Sex Determination in Animals: Genes or Temperature

Sex determination is the process that decides whether an animal develops as a male or a female. In most mammals, the decision is made by the chromosomes an embryo inherits: an embryo with an X and a Y chromosome becomes male, and an embryo with two X chromosomes becomes female. But that familiar rule is only one of several systems animals use. In birds, the female carries the two different sex chromosomes. In bees and ants, sex depends on whether the egg was fertilised. In many turtles and crocodilians, sex is decided by the temperature of the nest. And in some fish, an adult can change sex entirely, from female to male or from male to female, in response to the animals around it.

The variety is not a curiosity. It shows that sex is not one mechanism but a decision that evolution has solved many times, with different machinery, and that the different solutions often share the same genes. The same gene that makes a male bird, DMRT1, also helps make males in turtles, and the same hormone pathway that temperature controls in a turtle egg is the one that the Y chromosome controls in a mammal. Understanding sex determination therefore means understanding a small set of systems, genetic, environmental and mixed, and the single logic that connects them: a signal, received early in development, that switches the embryo onto the male or the female path. This article explains that logic, system by system, in plain language and at full depth, from the Mendel's laws of inheritance to the warming beaches where the sex of a whole generation of turtles is now being decided.

Sex determination is also a subject where the words matter. Determination is the decision itself, the moment when the embryo is committed to one sex. Differentiation is what follows, the building of testes or ovaries, ducts and external structures, which is mostly the work of hormones. The two are often confused, and the confusion causes real mistakes, such as the claim that temperature "changes" a turtle's genes, when what temperature changes is which genes are switched on. The sections below keep the two apart, follow each system from its signal to its result, and end with the question that now matters most: what happens to these systems as the world warms.

Sex Determination at a Glance

Every animal must solve the same problem: an embryo with the same genome must develop into either a male or a female, and the decision must be made reliably and early. The solution always has the same shape. A primary signal, genetic or environmental, is read by the embryo. The signal switches on a regulatory gene, and that gene starts a cascade that builds a testis or an ovary. Once the gonad exists, it produces hormones, and the hormones build the rest of the male or female body. The systems differ in what the primary signal is, but they converge on the same few genes and the same hormones, which is why a temperature can override a chromosome, and why a fish can change sex as an adult.

The genetic systems use chromosomes. In the XY system, one sex carries two different sex chromosomes and the other carries two of the same kind. In the ZW system, the pattern is reversed, and it is the female that carries the two different chromosomes. In the XO system, one sex has a single sex chromosome and no partner at all. In haplodiploidy, there are no sex chromosomes in the usual sense: sex depends on whether the egg was fertilised, so males have one set of chromosomes and females have two. The environmental systems use the world outside the egg. In temperature-dependent sex determination, the temperature during a sensitive window of incubation decides the sex. In social sex determination, the animals around an individual decide it. And in mixed systems, genes set the default and the environment can override it.

The systems are not evenly spread across the animal kingdom, and the pattern is itself interesting. Mammals are almost all XY, and birds are almost all ZW, but reptiles use every system there is: XY, ZW and temperature-dependent sex determination all occur in reptiles, sometimes within a single family. Fish use the widest range of all, including genetic systems, temperature effects and adult sex change. Amphibians are mostly XY or ZW, but temperature can shift their sex ratios too. Invertebrates add systems of their own, including haplodiploidy in the social insects and the strange arrangement of the marine worm Bonellia, where a larva that settles on a female becomes a male and a larva that settles elsewhere becomes a female. The table at the end of this article sets the systems side by side.

Determination Versus Differentiation

Determination is the decision, and differentiation is the construction that follows it. In a mammal, determination happens when the SRY gene on the Y chromosome switches on in the cells of the developing gonad, and those cells commit to becoming a testis. Differentiation is everything that happens next: the testis forms, it secretes anti-Müllerian hormone and testosterone, the anti-Müllerian hormone makes the female ducts regress, the testosterone builds the male ducts and the external genitalia, and the result is a male body. Without SRY, the gonad follows the ovarian pathway instead, driven by genes such as WNT4 and RSPO1, and the female ducts develop while the male ducts regress. The decision is made in the gonad, and the hormones then carry it to the rest of the body.

The distinction matters because the two stages can be separated in time and in cause. An embryo can be determined as male and then fail to differentiate as male, if the hormones are missing or the tissues cannot respond to them, and the result is an animal whose chromosomes say one thing and whose body says another. In humans, for example, an XY embryo whose cells cannot respond to testosterone develops as a female externally, a condition called androgen insensitivity, even though the testes are present. Conversely, differentiation can be redirected after determination: in the bearded dragon, a ZZ embryo determined as male by its chromosomes can develop ovaries instead, because high incubation temperature switches the gonad onto the female pathway. Determination sets the direction, but differentiation is where the body is actually built, and either stage can be altered.

The primary sex is the sex of the gonad, and the secondary sex is everything else: the ducts, the external genitalia, the body shape and the behaviour. In mammals the gonad decides the rest, because it makes the hormones. In birds, the gonad matters too, but the rest of the body also reads its own chromosomes directly: a bird's cells know whether they are ZZ or ZW, and that knowledge shapes the body independently of the hormones, which is why a gynandromorph bird can be half male and half female, with male plumage on one side and female plumage on the other. The lesson is that determination is not a single switch but a series of decisions, made first in the gonad and then, in some animals, in the tissues themselves.

