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

Overall effectiveness category Awaiting assessment
Number of studies: 3
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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 c rocodilian nests/eggs for incubation more generally, see Relocate nests/eggs for artificial incubation, 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; Snakes & lizards or Tuatara.

^{Booth D.T. (2006) Influence of incubation temperature on hatchling phenotype in reptiles. Physiological and Biochemical Zoology, 79, 274–281.}

Study locations

Key messages

Three studies evaluated the effects of altering incubation temperatures to achieve optimal/desired sex ratios on crocodilian populations. Two studies were in Argentina and one was in China.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (2 STUDIES)

Reproductive success (2 studies): Two replicated, randomized study in Argentina found that hatching success of broad-snouted caiman eggs was similar across all temperatures tested.

BEHAVIOUR (0 STUDIES)

OTHER (3 STUDIES):

Offspring sex ratio (3 studies): Two replicated, randomized studies in Argentina found that hatchling sex ratio of broad-snouted caimans was affected by temperature, and that warmer temperatures resulted in fewer females. One replicated study in China found that exposing Chinese alligator eggs to short periods of high temperatures during incubation resulted in fewer female hatchlings.

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

A replicated study in 1988–1989 in laboratory conditions in Anhui, China (Zhang 1995) found that short exposure to high temperatures during incubation of Chinese alligator Alligator sinensis eggs resulted in fewer female hatchlings compared to when temperatures were kept constant. Results were not statistically tested. Hatching success ranged from 90–100% (10–20 eggs/group). Less females were produced from eggs exposed to 34°C for 4–7 days (0 of 10 to 2 of 15, 0–13% female hatchlings) compared to when eggs were incubated at 31–32°C (15 of 20, 75% female hatchlings). Eggs were incubated at 31–32°C, and nine groups of 10 eggs were exposed to 34°C for four continuous days starting on the 14th and 24th day of incubation. One group of 16 eggs was exposed to seven days at 34°C from the 24th–31st day of incubation. An additional group of 20 eggs was incubated at 31–32°C for the whole duration. Tissue samples were assessed to determine the sex of hatchlings.
Study and other actions tested
Referenced paper
Zhang Z.D. (1995) Research on the sex sensitive period during the incubation of Chinese alligator eggs. Asiatic Herpetological Research, 6, 157-160.
A replicated, randomized study in 2013 in laboratory conditions in Santa Fe province, Argentina (Parachú Marcó et al. 2015) found that altering the incubation temperature of broad-snouted caiman Caiman latirostris eggs did not affect hatching success, but that females were only produced below a temperature threshold. Hatching success was similar across all temperatures (26 of 30, 88% at 31°C; 25 of 29, 85% at 33°C; 23 of 29, 78% at 34°C). Incubation at 31°C produced all females (46 eggs), whereas incubation at 33°C (45 eggs) and 34°C (43 eggs) produced all males. In 2013, a total of 134 viable eggs were collected from four wild nests and clutches were split evenly between three incubation temperatures (31, 33 or 34°C) with two groups/temperature. Eggs were incubated in moist vermiculite at high humidity. Forty-six eggs were dissected during development, just after the thermosensitive period when sex is determined. Sex was assessed by histological methods (46 embryos and 14 hatchlings) or by visual examination four months after hatching (74 hatchlings).
Study and other actions tested
Referenced paper
Parachú Marcó M.V., Piña C.I., Somoza G.M., Jahn G.A., Pietrobon E.O. & Iungman J.L. (2015) Corticosterone plasma levels of embryo and hatchling broad-snouted caimans (Caiman latirostris) incubated at different temperatures. South American Journal of Herpetology, 10, 50-57.
A replicated, randomized study (year not provided) in Santa Fe province, Argentina (Parachú Marcó et al. 2017) found that when altering incubation temperatures of broad-snouted caiman Caiman latirostris eggs, lower temperatures resulted in a higher number of female hatchlings compared to higher temperatures. At 31°C, all hatchlings were female, and at 33°C and 34°C all hatchlings were male (number of eggs/treatment not provided). At 32°C an average of 72% of hatchlings were female, but this varied from 17–100% depending on the nest of origin. Hatching success varied from 78–91% and was not affected by incubation temperature. A total of 172 eggs that were judged to be viable (by presence of opaque banding on egg) were collected from nine wild nests. Eggs were incubated at 32 or 33°C in the first year, and 31, 33 or 34°C in the second year. In both years, there were two groups/temperature, and eggs were split evenly between groups (number/treatment not provided). A total of 141 hatchlings were kept in captivity for four months, after which point sex was determined using histological methods (100 individuals) or by a visual examination (24 individuals).
Study and other actions tested
Referenced paper
Parachú Marcó M.V., Leiva P., Iungman J.L., Simoncini M.S. & Piña C.I. (2017) New evidence characterizing temperature-dependent sex determination in broad-snouted caiman, Caiman latirostris. Herpetological Conservation and Biology, 12, 78-84.

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.

Where has this evidence come from?

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This Action forms part of the Action Synopsis:

Reptile Conservation

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