Release reptiles born/hatched in captivity from wild-collected eggs/wild-caught females without rearing

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

Study locations

Key messages

  • Five studies evaluated the effect on reptile populations of releasing reptiles born/hatched in captivity from wild-collected eggs/wild-caught females without rearing. Four studies were in the USA and one was in Australia.



  • Reproductive success (1 study): One replicated, controlled study in the USA found that for plains gartersnakes released as newborns, two of over 350 released snakes were found to be gravid two years after release.
  • Survival (5 studies): One before-and-after study in the USA found that survival of captive-born desert tortoises released as hatchlings was similar over six months compared to hatchlings that were head-started in indoor or outdoor enclosures. One replicated, controlled study in the USA found that alligator hatchlings released into their mother’s home range had higher survival than those released outside her home range. Three replicated studies (including one controlled study) in Australia and the USA found that 11% of Murray short-necked turtles and 7% of plains gartersnakes survived for 1–3 years after release, and first year survival of gopher tortoise hatchlings released into a predator proof enclosure was around 30%.
  • Condition (2 studies): One before-and-after study in the USA found that captive-born desert tortoises released as hatchlings grew more slowly over six months than hatchlings head-started in an indoor enclosure. One replicated, controlled study in the USA found mixed effects on growth of alligator hatchlings released inside or outside of their mother’s home range compared to wild 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

  1. A replicated study in 1996–2000 in two lagoons in south-eastern Australia (Spencer & Thompson 2005) found that releasing captive-born Murray short-necked turtle Emydura macquarii hatchlings from wild-caught females resulted in some surviving for 1–3 years after release. The number of hatchlings that were recaptured over a three-year period was similar at both lagoons (38 of 328, 12% and 30 of 281, 11%) (number of released hatchlings taken from methods). In 1996–1997, gravid female turtles were captured and induced to lay their eggs (number of turtles and method not given). Eggs were artificially incubated, and hatchlings were released in to one of two lagoons (281 and 328 hatchlings each). Fox control was undertaken at one lagoon in May 1997 to January 1999 using poison baits and shooting. Recapture of turtles was carried out in 1998–2000.

    Study and other actions tested
  2. A replicated, controlled study in 1995–2001 on an urban river bank with a mix of mown lawns and riparian vegetation in Illinois, USA (King & Stanford 2006) found that captive-born plains gartersnakes Thamnophis radix from wild-caught, gravid females that were released as newborns had low survival in the wild, which was similar to wild-caught newborns. Twenty-seven of 362 (7%) captive-born snakes released as newborns and 2 of 15 (13%) wild-caught newborns were recaptured one or more years after release or initial capture. Seven snakes released as s reached maturity during the study, and two of these were gravid (aged 21–22 months old). In 1995–2001, gravid females were captured (number not given) and maintained in captivity until giving birth. Snakes born in 1998 (137 snakes), 2000 (188 snakes) and 2001 (71 snakes) were released within 2–27 days of birth. Recapture effort varied between months and years, but most snakes were recaptured by hand in April–June 1998–2001.

    Study and other actions tested
  3. A replicated, controlled study in 1998–1999 in three lakes in Florida, USA (Temsiripong et al. 2006) found that captive-born American alligator Alligator mississippiensis hatchlings from wild-collected eggs that were released in their mother’s home ranges had higher survival than those released outside of their mother’s home ranges and similar survival but less growth than wild hatchlings. The chance of recapture after nine months was similar for hatchlings released in their mother’s home range (22% recaptured) and wild alligators (23%), whereas hatchlings released outside their mother’s home range had a lower chance of recapture (15%). Hatchlings released in their mother’s home range were shorter (42 cm) than wild hatchlings (45 cm) nine months after hatching, but hatchlings released outside their mother’s home range were similar in length to both other groups (42 cm). Clutches of alligator eggs were collected in summer 1998 from three lakes and artificially incubated at 32°C and hatched. Hatchling alligators were held in captivity for 2–4 weeks before being tagged and released either near to the original nest site (34 clutches) or more than 1 km outside the mother’s home range (14 clutches). Wild hatchlings (22 clutches) were collected by hand from boats in September to November 1998, tagged, measured and released.  Clutches ranged from 8–41 hatchlings. Alligators were recaptured from May to July 1999 (347 hatchlings from 67 clutches).

    Study and other actions tested
  4. A replicated study in 2007–2010 in open mixed pine forest in Georgia, USA (Tuberville et al. 2015) found that approximately a quarter of captive-born gopher tortoise Gopherus polyphemus hatchlings from wild-collected eggs initially released into predator-proof cages, and then into the wild on an island, survived the first year. Results were not statistically tested. In three consecutive years, survival rates of captive-born hatchling gopher tortoises released into predator-proof cages were 20–29% (178 tortoises in total) in the first year following release. In 2007–2009, gopher tortoise eggs were collected from the wild (from nests or gravid females), or private collections and incubated at 28–30°C. After emergence, 178 gopher tortoise hatchlings were released shortly after hatching into temporary predator-proof release cages (190 cm long x 122 cm wide x 33 cm high, 10–15 individuals/cage) near to abandoned burrows on a 5,670 ha island. Hatchlings remained in cages for two–four weeks before being released. All hatchlings were monitored by live trapping for two weeks in September–October 2007–2010 as well as opportunistically during other trapping exercises in the same years.

    Study and other actions tested
  5. A before-and-after study in 2015–2016 in desert scrubland in California, USA (Daly et al. 2018) found that almost all released captive-born desert tortoise Gopherus agassizii hatchlings from wild-collected eggs survived at least six months in the wild, and that hatchlings that were head-started outdoors or indoors had similarly high survival during their head-starting. Survival rates of released hatchlings was similar to that of tortoises during head-starting over six months (released: 15 of 20, 75% survived; during outdoor head-starting: 20 of 20, 100%; during indoor head-starting: 29 of 30, 97%). After six months, released tortoises in the wild were a similar size compared to tortoises during outdoor headstarting (released: 49 mm long; during outdoor head-starting: 51 mm long), but were smaller than tortoises during indoor head-starting (78 mm long). The relative weights and body conditions of tortoises were similar after six months, regardless of rearing approach (see original paper for details). Eggs from 25 wild adult female tortoises were collected, incubated in artificial burrows outside and hatched in August–September 2015. In September 2015, seventy hatchlings (21–46 days old) were either released directly into the wild (20 hatchlings) or moved to either an indoor enclosure (30 hatchlings) or outdoor enclosure (20 hatchlings). Direct-release hatchlings were released in a 0.7 km2 unfenced area and monitored using radio telemetry twice weekly until November 2015, once a week in winter and twice weekly from March 2016. The indoor enclosure was climate controlled and hatchlings were fed five times/week and watered weekly (see paper for details). The outdoor enclosure (30 x 30 m) was semi-natural, predator-proof and hatchlings were provided supplemental food and water weekly until the end of the active season (November 2015). Hatchling morphometrics were assessed prior to their release or before being moved to their head-starting enclosure in September 2015 and again at least once in March–April 2016.

    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

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