the diversity of mating strategies within animal populations may be understood through evolutionary game theory concepts that assess the costs and benefits of reproductive decision-making. the evolutionarily stable strategy (ess) concept provides a particularly useful framework for considering alternative behaviours as they relate to fitness. given that a strategy describes a set of pre-programmed rules that specify particular behaviours, an evolutionarily stable strategy is one that persists in a population due to its benefits to fitness. an ess will be maintained in a population if it accords higher average fitness than other strategies, or a level of average individual fitness equivalent to all other strategies within the population.