The optimal clutch size revisited: separating individual quality from the costs of reproduction

Life-history theory, central to our understanding of diversity in morphology, behaviour and senescence, describes how traits evolve through the optimisation of trade-offs in investment. Despite considerable study, there is only minimal support for trade-offs within species between the two traits most closely linked to fitness - reproduction and survival - questioning the theory's general validity. We used a meta-analysis to separate quality effects from the costs of reproduction using studies of parental investment and survival in birds. Experimental enlargement of broods caused reduced parental survival. However, the effect of experimental enlargements was small and opposite to the effect of phenotypic quality, where individuals that naturally produced larger clutches also survived better. The opposite effects on survival in experimental and observational studies of parental care provides the first meta-analytic evidence for theory suggesting that quality differences mask trade-offs. Fitness projections using the overall effect size revealed that reproduction presented negligible costs, except when reproductive effort was forced beyond the level observed within species, to that seen between species. We conclude that there is little support for the most fundamental life-history trade-off, between reproduction and longevity, operating within a population. We suggest that within species, the fitness landscape of the reproduction-survival trade-off is flat until it reaches the boundaries of the between-species fast-slow life-history continuum. Our interpretation explains why the costs of reproduction are not apparent and why variation in reproductive output persists within species. ### Competing Interest Statement The authors have declared no competing interest.