Women After All: Sex, Evolution, and the End of Male Supremacy (10 page)

Darwin went on to write a book about all this, expanding the “few words” of
The Origin
. There, he described the process throughout
the animal world, and he proved its importance. But it would take another century, until 1972, before more neutral language was used to set out a general theory. This was in an article called “Parental Investment and Sexual Selection,” by Robert Trivers, who put the case this way:

Where one sex invests considerably more than the other, members of the latter will compete among themselves to mate with members of the former. Where investment is equal, sexual selection should operate similarly on the two sexes. The pattern of relative parental investment in species today seems strongly influenced by the early evolutionary differentiation into mobile sex cells fertilizing immobile ones, and sexual selection acts to mold the pattern of relative parental investment.

In fairness, this summarized a long paper in which the language was not so balanced, but in this passage at least we have a nongendered account. We start with the idea that one sex invests more in offspring and then reason that
that
sex is a scarce resource for which the second sex will have to compete. The latter will evolve either adaptations for victory in battle—size, muscles, claws, simple horns, and teeth—or things like songs, tail feathers, fancy antlers, manes, dances, or the ability to create bowers or generate light shows, mild electric shocks, or aromas that the first, more valued sex finds attractive.

If you are thinking that this more neutral language is just a way to cover up the same old male chauvinist claims, consider the cassowary, a gorgeous flightless Australian bird that is man-sized and dangerous, with knifelike claws and a kick that has maimed and killed people. That is, the
female
is man-sized; the male is about two-thirds as large. Both sexes have casques (the name derives from the French word for “helmet”), hornlike pointed crests rising up from the big bird’s brow like the bronze crest on a gladiator’s headgear.
The bird’s head is pale blue, the neck a lustrous darker blue, grading into a glimmering orange nape from which red wattles dangle. The female is more spectacular, and she attracts her diminutive males with a series of low-pitched grunts. When one shows up, she may display dramatically by splashing in a puddle, raising fountains of spray. They mate for up to two months, and the female lays her eggs—perhaps not all of them his—and moves on in search of other pliable males. He will brood the eggs and chicks all by himself, and for nine months, under his watchful eye, they will learn to make a living in the forest.

Writing in the September 2013
National Geographic,
biologist and writer Olivia Judson describes a male named Dad:

His chicks, which are about four weeks old and almost knee-high, make funny whistling-peeping sounds as they run about. He mostly stays silent—but from time to time clacks his bill, making a large banging noise. He burps too. And occasionally he booms. That is, he tucks his head down low, inflates his neck, and makes a series of low booming noises. As he does this, his feathers puff up. When he sits down, the chicks cuddle up to him, often snuggling into his feathers. . . . The chick also picks ticks off its father’s neck and eats them. Yum.

Meanwhile the errant mom is off to greener sexual pastures. As one (human) mother of five put it to Judson, “I’m coming back as a female cassowary.”

Cassowaries are the biggest birds with devoted dads and hit-and-run moms but not the only ones. Jacanas, also known as Jesus birds, trot on lily pads and so seem to walk on water, but that’s not their main claim to fame. What they are most known for is that, as in the cassowary, the sex that invests more in the young is the male. Before the female lays her eggs on her chosen lily pad, she collars a male who is bound to take care of them, and that’s the end of her interest. The male? Seduced and abandoned, while the female, twice his size,
goes off to mate with another male—as many as four in an hour, thirty in a season. Wham, bam, thank you . . .
sir
?

Only there’s not even that much of a thank-you in this water-lily world, where the females are too busy fighting off rivals and mating here, there, and everywhere to bother with the niceties. Their adversaries? Other females, who are trying to steal their males. By the end of the season they’ve a harem of cute little dads, each slavishly brooding a clutch of eggs, then doting for most of a year over a clump of chicks. But all of the father’s effort won’t necessarily stop the odd menacing female prowler, who can (when the boy-harem’s Amazon queen has her back turned) trot in, cow a pint-sized pa, peck open his eggs, and dispatch his hard-won babes.

