5.2 Differences between the sexes
In biology, 'sex' refers to a particular form of reproduction, sexual reproduction, that is distinct from asexual reproduction. As you know, sexual reproduction involves the production of eggs by females and sperm by males; eggs (or ova) and sperm are known as gametes. It is a universal feature of mammalian biology that in sexual reproduction there are two types of gametes and that progeny are produced by the fusion of two unlike gametes to form a single cell called the zygote. The zygote is the fertilised egg from which the young individual develops, passing through the blastocyst stage. Both partners to the sexual act make an equal contribution in terms of the amount of genetic material that the zygote receives.
It is a commonplace observation that half your genes came from your mother and half from your father. (This statement oversimplifies the issue, as you'll appreciate if you know of the distinction between alleles and genes; but for our purposes, the statement can stand.) What is not equal is the amount of effort that goes into the production of gametes. The eggs or ova contain more cellular material than the sperm; they are larger than the sperm. One consequence of there being two types of gametes is that, for a given amount of metabolic effort (i.e. internal synthesis), the female produces fewer gametes than the male. This fundamental difference between males and females means that a male's reproductive potential (that is the number of offspring he could potentially sire) is huge and he can improve his reproductive success by mating with many females. The female mammal is in a different situation.
What factors limit the female mammal's reproductive potential?
Her reproductive potential is limited by the number of eggs she can produce. She cannot increase her reproductive potential by mating with many males. You probably also thought about the production of milk and how suckling her young would limit the number of potential offspring and hence the reproductive potential of a female. Clearly, reproduction requires a lot of effort for a female mammal. If she puts too much effort into the production and nurturing of her offspring at the expense of keeping herself in good condition, she may exhaust herself and be unable to breed again for a while, thereby limiting her reproductive potential.
A male mammal also puts effort into maximising his reproductive potential. However, we might expect that as a consequence of differences in male/female gametes, his efforts are focused on competing with other males for the possession of as many females as possible. I'm sure you can think of many instances, from the TV programme(s) in particular, where you have observed exactly this behaviour.
But contrary examples of animals may have sprung to mind. Why are they monogamous?