Many species have dubious futures in the face of climate change. But sea turtles have a particularly pressing problem: their sex is determined by temperature.
Australia has ecologically and culturally significant populations of sea turtles, that also have value to the tourism industry. Some of them live out their lives in Australia, while others just nest here. Some of the nesting sea turtle populations in Australia are globally significant. These populations are among the largest in the world.
Climate change is expected to affect turtles in multiple ways and at all life stages. The most detectable impacts will happen during the part of their life turtles spend on land reproducing: the egg laying, egg incubation and hatchling success phases. There are clear, and relatively straightforward, effects of increased temperature, sea level rise, and cyclonic activity on sea turtles’ nesting sites and their reproductive output.
The sex of sea turtle hatchlings, hatching success, and hatchling quality are determined by incubation temperatures. Successful incubation of sea turtle eggs occurs within a tight range: 25 to 33°C. Incubation above the thermal threshold will result in babies with more physical abnormalities and lower hatching success.
Sea turtles’ sex is also determined by temperature; this is known as temperature-dependent sex determination (TSD). The ratio of males to females is determined by nest temperature during incubation. Warmer temperatures (higher than approximately 29°C) produce females. Cooler temperatures produce more males.
Higher sand temperatures - even small increases - also lead to shorter incubations, which decreases the young’s body size and mass. It is likely that smaller body size reduces hatchlings’ chances of survival, since some studies indicate that smaller offspring are more likely to be eaten as they cross the reef.
So a predicted increase in temperature could potentially cause lower hatching success and a gradual shift towards too many female turtles. Indeed, our recent research predicts hatchlings in the northern Great Barrier Reef green turtle population (the largest green turtle population in the world) will be feminised by 2030.
Predictions are bleaker for 2070, when some of the places these turtles nest could have temperatures near or above the upper thermal incubating threshold (33°C). This is likely to cause a decrease of hatching success. However, we did find that some nesting grounds will still produce male hatchlings, even under the most extreme scenario of climate change.
We don’t entirely understand what will happen if turtle nesting grounds are feminised. Some nesting beaches have persisted with strong female biases over a few decades or even longer. Further there is no evidence that a low production of male hatchlings has resulted in a low reproductive success within populations (although it is possible that the long-term population declines due to exploitation and other factors may mask such effects).
New evidence on sea turtle male mating patterns indicates that turtles may not be as vulnerable to warming temperatures and predicted feminisation as first anticipated. Wright et al found that when there are more females than males, males may mate more often than females, who tend to wait two to four years between reproducing. These researchers also found that males move between aggregations of receptive females and may visit multiple nesting grounds.
This behaviour will help to preserve genetic variation. This variation may be critical if marine turtles are to adapt behaviourally or physiologically to a warming climate. Most likely, genetic variation has contributed to their persistence through historical climatic upheaval.
Sea turtles’ reproductive rates will also be affected by cyclonic activities and sea level rise. Sea-level rise and cyclonic activity may wipe out or alter nesting beaches and destroy eggs.
The risk to sea turtle populations from climate change will depend on the turtles’ ability to adapt to changing conditions. Sea turtles have survived climate fluctuations during their evolutionary history. But their ability to do so again is uncertain, because current rates of climate change are much faster than historic rates. Today, there are many additional anthropogenic pressures - such as pollution and fishing - to cope with as well.
Nevertheless, sea turtles have evolved to be flexible. They may adapt to climate change through shifts in the distribution of their nesting areas or through changes in nest site selection, nesting season, and nest depths. They may also change their pivotal temperatures (that is, the temperature at which a nest produces a 50:50 hatchling sex ratio) and alter their migration routes.
We can’t be sure if sea turtles will adapt on their own. Precautionary human actions may be necessary to increase the turtles’ chances of adjusting. More than ever, we should try to reduce the impacts of other anthropogenic threats that sea turtles currently face and protect known habitats so that turtles can increase their resilience and have a better chance to adapt.