Action

Head-start wild-caught reptiles for release: Snakes & lizards

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

Study locations

Key messages

  • Nine studies evaluated the effects of head-starting wild-caught snakes and lizards for release. Five studies were in the USA, two were in Puerto Rico and one was in each of the Cayman Islands and Jamaica.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (9 STUDIES)

  • Abundance (2 studies): Two studies (including one before-and-after and one replicated study) in Jamaica and the Cayman Islands reported that the number of Jamaican iguanas found in the wild was higher after 23 years of head-starting and releasing compared to at the start of the programme and that there was a stable population of blue iguanas over four years during ongoing releases of head-started individuals.
  • Reproductive success (4 studies): Four studies (including two replicated studies) in Jamaica, Puerto Rico and the USA reported successful reproduction following release of head-started Jamaican iguanas (but not for 16 years) and Mona Island iguanas, and that timber rattlesnakes copulated or participated in pre-copulatory behaviour. One study also reported that 88–90% of Mona Island iguana eggs hatched successfully.
  • Survival (8 studies): Two of three controlled studies (including one replicated, randomized study) in the USA found that head-started plains gartersnakes and common water snakes were recaptured a similar number of times or had similar survival compared to resident snakes. The other study found that head-started northern water snakes had lower survival following release than resident snakes. One study also found that 76% of snakes survived the captive phase of head-starting. Three studies (including two replicated studies) in the USA and Puerto Rico reported that 22–40% of timber rattlesnakes or Mona Island iguanas survived for monitoring periods of eight months to six years. One replicated study in the USA found that head-started eastern massasaugas released in summer had higher survival than snakes released in autumn. One before-and-after study in Jamaica reported that 16% of Jamaican iguanas died during the captive phase of head-starting.
  • Condition (5 studies): Two of three controlled studies (including one replicated, randomized study) in the USA found that head-started northern water snakes and common water snakes grew more slowly than resident snakes. The other study found that head-started plains gartersnakes had similar growth rates to resident snakes. One study also found that head-started common water snakes had similar body condition to resident snakes. One controlled study in Puerto Rico found that body condition of head-started Mona Island iguanas was higher than wild iguanas before release, but similar at their first recapture after release. One replicated study in the USA found that more head-started eastern massasaugas released in summer gained weight before hibernation than snakes released in autumn.

BEHAVIOUR (3 STUDIES)

  • Behaviour change (3 studies): One of three studies (including one replicated, randomized, controlled study) in the USA found that head-started common water snakes showed similar behaviour to residents across a range of behaviour measures. One of the studies found that head-started northern water snakes had smaller home ranges and showed less surface activity than resident snakes. The other study found that head-started eastern massasaugas released in summer had larger home ranges than snakes released in autumn.

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 1993–1999 in a hardwood forest in eastern Texas, USA (Conner et al. 2003) found that some released head-started timber rattlesnakes Crotalus horridus survived for at least 2–6 years following release. Eight of nine released snakes survived for one year following release and at least two survived for six years. The status of a further three snakes was unknown after two years. Three of nine head-started rattlesnakes were observed mating or participating in pre-mating behaviour five years after release.  Nine young snakes (8 from a single adult female tracked near the eventual release site) were captured and housed in individual cages for six months (1 snake), 12 months (4 snakes) or 18 months (4 snakes). Snakes were released in March 1994 (1 snake), August 1995 (4 snakes) or February 1996 (4 snakes). All snakes were surgically implanted with transmitters and located weekly in March–November for 4–6 years.

    Study and other actions tested
  2. A replicated study in 1999–2001 in two sites of mixed wetland and scrub oak in Wisconsin, USA (King et al. 2004) found that head-started eastern massasaugas Sistrurus catenatus catenatus released in summer had lower mortality rates, larger home ranges and gained more mass compared to snakes released in autumn. Summer released snakes had lower mortality (7 of 15, 47% snakes died during hibernation) than autumn release snakes (14 of 15, 93% died either before, during or immediately after hibernation). Summer released snakes had larger home ranges (12 ha) than autumn release snakes (1 ha), and 65% (11 of 17) of summer released snakes gained weight prior to hibernation compared to 0% (0 of 15) of autumn-released snakes. Pregnant female snakes from three locations in Wisconsin USA were captured and 50% of each brood was retained for head-starting. Thirty-two head-started snakes were released with radio transmitters either in September 1999 (15 snakes, 1–3 years old) or July 2000 (16 snakes, two years old) and located daily after release until hibernation. All surviving snakes were re-captured in April 2001 and placed back in captivity.

