by Riskin, Dan
Forced copulation and traumatic insemination are instances where males hold the upper hand in the conflict between the sexes, but there are plenty of other examples where females have made things just as rough for males. Where females have a little more control over matings, they can choose some feature in males to judge them on, and then only mate with the males who meet their standards. Unfortunately for the males, females tend to choose a male feature that is most likely to get him killed.
For example, a female túngara frog will not mate with a male unless he woos her in the night with a loud, musical call.12 As a result, if you go out into the rainforests of Panama at night, you can hear males of the species calling as loudly and deeply as they can, each trying to attract a female. The problem is, other animals can hear that call too. One such eavesdropper is one of my favorite animals from that part of the world, a bat called Trachops or, more commonly, the frog-eating bat. That bat finds its food—yes, frogs—by listening for their calls.XV
Calling out into the night like that is basically a male túngara frog’s way of shouting “Eat me!” at the top of his lungs, but female túngara frogs have left him no other choice. Her logic is this: If a male can still survive despite the constant risk of bat attacks, he’s probably got great DNA. Any males that don’t take that risk aren’t worth mating with, and as for those males who get eaten by the bats? Well, they didn’t have very good DNA, now, did they?
This system of evaluation by the females can only work if they choose a task for males that really is dangerous. If they picked something easy, every male would do it equally well, so the females would end up mating with crappy males just as often as with good males. Females who chose an easy contest for males would therefore get no benefit from holding the contest in the first place. As a result, across species, the kinds of hoops males have to jump through to have sex are insane. The most studied examples come from birds.
Bright-colored birds doing intricate dances while singing loudly are beautiful to us, but when you think about it, those males have to use a ton of energy to keep themselves beautiful and perform those displays properly.XVI Besides, those displays have them basically doing everything they can do to get noticed by potential predators. Also, the bright, beautiful red feathers many males display are attractive to females because those yellow, orange, and red pigments are very metabolically expensive to produce, thus making it harder for the male who produced them to survive.XVII
Female spiders who eat their mates are another obvious example of females getting the upper hand on males, but male spiders often pay a high price before they get anywhere near a potential mate. For example, female golden silk spiders sit in their webs, merrily dining on insects, waiting for males to find them. While a female has only a 0.3 percent chance of being eaten by a predator on any given day, a male roaming the rainforests of Panama in search of her has about an 8 percent chance of dying each day. That’s more than twenty-six times higher. Once he finds her, they mate, and then she tries to eat him anyway.XVIII
Some females have done away with males altogether, becoming all-female species that use cloning to reproduce instead of sex. This has happened in more than eighty different species, including New Mexico whiptail lizards, several fishes, and a few salamanders. This is a fascinating phenomenon to start with, but it’s particularly interesting in some of the female-only salamanders, in which the females have absolutely no genetic use for males but still need sperm to stimulate the development of their eggs. In other words, they’re looking for a male to have sex with, but the male will get no genetic benefit whatsoever from the mating.13
This is such a rotten deal for the males of the species that . . . well . . . there aren’t any males. That’s what happens when you can’t pass on your genes. But that hasn’t slowed the females down. To get the sperm they need, females just have sex with males of other salamander species. That stimulates the development of eggs, so the females can clone themselves successfully. The babies aren’t hybrids of the two species, they’re just female clones of their mom, and the salamander she had sex with gets no benefit from the mating at all.XIX
Some animals out there possess both male and female parts, but that doesn’t make sex any easier on them. One striking example is the flatworm Pseudobiceros. Its name means “false two horns,” and you’ll see why in a moment. There are several different species of Pseudobiceros, all of which occur in the Great Barrier Reef of Australia. They’re only a few centimeters long, but they’re quite beautiful, like tiny undulating oval-shaped magic carpets, some with bright frills of color along their sides. All Pseudobiceros have two short bumps on their bellies. Those bumps are what give these flatworms their name. As you know from their Latin name, those aren’t really horns. Yup. They’re penises.
Each flatworm is a hermaphrodite—it has the penises but also the complete reproductive system of a female. That means that when two Pseudobiceros flatworms rendezvous to mate, they have to decide who will be the father and who will be the mother. The mother will be pregnant, which requires a lot of energy. The father only has to ejaculate. Since they’re both selfish, neither of them wants to get pregnant, so how do they settle it? Simple.
It’s called penis fencing.14
The two animals wrestle, parrying with their penises until one stabs the other with one of its two penises. The winner injects sperm into the other’s body. The loser gets pregnant. Problem solved.
If you look across all these lusty examples, it becomes clear that sex has a considerable cost for males and females. With males forcing females to copulate, females forcing males to put their lives on the line, and hermaphrodites fighting over who gets to be the daddy, it’s a wonder these animals bother at all. So why haven’t animals abandoned sex altogether?
There was a time on Earth before sex, when everything reproduced by cloning. Back then, there were no males or females; there weren’t even really parents and children. There were just organisms. Everyone had a single cell and lived in the water. Once in a while a cell would split in half, and the two cells that resulted would float apart. You couldn’t really call one of them the parent and one of them the child. They were just clones. For a selfish DNA strand, cloning worked perfectly well.
