If there is a message in all this, it must surely be that there are no simple rules that apply to all of the species all of the time. There are some key general principles that apply universally, but patterns of monogamy, divorce and polygamy vary both between and within species in response to local ecological and demographic conditions. But it is the availability of alternatives that makes shifts of strategy possible. Animals, every bit as much as humans, make choices about whom to pair with and for how long, and those decisions are influenced in large part by whether they will do better by staying with the current partner, by moving from one partner to another or by playing a more subtle kind of game. By contrast, most mammals have a lengthy gestation followed by lactation, which leaves males on the sidelines.
Male mammals are only monogamous under a limited number of conditions.
One is when females are so widely dispersed that a male would find it difficult to defend more than one at a time. In this case, he may do well to stick with one female to make sure that no other males mate with her when she does come into oestrus. However, many species are monogamous even when males could in principle defend several females. In these cases, it turns out that the male provides a service for the female. In South American marmoset and tamarin monkeys, this service amounts to full responsibility for everything to do with childcare, except lactation.