Reintroduce primates as single/multiple individuals

Supporting evidence from individual studies

1. A study in 1966–1985 on, a forested island in Tanzania found that reintroduced eastern chimpanzees Pan troglodytes schweinfurthii that were released as multiple individuals during four reintroduction events (from 1966 to 1969) alongside other interventions, bred and increased in numbers from 17 to at least 20 individuals over a 16-year time period. No statistical tests were carried out to determine whether this increase was significant. At least two males were shot after attacking game scouts. Two new-born infants were seen in 1968 and in 1985. Male-to-female ratios in the four release groups differed (4.7, 1.0, 1.0, 2.2) and few individuals within each group had met before. All 17 reintroduced chimpanzees had been born in the wild and spent various amounts of time in captivity. Their age at the time of release varied from four to 12. The island was free of predators and chimpanzees. The first release group was provided with supplementary food for two months. The study does not distinguish between the effects of the different interventions mentioned above.

   Study and other actions tested

   Referenced paper

   Borner M. (1985) The rehabilitated chimpanzees of Rubondo Island. Oryx, 19, 151-154.

2. A before-and-after trial in 1954–1985 in a degraded rain forest in Brazil found that a translocated captive-born golden lion tamarin Leontopithecus rosalia population, some of which were released as pairs alongside nine other interventions, decreased by more than half (57%) within the first year of release. No statistical tests were carried out to determine whether this decrease was significant. Of the 14 individuals released, seven died and two were removed. Three infants were born, one of which died. Eight individuals were released as a family group and six individuals were released as pairs one month later. Tamarins were placed in 15 x 4.5 x 3 m outside enclosures to acclimatize. They were habituated to humans and fostered to facilitate survival in the wild. The forest included natural predators. Sick or injured tamarins were captured and treated. Reintroduced tamarins were supplied with food for ten months after their release. Artificial nesting boxes, which were hollow logs provided to them during training, were also set up in the forest. The study does not distinguish between the effects of the different interventions mentioned above.

   Study and other actions tested

   Referenced paper

   Dietz L.A. (1985) Captive-born lion tamarins released into the wild: A report from the field. Primate Conservation, 6, 21-27.

3. A study, which was included in a review, in 1967–1970 on lar gibbons Hylobates lar reintroduced in pairs at two sites in Thailand along with other interventions found that populations decreased by 6-60% over three years. At one site the population of gibbons decreased by 60% over three years, while at the other population declined by 6%. No statistical tests were carried out to determine whether these decreases were significant. At the first site four infants were born (reproductive rate =20%), while at the second site there were no births. Before release anaesthetized gibbons were either kept in separate cages for 14 days before release, or laid out on the forest floor. At the first site, 20 gibbons were introduced into habitat that did not contain resident gibbons, while at the second site 31 gibbons were introduced in an area that contained wild gibbons. Monkeys were obtained individually from animal dealers and housed together in a laboratory for at least one month before release. Gibbons were fed supplementary food and water. Four gibbons joined wild groups. Injured animals were recaptured and treated. In 1961, gibbons were officially protected by the Thai government. Neither study distinguishes between the effects of the different interventions mentioned above.

   Study and other actions tested

   Referenced paper

   Eudey A.A. (1991) Captive gibbons in Thailand and the option of reintroduction to the wild. Primate Conservation, 12, 34-40.

4. A study, which was included in a review, in 1976–1977 in dry evergreen forest in Thailand on captive lar gibbons Hylobates lar that were partially released as single or multiple individuals, along with other interventions, found that their population decreased by 6% and no infants were born in the first 17 months after release. No statistical tests were carried out to determine whether this decrease was significant. Four gibbons joined wild groups. A total of 31 gibbons were introduced as individuals, pairs, or family groups into habitat with resident wild gibbons. Anaesthetized gibbons were either kept in separate cages for 14 days before release, or laid on the forest floor. Injured animals were recaptured and treated. In 1961, gibbons were officially protected by the Thai government. The study does not distinguish between the effects of the different interventions mentioned above.

   Study and other actions tested

   Referenced paper

   Bennett J. (1992) A glut of gibbons in Sarawak – is rehabilitation the answer? Oryx, 26, 157-164.

