. We investigated proximate cues affecting facultative paedomorphosis in order to understand how larval newts (Notophthalmus viridescens louisianensis) assess conspecific density. To isolate the effects of density cues from the effects of resources and agonistic behavior, we caged larval newts in mesocosms in a 2 × 2 factorial design that manipulated both background larval newt densities (high or low) and food levels (ambient or supplemented). We found strong effects of both food and density on caged individuals. Under high densities, caged larvae were more likely to become efts, a long-lasting juvenile terrestrial stage, across both food levels, while paedomorphs were more common under low densities. Though food levels increased growth rates, density had strong independent effects on metamorphic timing and phenotype. Competition for food and space are classical density-dependent processes, but density cues themselves may be a mediator of density-dependent effects on polyphenisms and life history responses.

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Acknowledgements

We would like to thank Tyson Research Center and Washington University in St. Louis, especially K. Smith and B. Schall, for allowing us to use their facilities and providing a stimulating environment. Also, B. Biro helped substantially with collecting, L. Eveland & M. Pintar contributed to the study in many ways, J. Hoeksema provided advice on mixed models, and L. Fuller and B. Mikah provided encouragement. Also, thanks to the National Science Foundation (DEB-0516298), Texas Tech University, The University of Mississippi, and the Henry L. and Grace Doherty Foundation. This research conformed to institutional guidelines (IACUC protocol 14–028) and all state and federal regulations (MDC permit no. 15680).

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Correspondence to Jason R. Bohenek.

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Appendix

Appendix

See Tables 2 and 3.

Table 2 Summary of alternative models in lme4 notation
Table 3 Summary of complete model parameters

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Bohenek, J.R., Resetarits, W.J. Are direct density cues, not resource competition, driving life history trajectories in a polyphenic salamander?. Evol Ecol 32, 335–357 (2018). https://doi.org/10.1007/s10682-018-9941-8

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Keywords

  • Crowding
  • Eft
  • Metamorphosis
  • Mesocosm
  • Neoteny
  • Paedomorphosis