Biological Demands of Reproduction

As a starting point for understanding reproductive strategies among primates, as well as in other mammals, the differences between male and female roles in reproduction can be examined. At the cellular level, the female gamete (egg) is metabolically expensive to produce in comparison to sperm, which are little more than a bundle of chromosomes. As a result, eggs are produced one at a time, whereas sperm are produced in much larger quantities (in the millions).

At the level of the organism, all female placental mammals carry and support the developing fetus within their wombs. Increased metabolic intake is necessary to offset the increasing weight (and concomitant energy expenditure required to carry while locomoting), as well as nutrition required for adequate support of both fetus and mother. Additionally, all mammalian infants require a period of nursing. This period is called lactation, whereupon the mother will again incur metabolic costs to provide adequate nutrition for the developing newborn. Contrary to this scenario, males generally do not have the same enforced biological costs incurred throughout the reproductive process.

Parental Investment

Because of this disparity between the sexes in terms of required parental investment, males and females will employ different strategies to further their reproductive success over the course of their lives. For example, since sperm is so plentiful and cheap to produce in males, their reproductive fitness is limited primarily by access to females. Therefore, if acquiring additional mates is relatively easy, males should preferentially adopt this activity - rather than investing in their current or soon-to-be offspring - to further their Darwinian fitness. Additionally, unless male parental contributions provide a substantial benefit to offspring, male investment remains low.

This disparity highlights a general point: the investing sex will tend to be the choosier sex, whereas the non-investing sex will tend to be more flamboyant, aggressive, and competitive in the realm of mate access. In the case of mammals, nursing prevents males from taking over duties as primary caretakers. This is not the case with many species of birds (e.g., rheas, sandpipers) or sea horses; in these situations, females deposit eggs in the males' care and provide little or no additional assistance. As predicted from disparities in parental investment, the females engage in much higher degrees of competition with each other for access to mates.

Sexual Selection and Competition

In primates sexual selection manifests itself in two scenarios:

• Intrasexual competition, where competition is among males for access to females

• Intersexual selection, where selection results from the mate choice by females

In male-male competition, selection generally favors those individuals with larger body size, physical weaponry (such as the large canine teeth of baboons), and other characteristics which expedite the male's control over access to female(s). For this very reason, a high degree of sexual dimorphism (difference in size between the sexes) has been strongly correlated with male-male combat over access to females. Additionally, this relationship holds true when examining degrees of sexual dimorphism within different mating systems (e.g., single-male/multi-female groups vs. monogamous groupings), particularly those in which the social environment engenders a high degree of male competition.

The second case of sexual selection - intersexual mate choice - must also result in higher reproductive success for an individual, but why certain traits are selected revolves around three points:

1. traits which increase the fitness of females
2. traits which are indicators of good genes, which would thereby improve the fitness of offspring
3. traits which have little adaptive significance, but were preferentially selected by females over many generations

The benefits conferred upon a female by a male who defends young, provides food, or defends territories better than other males are readily apparent. However, traits which indicate good genetic makeup act in an indirect manner. While good genes are not easily discerned, physical indicators of good genes may be easier to spot - perhaps brightly colored peacock tails or loud mating calls bear a relationship with genetic fitness. That peacocks would select mates on the conscious basis of good genes is not very likely, for example. However, the fact that brightly colored males tend to have more fit and vigorous offspring could affect mate selection, even if the genetic basis of this decision remains unknown to the actors.