Natural Selection and a Cost Ceiling on Reproductive Effort

Abstract

Where reproductive effort subjects the parent to a risk, the benefit of some breeding activity's contribution to present fitness must be weighed against the loss to future fitness. With risk and benefit defined in terms of life-table parameters, a simple model for a long-lived iteroparous organism permits calculation of the quantitative relation between risk and benefit necessary if an increase in reproductive effort is to be selected for. Constraints on the maximal real benefit establish a corresponding risk ceiling which represents that lowest increment to reproductive cost which can not be compensated by any realizable cost-benefit ratio. Application of this model to data for some offshore-feeding seabirds indicates that an increased reproductive effort will be selected for in these organisms only if the fractional amount by which fledging success increases is at least 19 times greater than the fractional amount by which parental survivorship is decreased. The calculated margin of possibly compensable mortality increment is .28 deaths per capita. These rather stringent conditions on reproductive effort help account for some otherwise puzzling features of the reproductive strategy of oceanic seabirds: namely, deferred maturity, delayed resumption of breeding after a failed attempt, and nest desertion and abondonment of courtship and nest building during periods of stress. While the model only considers Darwinian fitness at the level of the individual, the demographic properties selected for in this particular instance may significantly stabilize the population and create a misleading impression of selection for stability and reproductive restraint.