The XY System and the SRY Gene

In the XY system, females have two X chromosomes and males have one X and one Y, so it is the father who determines the sex of the offspring: a sperm carrying an X makes a daughter, and a sperm carrying a Y makes a son. The system is used by almost all mammals, including humans, and by many insects and some fish and plants. The Y chromosome is small and carries few genes, but one of them, SRY, is the switch. SRY, the sex-determining region of the Y chromosome, encodes a transcription factor, a protein that switches other genes on, and when it is expressed in the developing gonad, it starts the testis pathway. The discovery of SRY in 1990, by Robin Lovell-Badge and his colleagues, answered a question that had stood for a century: what, on the Y chromosome, makes a male?

SRY works by switching on SOX9, another transcription factor, and SOX9 then drives the cells of the gonad to become Sertoli cells, the supporting cells of the testis. Once SOX9 is on, it keeps itself on, so the decision is locked in, and the testis forms. The testis then secretes two hormones: anti-Müllerian hormone, which makes the female ducts, the Müllerian ducts, regress, and testosterone, which builds the male ducts, the Wolffian ducts, into the epididymis, vas deferens and seminal vesicles, and masculinises the external genitalia. Without SRY, SOX9 stays off, and the gonad follows the ovarian pathway, driven by WNT4, RSPO1 and FOXL2, which actively repress the testis pathway rather than simply letting it fail. Female development is therefore not a default but an active process of its own, and the ovary is built just as deliberately as the testis.

The evidence that SRY is the switch is unusually direct. An XY embryo with a damaged SRY gene develops as a female, a condition called Swyer syndrome, with female external genitalia but no functional ovaries, because the gonads never developed into testes or ovaries. An XX embryo with SRY moved onto an X chromosome, which happens when the X and Y chromosomes swap pieces during sperm production, develops as a male, with testes, even though there is no Y chromosome at all. And in mice, adding the SRY gene alone to an XX embryo is enough to make it develop as a male. The gene is the switch, and the rest of the Y chromosome is largely irrelevant to sex: most of its genes are concerned with sperm production, not with the decision itself.

Not every XY system uses SRY. In the fruit fly Drosophila, sex is decided not by a Y chromosome but by the ratio of X chromosomes to sets of autosomes: an embryo with two X chromosomes and two sets of autosomes, a ratio of one, is female, while an embryo with one X and two sets of autosomes, a ratio of one half, is male. The ratio is read by the gene Sex-lethal, which is switched on only in females, and Sex-lethal then controls the splicing of the gene doublesex, producing a female version of the doublesex protein in females and a male version in males. The Y chromosome of the fly is needed for sperm production but not for the decision. The fly shows that the XY system is not one mechanism but a pattern, male heterogamety, that different animals have built with different genes, and that the mammalian solution, a single switch gene on the Y, is only one way to run it.

The ZW System and DMRT1

In the ZW system, the pattern is reversed: males have two Z chromosomes and females have one Z and one W, so it is the mother who determines the sex of the offspring, because her eggs carry either a Z or a W. The system is used by almost all birds, by many butterflies and moths, and by some reptiles and fish. The Z chromosome is large and carries many genes, while the W is small and gene-poor, the mirror image of the mammalian X and Y. The ZW system is a reminder that there is nothing special about the male being the heterogametic sex: evolution has produced both arrangements, and the choice between them seems to be largely a matter of history, of which chromosome happened to acquire the sex-determining gene first.

In birds, the switch is DMRT1, a gene on the Z chromosome that is needed for testis development. The mechanism is dosage: a ZZ embryo has two copies of DMRT1 and develops as a male, while a ZW embryo has one copy and develops as a female. The evidence is as direct as for SRY: when DMRT1 is experimentally reduced in a ZZ chicken embryo, the gonad develops as an ovary instead of a testis, and the embryo is feminised. DMRT1 is a deeply conserved gene: versions of it are found in animals from worms to humans, and in mammals it is involved in testis development too, but in birds it carries the whole decision. The W chromosome, meanwhile, carries a gene called HINTW in chickens, which may help suppress the male pathway, but the main decision is made by the dose of DMRT1, not by a single dominant gene on the W.

The ZW system produces the same result as the XY system by a different route, and the two systems can be confused in the field, because the animals look the same either way. In butterflies and moths, the ZW system is ancient, and it is the female that is heterogametic, which has consequences for inheritance: a gene on the Z chromosome is passed from a father to all of his daughters and to half of his sons, but a mother passes her Z only to her sons. In some moths, the system has been modified further, and the females carry a single Z with no W at all, the ZO system, which is described below. The ZW system also matters for conservation, because the sex of a bird cannot be told from its chromosomes without a laboratory test, and many conservation breeding programmes for rare birds begin by sexing the birds with a DNA test on a feather.

XO and ZO Systems

In the XO system, females have two X chromosomes and males have a single X and no second sex chromosome at all. The male is therefore written as XO, with the O standing for the missing chromosome. The system is used by grasshoppers, crickets and many other insects, and by the roundworm Caenorhabditis elegans, one of the most studied animals in biology. In C. elegans, an XX animal is a hermaphrodite, producing both eggs and sperm, while an XO animal is a male, and the decision is made by the ratio of X chromosomes to autosomes, read by the gene xol-1, which is switched on in XO animals and off in XX animals. The XO system shows that a second sex chromosome is not necessary: one X is enough to make a male, and two are enough to make a female, provided the embryo can count them.

The ZO system is the mirror image, and it is rare. In some butterflies and moths, females carry a single Z chromosome with no W, and males carry two Zs, so the females are ZO and the males are ZZ. The system seems to have arisen when the W chromosome was lost, and the embryo compensates by counting the Z chromosomes, much as the XO system counts the X. ZO is a reminder that sex chromosomes are not fixed: they gain and lose genes, they fuse and split, and a chromosome that is a sex chromosome in one species can be an ordinary autosome in a close relative. In the platypus, for example, sex is determined by a chain of ten sex chromosomes, five X and five Y, which pair in a ring during sperm production, a system unlike any other mammal and a reminder of how recently, in evolutionary terms, the mammalian XY system was assembled.

