How an individual fecundity pattern looks in Drosophila and medflies.

Reproduction usually is characterized by a mean-population fecundity pattern. Such a pattern has a maximum at earlier ages and a subsequent gradual decline in egg production. It is shown that individual fecundity trajectories do not follow such a pattern. In particular, the regular individual fecundity pattern has no maximum so that experimentally observed maximums are average-related artifacts. The three-stage description of individual fecundity, which includes maturation, maturity, and reproductive senescence, is more appropriate. Data are presented for Drosophila and Mediterranean fruitfly females that clearly confirm this hypothesis. A systematic error between egg-laying scores and the regular individual pattern allows for evaluation of how close the random scores are to the pattern. The first finding of the analysis of the systematic errors is that they are consistent with the three-stage hypothesis and do not contradict the absence of the maximum in the regular individual pattern. The other finding is the existence of obvious dynamic properties of the systematic error. The slow decrease in egg-laying at the maturity stage might be the result of a cost of mating. It can also be a consequence of "structural" senescence, that is, a slow rate accumulation of oxidative damage in the gonads.