Heat waves turn female frogs into males, and the population could collapse within decades
In agile frogs, females exposed to a heat wave develop testes within days and become reproductively capable males — but their offspring are also more likely to sex-reverse, the compensation mechanism fails, and models predict population collapse within decades.
The video won't play here. Listen to the audio instead:
The argument · tap a timestamp to hear it
A few days of heat is enough to make female frogs grow testes
Agile frogs use the same XX/XY sex determination system as humans: XX is female, XY is male. But Veronica Bocconi says that simply exposing genetically female individuals to a heat wave of a few days makes them develop testes and become "apparently completely normal males." In fish and amphibians, heat turning individuals into males (female-to-male) is a fairly typical pattern, while most turtles are the exact opposite. That means the same warming pushes the sex ratios of the two groups in completely opposite directions.
— Veronica BocconiSex reversal was originally a stop-loss strategy for a bad childhood
Why would letting temperature determine sex be advantageous? Bocconi's explanation: it helps when offspring may land in wildly different environments, and some of those environments are especially bad for one sex. If an agile frog hits a heat wave in childhood, its development and growth are slowed, making it hard to reach the size and age needed to enter the adult population — it may never get to reproduce at all. But if it switches to a male phenotype, it can start reproducing earlier, because males typically start reproducing at a smaller size. This is driven by "sex-dependent fitness effects." The question is whether that benefit still holds once the environment becomes more extreme.
— Veronica BocconiPlastic buckets as honeymoon suites, experiments at the pond's edge
Sex reversal has been studied very little in the wild. Bocconi's approach exploits the fact that the frogs' breeding season is highly concentrated — for about three weeks a year they gather together, so she doesn't have to chase them around like colleagues studying Australian desert lizards. At the breeding pond she sets up "sex hotels": rows of plastic buckets, one room per pair of frogs. She pairs a sex-reversed male and a sex-consistent male each with a female, and watches whether the female accepts them, whether she produces fertile eggs and offspring, and how the tadpoles fare afterward. This semi-natural setup is the foundation of the whole series of experiments.
— Veronica BocconiSex-reversed males can reproduce, but they only produce daughters
Although XX males have chromosomes that don't match their phenotype, they still have the full equipment needed to reproduce and are fertile males. But an XX male paired with an XX female produces offspring that are all XX at the genetic level — in theory all should be female. The trouble is that these offspring themselves sex-reverse: experiments show that the offspring of sex-reversed fathers are more likely to sex-reverse. Bocconi describes it as a positive feedback loop: warming turns more females into males, and sex-reversed males could compensate by producing XX offspring, but those offspring reverse into males too, so the compensation fails and the sex ratio only skews further male.
— Veronica BocconiFemale frogs pick mates by laying fewer eggs
By measures like fertility, offspring survival and the number of eggs a female lays, sex-reversed males look completely normal. The only anomalous signal comes from "cryptic female choice": a female frog can't stop a male from clasping her, but if she doesn't like him she can lay only a small portion of her eggs, letting the male think she's done and release her, while she saves eggs for a future mate. Normally a female lays only one clutch per breeding season. The experiment was repeated over two years, and in both years some females did this partial laying with sex-reversed males, while they almost never did it with sex-consistent males. The sample size is small, but Bocconi thinks "something may really be going on."
— Veronica BocconiHeat wave plus virus, and the offspring of sex-reversed males can't take it
The researchers first gave tadpoles a heat wave, confirming that the offspring of sex-reversed fathers were more likely to sex-reverse; then they kept rearing them and tracked mortality — that year was the hottest on record in that country, so a heat-related mortality study came along by accident, but no difference in mortality showed up. Back in the lab, after infecting them with a ranovirus that is very hard on these frogs, a difference appeared: if the father was a sex-reversed male and the tadpole had gone through heat treatment, these juveniles carried a high viral load. In Bocconi's words, "heat first, then virus, and they just can't take it."
