Competition, the Fitness of Offspring, and Optimal Clutch Size

Abstract

The theory is developed which relates the optimal division of parental resources among offspring (hence clutch size) to the nature of the intraspecific competitive process, the rate of mortality, and environmental predictability. It assumes that natural selection results in parents adjusting resource investment per offspring in a manner which achieves the highest ratio of offspring fitness per unit of parental resource expended, given a certain relationship between the expected fitness of individual offspring and the investment per offspring. This fitness-resource relation can be constructed from knowledge or models of the frequency distribution of parental resource utilized by individual offspring in the population and from the nature of the competitive process. Model examples are given based on assumptions concerning territorial quality, and on the rules of the game governing contests for resources. These models of interference lead to strong optimization for high expenditures of resource per offspring. The mortality regime, by affecting the severity of competition, should affect the optimal level of allocation and result in selection for birth rates which seem to compensate for mortality. Environmental instability can lead to selection for parental mechanisms which increase growth variation in offspring and achieve a more optimal resource distribution. The theory thus explains how natural selection on individual parents can lead to population control through regulation of birth rates and behavioral mechanisms which seem to prevent overcrowding and starvation. Only fragmentary evidence for testing the theory is available.