4.10: Sexual Selection

It is not uncommon to see morphological and behavioral differences between the sexes. Sometimes the sexual dimorphism and behavioral differences are profound; they can even obscure the fact (at least for human observers) that the two sexes are members of the same species (→). Specific traits associated with one sex may even appear maladaptive; they might be expected to reduce rather than enhance an organism’s reproductive potential.^[207] The male peacock’s tail, the gigantic antlers of male moose, or the bright body colors displayed by some male birds are obvious examples. Darwin recognized the seriousness of this problem for evolutionary theory and addressed it in The Descent of Man and Selection in Relation to Sex (1871). Where the investment of the two sexes in successful reproduction is not the same, the two sexes may have different and potentially antagonistic reproductive strategies. Organisms of different sexes may be “looking” for different traits in their mates. In general, the larger parental investment in the production and rearing of offspring, the less random is mating and the more prominent are the effects of sexual selection, that is, the choice of who to mate with.^[208] It is difficult not to place these behaviors in the context of conscious choices, that is looking, wanting, etc., but they appear to be the result of selected behaviors and do not imply self-conscious decision making or moral judgements. Presumably, they arise from preferences based on costs and benefits. In humans, how consciousness, self-conscious-ness, social organization, ideological and theo-political choices influence sexual behavior (and selection) is even more complex and way beyond our scope here.

One of the most robust and reliable findings in the scientific literature on interpersonal attraction is the overwhelming role played by physical attractiveness in defining the ideal romantic partner. Both men and women express marked preference for an attractive partner in a non- committed short-term (casual, one night stand) relationship.

For committed long-term relationships, females appear to be willing to relax their demand for a partner's attractiveness, especially for males with high social status or good financial prospects.

Males also look for various personality qualities (kindness, understanding, good parental skills) in their search for long-term mating partners, but unlike females, they assign disproportionately greater importance to attractiveness compared to other personal qualities.

The paramount importance of attractiveness in males' mate choices has been recently demonstrated by using the distinction between necessities (i.e., essential needs, such as food and shelter) and luxuries (i.e., objects that are sought after essential needs have been satisfied, such as a yacht or expensive car) made by economists.

Using this method, Li et al., reported that males treat female attractiveness as a necessity in romantic relationships; given a limited "mating budget," males allocate the largest proportion of their budget to physical attractiveness rather than to other attributes such as an exciting personality, liveliness, and sense of humor.

Consider the situation in which females have help in raising their offspring. Selection would be expected to favor females who mate preferentially with the most robust, but not necessarily the most supportive males based on the reasonable assumption that the most robust appearing male will lead to the most robust offspring. This prediction has been confirmed experimentally in a number of systems; the robustness of offspring correlates with the robustness of the male, a win for evolutionary logic.^[209] In this context, the reproductive success of a male will be enhanced if it "advertises" its genetic robustness through visible and unambiguous features. A useful sign of robustness needs to be difficult to fake.^[210] The size and symmetry of a beetle’s or an elk’s antlers accurately communicate their state of health.^[211] Consider a male peacock's tail; a large, colorful and symmetrical tail is a sign of health an the ability to survive.

Another approach involves territoriality. Individuals, typically males, establish and defend territories. Since there are a limited number of such territories and females only mate with males that have established and can defend a territory, only the most robust males are reproductively successful. An alternative scenario involves males monopolizing females sexually. Because access to females is central to their reproductive success, males may interact with one another to establish a dominance hierarchy, typically in the form of one or more “alpha” males. Again, the most robust males are likely to emerge as alpha males, which in turn serves the reproductive interests of the females. This type of dominance behavior is difficult to fake. But, cooperation between non-alpha males can be used to thwart the alpha male’s monopolization of females.

Now consider how strategies change if the odds of successful reproduction are improved when a male can be counted on to help the female raise their joint offspring. In this situation, there is a reproductive advantage for females who can identify those males who will display reproductive loyalty.^[212] Under conditions of cooperative rearing of offspring, females may compete with other females for access to (perhaps rare) loyal males. Moreover, it can be in the male’s interest to cooperate with fertile females; often females (but not human females) advertise their state of fertility, through external signals.

There are of course, alternative strategies. For example, groups of females, including sisters, mothers, daughters, aunts, and grandmothers may cooperate, thereby reducing the importance of male assistance. At the same time, there can be conflicts. What happens if the most robust male is not the most "loyal" male? A female could maximize their reproductive success by mating with a robust male and bonding with a loyal male, who helps rear another male’s offspring. While not in the "loyal"male’s immediate reproductive interest, it could enhance their future reproductive "success". Selection could favor males that cooperate with one another to ward off robust but transient males. Since loyal males already bond and cooperate with females, it may be simple for them to cooperate with other males. Another possibility is that a loyal male may be attractive to multiple females, who in turn compete for his attention and loyalty. The outcome of such interactions is likely to be influenced by the number of females a male can protect and the impact the male has on a female's reproductive success.

It is critical to correctly read and/or respond to various traits, an ability likely to be selected. Males that can accurately "read" other males can determine whether they are likely to win a fight, while an inaccurate assessment could result in crippling injuries. A trickier question is how to determine whether a potential mate will be loyal? As with advertisements of overall robustness, we might expect that traits that are difficult or expensive to generate will play a key role. So how does one unambiguously signal one’s propensity to loyalty and a willingness to cooperate? One could use the size and value of nuptial gifts. The more valuable, that is, the more expensive and difficult the gift is to attain, the more loyal the recipient can expect the gift giver to be. On the other hand, once valuable gift-giving is established, one can expect the evolution of traits in which the cost of the gift given is reduced and by which the receiver tests the value of the gift, a behavior we might term rational skepticism, as opposed to naive gullibility.

This points out a general pattern. When it comes to sexual and social interactions, organisms have evolved to “know” the rules. If costs of cooperation are "expensive" there will be selective pressure to cheat. Cheating can be suppressed by making the signs of social or sexual cooperativity difficult or impossible to fake, or by generating counter-strategies that identify fakes. These biological realities produce many behaviors, some of which are disconcerting. These include sexual cannibalism, male infanticide, and various forms of infidelity, mentioned above. What we have not considered as yet is the conflict between parents and offspring. Where the female makes a major and potentially debilitating investment in its offspring, there can be situations where continuing a pregnancy can threaten the survival of the mother. In such cases, spontaneous abortion (ending the pregnancy) could save the female, who can go on and mate again. In a number of organisms, spontaneous abortion occurs in response to signs of reproductive distress in the fetus. Of course, spontaneous abortion is not in the interest of the offspring and we can expect that mechanisms will exist to maintain pregnancy, even if it risks the life of the mother, in part because the fetus and the mother, while related are not identical and can have conflicts of interest.^[213] There are many variations of reproductive behavior to be found in the world and a full discussion is beyond our scope here.

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