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Climate change is affecting gender ratios in fish, and could hamper their ability to return to a 50:50 balance. Author provided

Climate change can tip the gender balance, but fish can tip it back

As the world warms, it is crucial to understand how animals will respond to and cope with rising temperatures. Some species might be relatively unaffected by climate change, while others may be seriously disrupted.

One vital aspect is how populations maintain their numbers over time through reproduction. It might seem obvious, but an important component of successful reproduction is having the correct ratio of males and females ready to breed within the population.

Animals have evolved a number of different strategies to achieve and maintain the the optimal ratio, although some of these are dependent on the temperature in the ambient environment.

In some species, for example, the gender that juveniles become is influenced by temperatures experienced during early development. This is known as “temperature sex determination”, and is seen in a broad range of animals including turtles, alligators, lizards and fish.

Yet there is concern that global warming poses a threat to entire populations of these animals because of the effect of warming on the gender ratio of juveniles, which has a knock-on effect on breeding adults in the population.

If water temperatures warm over the Great Barrier Reef, then it could negatively impact breeding in some species of fish. Author provided

Out of balance

For many animals, including fish, a warmer environment means fewer females develop. Even relatively small increases in developmental temperatures – just 1.5°C above normal summer temperatures – can reduce the proportion of female offspring by more than 30%.

Previous research has investigated how parents can change the timing of breeding or move the location and depth of their nest to compensate for warming. For example, lizards can select nest locations with differing amounts of sunlight to obtain the correct nest temperature.

My colleague and I investigated the ability of parents to adjust the sex of their offspring back to the optimal ratio through other non-behavioural means.

We reared three generations of a marine fish, the Spiny Chromis coral reef fish, and found that when parents develop from hatching at elevated temperatures, they can adjust their offspring gender back to the ideal 50:50 ratio. This phenomenon is called “transgenerational plasticity”.

We were able to show that the gender balance of offspring was restored when parental fish were reared at 1.5°C temperature for their entire life and for two subsequent generations.

Some fish can correct for the effects of higher temperatures on sex ratio, but only to an extent. Author provided

Limited resilience

Unfortunately, it was not all good news. We found only a slight improvement in the sex ratio occurred when temperatures were 3°C above average conditions, even after two generations. This suggests a limitation on the ability of a species to maintain a balanced sex ratio when the water temperature is too high above normal.

We also found that for parents to compensate for the effects of higher temperatures on gender, they must develop from early life at the warm conditions themselves. Parents were not able to adjust the gender of offspring when they only reproduced in warm conditions.

No-one yet knows quite how the Spiny Chromis engineers these amazing adjustments, and it is something we are now investigating. What we do know is that oceans are warming. And transgenerational plasticity has been able to reduce the negative impacts of climate change on offspring gender of a coral reef fish, at least so far.

This represents good news for many species, with negative predictions made from other short-term experiments likely overestimating the true future impacts. However, in many cases the ability for parents to improve their offspring may be limited if warming is too great.

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