And it isn’t just the brooding. He may have risked his life to draw a snake or alligator away from them, instinctively flopping around as if he had a broken wing, so the not-too-bright predator would forget to eat his little ones. But now he sees a strange female—twice his size, true, but probably less dangerous to him than the reptiles were—and does little or nothing to stop her from doing in the chicks. Why?

Probably because of what happens next. She displays her (to him) intensely attractive rump, and soon enough he mates with her, fertilizing
her
eggs. She has won the prize away from the first female. The prize?
The sex that invests more in the young.
Although she has the upper wing, she is just, as ornithologist Stephen Emlen put it, an egg-making machine. The future dad instinctively starts pulling together a new nest on the lily pad, and in a week or so the once-menacing intruder returns, lays some eggs, and goes off to her next conquest. In due course, the pond is dotted with single-father families.

Eight species of jacanas circle the globe in tropical zones, and despite some fossils, we don’t understand their origins. It’s likely, though, that they evolved from pair-bonding birds, of which there are some
eight thousand
species. They, too, tend to have devoted dads but
not single ones, since parents share the burden of raising the young more or less equally. And it is a burden.

Watch the little perching birds in your eaves or garden. If they have a nest of chicks, they fly off nonstop in tandem—they practically need an air traffic controller at the nest—to bring back food. When they alight on the nest’s edge, they face a bevy of gaping mouths, often colored and marked in ways that, along with incessant peeping, turn mom and dad into doting slaves. If you’re a parent yourself, this may sound familiar. If not, take a moment to thank those who slaved away to keep you in earthworms and away from dangerous cats. Humans, we’ll see, are largely pair-bonding too, a fate that carries with it a suite of other adaptations under the logic of sexual selection.

In most pair-bonding birds, the sexes are about the same size, and usually neither has evolved extra weapons or great beauty compared to the partner. Each sex may have to fight off rivals, but not to the extent shown by jacanas, where males invest the precious resource that females have to fight for. In pair bonds, parenting is share and share alike, so once they’ve found each other, the lovebirds can concentrate on the kids. As with the male jacana, this is a full-time job, but in these species it’s a full-time job for two. Emperor penguins lay one egg at a time in their frozen Antarctic landscape, and the look-alike male and female take turns warming the egg and then the chick while the other goes out to swim and hunt in the icy sea, returning faithfully to regurgitate fish for the young.

This egalitarian kind of baby care, so common in birds, has the crucial result that male and female have roughly equal reproductive success. Together they mate, brood the eggs, raise the kids—repeat next season. Sure, there’ll be stolen copulations, sneaking around on both sides, and even occasional desertions—folklore aside, no pair-bonding species, not even geese, is perfectly loyal. But why not act the female jacana, trotting around corralling male after male, dumping eggs on them, and withholding child support? For
her
there’s a huge advantage: she can field dozens of young in a season, while the male yields far fewer of his own offspring.

Only there’s a catch. He can pretty much count on that lower number year in and year out. She has a chance at the breeding jackpot but an even greater chance of being squeezed out by a larger, tougher, even vicious female. Some jacanas can have harems of docile, willing males and leave their eggs in good care all over the pond top, but—barring a rare surplus of males—this has to mean other females have none. In fact, it’s this greater, all-or-none
variation
in reproduction that made the females big and rough in the first place. Much more than for their meek little males, it’s a zero-sum game. The result for females is more competition, faster evolution, and divergence from the size, shape, color, and behavior of the male.

In a great many other species, including large numbers of fish and countless insects, females are larger than males. We saw how the use and abuse of males stakes out an extreme in the black widow, where the female has her way with males and wraps them up for a midnight snack, and the praying mantis, whose females, in a way, pray for males and then prey on them in mid-copulation, not missing a beat. We also encountered pregnant male seahorses and asexual lizards that have completely gotten over the whole male thing. Compared to them, the jacana way is mild. But in birds and mammals, even that much female dominance is rare. There are those eight thousand species of pair-bonding birds, as well as quite a few mammals, on the egalitarian plan. Yet often in birds and generally in mammals, the jacana arrangement is turned upside down.

