The Reptiles Where Temperature Decides Gender|These Reptiles Have a Sex System You've Never Heard Of

Added: 2026-05-09

[00:00:00]There are animals on this planet whose sex is not decided [music] by genetics. It is decided by the weather. American Alligator. The American Alligator was one of the first reptiles to reveal to scientists [music] exactly how temperature controls sex. It lives across the swamps, [music] rivers, and marshlands of the southeastern United States, and it builds its [music] nest from decaying vegetation piled into a mound above the waterline. The rotting plant material generates heat, and the temperature inside that mound [music] during the middle third of incubation determines whether each egg produces a male or a female. Eggs incubated below 30° C >> [music] >> produce almost entirely females. Eggs incubated above 34° C produce almost entirely [music] males. The narrow window between those two numbers is where both sexes can appear in the same clutch. Mother alligators do not leave their nests unguarded. They actively monitor and adjust the nest material [music] to regulate heat. In a sense, the mother is choosing the sex of her offspring [music] without ever knowing it. Loggerhead Sea Turtle. The Loggerhead Sea Turtle nests [music] on sandy beaches across the Atlantic, Pacific, and Indian Oceans, and it travels [music] thousands of miles to return to the beach where it was born. The female drags [music] herself above the tideline, digs a flask-shaped chamber in the sand with her rear flippers, deposits up [music] to 200 eggs, covers the nest, and returns to the ocean without ever seeing her young. The sand temperature [music] does everything from that point forward.

[00:01:39]Cooler sand below 27° C produces predominantly males. [music] Warmer sand above 31° C produces predominantly females. Beaches in Florida have already begun producing more than 90% [music] female hatchlings in some seasons because the sand is warming. The males are disappearing from [music] certain populations not because of disease or predation, but simply because the beach is too warm.

[00:02:07]Green sea turtle. The green sea turtle is one of the most studied [music] reptiles in the world specifically because of what is happening to its sex ratios [music] right now. It nests on tropical beaches from Australia to Costa Rica and its eggs are exquisitely sensitive to temperature shifts.

[00:02:25]Research [music] conducted on the northern Great Barrier Reef found that more than 99% of juvenile green sea turtles born there in recent years were female. The population around the southern Great Barrier Reef, where [music] beaches are slightly cooler, still produces a more balanced ratio.

[00:02:43]The same species [music] on beaches separated by a few hundred kilometers is producing almost entirely opposite outcomes. [music] Scientists describe this as a natural experiment happening in real time, one that no [music] laboratory could have designed. Painted turtle.

[00:03:00]The painted turtle is the most widespread [music] native turtle in North America, found from the Atlantic coast all the way to the Pacific in slow-moving [music] freshwater habitats. It buries its eggs in loose soil near the water and then leaves [music] them entirely.

[00:03:16]No nest guarding, no parental monitoring, no intervention of any [music] kind. The soil temperature during the middle third of incubation makes every decision. What makes the painted turtle particularly interesting to [music] researchers is that it can tolerate a wider range of incubation temperatures than most [music] species without losing viability entirely. But even within that range, the sex determination [music] is absolute. The eggs do not negotiate.

[00:03:44]A clutch buried in a sunny exposed bank will produce different [music] sexes from a clutch buried 2 m away under the shade of a bush, even on the same [music] afternoon. Leopard Gecko. The leopard gecko is one of the most familiar reptiles [music] in the world because it is one of the most commonly kept as a pet. It originates from the rocky dry grasslands of Pakistan, [music] Afghanistan, and northwestern India. In the wild, females bury their eggs in warm soil and leave them [music] to develop. The sex determination in leopard geckos follows a more complex pattern than simple high female and low male. Low temperatures around 26° [music] C produce mostly females. Intermediate temperatures around 32° produce mostly males.

[00:04:33]High temperatures around 34° produce females again.

[00:04:37]But these high temperature females are physically [music] different from the low temperature ones. They are larger, more aggressive, and behave in ways that overlap with typical male behavior.

[00:04:49]Scientists [music] call this the hot female phenomenon, and it has changed how researchers understand [music] the relationship between temperature, hormones, and behavior across all reptiles. [music] Common Snapping Turtle.

