Emlen & Oring 1977: Ecology, Sexual Selection, and the Evolution of Mating Systems
The previous posts from our discussions on classic mating systems papers have shown how both empirical work and theory advanced knowledge of sex differences in the returns from relative investment in reproduction. In their 1977 paper, Emlen & Oring attempted to bring natural history observations and ecology into a general theory of mating systems evolution, and to discuss the ecological factors and selective forces that shape polygamous mating systems. Of particular relevance here is Trivers’ work on parental care, as the prevalence of polygamy is often related to whether one sex is freed from parental care duties (and thus have excess time and energy to seek additional mates).
‘Ecology, Sexual Selection, and the Evolution of Mating Systems’ was published 2 years after E. O. Wilson’s ‘Sociobiology’ had hit the shelves, and Emlen & Oring are careful to note early on that understanding mating systems requires the reader to abandon any thoughts of group- or species-level ‘adaptiveness’. Fitness is a measure of the reproductive success of an individual (or genotype) relative to other individuals (or genotypes) in the same or other populations, and so we must consider selection to be operating at the level of the individual . Intraspecific competition is a crucial aspect of sexual selection: essentially, when one sex becomes a limiting factor for the other, the result is an increase in intrasexual competition among members of the available sex for access to mates of the limiting sex. The authors hypothesise that an important cause of the differing intensities of sexual selection found both across species and between populations of the same species is “the ability of a portion of the population to control the access of others to potential mates”. This control may be enforced through physically excluding other members of the same sex from potential mates, or through controlling critical resources. Central to Emlen & Oring’s argument is a cost-benefit analysis: under what environmental circumstances is the defending of multiple mates, or of those resources necessary for gaining multiple mates, economically viable?
Two of the crucial components of this concept of an environment’s ‘polygamy potential’ are illustrated by the figure below, in which the height of the shaded area perpendicular to each diagonal line indicates the environmental potential for polygamy in relation to the spatial distribution of resources (on the x-axis) and the temporal availability of receptive mates (on the y-axis). Resource ‘clumping’ in space means individuals can monopolise critical resources, which always increases the potential for polygamy. Asynchrony of mate availability is required for polygamy, else the time to locate, attract or copulate will mean that other potential mates are no longer available. However, too much asynchrony means the cost of continual defence outweighs the benefits of gaining additional mates.
Another crucial piece of the sexual selection intensity puzzle is the realisation that the overall ratio of males to females in the population is less important than the operational sex ratio (OSR): the average ratio of fertilisable females to sexually active males at any point. The OSR is affected by spatial and temporal clumping of the limiting sex; the example given is of continuous long periods of male sexual activity alongside brief, asynchronous periods of female receptivity, producing a strong skew in the OSR.
The bulk of the paper outlines an ecological classification of mating systems, concentrating on whether and how access to potential mates and resources are controlled, and the effects of temporal and spatial clumping on the OSR. Detailed examples of mating system types are illustrated using examples of avian mating biology. Emlen & Oring also consider how changes in ecological parameters might disrupt the environmental potential for polygamy, and indicate that their framework should enable predictions of the form of mating system plasticity that occurs.