Alter incubation temperatures to achieve optimal/desired sex ratio: Snakes & lizards
Overall effectiveness category Awaiting assessment
Number of studies: 4
Background information and definitions
Incubation temperatures (for example warmer or cooler, constant or fluctuating) can influence the sex, size, shape, colour, behaviour, movement ability and post-hatching growth of reptile hatchlings and newborns (Booth et al. 2006). Practitioners carrying out conservation activities aimed at maximising hatching success, such as relocating eggs for artificial incubation or to on-beach hatcheries, will therefore need to consider the potential impact of temperature during incubation on hatchlings and populations. Human-induced climate change may also influence the sex ratios of some species of reptiles and limit the viability of populations over time. It may be possible to counter the impacts of climate change on affected populations by managing temperatures during incubation to create appropriate sex ratios.
This action includes studies that test the impact of different temperatures on the sex ratio of reptile hatchlings or newborns. For studies that discuss the effectiveness of relocating snake and lizard nests/eggs for incubation more generally, see Relocate nests/eggs for artificial incubation: snakes or lizards, Relocate nests/eggs to a nearby natural setting (not including hatcheries), and Relocate nests/eggs to a hatchery. For studies that discuss the effectiveness of captive breeding more generally, see Breed reptiles in captivity.
Due to the number of studies found, this action has been split by species group, though no studies were found for amphisbaenians. See here for: Sea turtles; Tortoises, terrapins, side-necked & softshell turtles; Crocodilians or Tuatara.
Booth D.T. (2006) Influence of incubation temperature on hatchling phenotype in reptiles. Physiological and Biochemical Zoology, 79, 274–281.
Supporting evidence from individual studies
A randomized study (year not provided) in laboratory conditions in California, USA (O’Donnell & Arnold 2005) found that maintaining pregnant female garter snakes Thamnophis elegans at intermediate temperatures in captivity increased overall embryo survival rates, male offspring stillbirths was also reduced at higher temperatures and temperature did not affect live hatchling sex ratios. When pregnant female garter snakes were maintained at 26.6°C, embryo survival rate was higher than at lower (21–24°C) or higher temperatures (28–33°C; data reported as model outputs). Rates of male offspring stillbirths reduced at intermediate and higher temperatures (data reported as model outputs). Incubation temperature did not affect the sex ratio of live offspring (see paper for details). Seventy-four wild pregnant female garter snakes were brought into captivity and maintained at one of nine constant temperatures (21, 24, 26, 27, 28, 29, 30, 32, and 33°C; 2–14 females/temperature) until giving birth. Females were on average in the 20th day of pregnancy when temperature management began. In total 504 snakes were born.Study and other actions tested
A replicated, randomized, study in 1998 in a laboratory in Zhejiang, China (Du & Ji 2008) found that altering the incubation temperature of stripe-tailed ratsnake Elaphe taeniura eggs did not affect the sex ratio of hatchlings. The ratio of males to females varied from 2:5 to 13:6 and was not influenced by temperature (result presented as statistical test). In 1998, thirteen captive-born gravid females were acquired and housed in a wire cage (200 x 80 x 80 cm) at 30°C. Eggs were incubated at 22, 24, 27, 30 or 32°C, with eggs from each clutch split evenly between temperatures. Eggs were incubated individuals in covered plastic jars in vermiculite and water at a ratio of 1:2. Hatchlings were euthanized by freezing to -15°C to allow their sex to be determined.Study and other actions tested
A replicated study in 2011 in Gansu, China (Tang et al. 2012) found that altering the incubation temperature of eggs from two species of toad-headed agamas Phrynocephalus przewalskii and Phrynocephalus versicolor did not influence the sex ratio of hatchlings. Sex ratio of hatchlings for Phrynocephalus przewalskii (61:53 ratio of females to males) and Phrynocephalus versicolor (50:36 ratio of females to males) were not affected by incubation temperature or moisture content of incubation medium. In addition, the highest temperature resulted in lower hatching success for both species (Phrynocephalus przewalskii: 34°C: 32–36%; 26–30°C: 40–53%; Phrynocephalus versicolor: 34°C: 11–22%; 26–30°C: 52–76%), although this result was not tested statistically. In 2011, wild female lizards of both species were captured and housed in groups of 15 in cages (800 x 360 x 400 mm) with a sand substrate. Temperatures of 25–37°C were available during the day and were 20°C at night. Eggs were collected (Phrynocephalus przewalskii: 263 eggs from 101 females; Phrynocephalus versicolor: 185 eggs from 66 females) and assigned to three temperature (26, 30, 34°C) and two moisture level (2 g water/5 g vermiculite, 2 g water/8 g vermiculite) treatments. Eggs were incubated in plastic containers (150 ml).Study and other actions tested
A replicated, randomized study in 2010–2011 in laboratory conditions in Iowa, USA (Telemeco 2015) found that the sex ratio of southern alligator lizard Elgaria multicarinata hatchings was not affected by incubation temperature and that hatching success was highest at intermediate temperatures. Sex ratio was not affected by incubation temperature, and overall, 15 of 21 (71%) hatchlings were male. In addition, hatching success was higher at intermediate temperatures (19 of 24, 79% at 26°C; 21 of 24, 88% at 28°C) than at the coolest (2 of 6, 33% at 24°C) or highest temperatures tested (11 of 25, 44% at 30°C; 0 of 6, 0% at 32°C), though this result was not tested statistically. Eggs were incubated in individual glass jars (140 ml), half buried in moist vermiculite (water potential of -150 kPa), and jars were covered with clear plastic wrap. Eggs were split between five temperature treatments: 24°C (6 eggs); 26°C (24 eggs); 28°C (24 eggs); 30°C (25 eggs); and 32°C (6 eggs). Sex was determined by assessing gonadal morphology (at six months) or histology (at 30 days).Study and other actions tested