The XO and ZO systems are useful for teaching, because they make the counting logic visible. The embryo does not need a special male gene or a special female gene: it needs a way to count, and a threshold. Two copies of a chromosome, or one copy of a gene, is enough to tip the balance, and the rest of development follows. The same counting logic appears in the XY system of the fruit fly and in the ZW system of the bird, where the dose of DMRT1 is the count. The systems look different, but the logic is the same: a small difference in the number of copies of a gene, read early in development, is amplified into the whole difference between a male and a female.

Haplodiploidy: Sex from Fertilisation

In haplodiploidy, sex is decided by whether the egg is fertilised. A fertilised egg, with two sets of chromosomes, develops as a female, and an unfertilised egg, with one set, develops as a male. The system is used by the Hymenoptera, the bees, ants and wasps, and by some beetles, mites and scale insects. A honeybee queen controls the sex of her offspring directly: she stores sperm from her mating flight and releases it, or withholds it, as each egg passes, so that she lays fertilised eggs in worker cells and unfertilised eggs in drone cells. The males of these species, the drones, have no father at all, and they have no sons, only daughters and grandsons, because a male's sperm carries his entire genome, unshuffled by meiosis.

The mechanism in the honeybee is one of the most elegant in genetics. Sex is controlled by a single gene, the complementary sex determiner, or csd. An embryo that is heterozygous at csd, carrying two different versions of the gene, develops as a female. An embryo that is hemizygous, carrying only one copy because it is haploid, develops as a male. An embryo that is homozygous, carrying two identical copies because it is diploid but inbred, also develops as a male, but a diploid male, and the workers of the colony usually eat diploid male larvae soon after they hatch, which is one reason inbreeding is so costly for a honeybee colony. The system means that a queen who mates with a closely related drone risks losing a large part of her brood, and it is one of the forces that drives honeybees to mate with many drones on their mating flights.

Haplodiploidy has a famous consequence for the social insects. Because a female receives half her genes from her mother and all of her father's genes, full sisters in a honeybee colony share three quarters of their genes with each other, while a female shares only half her genes with her own daughters. The geneticist William Hamilton pointed out in 1964 that this makes it more profitable, in genetic terms, for a worker to raise her sisters than to raise her own offspring, and he proposed that haplodiploidy helped drive the evolution of the sterile worker caste. The idea, called kin selection, is still debated: haplodiploidy also occurs in solitary species that never evolved social behaviour, and some highly social animals, such as termites and naked mole-rats, are not haplodiploid at all. But the arithmetic is real, and it shows how a system of sex determination can shape the whole social life of a species. The contrast with ordinary sexual reproduction in animals, where both sexes are diploid, is explained in our comparison of asexual and sexual reproduction.

Temperature-Dependent Sex Determination

In temperature-dependent sex determination, usually shortened to TSD, the sex of an embryo is decided by the temperature of the nest during a sensitive window in the middle of incubation, called the thermosensitive period. There are no sex chromosomes in a TSD species, or if there are, they do not decide the sex. The system is used by most turtles, by all crocodilians, by the tuatara, and by some lizards and fish. A sea turtle nest buried in warm sand produces mostly females, and the same nest in cool sand produces mostly males, and the difference between the two outcomes can be as little as one or two degrees. The temperature does not change the embryo's genes: it changes which genes are switched on, and the embryo's gonad follows the temperature's instruction instead of a chromosome's.

The mechanism is now understood in detail, and it runs through the same machinery as the genetic systems. During the thermosensitive period, temperature regulates the enzyme aromatase, which converts the male hormone testosterone into the female hormone oestradiol. At female-producing temperatures, aromatase activity is high, oestradiol is made, and the gonad becomes an ovary. At male-producing temperatures, aromatase activity is low, testosterone remains testosterone, and the gonad becomes a testis. The link between temperature and aromatase is epigenetic: in the red-eared slider turtle, a protein called Kdm6b removes a chemical mark from the DNA near the gene Dmrt1 at male-producing temperatures, which switches Dmrt1 on, and Dmrt1 then drives the male pathway. At female-producing temperatures, the mark stays, Dmrt1 stays off, and the ovary develops. Temperature, a gene and an epigenetic mark form a single chain, and the chain ends in the same testis or ovary that a chromosome would have made.

TSD comes in three patterns, and they are worth knowing separately, because they are often confused. In pattern Ia, low temperatures make males and high temperatures make females, and this is the pattern of most turtles, including the loggerhead and the green sea turtle. In pattern Ib, the reverse holds, low temperatures make females and high temperatures make males, and the tuatara, the ancient reptile of New Zealand, is the classic example. In pattern II, called FMF, females are produced at both extremes and males in the middle, and the American alligator and the leopard gecko are the best-known examples. Every TSD species has a pivotal temperature, the temperature at which the clutch is half male and half female, and a transitional range of temperatures around it in which both sexes are produced. Outside that range, the clutch is single-sex, and the pivotal temperature for most sea turtles is around 29 degrees Celsius, though it varies between species and between beaches.

