Introduction

Make Him a Monogamy Junkie

The Monogamy Method

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Most humans have the sense that males and females are different. Much has been made of battles between the sexes, of sexual dialectics, even of the possibility that men and women have different planets of origin. But we are hardly unique; any observant naturalist can list several species, in addition to our own, in which male-female differences are clear. Many of us can also produce an even longer list of species in which external sex differences are inscrutable. Songbirds, for example, often lack external sexual differentiation. A large number of marine species, such as fucoid brown algae, sea urchins, polychaete worms, and red snappers, also have separate sexes that are nearly indistinct. Sexual differences in land plants too are often obscure, although botanists may assert that this is because both sexes often exist within each individual. But why should monoecy cause monomorphism? And if it does, why should sex in both cottonwoods and junipers be apparent only upon close examination, whereas sex among marijuana plants, even for aficionados, is simple to diagnose? Invertebrate zoologists might now chime in with examples of physical uniformity among hermaphroditic barnacles, flatworms, and freshwater snails, as well as among gonochoristic comb jellies, kinorhynchs, and veneroid clams. What explanation can possibly exist for extreme sexual differentiation in some species, and for virtual monomorphism among others, especially those lacking gender?

The answer to this question is the mating system. That is, the circumstances in which reproduction occurs within individual species. It is here that sexual differences arise -or do not. In the current literature, mating systems are described in two distinct ways, and each description has different implications for how sexual differences may or may not appear. The first description of mating systems, one that is familiar to botanists and coevolutionary biologists, emphasizes the genetic relationships that exist between mating males and females. Random mating, positive assortative mating (inbreeding), and negative assortative mating (outbreeding), all are examples of mating systems described in terms of the genetic relationships that may arise among, or are imposed upon, breeding pairs. Because certain heritable traits will tend to co-vary between the sexes within each breeding scheme, genetic correlations may arise that lead to, or prevent, the appearance of sex-specific phenotypes. A second description of mating systems, familiar to behavioral ecologists, considers mating systems in terms of the numbers of mates per male or per female. For example, monogyny and polygyny are descriptions of mate numbers per male, whereas monandry and polyandry are descriptions of mate numbers per female. Monogamy, polygamy, poly-gynandry, and polyandrogyny, each describe male and female mate numbers in relative terms (Table 1). These relationships are fundamental to sex differences in fitness variance and thus to the strength of sexual selection - or to its absence as an evolutionary force.

When these two schemes for describing mating systems are combined, genetic covariances that may arise between male and female mating phenotypes, because of the circumstances in which mate number may vary among individuals, can be incorporated into discussions of mating system evolution. Such considerations provide simple explanations, that is, explanations that do not imply wilful intent on the part of the participants, for interspecific differences in promiscuity or mate guarding, for tendencies to aggregate or to release gametes synchronously, and for apparent coevolutionary arms races involving genitalic morphology or between prostatic and uterine secretions. This combined description of mating systems identifies the nature, as well as the rates, of possible runaway processes that may arise when

Table 1 A classification of mating systems based on male and female mate numbers

Variance in mate

number

Mating system

Definition

Females

Males

Monogamy

Each sex mates once

0

0

Polygyny

Females mate once;

0

++++

male mate numbers

vary

Polygynandry

Both sexes have

+

+++

variable mate

numbers; mate

numbers vary more

among males than

among females

Polygamy

Both sexes have

++

++

variable mate

numbers; mate

numbers are equally

variable within each

sex

Polyandrogyny

Both sexes have

+++

+

variable mate

numbers; mate

numbers vary more

among females than

among males

Polyandry

Males mate once;

++++

0

female mate

numbers vary

particular associations between mating individuals cause male and female traits to co-vary. Such considerations explain a larger range of the sexual dimorphism that is observed, within and among species, than descriptions of mate number alone.

