The Sex Lives of Female Jaguars

There are many ways of classifying mating systems in animals, but one of the most basic uses four main types. In polygynous species, the male mates with as many females as he can get away with, driving away or out-competing any potential rivals. This ensures he can sire as many children as possible, while the female gains the advantage of a strong father for her offspring. This pattern is perhaps seen most strongly in deer and seals, but it's also seen, for example, in lions and gorillas. 

Monogamy is somewhat less common. Sometimes, it happens only because the species is sufficiently widespread that any given male is unlikely to find more than one receptive female during the breeding season, but it can also occur by choice, typically where raising young is a sufficiently arduous task that the father has to stay around after the birth to help. This is commonly associated with birds, but many mammals also form pair bonds for raising young. These include species of gibbon and small antelope that, in paternity tests, have shown essentially 100% loyalty to their mates. The prairie vole is well-studied in this regard, with the formation of the pair bond through prolonged and repeated mating having been linked to, among other things, the "cuddle hormone" oxytocin.

The Tyranny of the Bruce Effect

Monogamy is a relatively rare phenomenon among mammal species. It tends to occur where it requires considerable investment to raise children, whether that be in terms of supplying food over a long period, or careful protection from external threats. If you can spread that effort between both parents, the child is more likely to survive and the species to be perpetuated.

That's common in birds, perhaps in part because they have to physically fetch and deliver food rather than producing milk with the calories from their own diet, not to mention the effort of incubating eggs. But, in mammals, while it certainly occurs in some species (as do the other two options) the most common pattern is polygyny. 

Leopard Cubs: The Struggle to Survive

It's obviously important to know, for conservation purposes, how populations of animals are changing. Are they increasing, or decreasing, and how rapidly? A clear factor in determining this is working out how many of them are dying, and ideally, what they are dying of. But we also need to know how many are being born, how often this happens, how likely they are to reach adulthood and have children of their own, and other factors more to do with birth than with death.

Probably the best way to really get a grip on births in an animal population is to study it for several generations, seeing what changes and when, and what factors are particularly important for infant survival. For the majority of small mammal species, a generation isn't a very long time, so it's practical to do that - although the sheer number of such species inevitably means that the great majority have never been studied. For larger, longer lived, animals, its a different matter, and the task can be far more difficult.