The research, conducted at the Ethological Station of the University of Bern in Switzerland, tracked 33 captive-bred tokay geckos from 16 breeding pairs across two breeding rounds. All eggs were incubated naturally within the parents’ enclosures, allowing temperature and humidity to vary around the clutches rather than being imposed by an incubator. Tiny waterproof data loggers called iButtons, tied close to each clutch, recorded temperature and humidity every 15 minutes for at least a week, giving the researcher a picture of the microclimate each embryo actually experienced. The average temperatures recorded across clutches ranged from about 26.1 to 29.6 degrees Celsius, a span of just 3.5 degrees.

That narrow range turned out to matter enormously. Warmer incubation temperatures were strongly associated with shorter incubation times, with hatching occurring anywhere between 78 and 138 days after eggs were laid. Bayesian statistical models, which estimate the probability that a predictor genuinely belongs in a model, gave strong support for this temperature effect. Higher temperatures were also linked to larger hatchlings, measured by snout-vent length, with very strong statistical support. Notably, no hatchlings in the study showed any of the deformities that are known to occur when reptile eggs are incubated at suboptimal temperatures, suggesting that the range experienced in the enclosures remained within a healthy developmental window.

Perhaps the most intriguing result concerns sex. Tokay geckos possess XX/XY sex chromosomes, which in textbook genetics should determine their sex. But the study found that a higher proportion of phenotypic males hatched at higher average incubation temperatures, and correspondingly more females at lower temperatures, a pattern echoing earlier observations in the closely related Schlegel’s Japanese gecko. The statistical evidence was comparatively weak, likely owing to the small sample size, but both the raw data and the model comparison pointed in the same direction. Sex was determined visually by the presence or absence of femoral glands, structures that males display prominently.

How can a species with sex chromosomes show a temperature-linked sex ratio? Herpetologists have documented similar puzzles before. Central bearded dragons, for instance, carry ZZ/ZW micro sex chromosomes, yet at incubation temperatures above 32 degrees Celsius genetically male embryos routinely reverse into functional females. For tokay geckos, two candidate explanations remain. One is temperature-dependent sex reversal, in which genetic males develop as phenotypic females at certain temperatures. The other is sex-biased embryo mortality: if female-producing temperatures kill male embryos, the surviving hatchlings skew female. In several clutches incubated at the cooler end of the range, mothers removed one of two eggs, leaving a single hatchling, an observation that could hint at selective mortality but could equally reflect simple disturbance. Without genetic sex testing, the study cannot distinguish between these mechanisms, and the author flags this as a priority for follow-up work.

Humidity, by contrast, showed no clear relationship with sex, incubation duration or hatchling size in the posterior estimates, even though model comparisons suggested the predictors deserved their place in the models. A recent meta-analysis across egg-laying reptiles found that wetter substrate conditions generally produce larger hatchlings, so a subtle humidity effect in tokay geckos cannot be ruled out with the current sample. What was unambiguous was nest choice. By placing loggers at matched but egg-free control locations in the same enclosures, the study showed that females consistently deposited their clutches at spots that were warmer and drier than the alternatives, with very strong statistical support. The same nest sites were even reused by different females across years, evidence that captive geckos actively seek out the microclimates that suit their embryos when given the choice.

The second half of the study asked whether the social environment after hatching matters as much as the environment before it. In commercial and laboratory settings, hatchling reptiles are typically raised in isolation, which simplifies cleaning and health monitoring but bears little resemblance to the lives of naturally social species. Tokay geckos are facultatively social, and adults perform biparental care of their offspring, making them an ideal species in which to test the costs and benefits of social rearing. The researchers therefore divided hatchlings between two treatments: some stayed in the family enclosure with their parents and zero, one or two siblings until six months of age, while others were removed immediately and raised entirely alone.

The outcome, at least for physical development, was a null result: group size had no detectable effect on snout-vent length, growth rate or body condition at six months, and tail loss from family interactions was observed in only two individuals. This contrasts with earlier work in other lizards. Juvenile veiled chameleons raised alone show deficits in social and feeding behaviour, while in tree skinks, housing with a social partner reduced growth and subordinate individuals were more likely to lose their tails. The author suggests that the spacious, naturalistic enclosures and scatter feeding used in the Swiss colony may have diluted competition, allowing family groups to develop without the resource conflicts seen in more confined setups. Notably, a companion study by the same researcher found that socially raised tokay geckos were bolder and more variable in their associative learning, hinting that the consequences of isolation may surface in behaviour rather than body size.

The body condition analysis did reveal some surprises of its own. Males at six months were in better condition than females, individuals from later clutches were in better condition than those from earlier ones, and hatchlings that began life larger tended to end their first half-year in better shape. Body condition was calculated using the scaled mass index, a method that adjusts an animal’s mass for its body length and is widely regarded as a robust measure of energetic reserves. An apparent difference in body condition across rearing group sizes in the raw data, backed up by model comparison, was tempered by the small and unbalanced sample, leaving the question open but tantalising.

For anyone breeding tokay geckos, the practical message is clear: provide options. Females that can choose among nest sites with a gradient of temperature and humidity will pick warmer, drier spots for their eggs, and those choices appear to influence how quickly embryos develop, how large hatchlings emerge and possibly what sex they become. For conservationists, the stakes extend beyond the terrarium. Captive breeding underpins reintroduction programmes for threatened reptiles worldwide, and animals raised under impoverished or unnatural conditions may be poorly prepared for life in the wild. The study also highlights a conspicuous gap in basic natural history: no one appears to have documented the natural range of nest temperatures for wild tokay geckos, information that would be essential for assessing how climate warming might affect this widespread but increasingly pressured species. Until that fieldwork is done, the humble cork bark panel on the back of a terrarium may be one of the best windows into what these geckos really need.

Subject of Research: Effects of incubation temperature and humidity and post-hatching social rearing on the development of captive-bred tokay geckos.

Article Title: The influence of the pre- and post-natal environment on Tokay gecko ( Gekko gecko , Squamata , Gekkonidae ) development in captivity

Article References: Szabo, B. (2026). The influence of the pre- and post-natal environment on Tokay gecko (Gekko gecko, Squamata, Gekkonidae) development in captivity. Herpetozoa, 39, 217-228. https://doi.org/10.3897/herpetozoa.39.e165454

Image Credits: AI Generated

DOI: 10.3897/herpetozoa.39.e165454

Keywords: tokay gecko, Gekko gecko, incubation temperature, temperature-dependent sex determination, reptile captivity, nest site selection, humidity, social rearing, captive breeding, animal welfare, herpetology, gecko development