TSD pattern	Low temperature	Middle temperature	High temperature	Example animals
Ia (MF)	Males	Mixed	Females	Most turtles: loggerhead, green turtle, hawksbill
Ib (FM)	Females	Mixed	Males	Tuatara, some lizards
II (FMF)	Females	Males	Females	American alligator, leopard gecko, snapping turtle

The existence of three patterns is itself evidence that TSD has evolved more than once, and that the temperature response is tuned to each species' life. A sea turtle that nests on a hot beach and a tuatara that nests in cool New Zealand soil face opposite problems, and their patterns are opposite too. The pattern also matters for conservation: a beach that warms by two degrees does not simply make more females, it can make a whole rookery female, and the effect depends on which pattern the species follows. A warming beach is a problem for a pattern Ia turtle, because it pushes the whole clutch above the pivotal temperature, and it is a different problem for a pattern II alligator, because it pushes the clutch through the male zone and out the other side into females again. The conservation section below returns to this.

Other Environmental Cues

Temperature is the best-known environmental cue, but it is not the only one. In the marine worm Bonellia viridis, a larva that settles on the proboscis of an adult female becomes a male, a tiny animal that lives inside the female and does nothing but make sperm, while a larva that settles on the open seabed becomes a female, a large green worm up to ten centimetres long. The cue is chemical: the female's proboscis releases a substance, bonellin, that masculinises the larva, and the difference between the sexes in Bonellia is one of the largest in the animal kingdom, a male a few millimetres long and a female a hundred times his size. The system is a reminder that sex determination can be social as well as thermal, and that the cue can be another animal entirely.

Social cues also decide sex in fish, and the slipper limpet Crepidula shows how precisely a social cue can be measured. Crepidula lives in stacks, with the animals piled on top of each other on a rock or a shell, and an individual's sex depends on its position in the stack. The animals at the bottom of the stack, the oldest and largest, are female, and the animals at the top are male, and as the stack grows, the males in the middle change into females. The cue is the presence of a female below: a male that has a female beneath him stays male, and a male that does not becomes female. The limpet's sex is therefore a property of its address, not of its genes, and a single animal can be male, then female, then male again as the stack changes around it.

Other cues are subtler. In the Atlantic silverside, a small fish of the American coast, sex depends partly on temperature and partly on the time of year: fish hatched early in the season, in cool water, are mostly female, and fish hatched later, in warm water, are mostly male, which spreads the sexes across the season. In some cichlid fish, the density of the population and the presence of dominant males can shift the sex ratio of the young. In the nematode Caenorhabditis elegans, the sex of an XX animal can be nudged by temperature and by the environment, although the main decision is chromosomal. And in some reptiles, the moisture of the nest and the gases inside it can shift the pivotal temperature, so that a nest that is warm and dry produces a different sex ratio from a nest that is warm and wet. The environment rarely acts through temperature alone, and the full picture is a set of cues, with temperature usually the strongest, acting together on the same gonad.

When Genotype Is Overridden by Temperature

The cleanest division in this article, between genetic systems and environmental systems, breaks down in the animals that have both. The Australian central bearded dragon, Pogona vitticeps, has a ZW system, with ZW females and ZZ males, but it also responds to temperature. At normal incubation temperatures, the chromosomes decide, and ZZ embryos become males. At high temperatures, above about 34 degrees Celsius, ZZ embryos develop as females instead, with ovaries and female behaviour, even though their chromosomes say male. The sex reversal is caused by temperature switching off the male pathway in the gonad, and the reversed females are fully fertile. When a sex-reversed ZZ female is mated with a normal ZZ male, all of the offspring are ZZ, and if the eggs are incubated at high temperature, the daughters are female and the sons are male, so the W chromosome can be lost from the lineage entirely, and a genetic system can become, in a single generation, a temperature-dependent one.

The bearded dragon is the best-documented case, but it is not the only one. The snow skink of Tasmania has an XX and XY system, and the sex of the offspring depends on both the chromosomes and the temperature, with the balance shifting between populations on the coast and in the mountains. In some cichlid fish, temperature can override a genetic system, and the offspring of a single pair can be mostly male or mostly female depending on the water temperature. In the Nile tilapia, high temperatures masculinise genetic females, and the effect is strong enough that fish farmers use it deliberately. In the European sea bass, temperature during the larval stage shifts the sex ratio, and the effect is large enough to matter for aquaculture. The pattern is always the same: the genes set a default, and temperature, acting through the aromatase and Dmrt1 machinery, can push the gonad onto the other path.

The reason temperature can override genes is that the two systems use the same machinery. The genetic systems end in Dmrt1, Sox9 and aromatase, and the temperature system ends in the same three. A chromosome and a temperature are two different ways of pressing the same switch, and when both are present, the stronger one wins. The bearded dragon shows the limit of the genetic system: the chromosomes propose, and the temperature disposes. It also shows why the distinction between genetic and environmental sex determination is a convenience rather than a law of nature. Most species sit somewhere on a spectrum, with pure genetic systems at one end, pure temperature systems at the other, and a large middle ground where the two interact, and the position of a species on that spectrum can change in a generation.

Sequential Hermaphroditism and Sex Change in Fish

Some animals do not settle the question of sex once, in the embryo, but answer it twice, as adults. A sequential hermaphrodite is an animal that is one sex first and the other sex later, and fish are the champions of it. There are two directions. In protogyny, the animal is female first and male later: the bluehead wrasse, a small reef fish of the Caribbean, lives in groups with one large dominant male and a harem of females, and when the male is removed, the largest female changes sex and becomes the male, taking over his colour, his behaviour and his territory. In protandry, the animal is male first and female later: in the clownfish, the breeding female is the largest fish in the anemone, and when she dies, the largest male changes sex and becomes the female, while a smaller male takes his place.

The change is fast and complete. In the bluehead wrasse, the largest female begins to change within a day of the male's removal, and within eight to ten days she is a functional male, with testes, male colouration and male behaviour. The change is driven by the brain: the loss of the dominant male removes a social brake, the stress hormone cortisol rises, the enzyme aromatase in the brain is switched off, and the gonad, freed from oestrogen, remodels itself from an ovary into a testis. The ovary is not discarded: its tissues are reorganised, the eggs are resorbed, and the same organ becomes a testis, producing sperm. In the clownfish, the change runs the other way, and the testis remodels into an ovary, but the logic is the same: a social cue, read by the brain, switches the gonad from one state to the other, and the whole animal follows.

