Does the two-sex life table for sexual populations invalidate those based solely on female cohorts?

Abstract

The age-stage two-sex life table procedure can be conducted using a software package, TWOSEX-MSChart. Subsequent to the publication of this procedure, papers employing conventional “female cohort” life table procedures have been challenged by some journals, on the grounds that the “two sex” method is superior. Here, we assess the validity of female cohort life tables by using a random normal probability function to evaluate 54 hypothetical female cohorts, and 10 male ones, with different means and variances of developmental time, reproductive success, and longevity. A number of variables were examined for their effects on the behavior of the system, including the type of survivorship curve, stage structuring, primary and secondary sex ratio, skewed or normal distribution of the reproduction curve, and protandrous versus protogenous emergence. Data were analyzed by both the female cohort and the two-sex method by including male cohorts in the latter analyses. The results revealed no impact of male cohorts or male characters on essential population growth parameters including net replacement rate, intrinsic and finite rates of increase, generation time, and doubling time, and the latter equation yielded exactly the same outputs as TWOSEX-MSChart. This could imply that Chi’s method is not fundamentally different from a female cohort procedure, but may include additional details that have no meaningful effect on the analysis. We conclude that the more complex TWOSEX-MSChart procedure may be considered an alternative to the female cohort method, rather than an obligatory replacement and that no grounds exist for discrediting the latter procedure.