Sex
Everything begins with a difference in size. A reproducing organism produces gametes of one of two shapes, and that single distinction is what the word sex describes. By convention, the organism that makes smaller gametes, the spermatozoa, is called male. The one that makes larger gametes, the ova or egg cells, is called female. An organism that makes both is a hermaphrodite. From this quiet asymmetry, Richard Dawkins has argued, all the differences between the sexes can be interpreted as stemming. How does a small motile cell and a large still one explain a peacock's tail, a clownfish that changes sex, or a platypus with ten sex chromosomes? How did this divide arise in the first place, and why has it spread across animals, plants, and fungi alike? The answers run deep into the cell, back more than a billion years, and out to creatures that determine their sex by the warmth of the sand around them.
A zygote forms when a male and female gamete fuse, and it carries half its genetic material from the mother and half from the father. This blending is exclusive to eukaryotes, where genetic traits are encoded in the DNA of chromosomes. The eukaryote cell holds paired homologous chromosomes, one set from each parent, a double-chromosome stage called diploid. To make gametes, a diploid organism runs meiosis, producing haploid sex cells that each carry a single set of chromosomes. Meiosis includes genetic recombination through chromosomal crossover, where regions of DNA are swapped between matched pairs to build new combinations of the parents' genes. The size difference between gametes is greatest in oogamy, where a small motile gamete combines with a much larger non-motile one. Not all organisms work this way. In the green alga Ulva the gametes are externally similar, a condition called isogamy, while anisogamy describes gametes that differ in size and other aspects. The most basic role of meiosis appears to be conserving the integrity of the genome passed to progeny, a quiet act of preservation hidden inside every act of reproduction.
A spermatozoon is an extremely reduced cell, stripped of many components and carrying a single long flagellum to propel it toward an egg. Produced in vertebrates within the testes, it is specialized for motility and for the fusion called fertilization. The egg cell is its opposite. Produced within the ovaries, it is large and immobile, packed with the nutrients and cellular components a developing embryo needs. Animals are usually mobile and seek out a partner of the opposite sex. Those that live in water can use external fertilization, releasing eggs and sperm to combine in the surrounding water, while most land animals use internal fertilization to keep the gametes from drying up. In most birds, excretion and reproduction share a single posterior opening called the cloaca. Males and females touch cloaca to transfer sperm, a process called cloacal kissing. Many other land animals rely on intromittent organs, and in humans and other mammals this male organ is the penis, which enters the vagina to achieve insemination. Motility carries a darker possibility too. Some insect species practice traumatic insemination, delivering sperm through a wound in the female's abdominal cavity, a process detrimental to her health.
A pollen grain landing on a stigma cannot walk to its destination, so it grows one. It germinates and sends a pollen tube down through the tissues of the style and into the carpel, delivering male gamete nuclei to fertilize an ovule that becomes a seed. In seed plants the male gametes travel inside pollen with hard protective coats, carried from the anthers to the stigma. The female gametes wait within ovules. Most flowering plants are hermaphroditic, with male and female parts in the same flower or on the same plant. Only about 5% of plant species have individual plants that are one sex or the other. Because seed plants cannot move, they depend on passive transport. Conifers and grasses produce lightweight pollen carried by wind, while some flowering plants make heavier sticky pollen suited to insects or to larger animals like hummingbirds and bats, drawn in by rewards of nectar and pollen. Plants have also evolved defenses against fertilizing themselves, including sequential hermaphroditism, molecular recognition systems, and morphological tricks such as heterostyly. Fungi complicate the picture further. Most fungal species are isogamous and lack male and female specialization, instead using mating systems with sexes that cannot be accurately described as male, female, or hermaphrodite. Baker's yeast carries mating types that decide compatibility, and yeasts sharing the same type will not fuse, only joining with a partner that carries another.
The Y chromosome carries factors that trigger male development, which makes XY sex determination mostly a matter of presence or absence. In humans and most mammals, XX individuals are typically female and XY typically male, and it is the male gamete that decides the sex of the offspring. The system bends in revealing ways. Individuals with XXY or XYY are males, while those with a single X or XXX are females. The platypus, a monotreme, abandons the simple pair entirely, carrying ten sex chromosomes. Birds invert the logic with the ZW system, where the W chromosome carries factors for female development and the default path is male. Here ZZ individuals are male, ZW are female, and it is the female gamete that determines the offspring's sex. Most butterflies and moths share this arrangement. The XO system drops a chromosome instead of swapping one. In field crickets, an insect with a single X develops as male and one with two develops as female. In the nematode Caenorhabditis elegans, most worms are self-fertilizing hermaphrodites with an XX karyotype, but rare inheritance errors produce XO individuals that are fertile males. Honey bees and ants take yet another route through haplodiploidy, where diploid individuals are generally female and haploid males develop from unfertilized eggs, producing highly biased sex ratios.
