Who Mates With Whom, and Why
Mating systems describe the patterns of mating within a population, shaped largely by the distribution of resources and each sex's parental investment — the time and resources a parent commits to each offspring. Monogamy (one male, one female) is common when offspring need substantial care from both parents. Polygyny (one male, multiple females) arises when males can monopolize resources or mates directly. Polyandry (one female, multiple males) is rarer and often comes with reversed parental roles. Promiscuity involves multiple partners for both sexes with no lasting bonds.
Parental investment theory (Robert Trivers) predicts that the sex investing more per offspring will be more selective (choosy) about mates, since a poor choice costs that sex more; the less-investing sex instead competes more intensely for mating access. This asymmetry drives sexual selection (Darwin), which operates through two mechanisms: intra-sexual selection (competition within one sex, usually male-male, for mating access) and inter-sexual selection (mate choice exercised by the other sex, usually female choice among displaying males).
| System | Pattern |
|---|---|
| Monogamy | One male, one female |
| Polygyny | One male, multiple females |
| Polyandry | One female, multiple males |
| Promiscuity | Multiple partners, no stable bonds |
Common pitfall: assuming the higher-investing sex is always female. Investment, not sex per se, is what predicts choosiness — in species like seahorses and jacanas, where males carry the eggs or the primary parenting burden, it is the males who become the choosier, more selective sex.