Hi, I'm Emily from MinuteEarth, and these two swans are in love. Or at least that's what it looks like. But the more we dig into mating in the animal world the less sweet it gets. Coming up: four short stories to prove it, starting with "Do these swans really mate for life?" Monogamy, the practice of mating with a single individual for an extended period of time, isn't that popular in the animal kingdom. Only about 3% of mammals are monogamous, and although 95% of birds pair off, at least for one breeding season, paternity tests have revealed that the avian world is chock full of cheaters. The least loyal bird species might be the superb fairy wren they form lifelong bonds and if you watched a pair of them from morning till night for an entire breeding season you'd think they were perfectly faithful. But that's only because female fairy wrens cheat under cover of darkness! Using radio transmitters to track their movements, researchers discovered that fertile females make daily pre-dawn flights to other territories. These trips only last about 15 minutes, but apparently that's more than long enough. DNA tests show that just 25% of baby superb fairy wrens are their father's biological offspring. Even though modern genetics might be deflating our romantic notions about lovebirds, from a biological standpoint, pairing up with one individual and then copulating with others on the side makes a lot more sense than absolute sexual loyalty. For birds, pairing up is a good strategy because they're young require a ton of care, so males increase their chances of successful reproduction if they stick around and lend a beak. On the other hand, putting all of one's eggs in a single basket is a risky proposition. So it also makes sense for males to try and slip some of their genetic material into a few other nests if they can. Females, of course, can't have more than one nest, but they can sneak around with other guys so their eggs have some variety. Cheating might also help explain the otherwise unexpected physical differences between males and females in apparently monogamous species. We've long had a solid explanation for this kind of male/female dimorphism in explicitly non-monogamous species because if a male plans to mate with many females, he needs to win their affection AND fend off other suitors. Over thousands of generations the traits that help him successfully mate can become more and more pronounced, even if they serve absolutely no other purpose. For example, male gorillas who fight each other for exclusive mating rights with the females in their clan are much larger than female gorillas while male and female gibbons, which are monogamous, are the same size. Which brings us to our favourite primate species, Homo sapiens. There are undeniable physical differences between males and females But it's unclear whether they're pronounced enough to suggest that our ancestors lived in harems, like gorillas, or whether our differences just stem from a monogamous, but adulterous society like the superb fairy wrens. But one thing is clear: among all the species on Earth, monogamy is rare, and monogamy without cheating is rarer. There is however at least one known example of perfect lifelong fidelity, and its name is Diplozoon paradoxum. When two of these young flatworms find each other They literally fuse together to form what looks like a single organism, and this adultery-free union lasts for their entire long and amorous lives which they spend sucking blood from fish gills, a truly romantic attachment. So, blood-sucking flatworms might be as romantic as the animal world really gets. To make matters worse, even when males and females appear to be getting along, from a biological perspective they're actually locked in an eternal battle of the sexes. Check it out. To pass their genes along to the next generation, most animals need a partner. So in one sense mating is cooperative, with each partner contributing half their DNA to the offspring. But mating also involves plenty of competition between the sexes. That's because both males and females are hardwired to try to make babies that can survive and reproduce. But they do this in totally different ways, ways that actually threaten the other sex's genetic legacy. For males, it's all about quantity. Sperm are easy to mass-produce and most males spend little time or energy parenting, so their best mating strategy is to breed in bulk. The more offspring they have, the better the odds that some will thrive. For most females, this strategy doesn't work, because they have a limited supply of eggs and spend lots of time and energy caring for their young. Instead, females want to make sure that they can get the best dad for every one of their offspring. Some choose only to mate with the biggest, flashiest, or best-behaved males. Others mate with multiple males and then choose a winner. Chickens for instance can selectively squirt out sperm from subpar suitors. But, the pickier females are, the tougher it is for the average male to make babies. So males have evolved ways to limit females' discretion. In some species, males take their mates through brute force. While males and other species take the slightly less unsavory approach of obsessively following their mates around to keep other suitors away. Male squirrels try to ensure fidelity by plugging females reproductive tracts with a fluid that hardens after sex, creating a sort of biological chastity belt. And male fruit flies go even further: along with their sperm, they release a chemical cocktail that deactivates other sperm, stimulates ovulation, and kills their partner's future sex drive with an anti-aphrodisiac. In response, females develop counter-counter strategies to regain the upper hand, or wing, or paw. Some female squirrels for instance have figured out how to pluck out their chastity plugs. And in other species like ducks and hyenas, females have evolved maze-like reproductive tracts that require their full cooperation to navigate. Males evolve in response, and the sexual arms race keeps escalating, which explains why some ducks have penises that look like this. In general the more promiscuous a species' males are, the choosier the females, and the more intense the sexual arms race. On the other hand, in species in which partners tend to have longer-term relationships and raise offspring together, like Homo sapiens, reproductive success for one partner also means success for the other. So the two sexes' baby-making strategies line up more closely. But even the possibility of non-monogamy is enough to spark some subtle subterfuge. Although seemingly romantic, a wedding ring is really just a gilded form of guarding our mate. So the next time you see a boy and girl duck together in the park, it's not cute―it's mate guarding. And speaking of ducks, that insane-looking duck penis we just showed you owes its weirdness in part to a quack―I mean a quirk―of bird genetics. The Argentine lake duck's spiraling penis extends more than 16 inches. The seagull, on the other hand, doesn't have a penis at all! For two animals with so much else in common, this is a pretty big spectrum. In fact, birds exhibit the biggest range and relative penis size of any class of vertebrates, and there are a couple of key reasons