Maintenance of polyphenism is mediated by inter-cohort interactions

Background/Question/Methods

Understanding the evolution of phenotypic variation in response to environmental change is a fundamental question in evolutionary ecology.  Polyphenisms provide excellent systems for testing such responses, and are considered consequences of spatiotemporal heterogeneity.  Some polyphenic salamanders express two adult morphotypes, terrestrial metamorphs and aquatic paedomorphs, which differ in morphology, physiology, life history, and behavior.  Paedomorphic salamanders live in size-structured populations, creating variation in demography such that early cohorts can inhibit the growth and development of later cohorts via cannibalism and competition.  This variation can potentially influence morph production and subsequent fitness, and thus impact the maintenance of the polyphenism.  Using data from long-term population monitoring of Arizona Tiger Salamanders (Ambystoma tigrinum nebulosum), we investigated how inter-cohort interactions influenced life history decisions and resulting fitness consequences.  Because previous results have suggested that most paedomorphs in this population were smaller as larvae than those becoming metamorphs, and because paedomorph dispersal is limited, we predicted that paedomorphs would exhibit increased reproductive effort to mitigate the fitness costs associated with smaller body size and reduced dispersal, and that these effects would be mediated by inter-cohort interactions such that cohorts appearing first would mature at larger sizes and older ages than those that followed.

Results/Conclusions

Overall, paedomorphs reproduced for the first time at a younger age and smaller size than metamorphs, and these responses depended on cohort position.  Specifically, paedomorphs from an early cohort (1998) were larger at first reproduction than those from later (1999, 2000, 2001) cohorts that may have experienced competition with the older cohort.  Moreover, we found evidence of sex-specific trade-offs between age and size at first reproduction, whereby increases in body size delayed first reproduction more in male metamorphs than male paedomorphs, which was also driven by cohort interactions.  Paedomorphs had a shorter interval between breeding events and produced larger embryos than metamorphs, but there were no morph-specific differences in their clutch size/body size relationships.  Metamorphs exhibited greater longevity and less variation in yearly survival than paedomorphs, and adult size impacted survival estimates more in metamorphs than paedomorphs, suggesting that density- and size-independent mortality, such as winter kill, may be driving paedomorphic survival rates.  These results indicate that paedomorphs in this population invest more in reproduction than metamorphs, and that this reproductive strategy strongly depends on body size differences that are mediated by inter-cohort interactions.  Thus, inter-cohort interactions provide spatiotemporal variation that helps maintain polyphenic life histories in size-structured populations.