For many species, increasing global temperatures are having detrimental effects.
Interactions between the environment, plants, animal physiology and behavior, and climate are all so complex that small changes can have a huge impact long term. For many species of reptiles, including the Tuatara, temperature plays a huge role in their physiology and life (Mitchell et al. 2008). Temperature sex determination is a phenomenon unique to reptiles where the sex of a growing embryo is determined by the outside temperature at a critical part of development called the thermosensitive period (Mitchell support for rare).
During this time, the gonads develop and lead to the embryo developing into a male or female (Mitchell support for rare). There are two types of sex determination (TSD); in Type I TSD, there is a pivotal temperature that determines whether the embryo will become male or female. Type I TSD is split into two smaller groups: Type Ia which exhibits a male-female pattern, and Type 1b which exhibits a female-male pattern. In Type II TSD, there are two pivotal temperatures where extreme temperatures will result in one sex, whereas “average” temperatures will result in the other.
Within both types, there is a lot of variation among different species when it comes to pivotal temperatures and patterns. The Tuatara is a very unique species of reptile because it exhibits Type 1b TSD, and shows several very primitive characteristics of ancestral reptiles including a full diapsid skull and no pairing organ (terranature). Tuataras are the only extant reptile in the Order Sphenodontia, found on islands off the coast of New Zealand, and have survived several mass extinctions since the Triassic and Jurassic period (Nelson)(Mitchell support for rare). The Tuatara is a very interesting species to research when it comes to the effects of climate change on temperature sex determination. Tuataras are one of the longest surviving reptiles, and have been able to overcome changes in climate in the past; however, with limited numbers, a female-male Type 1b TSD, and unbalanced sex-ratio increasing temperatures could have a huge impact on the Tuatara population.
Tuataras exhibit Type 1b temperature sex determination (TSD) which is when males are produced at higher temperatures and females at lower (Nelson). The pivotal temperature for producing males has been shown to be between 21.6°C and 22.0°C, with a thermosensitive period being between 0.25 and 0.55 for gonad development (Mitchell support for rare). Type 1b TSD is typically rare for reptiles, with Type II being much more common where a second, higher pivotal temperature produces females to have a female-male-female pattern (or vise-versa); however, various studies have shown that TSD Type II does not occur in the Tuatara and higher temperatures will continue to produce males. Currently, sex ratios in Tuatara populations on different islands in New Zealand range between 50-60% male (Nelson). Tuataras are long-lived reptiles with a life-span of up to 100 years. Females lay eggs every 2-5 years, eggs may take up to a year to hatch, and offspring will take on average 10 years to reach sexual maturity (Huey). Because of the long incubation period, slow reproduction rate, and rare Type Ib TSD, Tuataras may be extremely vulnerable to the changing climate (Huey).
Increasing temperatures have already been shown to impact Tuatara populations causing a rise in the percentage of males. Since males are produced at higher temperatures, and the pivotal temperature for Tuataras is already low for reptiles, increasing temperatures could lead to a male-biased, or even predominantly male population. Mitchell et al (from binder) predicted that as temperatures continue to increase, sex-ratio in Tuatara clutches could become all male. It is estimated that it takes less than a 1°C increase in temperature to produce male biased clutches, and if temperatures increase to 4°C, all-male clutches could be produced (Huey). Since Tuataras have such a long period until maturity, and slow reproductive rate, even just a few seasons of warmer temperatures could have a huge impact on overall sex-ratio and the population (Nelson). Furthermore, Tuataras already have such a small population range, and are currently restricted to islands, so the possibility of migration and movement to cooler temperatures — like some species have done — is not possible for Tuataras to do naturally. Having Type 1b TSD presents even more complications because creating more males in the population will not lead to higher reproductive rates.
There are a few different ways that Tuataras may be able to combat climate change and increasing global temperatures naturally (Huey). TSD species have been shown to be able to adapt both biologically and behaviorally. Tuataras may be able to change their thermosensitive period and pivotal temperature over time to increase the temperature that females can be developed (Huey). In some cases, TSD reptiles could possibly change to genetic sex determination (GSD) which would create a more balanced sex-ratio (Huey). However, even though these adaptations could happen, it is very unlikely that the Tuatara will be able to make these adaptations because of their breeding styles, long life span, and very stable genetic makeup (Huey). A more feasible, behavioral adaptation would be if Tuataras changed their nesting sites and selected for cooler nesting areas (Huey). Several reptile species with TSD have been shown to be able to do this; however, it is done more on a geographic scale. In any case, if Tuatara females were able to lay eggs in slightly cooler areas, better sex ratios could be restored and it could be a good way to survive the increasing temperatures.
Unfortunately, with the rate that global temperatures are increasing, and since the physiology and behavior of the Tuatara is so genetically fixed, scientists will most likely have to get involved to help save the population. Nelson et al. (2002) proposed that translocation of the species to its original habitat might be the only way to restore the population in a more natural way. Tuataras were originally found throughout New Zealand, but as the area became more populated, rat, cat, and dog predation wiped out the Tuataras there. Introducing them back to the mainland of New Zealand would require more conservation action to limit predation (Nelson). Nelson et al (2002) did a conservation study to test the success of lab-raised Tuataras. They collected eggs from the wild, reared them in the laboratory, and then reintroduced the young Tuataras back into the population. The study found that there was a 57% success rate after 5 years (Nelson e2002l).
Although intense conservation efforts and laboratory rearing may be the best way to save the Tuatara population, it will take many years to see real results and success of the study. Similarly, if human involvement and help from scientists can help balance out the sex-ratio in Tuatara populations short-term, it may take decades for Tuataras to be self-sustaining and