The reason sex change is common in fish is that size and sex are linked to reproductive success in different ways. In a species where a large male can defend a territory and mate with many females, it pays to be female when small and male when large, which is protogyny. In a species where a large female can produce many more eggs than a small one, it pays to be male when small and female when large, which is protandry, and the clownfish, where the female's size sets how many eggs she can lay and the anemone protects the group, is the classic case. The size-advantage model, proposed by Eric Charnov in 1982, explains both directions with a single idea: change sex when the other sex earns more offspring at your size. The model has been tested again and again, and it holds up well, which is why sex change is so common in reef fish and so rare in mammals, where the costs of changing a body built for one sex are far higher.

Direction	Meaning	Classic example	Trigger	Time taken
Protogyny	Female first, then male	Bluehead wrasse	Loss of the dominant male	8 to 10 days
Protandry	Male first, then female	Clownfish	Death of the breeding female	A few weeks
Simultaneous	Both sexes at once	Earthworm, many snails	Not a change; both systems present	Whole life

Simultaneous hermaphrodites, which carry both male and female organs at the same time, are the third case, and they are worth mentioning so they are not confused with sex changers. An earthworm is male and female at once, and two earthworms exchange sperm and both lay eggs. A garden snail is the same. These animals do not change sex, because they never had to choose: they are both, and the advantage is that any meeting with another member of the species can produce offspring, which matters for a slow animal that rarely meets a mate. The three arrangements, protogyny, protandry and simultaneous hermaphroditism, cover almost every way an animal can organise sex, and the choice between them is set by ecology: by how mates are found, by how size affects fertility, and by how much it costs to rebuild a body.

How the Systems Compare

The systems can be compared on four questions: what the signal is, who carries the deciding chromosome or cue, how the decision is made, and which animals use it. The table below sets them side by side, and the comparison brings out the pattern. The genetic systems are fast, reliable and independent of the weather, which is why they dominate in mammals and birds, animals that control their own body temperature and cannot afford to let the weather decide their children's sex. The environmental systems are flexible, and they let a species tune its sex ratio to the season and the place, which is why they dominate in reptiles, animals that lay their eggs in soil and sand and cannot control the temperature of the nest. Haplodiploidy is the most economical of all, because it needs no sex chromosome and lets the mother decide the sex of each offspring as she lays it.

System	Primary signal	Who decides	How the decision is made	Main animals
XY	Y chromosome, SRY gene	Father, through his sperm	SRY switches on SOX9, testis forms	Most mammals, fruit flies, some fish
ZW	Z chromosome dose, DMRT1	Mother, through her eggs	Two copies of DMRT1 make a male	Most birds, butterflies, moths
XO	Number of X chromosomes	Either parent	One X makes a male, two make a female	Grasshoppers, C. elegans
ZO	Number of Z chromosomes	Either parent	One Z makes a female, two make a male	Some moths and butterflies
Haplodiploidy	Whether the egg is fertilised	Mother, at egg laying	Fertilised eggs are female, unfertilised are male	Bees, ants, wasps
TSD	Nest temperature	The environment	Temperature sets aromatase and Dmrt1	Turtles, crocodilians, tuatara, some lizards
Social	Other animals	The group	Social cues switch the adult gonad	Clownfish, wrasses, slipper limpets

The comparison also shows what the systems share. Every one of them ends in the same few genes: Dmrt1, Sox9, aromatase and the hormones that follow them. The signal differs, but the machinery does not, and that is why a temperature can override a chromosome, and why a fish can change sex as an adult. The systems are not separate inventions but variations on one theme, and the theme is a switch: a small difference, in a chromosome, in a temperature or in a social cue, is amplified into the whole difference between a male and a female. The animal kingdom has found many ways to press the switch, and the variety is a measure of how important the decision is, and of how many ways there are to make it well.

The Shared Machinery Behind Every System

Behind the variety, there is a single set of genes that appears in every system, and Dmrt1 is the most widespread of them. Dmrt1 is a transcription factor with a DNA-binding region called the DM domain, and it is found in animals from nematode worms to humans. In the bird, its dose decides the sex. In the turtle, temperature controls it. In the mouse, it is needed for the testis to keep working after birth, and without it, a testis can turn into an ovary even in an adult. In the fruit fly, its relative, doublesex, is the final gene in the sex determination pathway, and it is spliced differently in males and females. Dmrt1 and its relatives are the nearest thing to a universal male gene that animals possess, and its history reaches back to the common ancestor of the bilaterian animals, more than half a billion years ago.

Sox9 is the second shared gene, and it is the one that SRY switches on in mammals. Sox9 drives the formation of Sertoli cells, the supporting cells of the testis, and once it is on, it maintains itself, which locks the decision in. Aromatase is the third, and it is the enzyme that converts testosterone into oestradiol, so that the balance between the two hormones sets the direction of the gonad. In a turtle nest, temperature sets aromatase, and aromatase sets the gonad. In a mammal, SRY sets Sox9, Sox9 sets the testis, and the testis sets the hormones. The three genes form a small circuit, and every system in this article is a different way of entering that circuit: through a chromosome, through a temperature, or through a social cue.

