Marriages, on the other hand, are social institutions only established within the human society. Marriage is exclusive to humans because it is a legal and spiritual tradition. One must note the different kinds of relationships that exist in different cultures: monogamy, and polygamy describe forms of marriage; "gamy" in Greek means marriage, not mating. Polygamy can be broken into polygyny (the marriage between one man and several women), and polyandry (the marriage between one women and several men). Another word that was commonly used in the book was "promiscuity." In discussing animal species other then humans, promiscuity means (male or female animals) quickly moving from one sexual partner to the next, an occurrence that is more normal. However, when discussing promiscuity in terms of humans, the meaning- although much the same- also means an act of consistent depersonalized sexual activity, an act that is not a norm, but an exception. .
Throughout the book, several examples, and references were made to birds, mammals (other than humans) and fish to further develop the argument about the differences in mating patterns. One fascinating example discussed the mating patterns of a little fist called a stickleback. The male, red-bellied fish courts the female, plain-colored bellied fish with a "zig-zag" swimming that leads the female to the nest the male has made to hold her eggs. When the female decides the nest is up to par, she lays her eggs in it and the male follows behind to fertilize the eggs with his sperm. The male is typically the guardian of the nest, and is aggressive towards competitors and chases them away. Another kind of stickleback fish has evolved. This fish is male, but it has a plain-colored belly resembling the female belly. This male fish patiently waits by another male's nest for a female to lay her eggs and then he unsuspectingly darts in and fertilizes the eggs before the rightful male can do so.