Reptiles exhibit diverse methods of sex determination, with external factors sometimes influencing offspring sex. While reptiles do not typically undergo a post-maturity “gender change” like some fish, their embryonic development is sensitive to environmental cues, leading to what is known as sex reversal.
How Sex is Determined
Sex in reptiles is primarily determined by two distinct mechanisms. Genetic Sex Determination (GSD) involves specific sex chromosomes dictating whether an individual develops as male or female.
The second mechanism is Temperature-Dependent Sex Determination (TSD), where temperature during a critical embryonic development period determines the hatchling’s sex. For many species, specific temperature ranges lead to the development of males or females. For example, in many turtle species, warmer temperatures produce females, while cooler temperatures result in males.
Environmental Factors Influencing Sex
Environmental Sex Reversal (ESR) occurs when external conditions override the genetic blueprint for sex, causing an individual to develop as the opposite sex from its chromosomal designation. This phenomenon primarily happens during embryonic development, not in adult reptiles. In species with GSD, extreme incubation temperatures can sometimes trigger ESR, leading a genetically male individual to develop as a functional female, or vice versa.
Beyond temperature, certain pollutants and chemicals can also influence sex development. Endocrine-disrupting chemicals (EDCs) interfere with hormonal pathways involved in sex differentiation, potentially altering sex ratios or leading to intersex individuals. For example, studies on alligators show that exposure to certain contaminants can cause abnormalities in reproductive organs and endocrine balance, impacting sex development.
Reptile Species Known for Sex Reversal
Crocodilians, including alligators and crocodiles, universally exhibit TSD, with specific temperature thresholds determining offspring sex. Some lizards, such as the leopard gecko, also display TSD patterns.
The bearded dragon (Pogona vitticeps) is a prominent example; genetically male (ZZ) bearded dragons can develop into functional females when their eggs are incubated at high temperatures, typically above 32°C. These sex-reversed females are reproductively viable. Another example is the eastern three-lined skink (Bassiana duperreyi), where cooler temperatures can turn genetic females into males.
The Biological Purpose of Sex Reversal
The existence of sex determination mechanisms like TSD and ESR offers adaptive advantages, particularly in fluctuating environments. These mechanisms allow reptile populations to adjust their sex ratios in response to environmental conditions, potentially optimizing reproductive success and population survival. If conditions favor the production of one sex, TSD or ESR can help ensure enough individuals of that sex are born, maintaining a balanced population over time.
However, the adaptive benefits of these mechanisms also present vulnerabilities in the face of rapid environmental changes. Global warming, for example, poses a significant threat to species with TSD and ESR. Sustained increases in incubation temperatures could lead to severely skewed sex ratios, potentially resulting in an overabundance of one sex and a scarcity of the other. Such imbalances can reduce breeding success and, in extreme cases, lead to population declines or even extinction if species cannot adapt quickly enough to the shifting thermal landscapes.