Provide supplementary food during/after release of captive-bred mammals

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • Fifteen studies evaluated the effects of providing supplementary food during/after release of captive-bred mammals. Four studies were in Australia, two were in each of the USA, China and Argentina, and one was in each of Poland, the UK, Oman and Saudi Arabia.



  • Abundance (5 studies): Four studies (one replicated, one before-and-after study) and one review in Poland, Oman, China and Australia found that following provision of supplementary food (and in one case water) to released captive-bred animals, populations of European bison increased more than six-fold over 20 years, Arabian oryx increased over 14 years, eastern-barred bandicoots increased for the first five years before declining, Père David's deer increased more than six-fold over 12 years and Przewalski’s horses (enclosed in winter) increased over 11 years.
  • Reproductive success (9 studies): Eight studies (including two replicated and one before-and-after study) and one review in Poland, the UK, China, the USA, Australia and Saudi Arabia found that following the provision of supplementary food (and in one case water or artificial nests) after release of captive-bred animals, some from holding pens, European bison, European otters, Père David's deer, eastern-barred bandicoots, Przewalski’s horses and some captive-bred red wolves successfully reproduced, Arabian gazelles started breeding in the year following releases and sugar gliders established a breeding population.
  • Survival (6 studies): Four of six studies (one controlled, before-and-after study) in the UK, USA, Argentina and Australia found that following the provision of supplementary food (and in one case water or nest boxes) after release of captive-bred animals, many from holding pens, 19% of red wolves survived for at least seven years, Eurasian otters survived for at least two years, over half the giant anteaters (some rehabilitated) survived for at least six months and hare-wallabies survived at least two months. Two of the studies found that red-tailed phascogales survived for less than a year and most Mexican wolves survived less than eight months.


  • Behaviour change (1 study): A controlled study in Argentina found that after being provided with supplementary food and kept in holding pens, released captive-bred giant anteaters were less nocturnal in their activity patterns than released wild-born rehabilitated individuals.

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 study in 1952–1973 in a mixed forest site in Białowieża, Poland (Krasinski 1978) found that captive-bred European bison Bison bonasus provided with supplementary food after being released into the wild bred successfully and the population increased more than six-fold over 20 years. The population increased to 253 individuals (112 males, 141 females) during 20 years in which 38 captive-bred bison were released. A total of 316 births and 67 deaths were recorded. In 1952–1972, thirty-eight captive-bred bison were released from reserves into the western Białowieża Primeval Forest (580 km2 area). Supplementary food (hay) was provided each winter. Numbers of bison and the number of births and deaths in the population were counted by observers each year in 1952–1973.

    Study and other actions tested
  2. A study in 1979–1981 at a young planted native forest reserve in Victoria, Australia (Suckling & Macfarlane 1983) found that released, captive-bred sugar gliders Petaurus breviceps provided with supplementary food and artificial nest hollows appeared to establish a breeding population. In the third year after releases began, approximately 37 sugar gliders were recorded. Of 17 females caught, 10 were over one year old. All six females that were over two years old had bred. Seven of the 32 animals caught had been wild-bred in the year after the first releases. Sugar gliders were almost all located near to where artificial nest hollows were installed and 58 of 70 were either occupied or showed signs of recent occupation. On a 130-ha island of planted native forest (trees ≤17 years old), 26 captive-bred juvenile gliders (12 male, 14 female) were released in February 1979. Thirty-four (21 male, 13 female) were released in January–February 1980. Twelve (six male, six female) were released in February 1981. Seventy artificial nest hollows (boxes, hollow branches and pipes) were installed. Supplementary food was provided at release points during winters of 1979 and 1980. Gliders were surveyed in May 1981, by live-trapping, using 54 traps for up to four nights, supplemented by sightings of animals flushed from nest hollows.

    Study and other actions tested
  3. A replicated study in summer 1983–1984 at a riparian site in East Anglia, UK (Wayre 1985) found that captive-bred European otters Lutra lutra provided with supplementary food after being kept in a pre-release pen bred successfully following release. Footprints of at least one otter cub were found in the year after release. Otters settled near the release site, but ranged along 32 km of river over the first 100 days after release. In July 1983, three 18-month-old captive-bred otters (one male, two female) were released. Before release, they were held together in a pen at the release site, for an unspecified period of time. After release, supplementary food was provided in the pens for 12 days. The male otter was radio-tracked for 50 nights after release. Local bridges were monitored for 100 days after release for signs of otter faeces.

