A process of pair formation leading to assortative mating: passive age-assortative mating by habitat heterogeneity

We present an individual-based model dealing with mating as a process of pair formation. Model simulations, based on data from a 19-year study of Spanish imperial eagles, Aquila adalberti, showed that the mating pattern of a population is not necessarily a direct consequence of the mating preferences of individuals; positive age-assortative mating, by which individuals of similar age are more likely to become paired, does not necessarily indicate homotypic mating preferences. For example, individuals of similar, young age may be constrained to a few low-quality territories, leading to passive assortative mating, independent of individual preferences. Confounding factors such as territory quality can affect the encounters between a male and a female available for mating, generating an age-assortative mating totally independent of mate preferences. Such a process may apply to many territorial species when spatial variation in territory quality is pronounced.