Can you imagine if the climate your mother lived in determined your gender in utero? Fortunately, for us mammals, this is not the case. However, nest temperature during incubation can influence the gender of many egg-laying reptile species, including turtles, crocodiles and lizards; this phenomenon is called ‘temperature-dependent sex determination’. If the nest is too hot, all the young might end up female but if it's too cold, they could all end up male – mothers aim for the perfect middle temperature, just like Goldilocks’ preferred porridge. Even though mums choose nesting sites with the best conditions for their eggs, they can't control the weather once the eggs are laid. Sexual differentiation doesn't occur in turtles until after about three weeks of age, which places the vulnerable clutch at risk of producing an uneven ratio of girls to boys, jeopardising their chances of reproducing in the future. As global warming continues and temperatures creep upwards, there is concern that this could spell disaster for species whose biology is influenced by developmental temperature.
But imagine if a reptile embryo could determine its own gender by moving from one side of the egg to the other. Wei-Guo Du from the Chinese Academy of Sciences and his collaborators from the same institution and Macquarie University, Australia, wanted to find out if this was possible for the eggs of an endangered freshwater turtle. They thought that if a turtle embryo could move and find the best temperature inside its egg, it could buffer the effects of unfavourable nest temperature. But is this behaviour possible on such a small scale? To test this, they ran a neat experiment. First, they measured the temperature at each end of eggs inside nests. Then, they gave the eggs a drug that stops embryos from sensing temperature and moving. They also located and marked the exact position of the embryonic turtles by holding up the eggs to a light at the beginning and end of the experiment. To complete the study, they predicted how hatchling sex ratios may differ 50 years into the future under a medium-level greenhouse gas and aerosol emissions climate change scenario.
The nest temperature that produced only females was a mere 5°C warmer than that resulting in only males, but the team found the temperatures at opposite ends of a tiny 34 mm turtle egg could differ by up to 4.7°C. Their main prediction also proved true: embryonic turtles moved to their preferred temperature inside the eggs, influencing their gender. Finally, the researchers’ computer modelling revealed that the sex ratios of nests with embryos that couldn't respond to temperature by moving would become more and more uneven with ongoing climate change. This is a big problem, because the survival of future turtle populations relies on there being enough females to reproduce and lay their own clutches of eggs.
Du and colleagues have revealed a fascinating and effective way that turtles can control their temperature during development to avoid the negative effects of less-than-ideal nest temperatures. However, it's worth remembering that turtles have been around for millions of years – more than enough time to have experienced numerous changes in climate – so is this behaviour really important? The team thinks so. The species-specific temperature that produces an even ratio of females to males can evolve and change over time in response to climate shifts, but it doesn't happen overnight. Coping with the swift speed of human-driven global warming needs faster reactions to temperature – such as those behaviours displayed by the turtle embryos in their eggs – to help animals survive in the short term while the evolutionary changes slowly catch up.