Then sex evolved, about 1 billion years ago, in a small group of single-celled organisms.15 It was such a successful strategy that the descendants of those creatures thrived. Today, every single plant, fungus, and animal on Earth is a descendant of those first sexual creatures, and sex remains an integral part of their continued success.
Sex is everywhere. There are a few animals that use cloning to supplement their sex lives, like jellyfish, starfish, and some worms, but only a tiny minority of animals have left sex behind altogether, to use cloning exclusively. Since asexual animals are so rare, one of the big questions biologists love to ask is, Why? What’s so great about sex? After all, sex gives you offspring with only 50 percent of your own DNA, instead of the 100 percent you get from cloning. So why is sex so popular?
So far as we can tell, the answer is twofold. First, sex helps an animal’s offspring deal with an unpredictable world better than clones would. If you have multiple kids from sex, they’ll be different from one another, the same way you’re different from your sisters and brothers. Let’s say, down the road, there’s a flood. Well, that might suddenly give an advantage to animals that are a little stronger, better at climbing, or better able to hold their breath. But if instead of a flood, there’s some new predator to deal with, the advantage might instead go to animals that run faster or are better camouflaged or maybe are just stinkier. There’s no way to know what the future holds, so having a diverse set of kids is the best way for a parent to end up with at least one kid who has the right stuff. It’s like spreading your bets across the roulette table. With cloning, all your kids would be identical. Sex shuffles the genetic deck in a way that cloning does not, and that’s useful in a changing and unpredictable world.
The second great advanta
ge of sex over cloning is that it gives the parent an opportunity to improve its DNA’s future odds of survival by mixing it with the DNA of someone else. Until sex evolved, cloning animals had no other option than to simply pass on whatever DNA they had been born with. With sex, an animal can look around for someone that looks like they’re doing well in the world, and then, by having sex with them (as opposed to some other animal), bring those favorable genes into the family. That’s why sex was a total game changer for life on Earth. Cloning animals can ignore one another, but with sex, everyone has to constantly check one another out. Your DNA doesn’t just want you to mate, it wants you to mate with the best possible partner (or partners) you can find. Whether or not your own babies will be able to compete will largely depend on the quality of your mate’s DNA. It’s not an exaggeration to say that your sex life is just as important to your DNA as your survival.
Ultimately, this urge to find great DNA forms the basis of the whole dating game. For humans, romantic love is an integral part of that, but romance isn’t really “natural” in that it’s not part of most animals’ sex lives. Raising a human baby to the point of self-sufficiency takes more than a decade, and having both parents work as a team to care for that child is a great strategy to ensure its success. As human meat robots, we have urges written into our DNA to help motivate us to work as partners with the people we have sex with. I would argue that the long childhood stage of humans is the only reason we have romantic love at all. Humans are still fundamentally selfish, for all the same reasons that other animals are selfish. But because human kids take so long to raise, a person can still be a good partner without giving up on the selfishness that goes along with being an animal.
I think it’s fun to look at my relationship with Sam’s mother in this context. When I first met Shelby, I had an urge to talk to her. When I talked to her, I wanted to spend more time with her, and then spending time with her made me want to do the things that my DNA had in mind in the first place. Meat robot, meat robot, meat robot.
Shelby impresses the hell out of me: she’s the kind of person who does things most people only ever forward emails about. She helped with the environmental cleanup after Hurricane Katrina in Biloxi, Mississippi. During grad school she performed with a professional dance company in Providence, Rhode Island, and before that she produced a dance show for the Minneapolis Fringe festival. She was even in one of YouTube’s most viral videos ever—the one from 2009 where the whole wedding party dances down the aisle, called “JK Wedding Entrance Dance.”XX
Shelby’s also a scientist like me, and while we were dating, she did the fieldwork for her PhD in the Amazon rainforest of Brazil.XXI She had no phone or Internet connection at her field site, so she’d make her way to the nearest town every other weekend, get a hotel room, and use Skype to fill me in. I was so jealous: she was there to collect soil and water samples but ended up seeing jaguars, pumas, anacondas, beetles, ants, giant anteaters, sloths, tapirs, rattlesnakes, and even some bats. That said, though, she also experienced nature’s rougher side while she was there, and I’m not quite so jealous of those experiences. She was attacked by swarms of bees, which got tangled in her hair as she ran away, and then kept stinging her while she tried to get them out. That happened three times. On another morning, she couldn’t quite seem to get her boot on all the way because something at the bottom was pressing against her toes. She removed the boot, slammed it against a wall, and a giant tarantula fell out of it. She once drank from the wrong tap at her field station and got stomach parasites called Giardia that made her so sick that, even though she nuked them with antibiotics, her digestive system didn’t get back to normal until about a year later.XXII
Shelby and I had lived together for only a few years before we decided we wanted to have a baby together—that is to say, before our DNA molecules coerced us to mate so that they could replicate. My body cut out twenty-three double-helix strings of DNA and placed them into a sperm cell. Shelby’s body, through an identical process, had already placed twenty-three of her own DNA strings into an egg. Then we had a special kind of sleep (wink, wink), and in the instant those two cells met, they both vanished. In their place was left a unique new being with (23 + 23 =) forty-six strings.
