Code from: The effects of intersexual interactions on survival can drive the evolution of female ornaments in the absence of mate limitation

The evolution of sexual ornaments in animals is typically attributed to reproductive competition. However, sexual ornaments also arise in contexts where the ornamented sex is neither mate nor gamete limited, and explanations for ornamentation in these cases remains incomplete. In many species, particularly those with slow life histories, lifetime reproductive success depends more strongly on adult survival than fecundity, and survival can depend on intersexual interactions. We develop a population genetic model to investigate how the effect of intersexual interactions on survival may contribute to ornament evolution in the absence of competition for mates. Using female ornamentation in polygynous mating systems as a case study, we show that, indeed, ornaments can evolve when the ornament functions to modify interactions with males in ways that enhance a female’s own survival. The evolutionary dynamics depend on qualitatively on the specific behavioral mechanism by which the ornament modifies social interactions. In all cases, the ornament’s long-term persistence is ultimately determined by the coevolution of the male locus that determines how males affect female survival. We outline the scenarios that are most likely to favor the evolution of female ornaments through effects of intersexual interactions on survival, and we urge empirical researchers to consider the potential for this social selection mechanism to shape traits of interest across taxa.

Methods
The archive includes a Mathematica notebook ("SurvivalModel_Clean_20240813.nb") that details the construction and analysis of a population genetic model to explore the potential for sexual ornamentation to evolve in female animals through effects of intersexual interactions on female survival. The notebook describes the premises and construction of the model, the analysis of direct selection arising from the model dynamics, and forward simulations of example evolutionary trajectories. The population genetic model is constructed in such a way that it may be used to explore multiple mechanistic hypotheses about how female ornaments might modify social interactions to increase their own survival. See README for additional details.