The question of whether lizards can change sex involves examining the complex biological mechanisms that determine sex in reptiles. Unlike mammals, where sex is rigidly set by chromosomes, many lizard species exhibit flexibility influenced by the environment. This phenomenon, known as sex reversal, demonstrates that an individual’s genetic blueprint for sex can be overridden during development. The science behind this involves an intricate interplay between an embryo’s genes and the temperature of its surroundings, which dictates the physical development of its reproductive organs.
Genetic and Environmental Sex Determination
A lizard’s sex is typically established by one of two primary biological systems: genetic sex determination (GSD) or environmental sex determination (ESD). GSD is the system most familiar in vertebrates, where specific sex chromosomes dictate whether an individual develops as male or female. Lizards that use GSD often follow the ZZ/ZW system, where males possess two Z chromosomes (ZZ) and females have one Z and one W chromosome (ZW).
Other lizard species rely on ESD, most commonly temperature-dependent sex determination (TSD). In TSD, the temperature experienced by the egg during a specific window of embryonic development determines the sex of the hatchling. Incubation at one temperature range might produce only males, while another range produces only females. This system highlights the direct influence of the external environment on development.
The distinction between GSD and TSD is not always absolute, as some species exhibit both systems. In these cases, the genetic sex is the default setting, but it can be overruled by specific environmental conditions. Understanding this dual mechanism provides the context for comprehending how sex reversal, where the environment trumps the genes, can occur.
Sex Reversal: When Biology Flips the Switch
Lizards in certain species can indeed change sex, but this is a developmental change known as sex reversal, not a change in adulthood. Sex reversal occurs when an individual possesses the chromosomes for one sex but develops the physical anatomy and reproductive function of the opposite sex. This is most famously observed in the Central Bearded Dragon (Pogona vitticeps), an Australian lizard species that naturally employs a GSD system.
In Central Bearded Dragons, genetically male embryos (ZZ chromosomes) can develop as functional females if their eggs are incubated at high temperatures, typically between 34 and 37 degrees Celsius. These sex-reversed individuals are genetically male but phenotypically and reproductively female, capable of laying viable eggs. This outcome is a clear demonstration of an environmental factor overriding the instruction set provided by the sex chromosomes.
The resulting sex-reversed females are reproductively functional, which is a distinction from sequential hermaphroditism, where an adult changes sex. In lizards, the change is established early in embryonic development, fundamentally altering the individual’s sexual fate before hatching.
Hormones and Temperature: The Mechanisms of Change
The mechanism by which temperature dictates the sex of a developing lizard embryo is regulated by the conversion of sex hormones. Temperature influences the activity of a specific enzyme called aromatase. Aromatase is responsible for converting androgens (male sex hormones like testosterone) into estrogens (female sex hormones).
High incubation temperatures trigger an upregulation in the production and activity of the aromatase enzyme within the embryonic gonads. This increased enzymatic activity leads to a higher concentration of estrogens during the thermosensitive period—the critical developmental window for sex determination. The resulting elevated estrogen levels effectively feminize the developing gonads, causing them to differentiate into ovaries even in a genetically male embryo.
Conversely, lower temperatures maintain a reduced level of aromatase activity, which limits the conversion of androgens to estrogens. This low estrogen environment allows the indifferent gonads to follow the default genetic pathway toward male development, resulting in testes. The sensitivity of the aromatase enzyme to temperature is the molecular switch that translates an environmental cue into a permanent sexual outcome.
The Ecological Advantage of Sex Flipping
The capacity for sex reversal in lizards is an adaptive trait that provides an ecological benefit, particularly in fluctuating environments. By allowing temperature to influence sex ratios, populations can maintain reproductive viability even when faced with climate variations. For species like the Central Bearded Dragon, high temperatures that induce sex reversal produce more females in the population.
This ability to produce a higher proportion of females under warmer conditions can buffer the population against skewed sex ratios. If a warming climate were to reduce the number of females, the population would face a reproductive bottleneck. The mechanism of sex reversal ensures that reproduction can continue by generating fertile females from genetically male eggs. This flexibility may also accelerate the evolutionary transition from a purely genetic system to a temperature-dependent system. This environmental sensitivity helps maintain population stability and reproductive fitness.