The hormones complete the picture. Once the gonad exists, it secretes the signals that build the rest of the body: testosterone and anti-Müllerian hormone from the testis, oestrogens from the ovary. The hormones act on tissues that are already prepared to respond, and the response is itself a matter of gene expression, switched on by the hormone in the target tissue. The whole process, from the first signal to the finished body, is a cascade of switches, and each switch is a gene being turned on or off. Sex determination is therefore best understood not as a single event but as a pathway, and the pathway is shared, even when the entry point is different. That is why the same drug, an aromatase inhibitor, can masculinise a turtle embryo and a fish, and why the same gene, Dmrt1, appears in the sex determination of a bird, a turtle and a worm.

Sex Determination and Conservation in a Warming World

Temperature-dependent sex determination turns a warming climate into a direct threat to reproduction, and the sea turtles are the clearest case. At Raine Island, the largest green turtle rookery in the world, on the northern Great Barrier Reef, researchers found that more than 99 percent of the hatchlings in recent warm years were female, because the sand had warmed past the pivotal temperature. A population that produces almost no males cannot sustain itself, and the effect is made worse by the fact that turtles return to the beach where they were born, so a hot beach stays hot, and its females keep returning to it. The same pattern is appearing in loggerheads, leatherbacks and hawksbills, and in the crocodilians, where a warming nest can push a whole clutch to one sex. The problem is not a future one: it is already measurable on the beaches where turtles nest today.

Conservationists have several answers, and all of them work by moving the nest back across the pivotal temperature. The simplest is shade: shading a nest with palm fronds or a cloth screen can cool the sand by one or two degrees, which is often enough to restore males to the clutch. A second is relocation: moving the eggs to a cooler beach, or to a hatchery where the sand temperature is controlled. A third is artificial incubation, in which the eggs are kept at a chosen temperature, so that the clutch is deliberately mixed. Each answer has costs: shading must be maintained, relocation disturbs the eggs and can lower hatching success, and a hatchery that produces a perfect sex ratio also produces turtles that have never chosen their own nest site. The work is therefore a balance, and the goal is not to control every nest but to keep enough males in the population for it to breed.

The bearded dragon adds a second conservation lesson. If high temperatures can turn ZZ lizards female, then a warming climate can erase the W chromosome from a population, and a genetic system can become a temperature system in a generation. The lizards would still breed, because the sex-reversed ZZ females are fertile, but the population would lose genetic diversity, and its sex ratio would then be set by the weather rather than by the chromosomes. The same logic applies to the snow skink and to the fish whose sex ratios shift with temperature. The lesson of sex determination for conservation is that a species' sex ratio is not a fixed property but a variable, set by genes and by temperature together, and that warming does not merely make a species hotter, it changes how many males and females it produces, and therefore whether it can reproduce at all.

What Sex Determination Is Not

The first misunderstanding is that sex determination and sex differentiation are the same thing. They are not: determination is the decision, and differentiation is the construction that follows, and the two can be separated. An embryo can be determined as male and then fail to differentiate as male, and a gonad can be determined one way and then redirected, as in the bearded dragon. The second misunderstanding is that temperature changes the genes. It does not: a turtle embryo in a warm nest has exactly the same DNA as its brother in a cool nest, and the temperature changes which genes are switched on, not the genes themselves. The change is epigenetic, a chemical mark on the DNA, and it is reversible in the sense that the next generation starts again from the same genome.

The third misunderstanding is that the XY system is the normal one and the others are exceptions. It is the normal one for mammals, and for the readers of most textbooks, but across the animal kingdom it is only one of several systems, and the ZW system is used by more species of birds than the XY system is used by species of mammals. The fourth is that sex is binary in the sense of being decided once and for all. In most animals it is, but in sequential hermaphrodites it is decided twice, and in simultaneous hermaphrodites it is never decided at all, because the animal is both. The fifth is that a single gene decides sex in every animal. No gene does: SRY decides it in mammals, DMRT1 in birds, the csd gene in honeybees, and temperature in turtles, and each is a local solution to the same problem. The systems are many, the machinery is shared, and the decision is made early, but the variety is the point, and it is what makes the subject worth a whole article.

Key Takeaways

• Sex determination is the decision of whether an embryo develops as a male or a female, and it is separate from differentiation, the building of the gonads, ducts and external structures that follows.
• In the XY system, used by most mammals, the SRY gene on the Y chromosome switches on SOX9 and builds a testis; without SRY, the ovary pathway, driven by WNT4 and RSPO1, proceeds instead.
• In the ZW system, used by most birds, the female carries the two different sex chromosomes, and the dose of the DMRT1 gene on the Z chromosome decides the sex: two copies make a male.
• In the XO system, one X chromosome makes a male and two make a female, and in the rare ZO system the pattern is reversed; both show that the embryo counts chromosomes.
• In haplodiploidy, fertilised eggs become females and unfertilised eggs become males, and in the honeybee the decision is made by heterozygosity at the csd gene.
• In temperature-dependent sex determination, the nest temperature during the thermosensitive period sets aromatase activity and Dmrt1 expression, producing three patterns: Ia, Ib and FMF.
• Genotype can be overridden by temperature, most clearly in the bearded dragon, where ZZ embryos develop as fertile females at high incubation temperatures.
• Sequential hermaphrodites change sex as adults: protogynous fish such as the bluehead wrasse change from female to male, and protandrous fish such as the clownfish change from male to female, driven by social cues read by the brain.
• Every system converges on the same machinery, Dmrt1, Sox9, aromatase and the sex hormones, which is why a temperature can override a chromosome and a fish can change sex as an adult.
• Warming threatens temperature-dependent species directly: green turtle rookeries are already producing more than 99 percent females in warm years, and conservation now works by shading nests, relocating clutches and incubating at chosen temperatures.

