|Panamanian night monkeys (Aotus lemurinus)|
Both of these situations lead to an inevitable conflict between males. If each male mates with several females, but females stay loyal to a single male (even if only for the season), a lot of males won't get a look in. There are strategies for some smaller males to get round this problem, but, in general, fighting is your best bet. And that means that male mammals are often larger than females, and with larger teeth, horns, antlers, or whatever else it may be that they do their fighting with.
But there are alternatives to polygyny. If females mate with lots of males, then male competition needs to work in a different way, whether by getting in first, or by producing more, and higher quality, sperm than your rivals. And then, of course, there's monogamy.
The advantage of monogamy is that nobody needs to do any fighting, and you have two parents available to look after the young. From the female's perspective, this is a great deal, but it's rather less so from the male's. A male can father lots of children by different females, if he puts his mind to it, so staying with just one means that, in the long run, he has less kids. (A female, by contrast, can only be pregnant with one set of children at a time). That's why monogamy is relatively rare among mammals.
When it does happen, it's usually in situations where care by both parents is particularly beneficial. If the mother needs help from a partner to raise her young, then the male has to stay with her if he wants any of his kids to survive at all. The pressure of incubating eggs and of rearing greedy chicks probably explains, for instance, why monogamy is more common in birds than it is in mammals. Still, there are monogamous mammals, and, because of the side benefit that you don't need to fight anyone, we would expect that males and females in such species will tend to be physically similar.
But it's not necessarily so simple, not least because females may well be competing for good quality mates as well. Are monogamous relationships necessarily all that we'd expect them to be, for instance? A recently published paper by Eduardo Fernandez-Duque and Maren Huck examined exactly what monogamy means for one particular species of monkey.
The monkey in question is Azara's night monkey (Aotus azarai), also known as "Azara's owl monkey". Formerly considered as part of the capuchin family, night monkeys are now generally regarded as distinctive enough to be given their own family. For one thing, as their name suggests, they are, unlike all other monkeys, nocturnal. This, of course, is the reason for the large, round eyes that give them such an appealing look. More importantly for this study, this particular night monkey is a firmly monogamous species, living in small family groups consisting of a mated pair and their children.
As we'd expect, males and females look much the same, and both parents share in looking after their young - if anything, the father carries the young about more often than the mother does. This is basically what we'd expect of a monogamous species, but how exactly do these pair bonds work? Do the monkeys "marry" for life, for instance? After all, while many birds are monogamous, sometimes those partnerships end in divorce - the pairs break up, often because they're not having enough luck making babies. Might night monkeys be the same?
The researchers followed eighteen families of night monkeys over a sixteen year period, checking up on them regularly to see how the partnerships endured. For nearly half of the families, the partners managed to stay together, at least until one partner died of natural causes. But that does mean that for over half, there was some sort of change going on. Azara's night monkeys, it seems, are more serial monogamists than life-long partners.
Furthermore, unlike many of the bird species in comparable situations, there was no evidence of "divorce". That is, couples weren't simply breaking up of their own volition. Indeed, they had little reason to do so. In birds, we often find that such divorces occur because the females are laying smaller clutches, and want a more compatible partner. If this were happening with the night monkeys, then break ups would be happening because the female was failing to get pregnant. But this was far from the case; on average, break-ups occurred ten months after the female had last given birth. Since they normally give birth once a year, that would often be too early for the male to have demonstrated any inability to father children.
So what is going on? In a word: violence. When the monkeys become old enough to leave home, they live on their own for a while, and then they break into an established family, violently ousting the same-sex parent in order to secure access to their partner. These are monkeys that, in other circumstances, aren't particularly aggressive. When rival families meet, there may well be a lot of screaming and posturing, but there's little physical combat. Yet these fights to steal mates are so vicious that, on at least one occasion, the ousted partner died of their injuries shortly after.
Those that survive such fights fare little better. The uninjured partner made no effort to find their mate, seemingly happy to stay with the intruder that had just mauled their spouse - who they had typically been with for many years. Only very rarely did an ousted individual ever manage to find a new partner; almost all died abandoned and alone. Clearly, this must be a risky strategy for the intruder, since he has no guarantee of victory; one can only imagine that the perils of living alone, without a family, are even greater.
But it's important to note that we're not just talking about males beating up husbands and stealing their wives. It's just as likely to be lonely females attacking "married" monkeys to steal their husbands. We had supposed that males and females of this species look so similar because they don't need to fight, but it's the other way about - they're similar because they all need to fight. It's just as important for a female to be powerful and have sharp teeth as it is for a male, because they're both doing exactly the same thing to secure mates.
It's not just serial monogamy, but a violent serial monogamy, one that doesn't fit so easily with our human preconceptions. Males are pitched against males, and females against females in battles that may only occur once every few years, but are vitally important when they do. Why do they do this? You'd think they would just find other solitary individuals (there must be several) and form their own partnership, but apparently that isn't the norm.
It's not that they have more young that way - as happens, for example, in oystercatchers - because they don't. Night monkeys that stay with the same partner are more or less guaranteed one child every year, but, according to this study, new partners take time to get used to one another sexually, commonly skipping a birth in their first year together and only breeding in their second. At least in the medium term, then, pairs would do better off staying together (although there's obviously an advantage in it for the intruder, if the alternative is no children at all).
It's a rare enough event that only occasionally does a monkey have more than two partners in the course of their life. Maybe there's some advantage in genetic diversity of the offspring, or something else that we're not seeing. Either way, it shows that monogamy isn't always the love-and-kisses that it's made out to be.
At least, not if you're a monkey.
[Picture by Birdphotos.com]