Researchers discover leopard gecko produces females at cooler incubation temperatures and mostly males at warmer ones. This clear sex-determination pattern of leopard gecko has established it as a key model for studying environmental effects on development.
Credit: Professor Shinichi Miyagawa from Tokyo University of Science, Japan.
Bible literalists insist that every word in the Bible is true and without error, yet the text itself contains statements that cannot all be true at the same time, so at least one of them must be false*. The Bible also contains factually incorrect statements, such as the assertion in Genesis 6:19 and 7:15–16 that males and females of every ‘kind’ were brought onto the Ark, reflecting the same assumption found in Genesis 1:27 that living creatures were created as male and female.
We now know, unlike the authors of Genesis, that not all species exist as fixed male–female pairs. There are many examples of hermaphrodite creatures that are both male and female; some species that are entirely female, such as the New Mexico whiptail lizard and the marbled crayfish; many aphids and some beetles such as the vine weevil; and species that can change sex during their lifetime, such as certain fish. And, as a recent paper in the journal Developmental Biology by a team of researchers led by Professor Shinichi Miyagawa from the Department of Biological Science and Technology, Tokyo University of Science, Japan, shows, there are also species such as the leopard gecko, Eublepharis macularius, in which sex is not determined by inherited chromosomes at all but by the temperature at which the embryo develops in the egg.
In other words, in some species the difference between male and female is not fixed at conception at all, but depends on something as mundane as the temperature of the nest.
The team showed that there is a discrete window, known as the temperature-sensitive period, during which temperature triggers temperature-dependent sex determination (TSD), activating specific sets of genes that control the development of either testes or ovaries, thereby determining the sex of the developing embryo.
Not All Species Are Male and Female.The team’s research is explained in a news item from Tokyo University of Science released through EurekAlert!.The assumption that all animals exist as fixed male–female pairs reflects everyday observation of mammals and birds, but it is far from universal in biology. Evolution has produced a remarkable diversity of reproductive systems in which sex may be combined, absent, or changeable.
Hermaphrodites: both sexes in one individual
Some species possess both male and female reproductive organs in the same individual.
Examples include:
- Earthworm – each worm produces both eggs and sperm and exchanges sperm during mating.
- Garden snail – simultaneous hermaphrodites that can fertilise each other during mating.
- Many flatworms and parasitic worms, which combine both reproductive systems in a single organism.
Hermaphroditism is particularly common among invertebrates and organisms that encounter mates infrequently.
All-female species and parthenogenesis
Some species reproduce without males at all, producing offspring from unfertilised eggs. This process is known as parthenogenesis.
Well-known examples include:
- New Mexico whiptail – an all-female lizard species that reproduces clonally.
- Marbled crayfish – a freshwater crustacean in which every individual is female.
- Many aphids, which reproduce parthenogenetically for several generations.
In these species there are no males at all, so reproduction occurs entirely without fertilisation.
Species that change sex
Some animals can change sex during their lifetime, depending on social or environmental conditions.
Examples include:
- Clownfish – individuals begin life as males, but the dominant male can become female if the breeding female dies.
- Bluehead wrasse – females can rapidly transform into males when a dominant male disappears.
- Many other reef fish such as groupers and parrotfish.
This phenomenon is known as sequential hermaphroditism.
Environmentally determined sex
In some species, sex is not determined genetically at fertilisation but by environmental conditions during development.
A well-known example is the Leopard gecko, where the temperature at which eggs incubate determines whether the embryo develops as male or female. This process is called temperature-dependent sex determination (TSD) and is also found in many turtles and crocodilians.
A diversity of reproductive strategies
These examples illustrate that the simple idea that all animals exist as fixed male–female pairs does not reflect biological reality. Instead, evolution has produced a wide variety of reproductive strategies adapted to different ecological conditions.
New study clarifies how temperature shapes sex development in leopard gecko
Researchers pinpoint when temperature determines sex and uncover early genetic changes guiding male or female development in leopard gecko
In reptiles, a simple temperature change can determine whether an egg develops into a male or female. This process is formally known as temperature-dependent sex determination (TSD), in which the sex of an embryo is determined by the temperature it experiences during a specific window of development known as the temperature-sensitive period. For example, in American alligators, incubation at around 30 °C typically produces females, while temperatures near 33 °C produce males, although extremely high temperatures can again result in females. Although TSD has been studied for decades, its mechanisms in squamates—the large and diverse group of reptiles that includes lizards and snakes—have been largely overlooked.
