3.1 Mating fundamentals

Mating is where selection's effects are most visible, because differential reproduction is the currency of evolution. The logic of mating begins with a single asymmetry: the unequal cost of reproduction to the two sexes.

Anisogamy: the original asymmetry

The very definition of the sexes is anisogamy — females produce few, large, costly gametes (eggs); males produce many, small, cheap gametes (sperm). This tiny asymmetry at the cellular level cascades upward into nearly everything else about mating behavior.

Parental investment theory

Robert Trivers generalized the insight: the sex that invests more in offspring becomes a limiting resource over which the other sex competes, and the higher-investing sex becomes choosier. In mammals, females invest enormously — gestation, lactation — so females tend to be selective and males tend to compete. Where the pattern reverses (as in seahorses, where males carry the young), so do the behaviors: females compete and males choose.

Sexual selection follows investment

Because the lower-investing sex competes for the higher-investing sex, the engines of sexual selection — male-male competition and female choice — follow directly from the investment asymmetry. This single framework predicts the direction of choosiness and competitiveness across the animal kingdom.

A grounding example

Angus Bateman's experiments with fruit flies established the principle now called the Bateman gradient: male reproductive success rose steeply with the number of mates, while female success plateaued after just one or two. A male who mated with many females had many more offspring; a female gained little from extra mates because her output was capped by her egg supply. This asymmetric payoff — more mates helps males far more than females — is the quantitative root of why the sexes pursue different mating strategies.