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.
Polyandry, in which females mate with multiple partners, but the males do not, is the rarest pattern in mammals, although it does exist in a few species, such as naked mole rats. And then, there is promiscuity, in which everybody has multiple, usually short-lived, sexual partners. It's pretty obvious what the male gets out of this, but perhaps less so what the benefit is to the female.
One argument put forward has been that it helps maintain genetic diversity in species with low population numbers, but it's far too common for this to be the only, or likely even the major, explanation. Instead, a much more common argument is that it helps prevent infanticide by males. The theory is that males in many species will kill young that they do not believe are theirs... but if the mother has mated with the male at some point, they can't be sure which of her children they have fathered.
Unpleasant as it may seem from a human perspective, infanticide is rife among wild animals. A 2018 survey identified 91 mammal taxa known to practice infanticide (albeit this is counting some subspecies separately) and the true number is surely higher. Sure, even allowing for the fact that we won't have checked with most of them that's out of over 6,000 species, so it's hardly universal. Nor is it evenly spread across the entire group - 40 of those 91 are carnivores, presumably because they're already adapted to killing things.
Among the big cats, we know that tigers, leopards, puma/cougar/mountain lions, and actual lions all practice infanticide. This, despite the fact that lionesses at least have the ability to gang up on an intruding male which the other, solitary, species do not. Quite how often it happens is hard to say, at least in the wild - cats are difficult to follow that closely at the best of times, and mothers put some effort into hiding their youngest cubs where wildlife photographers can't see them. Perhaps they do this well enough to hide from the males, since that's presumably part of the point, but if they didn't, we'd hardly know.
But what about jaguars (Panthera onca)? Are they sexually promiscuous, and, if so, does it relate to a defence against infanticide by males?
Well, there's a reason that you don't see jaguars as often in wildlife documentaries as you see lions, leopards, or cheetahs. Although they can live elsewhere, jaguars prefer to live in dense, lowland jungles that are much harder to film in than open savannah. That also makes them difficult for scientists to study in their natural habitat. We have detailed studies of the reproductive behaviour of all of the big cats mentioned above, plus cheetahs (which seem to be not at all infanticidal). But, while zoos can provide us with some of the basics, the first large-scale such study on wild jaguars was published only a few months ago.
The study was conducted on a cattle ranch and ecological refuge covering an area of 530 km² (205 square miles) in the Pantanal of southern Brazil. The Pantanal is an area of seasonally flooded grassland that is heavily vegetated and includes some areas of dense woodland; it's not quite the dense jungle that characterises most jaguar habitat, but it is rich in plant and animal life, has plenty of the water that jaguars love, and its rather more open than the depths of the Amazon, making them easier to spot.
For ten years, the researchers watched the jaguars living there or used automated camera traps to snap them as they went about undisturbed, using the photos to identify 180 individual animals. As they grew up and had young of their own, it became possible to put together the largest database of jaguar demographics so far created.
So what did we learn? Well, first the basics. Jaguar cubs in the area were mostly born at either the beginning or the end of the wet season, not quite what we see in other parts of the world. The fact that they're living in a seasonally flooded wetland may have a lot to do with this; you want the cubs to be old enough to move about before their den becomes submerged. Two-thirds of the births appear to be singletons, whereas twins are much more common in other big cats... but the mothers were good at hiding them for the first three months of life, so there may be some dying young that we just don't see.
Other findings were closer to what we'd expect both from smaller studies elsewhere and from large-scale studies of tigers, leopards, and so on. Mothers gave birth once every 22 months on average, becoming pregnant again shortly before or just after their previous cub(s) left home. Females seem sexually precocious compared with other large cats, the youngest mother was 25 months old, which, given the 3-month pregnancy period, means that she must have mated pretty much immediately after she left home. For that matter, some females were seen mating at just 15 months, when they were still living with their mother and effectively teenagers. Males waited longer, perhaps bulking up their weight, and not breeding before 36 months.
The oldest jaguar lived for 16 years - more than previously thought. Taken together with the precocious sexual activity of the females, and allowing for the occasional twins and triplets, this means that a typical female jaguar could reasonably expect to have eight cubs during her lifetime, which isn't bad for a large carnivore.
And, yes, it turns out that female jaguars are sexually promiscuous. The average female had six sexual partners during the study, with one having as many as eleven. What seems to be happening here is that when a female with cubs spots a male nearby who may be a potential threat, she enters a state of pseudo-oestrus and lures him away from her children to mate with him elsewhere even though, unbeknownst to him, she's still incapable of becoming pregnant. After spending a few days with him to reassure him that she really isn't going off with anyone else, she returns to her cubs.
That's a fairly high-risk strategy, involving leaving her helpless cubs alone and undefended for a while. But, if the choice is between that and the male killing them outright, it may be worth it. It's not as if she could fight him off, given that he'll be far more muscular than she is. So, even if there was no direct evidence of infanticide in this study, fear of it is a good explanation for the mother's behaviour - and there has been limited evidence from earlier studies that this does happen to jaguars from time to time.
One wouldn't exactly call female jaguars emancipated from the human perspective; they're essentially living in fear that somebody might come along and kill their children and are compelled to distract hostile intruders by offering them sexual favours. But they also mate with multiple males when they can get pregnant, which probably helps the gene pool. Unlike their African and Asian counterparts, jaguars are not considered a threatened species but there are still places where their numbers are dwindling due to the steady advance of farmland and widespread deforestation in the Amazon. If promiscuity keeps future generations alive, it may be the best bet that they've got.
[Photo by "Wolves201", from Wikimedia Commons.]
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