why. For one, male birds are uniquely set up to pass helpful sexual traits directly to their sons, traits like showy, mate-attracting tail feathers, or genitalia of certain dimensions. Animals generally hand these kinds of traits down through their sex chromosomes. But the sex chromosome male mammals pass on to their sons is pretty tiny and has relatively few genes. Male birds on the other hand have way bigger chromosomes with lots of genes, and therefore have higher chances of passing a mutation, like one for extra sexy tail feathers, directly to their sons, who in turn can attract more mates and potentially pass a mutation for extra, extra sexy tail feathers to their sons, and so on down the line. In short, male birds comparatively huge sex chromosomes are the reason much more exaggerated male traits have evolved in birds than in mammals, like the lake duck's reeeeally long penis. Male ducks often mate with unwilling females by force, and it happens that the males with the longest and twistiest genitals are the most successful, which means the longest and twistiest genital genes keep getting passed down from father to son. On the other hand in bird species where less well-endowed males do better, maybe because females prefer them or because big genitals can make flight tough, males can end up with little nubs or even no penises at all. One other reason the bird penis spectrum is so big is that it's really easy to make a bird penis shrink. A single mutation on the bird's large sex chromosome triggers production of a protein that basically erases the developing embryo's penis. All it then takes to make the entire species penis-less is for this mutation to get passed down the line from male to male. But given how the chromosomal arrangement of birds allows more male traits to change more easily, even penis-lessness isn't permanent. The chachalaca, a wild cousin of the chicken, went from having a penis to not having one, to growing one again, I wish I could say that we're done with animal penises for the day, but thanks to the female hyena, that's only "pseud" of true. First though, let's talk about the gentler sex. Which, in hyenas, is the males. This is Scarface, one of the hyenas I studied in Kenya, and like all male spotted hyenas, his life sucks. Babyhood is actually the pinnacle of a male hyena's life. Itty-bitty cubs of both sexes ranked just below their moms, who make up the entire top of the clan's social hierarchy. So cubs have decent access to food, sleeping spots, and everything else, but male cubs' glory days are short. Females generally don't like mating with males from the clan they grew up in. It's probably nature's way of avoiding inbreeding. So, once males turn 2 or so they leave their clan and strike out on a lonely, dangerous search for a clan with better mating prospects. Lots of other mammalian males do the same thing, but for them, the transition to a new group is an opportunity to challenge high ranking individuals and increase their social standing. Not so for male hyenas. They endure aggressive hazing by the new clan before even being allowed to join and once a male does gain acceptance, he becomes the very lowest-ranking hyena in the pecking―er―bone cracking order. The tastiest food rarely makes its way down to him, and there's no way to move up the social hierarchy besides simply waiting, either for new males to join the clan below him, or for males ahead of him to die. No adult male will ever top a single female, resulting in a female-dominated hierarchy. This is all... weird. Unlike most mammals, male hyenas are subordinate to females, which is probably because, unlike most mammals, female hyenas are bigger and meaner than males, which is probably thanks to their need to stand up for their offspring in this kind of crazy feeding frenzy. And, unlike pretty much all mammals, physically fit or socially capable individuals can't make their way to the top of hyena society to do most of the decision and baby-making. They're the only social mammals we know of with such a rigid hierarchy, and we don't yet know why. We do know the outcome, though. Male hyenas like Scarface end up plagued with injuries from harassment and hazing, endure prematurely worn-down teeth from eating too many bones, and on average only live half as long as females do. The best they can hope for is that, at the end of the day, they'll have left behind a few cubs, which is not the best for the ones that turn out to be males. So, female hyenas have some of the perks that nature usually gives to males. But also, some of the parts. And in this case, some of the perky parts! Take a look. Before we start, I want to let you know that this video covers the ins and outs of hyena genitalia. So if that's not something you're keen to learn about, you can check out some of our other videos. But, from the comments on our recent hyena video, we learned that a lot of people are really curious about what exactly is going on with hyena genitalia, and they're not alone. Everyone from Aristotle to Hemingway to a host of modern scientists has been interested in why exactly female spotted hyenas seem to have penises. Females urinate through these seven-inch long phalluses and they're fully erectile. But since they don't deliver sperm, they aren't actually penises. They're elongated clitorises. Spotted hyenas are the only hyena species to sport these so-called "pseudo-penises," and while a few other female mammals have male-like genitalia, the spotted hyena's is the most male-like by far, complete with a pseudo-scrotum. All the female reproductive parts are there, but the entrance is so unwieldy that females have to mate and give birth through what's essentially a penis. Mating via pseudo-penis is about as awkward as you might imagine. In order to make sex possible, the female actually has to retract her pseudo-penis so male hyenas can't force females to have sex. And, speaking of which, we got a lot of comments about female hyenas forcing sex on males, but there just isn't any evidence that this ever happens. Then, there's giving birth, which involves forcing a four pound cub through an inch-wide, 23-inch-long birth canal, which is not easy. For first-time moms, somewhere around 60% of cubs get stuck in this gauntlet and suffocate before they're even born, and a dead cub stuck in a mom's pseudo-penis can be fatal for her, too. A mom's pseudo-penis actually has to rip for her to give birth successfully, which leaves behind a stretchy patch that does make birth easier the next time. In our earlier video we talked about why it sucks to be a male hyena, but there is also a lot that sucks about being a female hyena. So why DO they have such incredibly unwieldy genitalia? Is fending off male suitors so critical that pseudo-penises evolved as protection? Doubtful. Females are dominant enough to keep suitors in check without any help. Are female hyenas trying to confuse others into thinking they're males? Probably not. Even experienced humans can tell what's pseudo and what's not. And hyenas have lots of other ways of distinguishing females from males. So far we just don't have a convincing explanation for why nature has also given female hyenas with a shaft. And that's why I'm glad I'm not a female hyena. Thanks for watching!