Suddenly, Sam was.
At first, Sam didn’t have a smile or eyes or even limbs. He was just a single cell, separated from the outside world by nothing more than a thin, oily, spherical layer—almost like a tiny soap bubble. You’d be able to see that bubble with your naked eye, but only just barely, and the DNA strands inside would be invisibly small. The sequence written into those strands, though, instantly made Sam absolutely unique: the sequence of DNA written across his forty-six strings has never existed before in the entire history of life. And it wasn’t just a jumble of letters either. It had meaning. That DNA sequence encoded the building instructions for the meat robot that now exists as Sam.
Shelby and I didn’t really create a life: our own cells were already alive when they merged. Sam is the next link in an unbroken chain of constant life that stretches backward through time—through his parents, through his grandparents, and beyond. Because each of Sam’s ancestors had a different sequence of DNA, the bodies built by their DNA molecules varied too. Sam’s body is similar to mine, and mine is similar to my father’s, but those small changes begin to accumulate as the story stretches back. Sam’s story goes back through me, through Ice Age hunter-gatherers, through apes walking on all fours, through squirrel-sized tree-climbing primates, through prehistoric reptiles, through alligator-sized amphibians, through strange, lobe-finned fishes in the ancient ocean, through wormlike protofishes in the ocean before that, and so on. In fact, by the time you go all the way back to the beginning of that chain, to the beginning of sex itself, there’s not much more to look at than a simple bag of DNA molecules floating in the primordial ocean—a single cell that looked an awful lot like Sam did on the first day of his life. All that’s really changed over those billion years are the meat robots that the DNA molecules have become able to build. The DNA itself has stayed pretty much the same that whole time.
But now he’s a child. When I hold him, Sam feels like a miracle. I love Sam—love him—and that’s helped me understand how DNA molecules coerce their meat robots to do their bidding. I’m experiencing the most powerful emotions of my life. If I were a bowerbird, I’d have decorated my nest with blue objects. If I were a bighorn sheep, I’d have smashed my head into anyone I could. But I’m a human, and I instinctively care more about Sammy than anything else in the whole world.
As for Shelby and me, we feel like a team. We look after one another, we support one another, and we’re happy together. Despite the horrible things animals do to one another in the name of sex, we’ve found a way to leave that conflict behind. You can call that natural, but it’s not really. It’s human.
Overall, sex has made me a pretty happy guy. Even if the testosterone that comes with that joy has cut my life shorter by a decade or two, I would say it’s been worth it.
* * *
I. The male builds his “bower” out of twigs. It’s a structure with two parallel walls, about four inches apart, with everything cleared off the ground around it. The male decorates that ground with blue objects, and the number of times females will mate with him depends largely on how many blue objects he can accumulate (Borgia 1985). One of the best places to find blue objects, of course, is the bower of another male bowerbird, so theft is a big part of competition among males (Wojcieszek et al. 2007).
II. Head-ramming behavior is essential to rams that want to mate (Martin et al. 2013), but collisions tend to be quite bad for mammalian brains. Rams avoid concussions, though, by having heads that are built in a way that mechanically dissipates energy when they ram (Maity and Tekalur 2011).
III. I’m talking about cortisol. It’s released in high doses by male antechinuses (Naylor et al. 2008), and by people with post-traumatic stress disorder, or PTSD (Steudte et al. 2011).
<
br /> IV. A lot of people assume male black widows are always killed by the female after sex, but males usually escape. In fact, male black widows can sometimes inseminate up to three females (Breene and Sweet 1985). The observation that females always kill males probably came from early experiments in closed containers that were so small the male couldn’t escape, which isn’t really how things would work in the wild (Vetter and Isbister 2008).
V. An example of a spider species that uses nuptial gifts is the nursery web spider (Stålhandske 2001). Sometimes he’ll play dead while he’s holding his gift, and just when the female comes to take it, he’ll spring into action and mate with her (Hansen et al. 2008).
VI. Maternal mortality per 100,000 births in a few Western countries: USA 21; Canada 12; Australia 7; New Zealand 15; UK 12. And in a few sub-Saharan countries: Chad 1,100; Somalia 1,000; Sierra Leone 890
You can look up other countries using an interactive map of World Health Organization (2012) data here: http://www.who.int/gho/maternal_health/en/index.html.
VII. Thousands of babies have died too. Worldwide every year 3.3 million babies are stillborn and more than 4 million die less than a month after being born. Those babies die for many reasons, but it’s clear that lack of access to medicine is a major factor because those deaths are heavily biased toward less developed countries (World Health Organization 2005).
VIII. Around 287,000 women died in childbirth in 2010 (World Health Organization 2012).
IX. The order Carnivora is split into two main groups: the “doglike” canimorphs and the “catlike” felimorphs. Canimorphs include dogs, bears, skunks, and sea lions. Felimorphs include cats, hyenas, fossas, and mongooses (Ignarsson et al. 2010).
X. For a complete rundown on the mating habits of spotted hyenas, including an adorable picture of a male cuddling with a female after sex, see Szykman et al. (2007).