A peacock struts his stuff slowly, arcing great turquoise plumes that dwarf his glistening blue body, raising a patch of iridescent gold coins, then sweeping a delicate green mesh up into a lustrous fan dotted by gorgeous, staring green-and-gold eyes, in which the bird stands onstage alone, radiating a gaudy spray with feathers like the sun’s rays, only in color. Another turn or two later, he enfolds
himself in drapery, collapsing his sumptuous feathers down into a sleek, pied multicolored tail that seems to loll along behind him endlessly. A drab female demurely watching this spectacle wouldn’t seem to stand a chance, but she does stand her ground, because he’s not the only male trying to wow and woo her. In fact, she has her pick of them, prancing, splaying, waggling, and dragging their stunning quills.

Like a breeder with a monocle at a dog show,
she
gets to choose which one’s genes are worthy of posterity. She sizes him up and matches him with her own calm and valued self—thus giving him a gene channel, a shot at the zero-sum. Over the eons, peacocks got grander and gaudier because they caught peahens’ eyes and the hens said,
You
or, at least,
Oh, as well him as another
. The rest is evolutionary history, the grandest display of male fashion ever to sidle and glide across an earthen runway, as if he’s thinking,
Don’t pass me by, girl, it doesn’t get better than this
, while she’s thinking,
Boy, get over yourself.
She may play hard to get or wait for an even spiffier male. Actually, in time she may choose several mates. But maybe in the end this one’s not so bad, and after all, she
is
in the mood.

As for just how good a mood she is in, we know, thanks to beautifully designed experiments by Marion Petrie and her colleagues over many years, that peahens lay more eggs for males with larger trains (possibly a product of their hormonal state) and that their offspring grow and survive better after release into the wild
even when the hens are randomly assigned to different males
and the offspring are reared under matching conditions. The suggestion is that a large train is an honest advertisement of overall quality, then imparted to offspring, but it could also be that the most impressive males (who don’t do any of the child care themselves) inspire hens to invest more in the young.

The peahen’s preference has turned out to be more complex than it seemed at first, but an elegant 2013 study by Roslyn Dakin and Robert Montgomerie proved decisively that one component of
the display matters to her a lot: “Our study shows that the blue-green eyespot color overwhelmingly influences peacock mating success.” It’s when those spots catch the sun at a certain angle that the hens are won over. The authors, who titled their paper “Eye for an Eyespot,” are continuing to examine other aspects of this stunning romantic dance—and for the most part confirming Darwin’s original surmise.

We’ve seen something like this process in jacanas, except in peacocks it’s the males who compete for the precious parental capacity owned by females. Peacock males contribute nothing to the care of the young. They put their energy into building beautiful tails and fanning them out in females’ view. According to zoologist Amotz Zahavi’s “handicap principle,” such useless or even detrimental appendages—peacock feathers puzzled Darwin mightily, because they seemed such a lure for predators—actually signal quality, because only a superior male could afford the cost and risk. Many studies have now shown that these add-ons do indeed signal male quality by other standards—measures of health, such as parasite load. In this case, the ornament and the antics that go with it are dubbed “honest advertisements” of excellence.

But according to mathematical biologist Ronald Fisher’s classic theory of “runaway selection” (also known as the “sexy sons” hypothesis), it may not really matter, because once the preference for prettier tails gets started (even by chance), females will keep choosing them because their sons will be better off having them, and this cycle will strengthen generation by generation. This should be most true in rich environments, where the cost of frills can be more easily borne and the gain for chosen males great. Whether the adornments evolved by handicap or runaway selection, they work; in most experiments, peahens prefer the cocks with more elaborate tails.

Similar effects have been shown in many species, including ones that to us look much less fine than peacocks. Although coloration also matters, tail length in male barn swallows can be manipulated
to influence mating success; under otherwise natural conditions, females mated to males who’d first had their tails lengthened are better at brooding eggs and steal fewer copulations with other males. This is why long tails have persisted despite their possible disadvantage in foraging and survival.