    Study and other actions tested
  4. A study in 1983–1985 along a river on the Norfolk-Suffolk border, UK (Jefferies et al. 1986) found that following the short-term provision of supplementary food after release from holding pens, captive-bred Eurasian otters Lutra lutra survived at the release site for at least two years and reproduced. The otters survived in the release area at least 28 months after release. Breeding was confirmed the summer after release and suspected again the following summer. On the first night, otters were fed prior to being released. They returned to feed on the second, third and fifth to seventh nights but after that food was untouched. Spraint analysis suggested they were catching fish from the fourth night. One male and two female otters (captive-bred and unrelated) were kept in a large pen with a pool where they had limited contact with humans from 10 months to 18 months of age. In June 1983, at 18 months, they were moved to a 9 × 15-m pre-release pen, 10 m from a river bank, on a river island. After 20 days, the pen door was fixed open. Food was placed in the pen daily for 12 days after release in diminishing quantities and uneaten food was cleared away. The male was radio-tracked for 50 days from 5 July 1983. Otter signs (especially spraints) were then monitored until 1985.

    Study and other actions tested
  5. A study in 1998 in a grassland, shrubland and forest reserve in Arizona, USA (Parsons 1998) found that most captive-bred Mexican wolves Canis lupus baileyi provided with supplementary food after being kept in holding pens and released in groups did not survive over eight months after release into the wild. Out of 11 captive-bred Mexican wolves released, six (55%) were illegally killed within eight months, three (27%) were returned to captivity and two (18%) survived in the wild for at least one year (long-term survival not reported). Three weeks after their release, three individuals from one family group killed an adult elk Cervus canadensis. Two females gave birth two months after release but only one pup survived. Eleven wolves in three family groups were released in March 1998. Before release, wolves were kept for two months in pre-release holding pens, where they were fed carcasses of native prey. Carcasses were provided as supplementary food for two months post-release when sufficient killing of prey was confirmed. The released wolves were fitted with radio-collars. No monitoring details are provided.

    Study and other actions tested
  6. A study in 1982–1996 of a large desert area in Oman (Spalton et al. 1999) found that a reintroduced captive-bred Arabian oryx Oryx leucoryx population initially provided with supplementary food and water grew in number over 14 years, but then declined, due to poaching. Oryx numbers in the wild peaked at >400 animals, 1–14 years after release of 40 animals. Poachers (capturing live animals, especially females, for international trade) then removed at least 200 oryx over the next three years. Animals were taken back into captivity to re-establish a captive breeding program. Seventeen years after releases began, the captive population was 40, and approximately 104 remained in the wild, with a high male:female sex ratio. Arabian oryx became extinct in Oman in 1972. Founders for the initial captive herd were sourced from international collections. Forty individually marked oryx were released in 1982–1995. A sample of wild-born animals was individually marked to retain the marked proportion at 20–30%. The original released herd was provided with food and water for seven months after release. Population estimates were derived from sightings using mark-recapture analysis.

    Study and other actions tested
  7. A replicated study in 1985–1997 in two grassland reserves in Jiangsu and Beijing, China (Jiang et al. 2000) found that captive-bred Père David's deer Elaphurus davidianus released into the wild and provided with supplementary food in the winter bred successfully and increased in number more than six-fold over 12 years. In one reserve, numbers of Père David’s deer were more than six times higher 12 years after release (127 deer) than at the time of release (20 deer). At a second reserve, numbers were more than seven times higher 11 years after release (302 deer) than at the time of release (39 deer). Average annual birth and death rates were 53% and 9% respectively at one site, and 54% and 3% at the other. Wild offspring translocated from the first site to another fenced area in China survived at least two years post-relocation and reproduced in the second year. In 1985–1987, thirty-seven captive-bred deer were released into a reserve (60 ha). In 1986, thirty-nine captive-bred deer were released into three fenced paddocks (each 100 ha) at a second reserve. In 1992–1996, twenty-one deer from one population and 134 deer from the other were moved to other sites. Supplementary food was provided in both reserves during the winter. The deer populations were monitored for 11–12 years after release in 1985–1997. Details of monitoring methods are not provided.