— Veronica BocconiThe model predicts: this kind of species has only decades left
Feeding these data into the theoretical model they had developed earlier, and plugging in the known parameters for agile frogs, the model predicts that no matter what, as long as the climate keeps warming, this kind of species has only decades left before population collapse. If sex-reversed males can't compensate for the skewed sex ratio — whether because females won't accept them, because their offspring all turn male, or because the offspring die of the virus — then extinction arrives sooner than that. Bocconi adds that what worries her more is the ponds drying up completely, weather that could make the extinction scenario come faster.
— Veronica BocconiThere's no universal conservation plan, only one population at a time
Sea turtles are the ready-made example of this worry: people are already discussing how to control beach temperatures so that not all hatchlings come out male. But Bocconi stresses that temperature reaction norms vary greatly between species — not just in which direction it goes (heat produces males or heat produces females), but also in the key absolute temperature at which it kicks in. So it's hard to give a universal strategy, but it is meaningful for a specific population: know the current water temperature of this pond or wetland network, predict what it will become, and then decide what kind of microhabitat to build so that offspring at least have a chance to choose an environment that produces males or females.
— Veronica BocconiIn their own words · checked verbatim
The researcher was telling me when they go out to the field, the hyaloma ticks are basically running after them. They're not like other ticks that wait in the vegetation.
Rich Stone4:18
One of the researchers I was talking to said the general mode of operation is to wait for a human case and then respond.
Rich Stone7:23
If you expose genetically female individuals to just a few days of a heat wave, they will develop testes and become, well, apparently perfectly normal males.
Veronica Bocconi14:37
if rising temperatures increase the proportion of males due to female to male sex reversal, and those sex reversed males could compensate for this by producing XX female offspring, but those are. spring them themselves undergo sex reversal as well, then there is no compensation and sex ratios would just get more and more male biased as the climate is warming.
Veronica Bocconi19:55
if they don't like that male is that they can withhold some of their eggs. So they just produce just a tiny amount, a small amount of eggs so that they might trick the male into thinking that they're done and the male would release them.
Veronica Bocconi20:57
if the father was sex reversed, then these froglets had high renovirus loads if they had been exposed to heat treatment when they were tadpoles. So first heat and then renovirus, they just can't cope.
Veronica Bocconi23:04
this model predicted that whatever happens, if this climate warming goes on, such species have a few decades left before population collapse.
Veronica Bocconi23:04
I must tell you that I'm more worried about the ponds totally drying out and the animals dying out because of this crazy weather that we see these days. So that might bring the whole extinction scenario even closer.
Veronica Bocconi24:06
Figures
| Apparent mortality rate of Crimean-Congo hemorrhagic fever in Balkan countries such as Kosovo | 20%–25% | 2:07 |
| Year of Portugal's first fatal Crimean-Congo hemorrhagic fever case | 2024 | 5:22 |
| Body length of the hyaloma tick | 5–6 mm | 4:18 |
| Length of the agile frog's concentrated breeding season | about three weeks a year | 17:47 |
| Remaining time to population collapse predicted by the model | decades | 23:04 |
Glossary
- temperature dependent sex reversal
- After genetic sex is already determined, temperature then makes an individual show the characteristics of the opposite sex.
- cryptic female choice
- When a female cannot refuse mating, she covertly screens mates by means such as laying fewer eggs.
- hyaloma tick
- The tick that transmits Crimean-Congo hemorrhagic fever and actively chases hosts for hundreds of meters.
- Crimean-Congo hemorrhagic fever
- A tick-borne hemorrhagic fever that starts like flu and in severe cases can cause fatal bleeding; there is no mature vaccine.
- digital twin
- Using AI or large models in place of real people for testing; this episode mentions that it has five kinds of distortion.
How to listen
Researchers studying the ecological impacts of climate change, people working on species conservation and population modeling, and readers who want to understand how sex reversal propagates into population collapse.
The autumn book recommendations at 9:23–10:23, unrelated to this episode's topic.