5. A before-and-after trial in 1976-1988 in a tropical forest in Malaysia found that of 87 captive, wild-born Müller's Bornean gibbons Hylobates muelleri that were reintroduced in pairs or as single individuals along with other interventions, at least 77 (90%) died after release. When possible, males and females were paired in cages to try to establish pair bonds before to release into habitat without resident wild gibbons. Confiscated gibbons had undergone veterinary checks and were placed in holding cages in a forest clearing. Müller's Bornean gibbons were fully protected under local law. Surveys of direct sightings and gibbon calls were conducted simultaneously by three or four observers on non-rainy days on eight mornings between 4 February and 31 March 1988. The study does not distinguish between the effects of the different interventions mentioned above.

   Study and other actions tested

   Referenced paper

   Andau PM, Hiong LK & Sale J.B. (1994) Translocation of pocketed orang-utans in Sabah. Oryx, 28, 263-268.

6. A study in 1994 in a tropical dry forest in Minas Gerais, Brazil found that that an abandoned infant muriqui Brachyteles arachnoides that was removed from its natural habitat and then returned as a single individual, along with other interventions, was retrieved by its mother and rejoined the wild group. Twenty-seven hours after detection of the infant, it was released close to its mother who retrieved it immediately. The 4-month old female infant muriqui was removed from the forest and was first given a blanket for warmth, fed with milk and food and some ectoparasites were removed for study. The mother answered to the infant’s cries and retrieved it immediately and then rejoined the group. The study does not distinguish between the effects of the different interventions mentioned above.

   Study and other actions tested

   Referenced paper

   Nogueira C.P., Carvalho A.R.D., Oliveira L.P., Veado E.M. & Strier K.B. (1994) Recovery and release of an infant muriqui Brachyteles arachnoides, at the Caratinga Biological Station, Minas Gerais, Brazil. Neotropical Primates, 2, 3-5.

7. A before-and-after trial in 1994-1995 in tropical forest in French Guiana found that most of the translocated white-faced sakis Pithecia pithecia that were partly released as single or multiple individuals, along with other interventions, survived for at least four months after release. One individual died after approximately 22 weeks. The female and the male that were released as single individuals bonded after release until the male died due to a parasite infection. Before release monkeys were captured by nets, were tagged with radio transmitters and underwent veterinary screens. Three out of six translocated wild sakis were monitored for 41 weeks after their release. The cause of death of dead sakis was clinically determined. The study does not distinguish between the effects of the different interventions mentioned above.

   Study and other actions tested

   Referenced paper

   Vié J., Richard-Hansen C. & Fournier-Chambrillon C. (2001) Abundance, use of space, and activity patterns of white-faced sakis (Pithecia pithecia) in French Guiana. American Journal of Primatology, 55, 203-221.

8. A controlled study in 2002-2003 in lowland forest in Kalimantan, Indonesia found that wild-born, captive-raised Bornean agile gibbons Hylobates albibarbis that were reintroduced in pairs, along with other interventions, shared a similar diet, spent similar amounts of time feeding, resting, and arm-swinging and at similar canopy heights as wild gibbons. However, wild gibbons spent more time singing, socializing and travelling. Before reintroduction gibbons were quarantined at a holding facility for at least one year, where they were screened by vets. They were kept in enclosures (3 x 3 x 3 m) to socialize and acclimatize to the natural environment and, during this time, were supplemented with vitamins and leaves once a week. Comparisons were made between a reintroduced pair of gibbons and a pair of wild gibbons at another site. The study does not distinguish between the effects of the different interventions mentioned above.

   Study and other actions tested

   Referenced paper

   Cheyne S.M., Chivers D.J. & Sugardjito J. (2008) Biology and behaviour of reintroduced gibbons. Biodiversity and Conservation, 17, 1741–1751.

9. A controlled study in 1967-2004 in tropical forest in Malaysia found that rehabilitated and individually reintroduced orangutans Pongo pygmaeus morio decreased in numbers by 33% over 33 years. Infant mortality (57%) was higher than in other wild and captive populations, and the sex ratio at birth was strongly biased towards females (proportion males=0.11) compared to other wild and captive populations. Time between births (6.1 years) was shorter than for other orangutan subspecies or species in the wild and in captivity, but similar to wild populations of the same subspecies. Average age at first reproduction (11.6 years) was lower than in other wild and captive populations. Orangutans were continuously provided with supplementary food. Before release at the site which contained other orangutans individuals underwent in-depth veterinary checks and were kept in quarantine for 90 days before they were released into the reserve. Individuals were captured and treated when they displayed signs of injury or illness. Staff and volunteers received medical checks and tourists were told to keep >5 m from animals at all times. The study does not distinguish between the effects of the different interventions mentioned above.