    Study and other actions tested
  3. 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 high survival during head-starting of plains gartersnakes Thamnophis radix, and that post-release survival was comparable to wild-caught snakes. Overall survival during head-starting was 76% (217 of 286 snakes). The number of snakes recaptured one or more years after release was similar for head-started (5% of 142 snakes and 26% of 53 snakes) and wild-caught snakes (20% of 80) (result not statistically tested). Growth rate was similar for head-started and wild-caught snakes (data reported as statistical model result). Three head-started females were gravid when recaptured (23–24 months old). In 1995–2001, gravid females were captured (number not given) and maintained in captivity until giving birth. Snakes born in 1995 and 1996 (53 snakes) were head-started for 327–335 days, while those born in 1999 (142 snakes) were head-started for 253–260 days. Recapture effort varied between months and years, but most snakes were recaptured by hand in April–June 1998–2001.

    Study and other actions tested
  4. A replicated study in 1999–2004 in a subtropical dry forest site on Mona Island, Puerto Rico (García et al. 2007) found that some released head-started Mona Island iguanas Cyclura cornuta stejnegeri survived in the wild and two females were observed breeding. Forty percent (4 of 10) of iguanas survived >1 year in the wild, and at least 30% (3 of 10) survived >2 years. At least 50% of the females (2 of 4) bred in 2004. Four of five (80%) iguanas released at their point of capture survived at least 96 days and two of five (40%) released at a new site survived at least 33 days. Hatchlings were collected from the wild in November 1999 and reared until they reached a target size (snout-vent-length: 25 cm; mass: >950 g). Iguanas were implanted with radio transmitters (12 g) and marked with PIT tags and coloured beads on the crest. In April–August 2002, five individuals were released at their point of capture, and five at a new site. All iguanas were monitored daily until radio transmitters failed, and monitored by active searching thereafter.

    Study and other actions tested
  5. A controlled study in 1999–2006 in a subtropical dry forest site on Mona Island, Puerto Rico (Pérez-Buitrago et al. 2008) found that some released head-started Mona Island iguanas Cyclura cornuta stejnegeri survived up to five years in the wild and at least two reproduced successfully. Twenty-five of 62 (40%) head-started iguanas (15 females, 10 males) were re-captured between 8–61 months following release. Two head-started females produced clutches of 8 and 11 eggs each, and 86–91% of eggs hatched successfully. Body condition of head-started iguanas was higher before release than wild iguanas but was similar at their first capture post-release (data reported as condition index). Sixty-two hatchlings were collected from the wild by digging up nests in October 1999 (8 nests) and 2000 (6 nests), and transported to fenced enclosures. Hatchlings were marked with PIT tags and weighed and measured every 4 months while in captivity. Iguanas were marked and released at their nesting site in April 2002 and October 2003 after reaching at least 620 g and 225 Snout to vent length (about 3 years of age). Seven mid-sized and 31 adult wild iguanas were also captured and measured. Intensive trapping was conducted during 2–3 months in 2003–2006.

    Study and other actions tested
  6. A controlled study in 2008–2009 in a site of mixed hardwood forest and scrub patches in Indiana, USA (Roe et al. 2010) found that head-started northern water snakes Nerodia sipedon sipedon had lower survival than resident snakes, as well as lower movement and growth. Following release, 8 of 12 head-started snakes survived until hibernation (67% survival over 5 months), but none survived one year, whereas seven of 12 resident snakes survived until hibernation (58% survival) and four survived to the end of the year (33% survival). Head-started snakes had smaller home ranges (head-started: 2 ha; resident: 5 ha) and grew less than resident snakes (head-started: 0.03 cm/day & 0.05 g/day; resident: 0.07 cm/day & 0.80 g/day). Head-started snakes also showed less surface activity than residents (reported as activity index). In July 2007, seven pregnant snakes were captured and gave birth in captivity before being returned to their capture site. Sixty newborn snakes (30 females, 30 males) were housed in small plastic boxes (20 x 65 x 13 cm) for 11 months and 12 snakes (9 females, 3 males) were chosen for release. Twelve resident snakes (matched in terms of size and sex) were captured in May 2008. All snakes were implanted with radio transmitters and were located once/week from May–September, every two weeks from October–November and March–April, and monthly from December–February.

