The sexual imperative. Part 2, Sexual contract.

The Sexual Imperative explores the reasons for sexual and asexual reproduction in the insect, animal and human world. This episode looks at monogamy and polygamy. Human relationships are generally based on monogamy between a male and a female, although some polygamous exceptions occur. However, the general human relationship can be defined as 'monogamy with infidelity', similar to lovebirds' relationships. Half the species of birds practice monogamous relationships, as often the chicks need much care after hatching, so both partners must be involved. In circumstances where the male provides only sperm, such as with baboons, the female is generally much more choosy about her mating partner, but in cases where males perform a service such as providing food, as with crickets, the male has some choice over which female it breeds with. There are also examples of assisted monogamy; African wild dogs live in packs, yet only one couple breed at a time and the entire pack raises the puppies.