Mating functions for 2 sex population dynamics: Effects of density and genetic diversity

Background/Question/Methods

Most population dynamics models explicitly track the density of a single sex. When the operational sex ratio can vary, two-sex models may be needed to understand and predict population trajectories. Various functions have been proposed to describe the relative contributions of females and males to recruitment, and these functions can differ qualitatively in the patterns that they generate. Which mating function best describes the dynamics of real populations is not known, since alternative two-sex models have not been confronted with experimental data. In addition to effects on local population dynamics, mating functions can affect rates of spatial spread by species with sex-biased dispersal (see abstract by Tom EX Miller et al).

Results/Conclusions

We conducted the first such comparisons of alternative mating functions, using laboratory populations of the bean beetle Callosobruchus maculatus and of the fly Drosophila melanogaster. For the beetle, experimental manipulations of the operational sex ratio and total density provided strong support for a demographic model in which the birth rate was proportional to the harmonic mean of female and male densities, and females, males, and their offspring made unique contributions to density dependent regulation. For the fly, harmonic mean functions also provided the best fit, and parameters differed among populations from seven different near-isogenic inbred lines.  Furthermore, parameters describing the shape of the mating function in populations with higher genetic diversity were not readily predictable from knowledge of the mating functions for the constituent isogenic lines.

We also offer guidelines for transferring our approach to other, less tractable systems in which possibilities for sex ratio manipulations are more limited. We show that informative experimental designs require strong perturbations of the operational sex ratio. Our experimental results and guidelines for design strategies promote synthesis of two-sex population dynamics theory with empirical data.