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 this episode 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.