Every year, millions of brides and grooms promise to love and care for each other ’till death do they part. It’s a profound promise. But marriage is also just one of the natural world’s many mating arrangements, as THE SEX CONTRACT, Part 3 of NATURE’s THE NATURE OF SEX shows. Throughout nature, males and females negotiate a wide range of carefully made plans for conceiving and raising offspring. Indeed, monogamy — two partners staying sexually faithful to one another — is one of the rarest kinds of sex contracts.

Overall, less than 3% of mammal and bird species practice monogamy, and in many of those species, cheating or alternative arrangements is common. Men in some cultures, for instance, have many wives. In others, many men share a single wife. A bird called the Jacana, or Lily Trotter, has a similar sex contract. A single female will have four or five male partners, each carefully caring for eggs laid by the female.

Still other creatures carry this idea to an extreme. African mole rats and termites, for instance, live in underground colonies with a single queen that produces all of the offspring in the colony.

As a result, almost all of the colony’s inhabitants are brother and sister. Male elephant seals, in contrast, play king, ruling over a harem that includes every female on a long stretch of beach.

These arrangements, however different, have the same goal: increasing the number of offspring carrying the parents’ genes — or, in the case of termites, the common genes of the entire colony. And in each case, the contracting partners work together for the common goal: the survival of a new generation.