Sexual Selection Theories

Parental Investment Theory

In a landmark paper published in 1972, one of the leading figures in evolutionary psychology, Robert Trivers, proposed that the sex contributing more parental investment (PI) will be a limiting resource for the less investing sex. Trivers defined PI as investment that increases the survival of existing offspring but decreases the parent’s ability to produce future offspring. This theory is key to understanding why there are significant differences between male and female reproductive strategies in some species.

Which sex contributes more PI across different animal species? In most species, but not all (see, for example, emperor penguins, where males and females share responsibility for taking care of the egg), it is the female who contributes more PI. Certainly, in human females this is true, starting with nine months of gestation, several months of lactation, and several years of child-rearing. In contrast, human males could walk away after a single act of copulation and choose not to participate in child-rearing at all.

Across the animal kingdom, where males and females invest equally in reproduction, sexual selection may not result in significant anatomical or behavioral sex differences. However, when one sex invests much more than the other, we are likely to see large sex differences in both anatomy and behavior.

The textbook provides some excellent examples of species where males invest as much as (if not more than) females in reproduction, and in those cases, the general trend is reversed—females tend to be larger, more brightly colored, and more aggressive and dominant than males.

Relative Reproductive Rate

Clutton-Brock and Vincent (1991) proposed that the sex that reproduces more slowly (the slow sex) is in short supply, so the fast sex competes for the slow sex. In humans, females are the slow sex, and males are the fast sex. Females release a single egg cell (ovulation) each month during their reproductive life span—from onset of menstruation in puberty to cessation of menstruation at menopause—and if that egg is fertilized, the female is involved in gestation, lactation, and so on for a significant period of time. So the number of offspring that females can produce is limited. Males, on the other hand, can produce millions of sperm cells with each ejaculation, and they don’t experience menopause—they can continue to produce sperm indefinitely. So, in theory, males can have numerous offspring if they can successfully mate with many females. However, in many species, dominant males will control access to large numbers of females, so subordinate males will often end up with no offspring at all (e.g., this occurs in elks and in several species of monkeys). This relative reproductive rate differs between the sexes and contributes to high or low reproductive success. Males in many species will be highly competitive with each other to gain increased access to the females.

The traits favored by sexual selection depend on whether an individual is a member of the fast sex or slow sex:

• Fast sex individuals tend to have the following characteristics:
  • tend to be competitive, bigger, flamboyant;
  • are more eager to mate; and
  • exhibit the Coolidge effect*.
• Slow sex individuals tend to be
  • less competitive for mates, slower to be available for reproduction;
  • the “ecological” sex (i.e., have traits that increase survival); and
  • choosier, or coy.

* The Coolidge effect is a term for the well-known phenomenon of arousal by a novel sex partner. It is associated with an anecdote about President Calvin Coolidge. The president and Mrs. Coolidge were once touring a poultry farm, and Mrs. Coolidge was shown the prize rooster, who mated several times a day. She requested that this be pointed out to the president when he came around. When the president was informed that the first lady wanted him to know about the rooster, he asked, “Same hen each time?” He was told, “Oh no, a different hen each time.” The president responded, “Please tell that to Mrs. Coolidge.”