Head-start wild-caught reptiles for release: Crocodilians

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

Study locations

Key messages



  • Abundance (2 studies): Two studies (including one replicated study) in the Philippines and Nepal reported that following releases of head-started crocodiles or gharials, wild populations increased in size over 8–9 years.
  • Reproductive success (2 studies): One replicated study in Argentina reported that released head-started broad-snouted caimans had similar clutch sizes and hatching success compared to non-head-started caiman. One replicated study in Nepal reported successful reproduction in all four rivers where head-started gharials were released.
  • Survival (5 studies): Three studies (including one replicated, controlled study) in Venezuela, the Philippines and Nepal reported that 88% of head-started Orinoco crocodiles survived 8–12 months and 53% of Philippine crocodiles or gharials survived for one year following release. One study also found that survival of Philippine crocodile hatchlings during the captive phase of head-starting was higher than for non-head-started hatchlings in the wild. One replicated study in Argentina reported that at least five released head-started broad-snouted caimans survived 9–10 years. One replicated study in Zimbabwe found that 38% of released head-started Nile crocodiles were recaptured at least once over four years. This study also found that hatching success of Nile crocodile eggs in the head-start programme was 74%, and that survival of hatchlings during the captive phase was lowest during the first year.
  • Condition (1 studies): One study in Venezuela found that released head-started Orinoco crocodiles grew at a similar rate to resident juvenile crocodiles.


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 1967–1974 in three sites along the Zambezi River in Zimbabwe (Blake & Loveridge 1975) found that some released head-started Nile crocodiles Crocodylus niloticus survived at least six months in the wild, and that mortality during head-starting was highest during the first year. Over seven years, hatching success of Nile crocodile eggs in a head-start programme was 74% (16,697 of 22, 697 eggs hatched). In one site, hatchling mortality from six annual cohorts was 8–52% in the first year, 1–14% in the second year and 0–4% in the third year (see original paper for further details). Twenty of 53 (38%) released head-started crocodiles were caught at least once in four years following release (see original paper for details). In 1967–1973, Nile crocodile eggs were collected from the wild, and hatchlings were head-started at three rearing stations (at Kariba, Binga and Victoria Falls) as part of a crocodile farming initiative (128–2,475 eggs collected/station/year). Eggs were artificially incubated in captivity (no details are provided). An annual quota set by the government required 5% of three-year-old crocodiles were returned to the wild. In total, 355 head-started crocodiles were returned to the wild by the end of 1973, of which 53 released into one site were monitored by twice-yearly recapture surveys in 1970–1974.

    Study and other actions tested
  2. A study in 1991–1992 in a river near San Jose, southwestern Venezuela (Muñoz & Thorbjarnarson 2000) found that after releasing head-started Orinoco crocodiles Crocodylus intermedius, some survived at least a year. Seven crocodiles survived for at least 235–352 days, and one was killed accidentally two weeks following release. Crocodiles moved an average of 4–5 km/month (maximum distance 12 km). The average growth rate of released crocodiles (4 of 8 released individuals) was 0.1 cm/day, which was comparable to some smaller, wild-caught juveniles (0.1 cm/day) (result not statistically tested). In 1987, eggs were collected from along the river and hatched in captivity. Eight male crocodiles were head-started (length range from 115–139 cm) and released in March or April 1991. Crocodiles were fitted with radio transmitters and located every 1–2 days from April 1991 to March 1992.

    Study and other actions tested
  3. A replicated study in 2001–2002 in Santa Fe province, Argentina (Larriera et al. 2006) found that some released head-started female broad-snouted caiman Caiman latirostris survived at least 9–10 years and bred in the wild. Seven released head-started female caiman (Five 9-year-olds and two 10-year-olds) nested within 1 km of their release sites. Clutch size and hatching success of wild-collected caiman nests was similar to wild nests left in situ (wild-collected clutch size: 26–41 eggs/nest and hatching success: 43–100%; wild in situ nests: no data provided). Since 1990, a head-starting programme collected caiman eggs from wild nests in December–January and artificially incubated the eggs.  Hatchlings were head-started for up to nine months and then released back into the wild at the collection site (see original paper for details). In austral summer 2001–2002, seven head-started female caiman were captured while guarding their nests. Eggs were collected from the nests and artificially incubated. Head-starter clutch size and hatching success data were compared with nests (clutch size comparison was with 31 nests; hatching success comparison was with 11 nests).