For many species, no inherited chromosome decides sex at all. Temperature does. In all crocodiles and most turtles, the warmth experienced by embryos during development sets their sex. In some turtles males emerge at lower temperatures than females, while Macroclemys females are produced below 22 degrees Celsius or above 28 degrees Celsius, with males appearing only in the range between. The environment can also reach into a creature's biology after development. In the fern Ceratopteris the default sex is hermaphrodite, but an individual growing in soil that once held hermaphrodites is pushed by the pheromone antheridiogen to develop as male. Some species refuse to settle the question once. Teleost fishes are the only vertebrate lineage where sequential hermaphroditism occurs. In clownfish the smaller fish are male and the largest becomes female, and when a dominant female is absent her partner changes from male to female. Many wrasses do the reverse, starting female and becoming male at a certain size. The plant Arisaema triphyllum changes sex as well, proof that the trait can be a stage in a life rather than a fixed inheritance.
Female southern black widow spiders are typically twice as long as the males, a gap that points to the cost of making eggs. Producing egg cells demands more nutrition than producing sperm, so larger females can produce more eggs, and females are the larger sex in a majority of animals. The pattern reverses where competition rules. In mammals with extreme size dimorphism, such as elephant seals, mating systems tend to be highly polygynous, presumably from selection for success in male competition. Dimorphism can run to extremes, with some anglerfish males living parasitically on the female. Among plants, the moss genus Dicranum and the liverwort genus Sphaerocarpos show females significantly larger than males. Color tells its own story. Male birds often look more colorful and may carry features like the long tail of male peacocks that seem to invite predators. The handicap principle offers an explanation. By surviving with such a burden, a male advertises his genetic fitness to females, a signal that benefits daughters who will not carry the handicap themselves. Behavior diverges as well. In most animal species females invest more in parental care and are choosier about mates, while males tend to compete more for mating, though some coucals reverse the parental pattern with males giving more care.
A 1.2 billion year old fossil of Bangiomorpha pubescens holds the oldest record of male and female reproductive types differentiating, showing that sexes appeared early in eukaryotes. The evolution of sperm and eggs left no other fossil evidence, so much of the story is read from living organisms instead. It is generally accepted that isogamy was ancestral to anisogamy, and that anisogamy evolved several times independently across protists, algae, plants, and animals. Each of those independent events was, in effect, the origin of male and the origin of female at once, and the first step toward sexual dimorphism. Studies on green algae have supplied genetic evidence linking sexes to mating types. The original form of sex was external fertilization, with internal fertilization evolving later and coming to dominate vertebrates after they moved onto land. The numbers reveal how unevenly this inheritance settled. About 95% of animal species are gonochoric with separate males and females, and roughly 99% of vertebrates follow suit, leaving the hermaphroditic remainder almost entirely among fishes. Among flowering plants only about 5% are dioecious, an arrangement reached through as many as 5000 independent origins, while about 65% of gymnosperm species are dioecious even as most conifers stay monoecious.
Common questions
What determines whether an organism is male or female?
Sex is defined by the size of the gametes an organism produces. Organisms that produce smaller gametes called spermatozoa are male, and those that produce larger gametes called ova or egg cells are female, while an organism producing both is a hermaphrodite.
What is the XY sex-determination system in humans and mammals?
In the XY system, the Y chromosome carries factors that trigger male development, so sex depends mostly on the presence or absence of the Y chromosome. XX individuals are typically female and XY typically male, and the male gamete determines the offspring's sex.
How does temperature-dependent sex determination work in reptiles?
In all crocodiles and most turtles, the temperature experienced by embryos during development determines their sex. In Macroclemys, females are produced below 22 degrees Celsius or above 28 degrees Celsius, and males are produced at temperatures in between.
Which animals can change sex during their lifetime?
Teleost fishes are the only vertebrate lineage where sequential hermaphroditism occurs. In clownfish the largest fish becomes female and a partner changes from male to female when a dominant female is absent, while many wrasses start female and become male at a certain size.
How do plants reproduce sexually without being able to move?
Seed plants depend on passive pollen transport, with conifers and grasses using lightweight wind-carried pollen and some flowering plants using heavier sticky pollen carried by insects, hummingbirds, or bats. When a pollen grain lands on a stigma, it grows a pollen tube down the style to deliver male gamete nuclei to an ovule.
When did the difference between male and female sexes first evolve?
A 1.2 billion year old fossil of Bangiomorpha pubescens provides the oldest record of male and female reproductive types differentiating, showing sexes evolved early in eukaryotes. Isogamy was ancestral to anisogamy, which evolved independently several times across protists, algae, plants, and animals.