Male mouse embryos have been turned into females by a procedure that involves eliminating the Y chromosome. The technique relies on CRISPR gene editing and has been used to create healthy and fertile female and male mice, both of which are clones of the father.

“We thought if we could remove the Y chromosomes immediately after fertilisation, we can change the sex,” says at the University of Yamanashi, Japan. “It is essentially sex reversal.”

The method may become useful for conserving animals that are so close to extinction they are down to just one male or only a few.

In animals that reproduce sexually, running out of males or females can mean the end of the line for a population or species. However, some species can turn to asexual reproduction, through budding off new organisms, such as jellyfish-like, or by parthenogenesis, in which unfertilised eggs of animals including lizards, stick insects and sharks can develop into a new animal. This produces offspring that are identical clones of the parent and the same sex. Clones created in labs, such as Dolly the sheep, are made through a similar process.

A few animals, like Okinawa rubble gobies (Trimma okinawae), can change sex to ensure sexual reproduction can continue. Inspired by these colourful marine fish, Ishiuchi and his colleagues came up with a method to change the sex of a male clone embryo, so sexual reproduction between individuals could take place – albeit exceedingly closely related ones.

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when the male-determining Y sex chromosome was deleted, leaving the specimen with just one X chromosome (known as XO), rather than an XY pair.

Ishiuchi and his colleagues developed a CRISPR-based tool called Y-CUT to reliably eliminate the Y chromosome from male mouse embryos.

First, they inserted DNA from the blood cells of a male mouse into mouse egg cells that had already had their own DNA removed – a technique called somatic cell nuclear transfer. They treated some of these embryos with Y-CUT and transferred them to surrogate female mice. 

All of the mice born after the use of Y-CUT were female clones – genetically identical to the father apart from lacking the Y chromosome. Other offspring, which hadn’t undergone Y-CUT, were all male clones.

“We call this dual-sex cloning,” says team member at the RIKEN BioResource Research Center in Tsukuba, Japan.

Introducing the father mouse to his clones was a very special moment, says Ishiuchi. “He could see the female version of himself, which is like sci-fi.”

The clones were fertile, and when they mated together, they produced offspring that seemed healthy. “This means sexual reproduction was initiated from a single male genome,” says Matoba. “The kids have survived over a year so far with no clear defects.”

Ishiuchi hopes that the Y-CUT technique could help revive species that are highly endangered or extinct, and for which only frozen tissue remains in cell banks.

“It would be incredibly valuable if this were successful in species other than mice,” says , also at the University of Yamanashi, who wasn’t involved in the study.

at wildlife conservation organisation Revive & Restore, which has been involved in cloning efforts with endangered species including and the , thinks similarly. “They are laying the foundations for a future of manipulating chromosomes that is probably going to be important in many cases for conservation,” he says.

Novak agrees the technique may be useful for frozen tissue samples from endangered or extinct species, where we have samples from only one or two individuals.

A previous study generated both , but it used a method that is unlikely to work in other species, unlike somatic cell nuclear transfer, which has been successfully applied to many mammal species and works with living or stored donor material.

One issue is that, though an XO female mouse is healthy and fertile, that isn’t the case in many other species.

“Y-deleted offspring are not fertile in humans and horses, and therefore this is likely to be the case in multiple wildlife species,” says of SEZARC, a group of zoos and conservation centres based in the US. She also says there could be ethical concerns with producing Y-deleted animals.

Ishiuchi accepts this, but notes that there is research showing that , so it might be possible to do this in combination with Y-CUT to make resulting females fertile in a wider range of species.

Penfold also warns that breeding from clones of a single animal would create genetic issues. “There is a lack of genetic diversity, which is critical for healthy populations,” she says.

Novak is more positive about that aspect. “Sexual reproduction with one individual is certainly better than no sexual reproduction,” he says, but he adds that we should do everything we can now to never have to use such tools. “While they’re awesome, their use indicates that you’ve gotten to a situation that’s so dire that you need sex reversal,” he says.

Habitat protection and reducing hunting will always be much more effective at conserving species, says Penfold.

Within just a 17-kilometre radius in one Indonesian archipelago, there is an explosion almost every hour as locals practise “bomb fishing”, destroying coral reefs and decimating marine life.

Over 483 days between May 2023 and September 2024, at University College London and his colleagues deployed an acoustic monitoring system at Spermonde Archipelago, south-west of Sulawesi, Indonesia.

As well as being a global coral reef biodiversity hotspot, it is a location where fishers detonate explosives underwater, stunning and killing fish that can then be easily harvested.

“The sound of bomb fishing is so loud that if you’re on a dive it can shake you out of your skin,” says Williams. “You feel it more than you hear it. But if you put your head up on the surface, you likely won’t even see the boat responsible because the sound of bomb fishing travels such a huge distance underwater.”

The recording equipment sat at a depth of 17 metres and was able to detect explosions inside a radius of almost 17 kilometres. The team used AI to analyse their 3650 hours of raw audio. They recorded 3567 explosions, or just over 23 per day while the device was underwater.

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“We have shown the true extent of this practice in a known hotspot for the first time ever, which we hope will drive action to curb this destruction,” says Williams.

One of the biggest issues is that there is an illegal supply chain underpinning the bomb fishing, usually involving the sourcing of fertilisers and fuses or detonators, he says. “In other regions these may be stolen mining explosives or even military explosives.”

Once the fishers have the explosives, enforcement at sea is sparse. “It’s like Whac-A-Mole targeting the bomb fishers. The best approaches should tackle the illegal supply chain that gets the materials needed to make explosives to bomb fishers or sellers of these,” says Williams.

