Temperature-dependent sex determination shows how environmental conditions can direct developmental pathways. In several reptiles, incubation temperature influences whether embryos develop testes or ovaries, linking gene regulation, hormones, and ecological context.

Core idea: temperature can determine sex

In many animals, sex is set primarily by sex chromosomes. In contrast, some reptiles use environmental sex determination, where incubation temperature during embryonic development biases sex outcomes.

TSD is a clear example of a genotype–environment interaction: the embryo’s genes provide the developmental machinery, but temperature alters gene expression and hormone signalling, shifting the developmental outcome.

When temperature matters: timing and thresholds

Temperature does not usually affect sex equally throughout development. Instead, sensitivity is concentrated in a limited window.

Outside the TSP, temperature may still affect growth rate or survival, but it has much less influence on sex fate.

Pivotal temperature and mixed outcomes

Many TSD species show a characteristic sex ratio curve across temperatures, often with:

Empirical sex-ratio curves for a TSD lizard incubated at multiple constant temperatures, illustrating how sex ratios shift across a thermal gradient. Vertical markers indicate pivotal temperature(s), while shaded bands show the transitional range of temperatures (TRT) where both sexes occur. Multiple panels/populations highlight that local adaptation and nesting environments can shift these thresholds and curve shapes. Source

• A pivotal temperature (or narrow range) that produces roughly 1:1 sex ratios

• Increasingly skewed sex ratios as temperature deviates above or below that point

• Variation among populations that reflects local climates and nesting conditions

Common TSD patterns seen in reptiles

Different species show distinct relationships between incubation temperature and sex ratio.

Reaction norms for temperature-dependent sex determination (TSD) showing how the proportion of males changes across incubation temperatures. The shaded regions indicate the transitional range of temperatures (TRT) that yields mixed sex ratios, and $T_{piv}$ marks the temperature producing an approximately 1:1 sex ratio. The panel contrasts Type Ia, Type Ib, and Type II patterns, reinforcing that TSD is species-specific rather than a single universal rule. Source

Frequently discussed patterns include:

• Type Ia: lower temperatures produce mostly males, higher temperatures produce mostly females

• Type Ib: lower temperatures produce mostly females, higher temperatures produce mostly males

• Type II: females are produced at both temperature extremes, with males most common at intermediate temperatures (or the reverse, depending on species)

These patterns reinforce that “temperature determines sex” is not a single rule; it is a species-specific reaction norm shaped by evolution.

Mechanistic basis: how temperature changes developmental pathways

TSD reflects temperature-driven shifts in molecular signalling that regulate gonad differentiation. Key mechanistic themes include:

• Temperature influences gene expression in developing gonads (turning certain regulatory genes “up” or “down”)

• Temperature affects enzyme activity and hormone balance, especially pathways involving estrogens and androgens

• Temperature-associated changes can involve epigenetic regulation (for example, altered DNA methylation patterns that influence transcription during the TSP)

A common mechanistic focus in reptiles is the role of aromatase, an enzyme that converts androgens into estrogens. In many TSD systems:

• Conditions that increase estrogen signalling tend to promote ovary development

• Conditions that reduce estrogen signalling or favour alternative pathways tend to promote testis development

Because these effects depend on both the embryo’s genetic toolkit and its thermal environment, TSD provides a concrete illustration of developmental plasticity.

Ecological and evolutionary significance

TSD can be adaptive when the fitness of males and females differs across thermal environments. Potential advantages include:

• Producing the sex that performs best under the typical conditions associated with certain nest temperatures (for example, differences in body size, growth, or future reproductive success)

• Matching sex ratios to local environments through maternal nesting behaviour (nest depth, shade, timing, and site choice), which can fine-tune incubation temperatures

However, TSD also creates vulnerability when environmental temperatures shift rapidly.

Climate and conservation relevance

Because sex ratios can be strongly temperature-sensitive, climate warming and increased heat extremes may:

• Skew population sex ratios toward one sex

• Reduce effective population size if one sex becomes rare

• Interact with habitat changes that alter nesting microclimates (loss of shade, altered soil moisture)

Understanding TSD therefore links developmental biology to population dynamics: small temperature changes during the TSP can scale up to large effects on future breeding structure.