However, two other issues have led to controversy for the study of animal mating systems: (1) the source of sexual selection and (2) the intensity of sexual selection. These issues lie at the foundations of mating system research because the emphasis taken to explore them determines (1) the processes that are presumed to underlie sexual selection, (2) the procedures that are undertaken to observe these processes, and (3) the variables that are measured to test specific hypotheses regarding mating system evolution.

Until recently, the analysis of mating systems, particularly for animals, has emphasized sex differences in parental investment as the source of sexual selection. According to this view, female reproduction is limited by the availability of resources required for energetic investment in ova and young. Because resource abundance may vary in space and time, male reproduction is presumed to be limited, in turn, by the spatial distribution of materials required by females, and by the temporal distribution of sexually receptive females themselves. Stated differently, males are expected to compete for females because male reproduction is limited by the availability of parental care that only females can provide. Parental investment theory (PIT) thus holds that the intensity of male-male competition reveals the intensity of sexual selection.

As a means for determining how female spatiotemporal distributions may influence this selection intensity, two measures have been defined: the operational sex ratio (OSR) and the environmental potential for polygamy (EPP). The OSR was originally defined as the ratio of potentially receptive males to receptive females at any time. The simplest quantitative description of OSR in these terms can be expressed as N?/N, = RO, where N? and N, indicate the total number of males and females in the population, respectively. However, many researchers have focused instead on instantaneous measures of OSR that include only the individuals who are receptive at any particular time. With this emphasis, when OSR>1, females are numerically rare, and male competition for mates can appear to be intense, although this assumption depends on the degree to which male mating success is consistent among males throughout the breeding season. When OSR<1, females are numerically abundant, and male competition for mates can appear to have relaxed; but again, depending on the cause of the surplus in females and how males respond to it, such conditions may still allow certain males to contribute disproportionately to the next generation.

The EPP identifies the degree to which social and ecological conditions allow males to monopolize females. However, standardized methods for quantifying female distributions, or the scale on which EPP can be consistently measured, have never been clearly defined. As a result, while serving as a conceptual surrogate for the intensity of sexual selection, the uncertain relationship between EPP and selection intensity itself, within as well as among species, makes the practical use of this measure imprecise.

Researchers emphasizing PIT have encountered further difficulties in putting its assumptions to rigorous empirical tests. Despite PIT predictions, a sex difference in relative parental investment has proven extremely difficult to compare within and among species. Not only are the amounts of energy, cost, and risk associated with relative parental investment difficult to quantify, but the correlation between sex differences in parental investment and sexual dimorphism itself is often poor, particularly in species with reversed sex roles. Measures of sexual selection intensity based on PIT, such as comparisons of potential reproductive rates among males and females, require laboratory conditions that are rarely encountered in nature. Other PIT estimates, which emphasize the number of individuals qualified to mate, require assumptions about who is breeding and who is not, that may underestimate the actual variance in mating success within the population. Like other research paradigms grounded in optimality theory, PIT also has an unfortunate tendency to emphasize adaptive outcomes. Thus, researchers may find themselves first identifying traits they consider likely to evolve due to sex differences in parental investment or in expected fitness returns, and then searching in earnest for evidence of adaptations that are consistent with their initial predictions. As creationists, astrologists, and politicians have shown, research methods aimed at hypothesis confirmation tend to be less rigorous than those aimed at hypothesis falsification.

Below, an approach for measuring the source and intensity of sexual selection that provides such empirical rigor is described. This framework emphasizes measurement of the actual selective forces responsible for shaping male-female differences instead of ad hoc proxies for sex differences in parental investment. Using data commonly available from ecology, life history, and behavioral analyses for sexual species, the author shows how the magnitude of the sex difference in fitness variance, estimated by measuring male and female offspring numbers, can be used to classify the mating systems of sexual species. The author also shows how the sex difference in the opportunity for selection can be influenced by runaway processes caused by genetic correlations. This approach provides an explicitly quantitative and easily interpreted means for classifying mating systems and for predicting sexual differences in adult phenotype.

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