[00:05:02]The common snapping turtle is a large aggressive freshwater turtle found across eastern [music] North America and into Central America. It is one of the few turtle [music] species where the sex determination pattern runs in the opposite direction from most. Most turtles produce [music] females at high temperatures. The snapping turtle produces females [music] at both the low and high ends of the temperature range, with males appearing only in the middle band. This U-shaped pattern is unusual and took [music] researchers time to establish because early studies only tested one part of the temperature range [music] and assumed the pattern was simpler than it was. The snapping turtle is a reminder that even within a group of animals that all use temperature [music] to determine sex, the specific rules are not universal. Each species has written [music] its own version of the same instruction.

[00:05:54]Tuatara The tuatara is not technically a lizard, >> [music] >> not a snake, and not a turtle. It is the sole surviving member of an ancient reptile order that diverged from all other reptiles over 200 million years ago. It lives [music] only on small islands off the coast of New Zealand and is considered a living fossil. An animal whose body plan has remained largely unchanged since the age of dinosaurs.

[00:06:19][music] The tuatara uses temperature-dependent sex determination and its sensitivity [music] to temperature is extreme.

[00:06:27]Males are produced at temperatures above 22° C.

[00:06:32]>> [music] >> Females are produced below that threshold. Because the tuatara lives at one of the cooler ends of the reptile temperature range and [music] its population is already small and island-bound, even a 1° rise in average nest temperature [music] across those islands could tip entire breeding seasons toward producing only males, slowly starving the population of females until reproduction itself [music] becomes impossible. Three-lined skink The three-lined skink, found [music] across parts of Australia, represents one of the most scientifically fascinating cases in [music] the entire field of reptile sex determination because it does not fit neatly into either category. Most reptiles [music] either use temperature-dependent sex determination or chromosomal sex determination.

[00:07:18]The genetic system [music] used by mammals and birds, where sex chromosomes decide everything at the moment of fertilization.

[00:07:25]>> [music] >> The three-lined skink uses both simultaneously.

[00:07:29]Individuals [music] in cooler highland populations rely on temperature to determine sex. Individuals [music] of the same species in warmer lowland populations rely on chromosomes.

[00:07:41]Populations in intermediate zones show a mixture of both systems operating at once. This single species is showing scientists [music] that the two systems are not opposites.

[00:07:52]They are points on a continuum, and the transition between [music] them is still happening in real time inside a living animal.

[00:07:59]Nile crocodile. [music] The Nile crocodile is one of the largest reptiles on Earth, growing to over 5 m and [music] living across sub-Saharan Africa and Madagascar. It builds its nests in sandy riverbanks and guards [music] them aggressively through the entire incubation period.

[00:08:17]One of the most dedicated nest-guarding [music] behaviors of any reptile. The mother will defend the nest against almost any threat, and when the eggs are ready to hatch, she will gently carry the hatchlings [music] in her jaws to the water. Despite this intense parental investment, she has no genetic [music] control over the sex of her offspring.

[00:08:37]Everything is temperature.

[00:08:39]Males are produced in [music] the middle temperature range.

[00:08:43]Females are produced at the cooler and warmer extremes. A mother who builds [music] her nest in slightly different sand, on a slightly different bank angle, on a slightly [music] cloudier or sunnier season, produces an entirely different set of sons and daughters without any mechanism to know or correct it. Red-eared slider.

[00:09:02]The red-eared slider is one of the most widely distributed freshwater turtles in the world and is also one of the most invasive, having been released into waterways across Europe, Asia, and Australia [music] after being kept as pets. It originates from the Mississippi River Valley of the United States. Like most turtles, [music] it produces females at high temperatures and males at lower temperatures. [music] What makes it notable in the context of climate research is that it has a relatively narrow pivotal temperature, the point at which a 50/50 sex ratio [music] is produced, sitting around 29° C in urban environments where heat island [music] effects raise soil temperatures even slightly above surrounding rural areas.

[00:09:47]Populations of red-eared sliders have [music] already begun showing skewed female ratios in city park ponds, while nearby rural populations remain balanced. [music] The city itself is altering the sex of wild animals living [music] inside it.