    Study and other actions tested
  4. A replicated, controlled study in 2005–2009 in a captive facility and ponds, creeks and rivers in northern Luzon, Philippines (van de Ven et al. 2009; same experimental set-up as van Weerd et al. 2010) found that most head-started Philippine crocodiles Crocodylus mindorensis survived rearing in captivity and at least half survived their first year in the wild. After one year in captivity, head-started Philippine crocodile hatchling survival was 72% (63 of 88), compared to 47% (17 of 36) for wild hatchlings (results were not statistically tested). After one year in the wild, at least 17 of 32 (53%) head-started hatchling crocodiles were still alive. Authors reported that the released head-started crocodiles adapted well to natural conditions and increased in size. In 2005–2008, crocodile hatchlings were collected from the wild just after hatching (88 individuals) and 32 crocodiles were released back into their natural habitat after being head-started for 14–18 months (31 still held in captivity in 2009). Two ponds (75–450 m2) were created to provide suitable release habitat. Growth and survival was monitored by night surveys for one year after release. In 2000–2006, thirty-six wild hatchlings were monitored every three months for one year to compare survival rates.

    Study and other actions tested
  5. A study in 2002–2004 on the Narayani and Rapti rivers, Chitwan National Park, Nepal (Ballouard et al. 2010) found that approximately half of released captive reared gharials Gavialis gangeticus survived at least a year following their release. Nineteen of 36 gharials released survived approximately one year, with two surviving at least two years. Captive reared gharials were released into two different river sections in March 2002 (10 gharials) and March–April 2003 (26 gharials). Individuals were monitored by kayak in November 2002–April 2003 and November 2003–May 2004. Tags and notches on tail scales were used for identification. Gharials were from wild-collected artificially-incubated eggs. Hatchlings were reared until 4–7 years (average body size 1.5 m long) before being released.

    Study and other actions tested
  6. A study in 1999–2009 in freshwater and riparian zones in northern Luzon, Philippines (van Weerd et al. 2010; same experimental set-up as van de Ven et al. 2009) found that after releasing head-started Philippine crocodiles Crocodylus mindorensis, wild crocodile populations in sanctuaries managed by local communities increased in size. Following regular releases of wild-born Philippine crocodiles head-started in captivity, the crocodile population increased to 65 individuals in 2009 from 12 individuals in 2000. The authors reported that survival rates were high (no data are provided) and released crocodiles had no problems adapting to living in the wild. The authors reported that most people in the area knew that crocodiles are legally protected and no crocodiles were killed in the sanctuaries since 2007. After a small population of crocodiles was discovered in 1999, three crocodile sanctuaries were created. Between 2000 (start year not provided) and 2007, wild-born hatchling crocodiles were head-started in captivity for 14 months and then released into the wild. Details of numbers of crocodiles released each year and monitoring were not provided. A communication, education and public awareness campaign about the risks facing crocodiles was carried out in the local rural communities. Crocodile sanctuaries were protected by paid local community members.

    Study and other actions tested
  7. A replicated study in 2004–2016 along four rivers in lowland Nepal (Acharya et al. 2017) found that in three of four rivers where head-started gharials Gavialis gangeticus had been released, more gharials were counted after eight years and there was evidence of breeding in the wild in all rivers. Results were not statistically tested. In three of four rivers where head-started gharials were released, more gharials were counted in 2016 (Narayani river: 84 individuals; Rapti river: 82 individuals; Babai river: 31 individuals; Karnali river: 1 individual) compared to eight years previously (in 2008, Narayani river: 34 individuals; Rapti river: 23 individuals; Babai river: 10 individuals; Karnali river: 6 individuals). Over the same time period, subadults were observed in all four rivers and hatchlings in three of four rivers (see original paper for details). In 1981–2016, eggs were collected from wild nests and gharials were hatched and head-started in captivity and released aged 4–7 years into the Narayani (~397 individuals), Rapti (~477 individuals), Karnali (~41 individuals) and Babai (~111 individuals) rivers. Gharials were surveyed by boat in November–March over several years between 2004–2016 in Narayani-Rapti rivers or 2008–2016 in Karnali-Babai rivers. Observed gharials were grouped into age classes.

    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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