To address this gap, a team of researchers led by Professor Shinichi Miyagawa from the Department of Biological Science and Technology, Tokyo University of Science, Japan, investigated TSD in the leopard gecko (Eublepharis macularius), a lizard species where lower temperatures of about 26 °C produce females, while higher temperatures around 32 °C produce mostly males. Their study was made available online on February 18, 2026, and will be published in Volume 533 of the journal Developmental Biology on May 01, 2026.
Our study is the first to provide a comprehensive histological and transcriptomic analysis of gonadal development in the leopard gecko with a TSD system.
Professor Shinichi Miyagawa, corresponding author
Department of Biological Science and Technology
Faculty of Advanced Engineering
Tokyo University of Science
Katsushika, Tokyo, Japan.
In this study, the team incubated eggs at either 26.5 °C, a female-producing temperature, or 31.5 °C, a male-producing temperature. To determine the exact window when temperature influences sex, they performed shift experiments, moving eggs between the two temperatures on different days after they were laid.
When the eggs were close to hatching, the researchers examined their gonads, the organs that develop into ovaries or testes, to determine their sex. Notably, incubation at the cooler temperature produced 100% females, while the warmer temperature produced 91% males.
Early in development, embryos from both temperature groups appeared similar, with no obvious external differences. The first clear structural differences appeared later, when ovaries became more spherical and testes elongated and formed seminiferous tubules. However, gene expression analysis showed that male and female developmental pathways had already begun to diverge before these visible changes appeared. Important testis-related genes such as AMH, DMRT1, and SOX9 were activated earlier at male-producing temperatures, while ovarian genes such as FOXL2 and CYP19A1 became more active at female-producing temperatures.
The team determined that the temperature-sensitive period ends at embryonic stage 36. Before this stage, changing the incubation temperature could still alter the sex. However, after this point, the sex was set and could no longer be changed by temperature.
The study also uncovered features unique to the leopard gecko. For example, the gene KDM6B, which plays a key role in male determination in turtles, showed a different pattern of regulation in this species. In addition, it identified early temperature-responsive genes involved in RNA splicing and cell adhesion, suggesting that changes at the molecular level begin before any physical differences between males and females become apparent.
The researchers add that factors such as the mother’s body temperature before laying egg may influence early development and could vary between laboratories. However, their findings show that the genes responsible for forming gonads are largely shared across reptiles, but the way temperature controls these genes has evolved differently in different reptile groups.
Our study addresses a critical phylogenetic gap in TSD studies and contributes significantly to the broader understanding of the evolutionary plasticity and molecular complexity by which environmental cues direct biological fate.
Professor Shinichi Miyagawa.
Publication:
What the work by Professor Miyagawa and his colleagues shows is that biological reality is far more complex than the ancient authors of Genesis could possibly have imagined. In species such as the leopard gecko, the difference between male and female is not fixed at conception at all but depends on environmental conditions during a critical stage of development. Something as simple as the temperature of the nest can determine whether the embryo develops testes or ovaries.
This is just one example of the extraordinary diversity of reproductive systems that evolution has produced. Some species combine male and female reproductive organs in the same individual; some reproduce entirely without males; others can change sex during their lifetime; and in many reptiles the environment determines whether offspring become male or female. These systems make perfect sense in evolutionary terms, where natural selection favours whatever reproductive strategy works best in a particular ecological context.
What they do not reflect is the simplistic view of biology assumed in ancient mythology, where all animals were thought to exist as fixed male–female pairs created at the beginning of time. Like so many other claims in the Bible about the natural world, that assumption turns out not to be a statement of fact but merely a reflection of the limited understanding of the people who wrote it.
And once again, it is modern science—not ancient scripture—that reveals how the natural world really works.
- People have seen God:
- Genesis 32:30. And Jacob called the name of the place Peniel: for I have seen God face to face, and my life is preserved.
- Exodus 24:9-10. Then went up Moses, and Aaron, Nadab, and Abihu, and seventy of the elders of Israel: And they saw the God of Israel: and there was under his feet as it were a paved work of a sapphire stone, and as it were the body of heaven in his clearness.
- Exodus 33:11. And the LORD spake unto Moses face to face, as a man speaketh unto his friend.
No they haven't:
- Exodus 33:20. And he said, Thou canst not see my face: for there shall no man see me, and live.
- John 1:18. No man hath seen God at any time, the only begotten Son, which is in the bosom of the Father, he hath declared him.
- 1 John 4:12. No man hath seen God at any time.
Clearly, if the first three verses are true, the last three can't be.
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