Delayed life history effects, multilevel selection, and evolutionary tradeoffs: Mass and date of metamorphosis in the California tiger salamander

Background/Question/Methods

Delayed life history effects (DLHEs) occur when fitness in one life stage affects fitness in subsequent life stages. Given their biphasic lifecycle, pond-breeding amphibians provide a natural system for studying DLHEs. In this study, we use numerous mark-recapture techniques (visual implant alphanumeric tags, visual implant elastomer, and photographic pattern recognition) and a large drift fence array (221 fences monitored over a six-year period) to document selection in a population of the endangered California tiger salamander (Ambystoma californiense).

Results/Conclusions

We find that DLHEs are prominent across all life stage transitions and that there is variation in whether selection acts at the individual or cohort level. We also demonstrate that there is large variation in average cohort fitness (> one order of magnitude), which is the variation that DLHEs act upon. A literature review reveals that this high level of intercohort variation is not restricted to A. californiense, but also occurs in numerous other pond-breeding amphibians, and that appropriately documenting the magnitude of intercohort variation requires long-term studies (~two population turnovers). Given the profound effect that DLHEs can have on population dynamics, understanding intercohort variation in average metamorph quality and the action of selection at the individual or cohort level is critical for developing realistic models of population dynamics. When developing models of population dynamics, greater attention should be paid to variation in average fitness rather than focusing exclusively on variation in total numbers. Finally, we empirically document an evolutionary tradeoff between mass at metamorphosis and date of emergence. This tradeoff may play a role in maintaining the variation in mass (fitness) at metamorphosis.