Sexual dimorphism with female demographic dominance: Age, size, and sex ratio at maturation

Under the influence of natural and sexual selection, males and females will often differ in the costs and benefits of achieving some particular body size; we might therefore expect to find them growing at different rates for different times, and dying on different schedules. The analysis of sexual dimorphism presented here assumes that life histories are optimized over three key variables: activity level (or 'effort') during the juvenile stage, duration of the juvenile stage, and size at maturation. Reproduction is taken to be semelparous (or monocarpic), reproductive success is size dependent, and females are demographically dominant. Juvenile male and female life histories are assumed to be 'structurally identical' (i.e., governed by identical functions of activity, size, and development time). Under these conditions, sexually dimorphic life histories readily arise when features of juvenile behavior and development are adjusted to maximize r in response to gender-specific selection pressures on adults. Some conclusions from the analysis are as follows: 1) Seasonal constraints that force males and females to have equal development times cause differences between the sexes in other characteristics. 2) The survival cost of greater body size generates a sex ratio at maturation favoring the smaller sex, consistent with much empirical evidence. 3) The relationships among size, growth rate, and development time can be represented graphically in a way that permits sexually dimorphic life histories to be depicted and compared. Cases considered here indicate considerable diversity among these patterns. 4) An allometric growth function is derived to facilitate graphical and mathematical analysis of body size. 5) The reproductive size quotient, an indicator of reproductive potential relative to body size, is defined and shown to be a significant life history characteristic. Except when mortality rate is strongly related to size at maturation, or when generation time is fixed, the reproductive size quotient should be maximized, and these maxima should be the same for males and females.