    Study and other actions tested
  8. A study in 1987–1994 in a grassland site in North Carolina, USA (Phillips et al. 2003) found that having provided supplementary food after release (after some animals were kept in holding pens), 12 of 63 captive-bred red wolves Canis lupus rufus survived for at least seven years, and some animals successfully reproduced. Seven years after wolves were first reintroduced, 12 of 63 translocated animals were still alive. By the same time, at least 66 pups had been born. Between October 1987 and December 1994, sixty-three captive-bred wolves were released. Twenty-nine wolves were held in pens (225 m2) on site before release (duration: 14 days-49 months), and 34 animals were released on arrival at the site. An unspecified number of wolves were fitted with radio-collars. From October 1987 to December 1994, wolves were radio-tracked from the ground and from an aeroplane. Monitoring frequency was not specified. Supplementary food (deer carcasses) was provided for 1-2 months after release from the ninth release onwards.

    Study and other actions tested
  9. A controlled, before-and-after study in 2001 in five shrubland sites in Western Australia, Australia (Hardman & Moro 2006) found that most captive-bred banded hare-wallabies Lagostrophus fasciatus and rufous hare-wallabies Lagorchestes hirsutus provided with supplementary food and water (and in some cases having been in holding pens) survived at least two months after being released into a fenced peninsula where predators had been controlled. After 1-2 months, 10 of 16 rufous hare-wallabies and 12 of 18 banded hare-wallabies were still alive. Overall both rufous and banded hare-wallabies recaptured had similar body conditions to when they were released, although rufous hare-wallabies lost 12% of body condition while waiting for release in holding pens (data presented as a body condition index; see paper for details). Sixteen captive-bred rufous hare-wallabies and 18 captive-bred banded hare-wallabies were released at five sites in August 2001. Six rufous and nine banded-hare wallabies were placed in separate 3-ha enclosures with electrified fencing for 10–19 days before being released. Remaining animals were released directly into the wild. Supplementary food (kangaroo pellets, alfalfa) and water were made available to all hare-wallabies (those in holding pens and those not; duration of feeding not given). Hare-wallabies were monitored by radio tracking (once per week for 1.5 years after release) and live-trapping (at 4 and 8-9 weeks after release). Release areas were within a fenced peninsula where multiple introduced mammals were controlled or eradicated.

    Study and other actions tested
  10. A review of eight studies in 1989-2005 in eight grassland and woodland sites in Victoria, Australia (Winnard & Coulson 2008) found that in two studies where captive-bred eastern-barred bandicoots Perameles gunnii were given supplementary food as part of a release program, the populations survived and bred in the wild, increasing for the first five years prior to declining. Two captive-bred bandicoot populations provided with supplementary food increased for at least five years after releases began and there was evidence of breeding and wild-born pouch young maturing to adults. These populations subsequently declined to low numbers 12-15 years after the original releases began. Between 174 and 207 bandicoots were released into 100-300 ha fenced predator-free enclosures in 1989-2004. Bandicoots were released in stages in each site. Supplementary food was provided in both sites (in one for 6-10 days after release, the other was not specified). Red fox Vulpes vulpes were controlled in both sites. Bandicoots were monitored by live-trapping but frequency and methods are not detailed.

    Study and other actions tested
  11. A study in 2011–2014 of a dry dwarf-scrubland site in Saudi Arabia (Islam et al. 2014) found that captive-bred Arabian gazelles Gazella arabica provided supplementary food and water after release into a fenced reserve started breeding in the year following the first releases. Seven females gave birth in August–September of the year after the first releases and all calves survived to the year end at least. Of 49 gazelles released over three years, 10 had died by the time of the final releases. In 2011–2014, three groups of captive-born gazelles, totalling 49 animals, were released in a 2,244-km2 fenced reserve. They were moved from a wildlife research centre and kept for 23 days to a few months in holding pens (500 × 500 m) prior to release at the reserve. Water and food was provided for three weeks following release. Released gazelles were radio-tracked from the ground and air.

    Study and other actions tested
  12. A before-and-after study in 1985–2003 on a nature reserve in Xinjiang, China (Xia et al. 2014) found that following release of captive-bred Przewalski’s horses Equus ferus przewalskii into the semi-wild (free-roaming in summer, enclosed in winter and provided with food), animals reproduced and numbers increased. The first foals were born two years after the first releases. Over the following 11 years, 107 foals were born in the semi-wild with first-year survival of 75%. At this time, released animals formed 16 groups, comprising 127 individuals. From 2001–2013, eighty-nine horses from a captive-breeding centre were held in a pre-release enclosure (20 ha) for an unspecified period of time before being released into semi-wild conditions. Released animals roamed freely from spring to fall, but were kept in a coral in winter, to enable supplementary feeding and to reduce competition with domestic horse herders. The founders for the captive population were sourced from zoos in Europe and North America. The release site (and adjacent areas of Mongolia) were the last refuge of Przewalski’s horse, before extinction in the wild in 1969.

