Lonely hearts or sex in the city? Density-dependent effects in mating systems

Abstract

Two very basic ideas in sexual selection are heavily influenced by numbers of potential mates: the evolution of anisogamy, leading to sex role differentiation, and the frequency dependence of reproductive success that tends to equalize primary sex ratios. However, being explicit about the numbers of potential mates is not typical to most evolutionary theory of sexual selection. Here, we argue that this may prevent us from finding the appropriate ecological equilibria that determine the evolutionary endpoints of selection. We review both theoretical and empirical advances on how population density may influence aspects of mating systems such as intrasexual competition, female choice or resistance, and parental care. Density can have strong effects on selective pressures, whether or not there is phenotypic plasticity in individual strategies with respect to density. Mating skew may either increase or decrease with density, which may be aided or counteracted by changes in female behaviour. Switchpoints between alternative mating strategies can be density dependent, and mate encounter rates may influence mate choice (including mutual mate choice), multiple mating, female resistance to male mating attempts, mate searching, mate guarding, parental care, and the probability of divorce. Considering density-dependent selection may be essential for understanding how populations can persist at all despite sexual conflict, but simple models seem to fail to predict the diversity of observed responses in nature. This highlights the importance of considering the interaction between mating systems and population dynamics, and we strongly encourage further work in this area.

A large population parental care game: Polymorphisms and feedback between patterns of care and the operational sex ratio

“…This parameter can be thought of as a measure of the density and mobility of the population. As Kokko and Rankin (2006) argue, density effects may be very important in the evolution of behaviour. By defining λ 1 and the rate of transition of females from the non-receptive state to the receptive state to be arbitrarily large, we essentially obtain the model of Yamamura and Tsuji (1993).…”

Optimal mate choice patterns in pelagic copepods

“…It is studied extensively in vertebrates and terrestrial invertebrates, but rarely in marine organisms and especially not in plankton. This is astonishing given that marine plankton provides the basis of the marine food web, and that sexual selection are known to influence population dynamics (Kokko and Rankin 2006).…”

Adaptive value of same-sex pairing in Laysan albatross

“…Alternative reproductive strategies, such as 'sneaker' and 'satellite' males, have been observed most often in males [29,30]; the occurrence of an alternative mate acquisition strategy in female Laysan albatrosses, skipping, suggests a broadening of the circumstances in which alternative reproductive behaviour evolves, particularly in a sexually monomorphic species with internal fertilization and obligate biparental care. The consequences of demographic processes shaping mating systems is of increasing interest [27,31,32] and our results contribute some rare empirical data about the interactions among population dynamics, demography and social organization to the prevalence of a particular mating system. Same-sex pairing appears to be part of a flexible breeding strategy that female Laysan albatrosses employ in response to dynamic social conditions, for example sex-ratio fluctuations.…”

“…Female-female competition is observed most often when there is a skewed operational sex ratio that limits access to males or the opportunity to mate [27]. Sex-ratio theory indicates that the number or availability of potential mates can strongly influence evolutionary strategies as a result of this competition [28].…”