The climate is changing, the ice caps are melting, the fungi are fruiting later, and winter coats are no longer needed in November. We are all so closely connected to the change in our climate. If humans continue to take a “business as usual” stance in the way we use the world’s resources, temperatures are predicted to rise by 3.7 to 4.8°C over the next century (IPCC). For certain species, especially those whose biological processes depend heavily on specific temperatures, this could pose as a great problem.
What is TSD?
Certain species of reptile (among other vertebrate taxa) are very closely connected to their external temperatures, not just because of their ectothermic nature but also because of a certain temperature-sensitive reproductive strategy that some tend to demonstrate.
This strategy is called temperature-dependent sex determination (TSD).
It differs from the more common method of genotype sex determination (e.g. XX = female, XY = male in humans) in that it is not determined by the combination of different sex chromosomes. Instead, this phenomenon occurs when the gender of a developing embryo is determined by its external temperature, which can be further influenced by such factors as the mother’s choice of location, nesting material and structure. Males typically occur when eggs are incubated at cooler temperatures and females at warmer temperatures, however, as always in nature, there are exceptions. During the critical window of time called the thermosensitive period, specific temperatures are needed for an equal balance of sex ratios within the population, meaning that if these temperature points happen to change by even a few degrees, we are likely to see some skewed gender ratios appearing.
Enter: the changes
A paper published by Allen et al, (2015)1 assessed the population sex ratio of the threatened East Pacific green turtle (Chelonia mydas), finding a female skewed sex bias of 2.83 females: 1 male. Although they suggest that the skew may be beneficial for the endangered population, due to the increase in number of breeding females, the authors expressed concern about a future increase in ratio skew, or “feminization” in the context of climate change.
Changes in temperature may not only alter sex ratios but may cause problems to individual reproductive abilities. A study by Warner & Shine (2008)2 looked at the adaptive significance of TSD using Jacky Dragon lizards. By manipulating the sex-determining hormones testosterone and oestrogen, they artificially produced males in female-suitable temperatures and females in male-suitable temperatures. The study followed all the offspring throughout their lives and saw that those unlucky individuals who had been forcibly subjected to a sex change had severely worse reproductive abilities as a result.
What does all of this mean? That we may not only see some abnormal sex skews in the future, which may lead to a change in reproductive strategies, but that offspring resulting from the temperature change may have a hard time making babies.
But it’s not all ‘doom and gloom’
We should perhaps remember, however, that climate change and global warming are natural phenomena that have occurred multiple times throughout the earth’s history, and we can expect species to respond in a number of ways. One method for these awkwardly adapted reptiles would be to adapt their reproductive strategy from TSD to GSD instead to stabilise sex ratios. This is feasible because TSD and GSD have independently evolved multiple times in vertebrates 3. Another option is for their distribution to shift to more suitable environments, though this may not be possible for creatures requiring specific niches or living on islands, such as the tuatara in New Zealand. Individual plasticity in breed timing has also been identified as a possible off-setter to the effects of climate change on sex ratio determination. However, a study using painted turtles (Chrysemys picta) suggests that the ability to change nesting phenology is not likely to be quick enough to offset the effect of changing temperature 4.
So what of the future of our reptiles?
Failing these more plausible adaptations, things could start to look a bit more interesting. Another reproductive method performed by some reptiles is the “asexual” process of parthenogenesis, which doesn’t always require a male mating partner. So who knows? We may see an end to sexual reproduction all together and begin to see more female East Pacific green turtles breeding “Russian-doll” style. Hypothetically. Personally I think that would be a more interesting evolutionary pathway.
However, some studies have suggested that the use of a TSD strategy has lead to the loss of species in the face of climate change, and this has been posed as a potential reason for the decline of extinct reptiles like the dinosaurs 5. So for the sake of the wonderful extant creatures currently exhibiting this unique strategy of TSD, one can only hope that the evolutionary “adapt or die” scenario will fall towards the former option, (unlike our well-loved dinosaurs).
- Allen CD, Robbins MN, Eguchi T, Owens DW, Meylan AB, Meylan PA, et al. First Assessment of the Sex Ratio for an East Pacific Green Sea Turtle Foraging Aggregation: Validation and Application of a Testosterone ELISA. PLoS One 10, (2015).
- Warner, D. & Shine, R. The adaptive significance of temperature-dependent sex determination in a reptile. Nature 451, 566–568 (2008).
- Tokarz, R. R. & Summers, C. H. Hormones and Reproduction of Vertebrates. Hormones and Reproduction of Vertebrates 1, (2011).
- Schwanz, L. E. & Janzen, F. J. Climate change and temperature-dependent sex determination: can individual plasticity in nesting phenology prevent extreme sex ratios? Physiol. Biochem. Zool. 81, 826–834 (2012).
- Miller, D., Summers, J. & Silber, S. Environmental versus genetic sex determination: A possible factor in dinosaur extinction? Fertil. Steril. 81, 954–964 (2004).