    Study and other actions tested
  7. A replicated study in 2004–2013 in a dry shrubland site in a reserve on Grand Cayman, Cayman Islands (Burton & Rivera‐Milán 2014) found that releasing head-started blue iguanas Cyclura lewisi resulted in a stable population over four years, but that the population size remained lower than the number of iguanas released. Six years after the first head-started iguanas were released, but while releases were ongoing, 46 and 42 iguanas were re-sighted in 2010 and 2013 respectively, and densities were estimated at 5–6 iguanas/ha. Authors reported that released iguanas were also sighted outside of the study area. In 2004–2009, a total of 307 head-started iguanas were released, and in 2010–2012, a further 98 iguanas were released. All iguanas were tagged at time of release with unique coloured glass bead combinations (as piercing on the neck crest) and PIT tags. In the first three weeks of March 2010 and 2013, iguana surveys were conducted twice a day by teams of two observers on 12 transects of unequal length (range 323–432 m). All offspring that were captured were also tagged.

    Study and other actions tested
  8. A replicated, randomized, controlled study in 2007–2010 in mixed wetland, shrubland and hardwood forest in Indiana, USA (Roe et al. 2015) found that common water snakes Nerodia sipedon sipedon that were released following two methods of head-starting had similar survival rates and showed similar behaviour compared to resident snakes, but grew more slowly. Annual survival following release was similar for head-started snakes (basic conditions: 64%, semi-natural conditions: 50%) and resident snakes (46%). A range of behaviour and activity measures, including post-release movement, habitat use, and hibernation date were also similar between head-started and resident snakes (see paper for details). Head-started snakes grew more slowly than resident snakes (head-started: 0–0.05 cm/day; resident: 0–0.11 cm/day), but body condition remained similar between all groups (data presented as statistical model result). In July 2007, seven pregnant female snakes were captured and brought into captivity. Sixty offspring were raised for 18 months in individual plastic tubs (20 x 64 x 13 cm) containing a water bowl and hide. In February 2009, snakes were divided in to two groups and raised for a further four months in either basic conditions (remaining in the plastic tub) or semi-natural conditions (see paper for details). After 22 months in captivity, head-started snakes (basic conditions: 12 snakes; semi-natural conditions: 10 snakes) and an additional 15 resident wild snakes were released, and radio-tracked 1–4 times/month throughout the year. Of the resident snakes, eight were tracked in 2008–2009, three in 2009–2010, and four during both seasons.

    Study and other actions tested
  9. A before-and-after study in 1991–2015 in old-growth tropical dry forest in Jamaica (Wilson et al. 2016) found that after releasing head-started Jamaican iguanas Cyclura collei (along with associated actions), the number of nesting female and hatchling iguanas increased over 23 years. Results were not statistically tested. After 23 years of head-starting and releasing Jamaican iguanas, 321 iguana hatchlings and 63 nesting female iguanas were counted compared to 31 hatchlings and 9 nesting females at the start of the programme. The first new wild-born female iguana joined the breeding population after 16 years. The authors reported that health of head-started individuals was generally good but that 16% died or were lost prior to being released. In 1991–2015, Jamaican iguana eggs and hatchlings were collected from the wild and head-started in a zoo. Head-started individuals were released in 1996 (278 total iguanas released, usually 6–8 years old or 1–2 kg). In 1997–2014, non-native mammalian predators (mongoose Herpestes javanicus, cats Felis catus, dogs Canis lupus familiaris and feral pigs Sus scrofa) were removed using baited cage traps, snares and leg-hold traps (around 1,500 predators in 350,000 trap days over 17 years using 20–300 cage traps). In 2011–2012, an artificial nesting site was constructed 40 m south of the main nesting area. During the nesting season in 1991–2015, nests were checked daily and adult female iguanas were monitored by live trapping, observation and camera traps.

    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.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

Reptile Conservation

This Action forms part of the Action Synopsis:

Reptile Conservation
Reptile Conservation

Reptile Conservation - Published 2021

Reptile synopsis

What Works 2021 cover

What Works in Conservation

What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, mammals, forests, peatland and control of freshwater invasive species. More are in progress.

More about What Works in Conservation

Download free PDF or purchase
The Conservation Evidence Journal

The Conservation Evidence Journal

An online, free to publish in, open-access journal publishing results from research and projects that test the effectiveness of conservation actions.

Read the latest volume: Volume 19

Go to the CE Journal

Discover more on our blog

Our blog contains the latest news and updates from the Conservation Evidence team, the Conservation Evidence Journal, and our global partners in evidence-based conservation.


Who uses Conservation Evidence?

Meet some of the evidence champions

Endangered Landscape Programme Red List Champion - Arc Kent Wildlife Trust The Rufford Foundation Save the Frogs - Ghana Bern wood Supporting Conservation Leaders National Biodiversity Network Sustainability Dashboard Frog Life The international journey of Conservation - Oryx British trust for ornithology Cool Farm Alliance UNEP AWFA Butterfly Conservation People trust for endangered species Vincet Wildlife Trust