There is enormous diversity in every aspect of animal life histories, including the age at maturity, frequency of reproduction, number and size of offspring, and the effort invested by parents in individual offspring. What factors in the environment selected for these differences among and within species? The goal of the proposed research is to evaluate the role of specific environmental influences on life history evolution of natural populations of guppies on the island of Trinidad. Natural populations of guppies from the Northern Range of Trinidad have life history differences associated with differences in predation. Guppies that co-occur with the pike cichlid Crenicichla alta and other predators mature at an earlier age, devote more of their available resources to reproduction, and tend to produce more and smaller offspring than guppies that co-occur with just the killifish Rivulus harti. Crenicichla and some of the associated species of predators prey predominantly on large, sexually mature size classes of guppies. Rivulus prey predominantly on small, immature size classes of guppies. The size specificity of these predators suggests that they select for life history changes by altering the age-specific survival of guppies. By preying on large guppies, Crenicichla should reduce adult survival. By preying on small guppies, Rivulus should reduce juvenile survival. The influence of such differences in age-specific survival on life history evolution has been investigated theoretically; the predicted responses in life history evolution correspond precisely with the differences observed between Crenicichla and Rivulus localities. The key field experiments in the proposed research involve introducing guppies and guppy predators into sections of stream where they had previously been excluded by waterfalls. The effects of predators and the environment, most notably resource availability, will be evaluated with direct observations of growth and survival, plus indirect estimates of growth and survival using otolith. Otolith are bone-like structures in the inner ear which record daily rings, similar to the annual rings commonly seen in cross-sections of tree trunks.
