Alter incubation temperatures to achieve optimal/desired sex ratio: Tuatara

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

  • Two studies evaluated the effects of altering incubation temperatures to achieve optimal/desired sex ratios on tuatara populations. Both studies were in New Zealand.





  • Offspring sex ratio (2 studies): Two replicated studies (including one controlled study) in New Zealand found that hatchling sex ratio of tuatara was affected by temperature, and that warmer temperatures resulted in more males.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated study in 1998 in a captive setting in New Zealand (Nelson et al. 2004) found that incubating tuatara Sphenodon punctatus eggs at higher temperatures resulted in more male hatchlings compared to cooler temperatures. Results were not statistically tested. More male hatchlings were produced at the highest incubation temperature (22°C: 100% of 113 hatchlings were male) compared to the intermediate temperature (21°C: 4% of 80 hatchlings were male) and lowest temperature (18°C: 0% of 105 hatchlings were male). In 1998, a total of 320 eggs were collected either from natural nests (154 eggs from 29 clutches) or by inducing females to lay eggs with oxytocin (166 eggs from 21 clutches). Eggs were incubated in moist vermiculite in plastic containers, with clutches divided equally for incubation at 18°C, 21°C or 22°C. The sex of young tuatara was determined one year after hatchling using a surgical procedure.

    Study and other actions tested
  2. A replicated, controlled study in a captive setting in Wellington, New Zealand (Mitchell et al. 2006) found that incubating eggs of two populations of tuatara (Sphenodon guntheri and Sphenodon punctatus) at higher temperatures produced more male hatchlings. Incubating eggs at higher temperatures resulted in more male offspring (22.1–24°C: 100% of 7–113 eggs produced males) compared to at lower temperatures (18–18.3°C: 0–8% of 12–105 eggs produced males). For one population (Sphenodon guntheri), males were produced above 21.6°C, and for the other population (Sphenodon punctatus), males were produced above 22.0°C. In 2000, a total of 71 Sphenodon guntheri eggs were collected from North Brother Island by inducing gravid females to lay eggs with oxytocin (49 eggs) or removing eggs from nests (22 eggs). Eggs were placed in moist vermiculite and randomly assigned to incubate at 18°C, 21°C, 22°C or 23°C. The sex of these hatchlings was determined via a surgical procedure (see paper for details). In 2003, fifteen eggs from a captive female (Sphenodon punctatus) were incubated at 18°C for seven weeks before being moved to 21.5°C (7 eggs) or 24.1°C (8 eggs). For eggs that failed to develop fully, sex could still be determined in some cases. Data from a number of other studies on incubation temperatures and sex ratios from 1989–1991 and 1999 were also included for comparison (see paper for details).

    Study and other actions tested
Please cite as:

Sainsbury K.A., Morgan W.H., Watson M., Rotem G., Bouskila A., Smith R.K. & Sutherland W.J. (2021) Reptile Conservation: Global Evidence for the Effects of Interventions for reptiles. Conservation Evidence Series Synopsis. University of Cambridge, Cambridge, UK.

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Reptile Conservation

This Action forms part of the Action Synopsis:

Reptile Conservation
Reptile Conservation

Reptile Conservation - Published 2021

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