PHENOTYPIC PLASTICITY IN LIFE-HISTORY TRAITS OF FEMALE THALASSOMA BIFASCIATUM (PISCES: LABRIDAE). 1. MANIPULATIONS OF SOCIAL STRUCTURE IN TESTS FOR ADAPTIVE SHIFTS OF LIFE-HISTORY ALLOCATIONS.

Optimal-life-history theory is based on the relative benefit of immediate versus future reproduction. We apply this theory to the life-history tactics of female Thalassoma bifasciatum, a sex-changing coral-reef fish. Local social structure varies in this species and influences a female's chances of achieving high future reproductive success as a territorial terminal-phase male. We predicted that female life-history allocations would be flexible and responsive to variation in future reproductive prospects. We altered population size structure on four reefs, removing large fish and adding small fish. These alterations enhanced the residual reproductive value of the remaining larger female residents. The predicted response to the manipulation was a reduction in reproductive activity and an increase in growth. Our results do not support this hypothesis. Possible interpretations are that: 1) the theory, or our application of the theory, is flawed; 2) female Thalassoma are unresponsive to changes in future reproductive prospects; or 3) the design of our study was ineffectual at detecting a response. We report here an approach to the study of adaptive life-history strategies that deserves implementation in other systems. © 1989 The Society for the Study of Evolution.