    Study and other actions tested
  13. A study in 2007–2014 in a grassland reserve in Corrientes Province, Argentina (Di Blanco et al. 2015; same experimental set-up as Di Blanco et al. 2017) found that over half of released captive reared or rehabilitated giant anteaters Myrmecophaga tridactyla, some of which were provided supplementary food and initially kept in holding pens, survived for at least six months. At least 18 of 31 (58%) released giant anteaters survived for a minimum of six months. Long term survival and the fate of the other 13 anteaters is not reported. In 2007–2013, thirty-one giant anteaters (18 males, 13 females; 1–8 years old) were released into a 124-km2 private reserve. Hunting within the reserve was prohibited and livestock were absent. Twenty-two anteaters were wild-born but captive-reared, six were from zoos (origin not stated) and three were wild-born but rehabilitated in captivity from injuries. Of the 18 surviving anteaters, six had been released after a short period in a 0.5-ha pen at the release site and 12 after 7–30 days in a 7-ha pen. Supplementary food was provided for several weeks after release. In 2007–2014, thirteen anteaters were tracked for less than six months, and 18 were tracked for 6–46 months.

    Study and other actions tested
  14. A study in 2006–2008 in a woodland and shrubland site in Northern Territory, Australia (Short & Hide 2015) found that captive-bred red-tailed phascogales Phascogale calura that were initially given supplementary food when released into a fenced area with nest boxes, having been kept in pre-release pens, survived for less than a year. Six captive-bred females survived for at least three months after release, with at least two of them carrying young. However, there were no sightings after the first year post-release, and the population is believed to have died out. Authors suggest that there may have been a shortage of tree hollows for nesting. In July 2006 and January–February 2007, thirty-two captive-bred phascogales were released into a 26-ha fenced reserve after spending either 10 days or over four months in a pre-release pen (3×6×2 or 4.5×3×2.2 m). Supplementary food was provided for one week after release. Feral cats were abundant outside of the fence. Eleven nest boxes were provided within 150m of the release pen. No information on monitoring is provided.

    Study and other actions tested
  15. A controlled study in 2007–2012 in a grassland reserve in Corrientes, Argentina (Di Blanco et al. 2017; same experimental set-up as Di Blanco et al. 2015) found that after being provided with supplementary food and kept in holding pens, captive-bred giant anteaters Myrmecophaga tridactyla released into the wild were less nocturnal in their activity patterns than were wild-born rehabilitated individuals. Captive-bred giant anteaters were proportionally less active at night (43% activity records were at night) than wild-born animals (70% of activity records). During 2007–2012, three captive-bred and four wild-born adult giant anteaters were released into a 124-km2 private reserve. Wild-born animals were rehabilitated after being injured by hunters or in road accidents. Six anteaters (all wild-born and two captive-bred anteaters) were released after spending a short period of time in a 0.5 ha acclimatisation pen. The remaining 12 anteaters spent 7-30 days in a 7 ha holding pen at the release site prior to release. Supplementary food was provided in the holding pen and for several weeks after anteaters were released. Each of the seven anteaters was fitted with a radio-transmitter and tracked for 1–2 x 24 h periods/month in 2007 and 2011. The released anteaters were further monitored using 14 baited camera traps for an average of 336 days/trap in 2008–2012.

    Study and other actions tested
Please cite as:

Littlewood, N.A., Rocha, R., Smith, R.K., Martin, P.A., Lockhart, S.L., Schoonover, R.F., Wilman, E., Bladon, A.J., Sainsbury, K.A., Pimm S. and Sutherland, W.J. (2020) Terrestrial Mammal Conservation: Global Evidence for the Effects of Interventions for terrestrial mammals excluding bats and primates. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

Terrestrial Mammal Conservation

This Action forms part of the Action Synopsis:

Terrestrial Mammal Conservation
Terrestrial Mammal Conservation

Terrestrial Mammal Conservation - Published 2020

Terrestrial Mammal Conservation

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 21

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 ProgrammeRed List Champion - Arc Kent Wildlife Trust The Rufford Foundation Save the Frogs - Ghana Mauritian Wildlife Supporting Conservation Leaders
Sustainability Dashboard National Biodiversity Network Frog Life The international journey of Conservation - Oryx Cool Farm Alliance UNEP AWFA Bat Conservation InternationalPeople trust for endangered species Vincet Wildlife Trust