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| For the white-tailed prairie dog, one male is quite enough, thank you |
The general pattern for mammals is that males seek to mate with as many females as possible, while females prefer to keep a single partner, at least in any one mating season. This is because females can't be pregnant with more than one litter at a time, so they may as well pick the fittest males around and get them to father all their children. Males, on the other hand, have no such investment, so the more females they mate with, the more children they have, and the better their genes survive.
But this is really a gross oversimplification. It's the basis of the polygynous mating system, where a powerful male drives off rivals and attracts a harem, or acquires multiple mates by some other means. This sort of thing is most apparent in animals like seals and deer, but it's by no means universal. For one thing, many mammals are more or less monogamous. This typically occurs where the male has to help to look after his children if they're to survive, thus putting him under the same pressure as the females.
But what are we to make of polyandry, where one female mates with multiple males in the same breeding season? It's surprisingly common, so there must be some reason for it. One, of course, is that they might not get much choice in the matter - if there's lots of randy males around, then it may be easier to give in than to put up a fight. It's hardly acceptable behaviour for humans, but, then, animals aren't human.