Short Questions

1. What is sex determination in simple words?

See answer

Sex determination is the process that decides whether an animal develops as a male or a female, and it happens early in development, usually while the embryo is still forming its gonads. In mammals, the decision is made by the chromosomes: an embryo with an X and a Y chromosome becomes male, and an embryo with two X chromosomes becomes female. In other animals the decision is made differently: in birds it depends on the dose of a gene called DMRT1, in bees it depends on whether the egg was fertilised, in many turtles it depends on the temperature of the nest, and in some fish it can be changed in adulthood. Determination is the decision itself, and it is different from differentiation, which is the building of the testes or ovaries and the rest of the male or female body that follows.

2. What does the SRY gene do?

See answer

The SRY gene, the sex-determining region of the Y chromosome, is the switch that makes a male mammal. When SRY is expressed in the developing gonad, it switches on another gene called SOX9, and SOX9 drives the gonad to become a testis, whose Sertoli cells then support sperm production. The testis secretes anti-Müllerian hormone, which makes the female ducts regress, and testosterone, which builds the male ducts and masculinises the external genitalia. Without SRY, the gonad follows the ovarian pathway instead, driven by genes such as WNT4 and RSPO1. The evidence is direct: an XY embryo with a damaged SRY gene develops as a female, and an XX embryo with SRY moved onto an X chromosome develops as a male, so the gene, not the rest of the Y chromosome, is the switch.

3. What is haplodiploidy?

See answer

Haplodiploidy is a system of sex determination in which sex depends on whether the egg is fertilised. A fertilised egg, with two sets of chromosomes, develops as a female, and an unfertilised egg, with one set, develops as a male. It is used by the bees, ants and wasps, and a honeybee queen controls the sex of her offspring by releasing or withholding sperm as each egg is laid. In the honeybee, the decision is made by a single gene called the complementary sex determiner, or csd: an embryo with two different versions of csd becomes a female, and an embryo with only one copy becomes a male. The males have no father, and they pass their whole genome, unshuffled, to their daughters, which is why full sisters in a honeybee colony share three quarters of their genes.

4. What is temperature-dependent sex determination?

See answer

Temperature-dependent sex determination, or TSD, is a system in which the sex of an embryo is decided by the temperature of the nest during a sensitive window in the middle of incubation, called the thermosensitive period. It is used by most turtles, all crocodilians, the tuatara and some lizards and fish. During that window, temperature regulates the enzyme aromatase, which converts testosterone into oestradiol: at female-producing temperatures aromatase is active and the gonad becomes an ovary, and at male-producing temperatures it is inactive and the gonad becomes a testis. TSD comes in three patterns: pattern Ia makes males at low temperatures and females at high ones, pattern Ib does the reverse, and pattern II makes females at both extremes and males in the middle. Every TSD species has a pivotal temperature at which the clutch is half male and half female, and for most sea turtles it is around 29 degrees Celsius.

5. How does a clownfish change sex?

See answer

A clownfish changes sex because it is a protandrous sequential hermaphrodite, meaning it is male first and female later. A clownfish group living in a sea anemone has one breeding female, the largest fish, one breeding male, the second largest, and several smaller non-breeding males. When the breeding female dies, the largest male changes sex and becomes the female, and the next largest male takes his place as the breeding male. The change is driven by the brain: the loss of the female removes a social brake, the gonad remodels itself from a testis into an ovary, and within a few weeks the former male is laying eggs. The arrangement works because a large female can lay many more eggs than a small one, so it pays to be male while small and female when large.

6. Why does warming worry turtle conservationists?

See answer

Warming worries turtle conservationists because most sea turtles use temperature-dependent sex determination, and a warmer nest produces more females. At Raine Island, the largest green turtle rookery in the world, more than 99 percent of the hatchlings in recent warm years were female, because the sand had warmed past the pivotal temperature of around 29 degrees Celsius. A population that produces almost no males cannot sustain itself, and the effect is made worse because turtles return to the beach where they were born, so a hot beach keeps producing females. Conservationists answer by shading nests, moving eggs to cooler beaches, and incubating them at chosen temperatures, but each of these is work that must be done nest by nest, and the underlying warming continues.

Frequently Asked Questions

Is sex always decided by chromosomes?

No, chromosomes decide sex in many animals but not in all of them. In mammals, the XY system decides it, and in birds the ZW system does, but in bees and ants sex depends on whether the egg was fertilised, in many turtles and crocodilians it depends on the temperature of the nest, and in some fish it can be decided in adulthood by the animals around the individual. Even in animals with chromosomes, temperature can sometimes override them, as in the bearded dragon, where ZZ embryos develop as females at high incubation temperatures. Chromosomes are one way of deciding sex, and they are the commonest way in mammals and birds, but they are not the only way.

Can temperature really change an animal's sex?

Temperature can decide an animal's sex, but it does not change the animal's genes. In temperature-dependent sex determination, the temperature of the nest during a sensitive window in the middle of incubation decides whether the gonad becomes a testis or an ovary, by controlling the enzyme aromatase and the gene Dmrt1. A turtle embryo in a warm nest has exactly the same DNA as its brother in a cool nest; the temperature changes which genes are switched on, not the genes themselves. The change is epigenetic, and it is also why a warming climate can skew the sex ratio of a whole rookery without changing a single gene.

How does a clownfish change sex?

A clownfish changes sex because it is a protandrous sequential hermaphrodite, meaning it is male first and female later. A group living in a sea anemone has one breeding female, the largest fish, one breeding male, the second largest, and several smaller non-breeding males. When the breeding female dies, the largest male changes sex and becomes the female, and the next largest male takes his place. The change is driven by the brain, the testis remodels into an ovary, and within a few weeks the former male is laying eggs. It works because a large female can lay many more eggs than a small one, so it pays to be male while small and female when large.

