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Today I wanna talk to you about monogamy: what it is, why some animals do it, and how common it is in the wild. One of the animals I'm going to be talking about in this video is humans, but listen to me very carefully. Are you listening? I am not, I repeat not, giving relationship advice in this video.

(Intro)

Now, you've heard of monogamy, and you probably have a pretty good idea of what it is, but I'm gonna hit you with some definitions just in case.

Monogamy: the breeding strategy in which two animals mate only with each other during a given period of time.

Sometimes that period is a single breeding season, sometimes it's for an animal's entire adult life. And also scattered throughout the animal kingdom we see examples of social monogamy - that's what happens when a pair of animals live together, has sex together, and cooperates in nailing down the basic necessities of life.

There are not a lot of socially monogamous animals. 
There are some insects, a handful of fish, a whole bunch of bird species, a few mammals, and even a parasitic flatworm.

The diversity of species that practice social monogamy leads us to believe that instead of having some kind of common, socially monogamous ancestor, they all sort of arrived at the strategy each individually. I should also mention the obvious here, uh, monogamy is not the only game in town.

Polygamy: which is when one or the other or both mates is exclusive with a set of additional partners.

When it's the male that's doing the extracurricular partnering, it's called polygyny, which is like 'many wives,' and when it's female, it's called 'polyandry,' or many men. And when it's both male and female partners it's called polygynandry.

And let's not forget good, old-fashioned promiscuity, which is basically a mating free-for-all, wherein one individual of either sex can pretty much mate with any member of the opposite sex it can talk into the sack.

But back to monogamy. You'll notice that before I was referring to social monogamy. This is a delicate way of saying that this pair, by all outward appearances, shares resources and works together and only has sex with each other. This isn't, necessarily, the same thing as sexual monogamy, which means that these two, seriously, are only having sex with each other, ever, seriously.
Sexual monogamy is actually pretty rare, because even among socially monogamous animals, there's a lot of what's called extra-pair copulations going on.

This is a fairly recent discovery actually, where we've been using DNA analysis to catch these extra-pair copulations red handed. 'Cause apparently, if you're gonna wanna do some extra-pair copulation, you're gonna wanna be a little bit sneaky about it.

Let's look at some examples.

Birds are a good place to start, because monogamy makes a lot of sense if you are a bird. In fact, over 90% of bird species are socially monogamous, at least within the breeding season, and many go beyond one breeding season. This is something that only 3% of mammals can claim. The reason birds like monogamy is because they lay eggs, and eggs require a lot of sitting on. And when these eggs hatch, the baby birds need to be fed almost constantly, and they're taught to fly, and also have to be protected from predators. Being bird parents is frickin' complicated.

So, for a lot of birds, being a so-called 'bonded pair' makes a lot of sense, making it possible for both parents to ensure that their genes get passed on in a healthy, happy offspring without, y'know, starving to death or having some kind of stress-induced aneurysm.

For a long, long time scientists assumed that these socially monogamous bird species were also sexually monogamous.

Take, for example, the dunnock. A cute, little brown sparrow that lives in English hedgerows. A Victorian naturalist, the reverend F.O. Morris was so impressed by these birds' seeming devotion to each other - "their humble behavior and their drab and sober dress" - that he often recommended his parishioners emulate their snooze-festival of an existence. 

It wasn't until the 1990's, through DNA analysis that scientists realized that the dunnock has some of the naughtiest sex in England.

It turns out that the dunnocks pair up just like the reverend observed. However, the female dunnock then turns 'round, encourages other males to invade her male's territory. Once the other male gets to her, she takes him behind a shrub, does a little tail-spinny grindy dance, and he can't handle it, so male #2 does the deed. Then male #2 goes off to find her a nice, juicy worm, feeling quite pleased with himself.

In the meantime, the alpha male, or male #1, has gotten the feeling that something's not quite right on the home-front, so he goes over to his lady and pecks at her private parts until she ejects the sperm packet that male #2 left. And then alpha male must assert his dominance and so he mates with her again. And this can happen all over and over again, about 100 times per day- I don't think that the reverend would have approved.

So why would Mrs. Dunnock cuckold alpha male like this?  Well, it's hard to say why animals do anything, but one reason is that she now has two males bringing her juicy worms, and even potentially two males to help her raise her children, though one of them has to be kind of sneaky about it.
And, in fact, if you do paternity tests on all the little baby dunnocks in a dunnock nest - turns out, often several daddies involved in one nest. 

So, social monogamy, even among the birds, turns out to be a lot less monogamous and a lot more social than anyone ever thought. Even swans, until recently considered the paragon of avian monogamy because they mate for life - upon closer inspection get divorced a lot more often than scientist ever gave them credit for. And though they might not be as promiscuous as Mrs. Dunnock, they do their fair share of mating outside their bonds. And so the distinction between social monogamy and sexual monogamy becomes quite clear: it's not cheating if you don't get caught by a scientist. 

Of course, there are a few real deal sexually monogamous species out there - there are the beavers, and there are the California mice, and the Malagasy giant jumping rat - they remain faithful to each other throughout their entire lives, either because it helps them fight off predators, or establish a giant territory, or raise scads and scads of giant jumping rat offspring. It's also worth noting, all of these animals are extremely territorial, so there's not a lot of opportunities for promiscuity, but still.

And now to answer the question that you've been asking yourself this entire episode: what about humans? Aren't we monogamous?

Uhh, not strictly speaking. I mean, if a zoologist came down from another planet, looked at us, they would say, "Human kind - mildly polygamous, y'know, kind of socially monogamous, but occasionally engaging in extra-pair copulations."

Monogamy is possible, even potentially very advantageous for humans, but our genes say that for a long time, polygyny, or the practice of men mating with multiple, like, a set group of women, was very much the norm for a very long time in our history.

Studies are showing that polygyny may have become popular with early humans because women actually, on average, pass along more of their genes to their offspring. And even though monogamy is more popular with humans than it once was, a lot of modern societies still engage in some forms of polygamy. And it's true that very few American men these days take multiple wives, but over the course of their lifetimes, men tend to father more children with multiple women than women with multiple men. You can call this whatever you like, but anthropologists call it effective polygamy. 

Another reason that our ancient ancestors might have been less interested in monogamy is that monogamy is hard to pull off. It takes an animal with an enormous brain - a big ol' cerebral cortex to figure out how to do it. Because monogamy requires us to not only know what we want, but also to figure out what our mate wants, and provide for that want, that is not our own, on a day to day basis.

It also takes a huge about of brain power, not only to figure out if a mate is fertile, but also if it will be a good mate in the long-term - if it will be able to not only produce offspring, but take care of them. It takes a lot of calculating, so monogamy's difficult for stupider animals. But we are not stupider! And it could be, in fact, that monogamy has been one of the driving forces for making human-kind smarter - for making us more human.

Some scientists have gone as far as to speculate that our super-sophisticated social intelligence - our ability to navigate tricky social situations and maintain our status within complicated social groups - was made possible by the ultra-challenging task of how to make monogamy work for us.

So while monogamy might not be normal, it might not be the strictly natural state for humans, it can be argued that most of the great things that we do as humans aren't natural: playing the violin, sending people to space, making really awesome Youtube videos - none of those things are natural. But they're awesome. We do these and thousands of other things every day that go against our instincts because our giant cerebral cortexes are telling us that it's worth it. 

Thank you for watching this SciShow Infusion. Like good little scientists, our citations are in the description if you would like to read more about the sex practices of various animals, including humans. If you have questions or suggestions for us, we'll be on Facebook, Twitter, and of course the YouTube comments below, goodbye.