Why there are two sexes

A dragon with no mate

In 2006 a Komodo dragon named Flora, at Chester Zoo in England, laid a clutch of eggs. Flora had never been housed with a male. At London Zoo another female, Sungai, had laid eggs two and a half years after her last contact with a male, and her keepers first suspected stored sperm. DNA fingerprinting settled it. The young had no father at all. Phillip Watts and his colleagues reported both cases in Nature in December 2006: parthenogenesis, from the Greek for “virgin birth.” Five of Flora’s eggs had hatched by late January 2007, with others still to come, and every young dragon from both zoos was male.

The reason is the way sex is determined in these lizards. In birds and in many reptiles, including the Komodo dragon, females carry two different sex chromosomes, Z and W, and males carry two Zs. A parthenogenetic egg ends up with two copies of one of its mother’s chromosomes: ZZ, a male, or WW, which does not survive. Sungai’s story shows that this can be a backup rather than a way of life. She later mated with a male, and one of her eggs hatched.

Komodo dragons are not alone. Several whiptail lizards of the American Southwest are all-female species; a bonnethead shark gave birth without a mate at a zoo in Omaha in 2001; wild copperhead and cottonmouth snakes have been found doing it; dandelions make seeds without fertilization, and aphids raise generation after generation of daughters. A single-celled amoeba simply divides in two. The one group where it never happens naturally is the mammals, because genes that are switched on or off according to which parent they came from block it. For many organisms, then, reproduction without sex is an option. For a lineage, it looks efficient: every individual can have young. So why does nearly everything with a complex body bother with sex?

The cost of sex

The British biologist John Maynard Smith put the puzzle in a famous way in 1978. Imagine a population in which every mother raises two surviving young, one daughter and one son. Now a mutant mother appears who reproduces without sex and raises two daughters. A sexual mother has one daughter to carry on the line, because her son cannot bear young; the mutant mother has two. The asexual lineage doubles each generation; the sexual one holds steady. Within a few dozen generations the mutants should take over. This is the “twofold cost of sex”: the cost of making males, who in many species contribute genes and little else. The activity below lets you set how much better a sexual mother’s young would have to do to pay for it.

Sex has other costs too: finding and winning a mate takes time, courtship can attract predators, mating can spread disease, and sex breaks up gene combinations that were working. Yet sex is almost universal among animals and plants. That is a real puzzle, and the surprising answer is that biologists do not agree on a single solution.

Why sex persists anyway

Several ideas each have evidence behind them. The first is about parasites. Germs and parasites evolve to attack whatever is common, so a host that is genetically the same as its neighbors is an easy target. Sex shuffles genes into new combinations, and a rare combination may escape the parasites. The idea is called the Red Queen hypothesis, after the character in Through the Looking-Glass who says that it takes all the running you can do to stay in the same place. The best evidence comes from the New Zealand mud snail, which comes in both sexual and cloning forms. Curt Lively found in 1987 that sexual snails were more common in places where a parasitic worm was common. In a later seven-to-ten-year study by Jukka Jokela and colleagues, the commonest clones were replaced by rarer ones while the sexual snails held their own. In a laboratory test, a roundworm forced to coevolve with a disease-causing bacterium reproduced by outcrossing more, while worms that could only fertilize themselves died out.

A second idea is about harmful mutations. In 1964 Hermann Muller argued that an asexual lineage can only ever add new harmful mutations and cannot shuffle them away, like a ratchet that clicks in one direction. Sex lets a lineage make some offspring with fewer mutations than their parents. Studies of mitochondrial DNA in asexual water fleas and snails show the faster build-up of harmful changes that the idea predicts. A third idea is about repair: the cell machinery of meiosis, the special cell division that makes eggs and sperm, includes repair of damaged DNA, and some biologists think this is why sex arose. A fourth is about changing worlds: a sexual species can generate new combinations quickly when conditions change.

These ideas are not rivals so much as parts of a puzzle, and the weights differ between species. Textbooks now say that there is no complete consensus. The popular “gene repair shop” explanation of sex, in which sex exists to fix copying errors, is one version of the third idea. It is a respectable hypothesis, and not the settled answer.

What makes a male

The word “male” has a precise meaning in biology, and it has nothing to do with chromosomes or behavior. The first sexual organisms did not have males and females at all. Like some algae today, they released sex cells of one size, and cells of different mating types could fuse. Then, probably more than once, the cells split into two sizes. Geoff Parker, Robin Baker, and Vic Smith showed in 1972 why this is likely. A fertilized cell needs a supply of food to get started, which favors cells that carry a big supply. But if a few cells carry the supply, other cells do better by being tiny and numerous, so they can find the big ones. Selection pulls the two kinds apart. The result is anisogamy, “unequal gametes.” A female is the sex that makes the large sex cells, the eggs. A male is the sex that makes the small ones, the sperm.