Why do honeybee colonies have so few males?

Honeybee colonies have few males because of haplodiploidy, the system in which fertilised eggs become females and unfertilised eggs become males. The queen controls the sex of each egg by releasing or withholding sperm as she lays it, and she lays mostly fertilised eggs, which become workers, and only a small number of unfertilised eggs, which become drones. The drones are needed only for mating, and they are produced in larger numbers only in the swarming season. The decision is made by a single gene, the complementary sex determiner: an embryo with two different versions of the gene becomes a female, and an embryo with one copy becomes a male.

What is the difference between sex determination and sex differentiation?

Sex determination is the decision of which sex an embryo will develop as, and sex differentiation is the building of the male or female body that follows the decision. In a mammal, determination happens when the SRY gene switches on in the developing gonad and commits it to becoming a testis, and differentiation is everything after that: the testis forms, it secretes anti-Müllerian hormone and testosterone, and those hormones build the ducts and the external genitalia. The two can be separated: an embryo can be determined as male and then fail to differentiate as male, and a gonad can be determined one way and then redirected, as in the bearded dragon. Determination sets the direction, and differentiation is where the body is actually built.

Will warming make all sea turtles female?

Warming is already making most sea turtle hatchlings female on the hottest beaches, but it will not make every turtle on every beach female, because beaches differ in temperature and species differ in their pivotal temperature. At Raine Island, the largest green turtle rookery in the world, more than 99 percent of the hatchlings in recent warm years were female, and the same pattern is appearing in loggerheads and hawksbills. Conservationists answer by shading nests, moving eggs to cooler beaches and incubating them at chosen temperatures, which can restore males to the clutch. The risk is real and measurable, but it is also manageable nest by nest, and the goal is to keep enough males in the population for it to keep breeding.

Related reading

• DNA Structure and Replication Explained Simply
• Mendel's Laws of Inheritance Explained
• All Genetics Articles

MCQs

1. What is the main job of the SRY gene in mammals?

• A. Building the placenta
• B. Making eggs in the ovary
• C. Switching on testis development in XY embryos
• D. Controlling shell colour

Answer: C

Explanation: SRY, the sex-determining region of the Y chromosome, switches on SOX9 in the developing gonad, which drives it to become a testis, and the testis then secretes the hormones that build the male body.

2. In birds, which sex carries two different sex chromosomes?

• A. The male
• B. The female
• C. Both sexes
• D. Neither sex

Answer: B

Explanation: Birds use the ZW system, in which males are ZZ and females are ZW, so it is the female that carries the two different sex chromosomes, and the mother who determines the sex of the offspring.

3. In haplodiploidy, which eggs develop as males?

• A. Unfertilised eggs
• B. All eggs
• C. Only fertilised eggs
• D. Eggs kept at high temperature

Answer: A

Explanation: In haplodiploidy, fertilised eggs, with two sets of chromosomes, become females, and unfertilised eggs, with one set, become males, which is how honeybee queens control the sex of their offspring.

4. In most sea turtles, warm nests produce mostly which sex?

• A. Mostly males
• B. Equal numbers of both sexes
• C. No hatchlings at all
• D. Mostly females

Answer: D

Explanation: Most sea turtles follow TSD pattern Ia, in which low temperatures make males and high temperatures make females, so a warm nest produces mostly females.

5. Which fish changes from male to female when the breeding female dies?

• A. Clownfish
• B. Bluehead wrasse
• C. Earthworm
• D. Grasshopper

Answer: A

Explanation: The clownfish is a protandrous sequential hermaphrodite, so when the breeding female dies, the largest male changes sex and becomes the female, while a smaller male takes his place.

6. What is the pivotal temperature in temperature-dependent sex determination?

• A. The hottest temperature an egg can survive
• B. The temperature at which eggs hatch fastest
• C. The temperature producing a half-male, half-female clutch
• D. The average temperature of the nest

Answer: C

Explanation: The pivotal temperature is the temperature at which a clutch is half male and half female, with a transitional range around it in which both sexes are produced, and for most sea turtles it is around 29 degrees Celsius.

7. In the bearded dragon, what happens to ZZ embryos incubated at high temperature?

• A. They become sterile males
• B. They develop as fertile females
• C. They develop no gonads at all
• D. They become hermaphrodites

Answer: B

Explanation: At incubation temperatures above about 34 degrees Celsius, ZZ bearded dragon embryos develop as fertile females, even though their chromosomes say male, because temperature switches the gonad onto the female pathway.

8. Which gene's dose decides sex in birds?

• A. SRY
• B. The csd gene
• C. SOX9
• D. DMRT1

Answer: D

Explanation: DMRT1 sits on the Z chromosome, and a ZZ embryo, with two copies, develops as a male, while a ZW embryo, with one copy, develops as a female, which was shown by reducing DMRT1 in ZZ chicken embryos and feminising them.

9. What is sex determination, as opposed to sex differentiation?

• A. The decision of which sex the embryo will develop as
• B. The building of the testes or ovaries
• C. The production of sperm or eggs
• D. The growth of the embryo

Answer: A

Explanation: Determination is the decision itself, made early in the gonad, while differentiation is the construction that follows it, the building of the gonads, ducts and external structures, mostly under the control of hormones.

10. In the fruit fly Drosophila, sex is set mainly by what?

• A. The presence of a Y chromosome
• B. The temperature of the egg
• C. The ratio of X chromosomes to autosomes
• D. The number of legs

Answer: C

Explanation: In Drosophila, an embryo with two X chromosomes and two sets of autosomes is female, and one with one X and two sets of autosomes is male, and the ratio is read by the gene Sex-lethal, while the Y chromosome is needed only for sperm production.