For most animals, the sex of their offspring is determined by genetics. However, for tuatara, a lizard-like reptile that inhabits select New Zealand islands, the number of males versus females is related to the temperature during a specific period of the egg’s development.
The name for the scientific concept of temperature influencing the sex of your offspring is environmental sex determination (ESD) and is a trait found in many reptiles. For tuatara, the warmer it is, the more likely an egg is to develop into a male, and an incubation temperature of 69.8˚F gives about an equal chance of either gender developing.
What’s so special about these creatures? Tuatara are native to New Zealand, the only place in the world that they can be found. They are special for another reason, too: they are the only living members of the order Rhynchocephalia, which has earned them the nickname “living fossils.”
Tuatara stand out reproductively as well. It can take them up to 20 years to reach the age of reproduction, and once they do, the females only lay eggs every 2 to 5 years. They can also live to be more than 100 years old.
In a recent PLOS ONE study, the authors looked at a population of Tuatara living on North Brother Island in New Zealand to investigate the current male-to-female ratios and predict what these numbers may look like if temperatures increase based on current climate models.
For this study, the authors first looked at surveys conducted between 1988 and 2001 and found that approximately 60% of the population was male. They then looked at more recent surveys conducted between 2005 and 2011 to compare temperatures and sex ratios, the results of which revealed a 70% male population.
The authors then used a model to predict the ratio of males to females that might be born in future years, based on the soil temperature in various nesting sites. From this, they estimated that an increase of less than 1 degree Celsius would over time shift the population to 57% male, and an increase by 3-4 degrees Celsius could eventually shift the entire population to be male. Unfortunately, a large sex bias, especially toward males, has the potential to put tuatara at risk for extinction.
Though this population of tuatara has not yet been classified as endangered, the authors suggest that the potentially large effect a small temperature change could have on the population means that further population monitoring will be important. Increases in the temperature of their environment, combined with their slow reproductive cycle, means that it could be difficult for them to “bounce back” from population shifts toward fewer females. The authors note that in such a case, human interventions, such as artificially incubating eggs or modifying habitat, may be necessary to save the species.
The loss of tuatara would not only result in an extinct species, but the entire order Rhynchocephalia would be lost. However, since this species has not yet reached an endangered status, we are provided with an opportunity to take action. Future studies and monitoring of tuatara may allow us to prevent the grim fate that all other species of this order have reached.