This explains several things the popular account gets wrong. Life did not begin with females, with males added later as a repair kit; before gametes differed in size, “female” and “male” had no meaning. There is a real hypothesis in the neighborhood of the claim that males “give up” part of the cell. Cosmides and Tooby suggested in 1981, and Laurence Hurst and William Hamilton in 1992, that when two cells fuse it is safer for only one parent’s mitochondria, the cell’s power plants, to be passed on, since two sets could compete. In mammals the father’s mitochondria are tagged and destroyed after fertilization. But no biologist thinks men surrender their immune system, and researchers still dispute whether this idea alone explains why two kinds of gamete arose. What the size difference does explain is a great deal of what follows in this course: eggs are costly to make and sperm are cheap, which is where the story of mate choice begins.

Many ways to be a sex

In people, the sexes are decided by a gene. In 1990 a team found a gene on the Y chromosome, called SRY, that sets the embryo toward developing testes; in 1991 Peter Koopman and colleagues showed that female mice carrying the mouse version of the gene developed as males. The Y is small, with a few dozen protein-coding genes, about two dozen different ones, against roughly 850 on the X. The X is a sex chromosome too, and is not an “ordinary” one. A son gets his X from his mother and his Y from his father; a daughter gets an X from each parent. It is sometimes said that daughters resemble their fathers more because they receive his X. But every child gets half of its other chromosomes from each parent, and the X is only about 5 percent of the genome, so the difference is small.

Other animals use other systems. Birds and Komodo dragons use Z and W. Many reptiles use temperature. In alligators, eggs kept at 30°C or below hatch as females and eggs at 34°C or above as males. In sea turtles, warm nests produce females: in a 2018 study of green turtles on the northern Great Barrier Reef, about 99 percent of the young turtles were female. Leopard geckos show a mixed pattern, with mostly females at both the cool and the very warm ends of the range.

Some single-celled organisms and fungi have mating types, not sexes, with far more than two. The pond protozoan Tetrahymena has seven. The split-gill mushroom Schizophyllum commune has well over ten thousand. Among humans, nearly everyone develops clearly as female or male, and a small number are born with natural variations in how sex develops. One example is complete androgen insensitivity, in which a person with XY chromosomes has a female body because the cells cannot respond to the hormones that drive male development. Biology shows a clear pattern here, with a few rare exceptions.

Fish that change sex

In some fish a single animal may be both sexes in the course of its life, and the trigger is social. In the bluehead wrasse, a Caribbean reef fish, the large, brightly colored males are former females. In a 1991 study, Robert Warner and Stephen Swearer removed the big males from reef patches and found that the largest females took over almost one for one: their behavior changed within minutes, their color within a day, and sperm could appear in about a week.

Clownfish do it the other way round, from male to female. A clownfish group lives in one sea anemone and is a queue. The largest fish is the breeding female, the second largest is her breeding mate, and smaller fish wait in line, not yet breeding. If the female is lost, the breeding male becomes the female and the largest waiting fish becomes the breeding male. In Pixar’s Finding Nemo the mother clownfish is eaten at the start, and the father, Marlin, goes on as a father. In real clownfish biology, he would be expected to change into the female. In anthias, a reef fish that lives in harems of one male and many females, removing the male leads the largest female to change into a male soon afterward.

These fish do not look for a replacement. The roles are tied to rank, and the ranks move up. It is sometimes said that the male goes off to find a smaller fish and turns into the female himself. In fact the male changes, and the next fish in the group takes his old place.

A need that isn’t thirst

Many people would call sex a physical need, like food. This course’s own starting point argues the opposite, and it has a point. Someone who goes without water dies in a matter of days; no one has died from an unmet sex drive. Psychologists, however, have long listed sex among the basic needs. Abraham Maslow’s 1943 paper on motivation discusses it with hunger and thirst, though the pyramid that textbooks draw from it is not in his paper and he never drew one. In 2010 Douglas Kenrick and colleagues proposed a renovated pyramid in which the top levels are not self-actualization but the needs to attract a mate, to keep a mate, and to care for children. In their picture, sex belongs to the family of social needs, the needs we can only meet with other people.

That puts it alongside a need that can be measured in lives. In 1995 Roy Baumeister and Mark Leary argued that people have a basic “need to belong,” and in 2010 Julianne Holt-Lunstad and colleagues pooled 148 studies of 308,849 people and found that those with stronger social relationships were about 50 percent more likely to be alive at the end of a follow-up of seven and a half years on average. The data are observational, so the link may not be simply causal. They do not tell us how much sex anyone should want, and they say nothing at all about sex in particular. They suggest that wanting closeness is no luxury.