Bomb fishing is a problem across the tropics, and part of the solution is retraining locals in sustainable tourism and reef restoration, he says. “It’s been documented in 34 countries, from South America to the Mediterranean to the East Coast of Africa and South-East Asia.”

, who wasn’t part of the work, is currently studying at the University of Queensland, Australia, and has worked extensively with Indonesian non-governmental organisations in reefs targeted by bomb fishing. He says he wasn’t surprised at the scale of the problem.

“I expected the frequency to be high, but studies regarding bomb fishing are relatively scarce because it is notoriously difficult to quantify,” Gotama says.

He says the new method employed by the researchers might help combat the practice. “If this acoustic method could be replicated elsewhere, it could help with detecting and quantifying blast fishing and help identify hotspots of activity,” says Gotama.

“That could be valuable not only for ecological monitoring, but also for targeting enforcement and evaluating whether management interventions are actually reducing blast fishing.”

Two of Antarctica’s most iconic species, the emperor penguin (Aptenodytes forsteri) and the Antarctic fur seal (Arctocephalus gazella), have declined so dramatically and rapidly that they have been classified as endangered on the IUCN Red List.

A third Antarctic species, the southern elephant seal (Mirounga leonina), has had its status updated from “least concern” to “vulnerable”.

The International Union for Conservation of Nature (IUCN) runs the Red List, which is regarded as the world’s most comprehensive listing of the global conservation status of animal, fungus and plant species.

Satellite images show that around 10 per cent of the emperor penguin population was lost between 2009 and 2018, equating to more than 20,000 adults, according to the IUCN. Projections suggest that the population will halve by the 2080s.

“After careful consideration of different possible threats, we concluded that human-induced climate change poses the most significant threat to emperor penguins,” said , at the British Antarctic Survey, and a member of the IUCN Species Survival Commission, in a statement. “Early sea-ice break-up in spring is already affecting colonies around the Antarctic, and further changes in sea-ice will continue to affect their breeding, feeding and moulting habitat.”

The Antarctic fur seal population has decreased by more than 50 per cent from over 2 million mature seals in 1999 to 944,000 in 2025, also due to the impacts of climate change.

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Meanwhile, southern elephant seal populations are being devastated by avian flu, which is killing more than 90 per cent of newborn pups in some colonies, the IUCN said.

at the University of Wollongong, Australia, identified that emperor penguins were the most threatened Antarctic species and were likely to go extinct by 2100.

“As global heating warms the oceans and melts the sea ice, this removes the breeding places which allow emperors to reproduce successfully,” says Robinson. “Like most birds and mammals, penguin chicks need a safe place to develop, and human actions are removing that stable platform at a rapid pace.”

Both Robinson and , also at the University of Wollongong, worked on that had dire warnings about emperor penguins and other Antarctic species.

“Of the 60-plus known emperor colonies around the coast, about half have experienced increased or complete breeding failure events since 2016 due to early fast-ice loss, and 16 colonies have suffered two or more such events,” says Bergstrom. Fast ice refers to sea ice that is attached to the coast or seabed.

“This adds an Antarctica-wide context to the more extreme picture occurring on the Antarctic Peninsula, where we have seen chicks drown through early sea-ice breakout,” she says.

The World Wide Fund for Nature said in a statement the fate of the emperor penguin is “inextricably linked to climate change policy”. “Transitioning away from fossil fuels and limiting global temperature increases to as close as possible to 1.5°C is critical in avoiding the worst effects,” the WWF said.

A camera trap survey has identified more than double the number of critically endangered Sumatran tigers than studies have found elsewhere on the Indonesian island, in a positive sign for protection efforts.

Sumatran tigers are the last remaining population of the subspecies Panthera tigris sondaica, one of two tiger subspecies according to

Working with the Gayo Indigenous people, researchers attached 60 sets of camera traps to trees in a previously unstudied part of the Leuser ecosystem in Sumatra, Indonesia, one of the largest intact rainforests in South-East Asia.

The traps captured images of 17 tigers over a 90-day period in 2023 and 18 tigers in 2024, while previous 90-day surveys on Sumatra spotted only seven tigers on average. In total, the survey identified 14 adult female tigers, 12 males and three sets of cubs, as well as one adult tiger of unknown sex.

“It surpassed my expectations, just because the literature up until this study said that we weren’t supposed to find tigers in these densities,” says at the conservation organisation Hutan Harimau. “So it was a very promising result, testament to a lot of efforts that have been going on in that region.”

Sumatran tigers are threatened by poaching for trophies and traditional Chinese medicine, as well as logging for dipterocarp trees, a valuable hardwood. Because of limited data, the population estimate varies from 173 to 883.

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While the 90-day periods were used for comparison, the researchers ultimately surveyed for 180 days in both 2023 and 2024, which provides a more reliable record across seasons, says Deborah Martyr at the NGO Flora and Fauna International, who was not involved in the study.

The higher-than-expected tiger count in the Leuser ecosystem shows the lowland forests there are rich in prey, especially sambar deer, says Figel. It also suggests monthly rounds made by rangers have deterred poaching.

Near Gunung Leuser National Park, the study area is patrolled by rangers paid by the NGOs Forum Konservasi Leuser and Hutan Harimau, in partnership with the Environment and Forestry Service of the previously war-torn Aceh region.

But research suggests more rangers are necessary. In recent years, tigers have gone extinct in Vietnam, Cambodia and Laos.

“What more of a wake-up call do we need?” says Figel. “The tiger in South-East Asia right now is really in dire straits.”