Mate Density, Predation Risk, and the Seasonal Sequence of Mate Choices: A Dynamic Game

We describe and analyze a computer-simulation model of mate choice, featuring two different quality groups (based on offspring per mating) in each sex. Mating between quality groups results from two-dimensional random encounter and mutual assent, where assent reflects an attempt to maximize expected lifetime reproductive success, E(LRS). Premating predation (via random encounter with predators) and other mortality also influence E(LRS). Given potentially conflicting optimal choices, the model finds the evolutionarily stable patterns of choosiness for the four quality groups. When there are multiple mating episodes by individuals through the season, the resulting dynamic game is solved to obtain a seasonal pattern of mate choice and reproduction. The model generates seven different mating patterns among quality groups. These patterns imply different opportunities for selection, as indicated by the variance components of normalized lifetime reproductive success, var(LRS). The changes in E(LRS), var(LRS), and mating patterns in response to different densities of predators and of potential mates are explored in detail. Decreasing predation risk or increasing mate availability tends to increase E(LRS), choosiness, and assortative mating. Var(LRS) and thus the opportunity for selection for mate quality is highest at intermediate densities of predators and of potential mates. When density remains constant throughout the mating season, choosiness increases late in the season, a time at which less of the potential E(LRS) is jeopardized by the greater predation risk associated with choosiness. Reproductive success of the low-quality group of the less choosy sex is particularly sensitive to changes in density and other parameters. When seasonal density patterns of predators and potential mates are predictable, these low-quality individuals should do better and may thus be more numerous when mate densities are high and predator densities are low, or when predator densities are high and mate densities are low, than for other combinations of relative densities.