Can you tell the sex of the hatchling in the photo above just by looking at it? No! In most species, sex is determined by the genes that are passed down from parents to offspring. But, this is not the case for every animal. Many species of reptiles, like some lizards and turtles, use a different method, called temperature-dependent sex determination (TSD). This is a process where the ambient temperature determines the sex of an offspring without the use of sex chromosomes. TSD is the differentiation of gonads depending on the incubation temperature of the eggs (Tezak et al., 2020). Specific incubation temperatures produce males or females, and intermediate temperatures can result in the development of either one (Spotila et al., 1987). There are also three distinct types of TSD, and they affect hatchlings differently.
For most turtle species, temperature plays a huge role in determining the sex of their hatchlings. There are three patterns of TSD that have been discovered so far (Spotila et al., 1987). In the first pattern, ‘Ia’, females are produced at higher temperatures, and this pattern is the one most often seen in turtles. In pattern ‘Ib’, the opposite happens, and higher temperatures produce males. The final pattern ‘Il’ produces females at both high and low temperatures, meaning males are only found at intermediate temperatures (Hulin et al., 2009). It should be noted that there are a few turtle species that use this latter pattern as well. These patterns are determined as they relate to a “pivotal temperature”; an important value representing the temperature where males and females are produced in equal proportions (Hulin et al., 2009).
Research has been done whereby scientists have tested cooler nests versus warmer nests (McCoy et al., 1983). To date, mostly all turtles are known to use pattern ‘Ia’, where warmer temperatures produce females and colder temperatures produce males (Spotila et al., 1987). However, because pattern ‘II’ occurs in some turtles and lizards, it is thought to possibly be the ancestral form which ‘Ia’ and ‘Ib’ originated from (Santidrián Tomillo et al., 2015).
Some turtles that display TSD include many sea turtles such as Green, Loggerhead, and Leatherback (Standora and Spotila, 1985), but also in freshwater turtles such as snapping turtles, Map turtles (Spotila et al., 1987), Painted turtles (Hulin et al., 2009), and Red-eared sliders (Ramsey et al., 2007).
There is a precise period of embryonic development at which this temperature is needed for the development of either ovaries or testis (Hulin et al., 2009). Studies have pinpointed this critical period to the middle-third section of incubation (Spotila et al., 1987). The temperature of incubation needs to be quite precise for this determination. For example, the Red-eared slider needs temperatures of above 31°C to give females, where temperatures under 26°C give males (Ramsey et al., 2007). Any temperature in between gives mixed ratios.
The nesting location therefore has an influence on temperature and therefore the development of turtle hatchlings. In a study done on Green turtle nests, it was observed that the nests located on open beaches mainly produced females, while the nests shaded under vegetation produced 94% males (Standora and Spotila, 1985). This is because the location of these nests, being either in full sun or in full shade, controlled the ambient temperatures experienced by the eggs.
Varying temperature conditions are also known to affect the ratio of males to females in a given nest. In nesting Map turtles on the Mississippi river, this variation based on temperature has been observed. In this study, four different sites produced various ratios spanning from mostly females to mostly males (Spotila et al., 1987). Another variation that may have an impact within the nest itself is pivotal temperature. At this pivotal temperature, nests “react” keenly to slight changes and therefore if nests are near the pivotal temperature generally, simply the metabolic heat can effect sex and cause females to hatch from the eggs at the centre where there is more heat, and males will hatch from along the outskirts of the nest (Standora and Spotila, 1985).
But one cannot determine the sex of hatchlings in their nests easily. There are limited physical or morphological markers and there are considerable ethical issues when sampling nests and hatchlings as well. And while scientists can now guess-timate ratios based on nest and air temperatures, these methods are not reliably accurate. A recent study by Tezak et al, 2020, has helped uncover a method of testing for hormones in the blood of hatchlings, to accurately determine their sex at early ages and with minimally invasive techniques. Red-eared slider hatchlings were tested for Anti-Müllerian hormone (AMH), and found that only male hatchlings possessed this hormone, as it plays a critical role in male differentiation. AMH was found to be an accurate marker of turtle sex in both the red-eared sliders and loggerhead turtles in the study (Tezak et al., 2020).
Something that cannot be ignored when talking about TSD is the dangers that temperature changes can have on populations, especially during climate change. The predicted rate of temperature change is very high over an unusually short timeframe, and can have disastrous consequences on different species, making it more crucial that researchers understand the effects of climate changes on our wildlife and ecosystems. These new extremes in temperatures that are predicted to result from climate change, may modify some life history traits for TSD species, or result in strong sex ratio biases that skew the population towards female majority.
In a study performed on painted turtles, sex ratios were directly correlated with air temperatures (Hulin et al., 2009). This would mean that increasing temperatures produced increasing numbers of females. Having more females than males is not inherently bad, as males reach sexual maturity and can reproduce much quicker than females (Santidrián Tomillo et al., 2015). However, climate change could mean an overabundance of females, which would eventually lead towards extinction (Hulin et al., 2009; Tezak et al., 2020). It is also possible that the ratios could be balanced out over time (i.e. one season of mostly females followed by one of mostly males) (Spotila et al., 1987), but there is no guarantee that the warming temperatures would allow this.
There are a few ways that turtles may adjust to a degree due to increasing temperatures including attempting to choose alternate temperature-specific nesting sites or adjust locations within their territories entirely (phenotypic plasticity; changes made in response to the environment). Other possibilities are microevolution (changes of numbers of genes in a population) (Hulin et al., 2009). However, turtles are generally slow-moving and highly attuned yet also reliant on patterns in the environment, therefore the rate of any adaptations may not be sufficient.
Turtles that can nest multiple times throughout the year also play an important role in balancing these changes, as they can also nest during cooler months (Spotila et al., 1987), however not all species double-clutch (lay twice during the season).
Therefore, human-intervention and habitat restoration becomes more essential; Mitigation techniques development could potentially help reduce the effects of climate change on the nesting turtles, some of which may include nest shading, nest-irrigation, both which could be facilitated through native plantings to extend shade areas and habitat integrity; and also climate-controlled hatcheries (Santidrián Tomillo et al., 2015).
As we’ve seen through all the examples given above, temperature is one of the most important factors when determining the sex of turtle hatchlings. Things like next placement, ambient air temperatures, and the specific patterns in nest construction can also sway the ratio of males and females in each nest individually. It is important for us to remember that while climate change is a growing reality, we can have an impact on these nests and future populations of turtles; be mindful of this when discovering a nest and alert your local conservation organization, such as Turtle Guardians which may have incubation permits; maintain natural plants and habitats to ensure there is shade, moisture and features that support native wildlife too.
Written and Researched by Kiara Duval, December 2021
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