   Study and other actions tested

   Referenced paper

   Kuze N., Sipangkui S., Malim T.P., Bernard H., Ambu L.N. & Kohshima S. (2008) Reproductive parameters over a 37-year period of free-ranging female Borneo orangutans at Sepilok Orangutan Rehabilitation Centre. Primates, 49, 126-134.

10. A study in 2009 in savanna in Senegal found that a confiscated 9-month old female infant chimpanzee Pan troglodytes verus that was reintroduced, along with other interventions, was reunited with its mother in the wild. Four days after confiscation, the chimpanzee was released close to its natal group, which retrieved it immediately. Researchers wore surgical masks and sanitized their hands when handling the infant and its food. The infant’s natal group was located with the aid of poachers, after which the infant was released close to the group. The infant was also treated for its injured eye. The study does not distinguish between the effects of the different interventions mentioned above.

    Study and other actions tested

    Referenced paper

    Pruetz J.D. & Kante D. (2010) Successful return of a wild infant chimpanzee (Pan troglodytes verus) to its natal group after capture by poachers. African Primates, 7, 35-41.

11. A site comparison in 2008-2012 in two forests in South Vietnam found that several pygmy slow lorises Nycticebus pygmaeus that were reintroduced as multiple individuals, along with eight other interventions, survived for at least two months. Four out of eight lorises survived for at least two months after release, whereas the remaining animals either died or their radio collar signal was lost at an early stage after release. Lorises were released during the wet season after all animals had undergone a 6-week quarantine, veterinary screens, and treatment for parasites. Both release sites were protected and no wild resident lorises occurred there, but predators were present. Lorises were kept in a cage for between two days and two months and were subsequently supplemented with food for between seven and 30 days. Bodies of dead animals were examined to determine the cause of death. The study does not distinguish between the effects of the different interventions mentioned above.

    Study and other actions tested

    Referenced paper

    Kenyon M., Streicher U., Loung H., Tran T., Tran M., Vo B. & Cronin A. (2014) Survival of reintroduced pygmy slow loris Nycticebus pygmaeus in South Vietnam. Endangered Species Research, 25, 85-95.

12. A site comparison in 2008-2012 in two forest sites in South Vietnam found that all pygmy slow lorises Nycticebus pygmaeus that were reintroduced as individuals, along with other interventions either died or disappeared. All five lorises died or their radio collar signal was lost at an early stage after release. Before release, lorises were quarantined for six weeks, screened by vets, and treated for parasites. No wild resident lorises occurred at either of the release sites, but predators did. Three lorises were released at one site during the dry season. Another two individuals were held in a semi-wild enclosure for one month to allow them to learn natural behaviour that would aid their survival in the wild. The latter were released during the wet season. Bodies of dead animals were examined to determine the cause of death. The study does not distinguish between the effects of the different interventions mentioned above.

    Study and other actions tested

    Referenced paper

    Kenyon M., Streicher U., Loung H., Tran T., Tran M., Vo B. & Cronin A. (2014) Survival of reintroduced pygmy slow loris Nycticebus pygmaeus in South Vietnam. Endangered Species Research, 25, 85-95.

13. A before-and-after trial in 2006-2011 in tropical forest in Indonesia found that few reintroduced Javan slow lorises Nycticebus javanicus and greater slow lorises N. coucang that were released as single individuals, along with other interventions, survived for at least 146 and 22-382 days, respectively. One of five reintroduced greater slow lorises survived for at least 146 days and five of 18 reintroduced Javan slow lorises individuals survived for at least 22-382 days. Before release, lorises underwent quarantine and veterinary screens. Sick individuals were recaptured and treated. All but two lorises were held in enclosures at the release site to adapt to local habitat where resident lorises and predators occurred. Bodies of dead lorises were examined to determine their cause of death. The study does not distinguish between the effects of the different interventions mentioned above.

    Study and other actions tested

    Referenced paper

    Moore R.S., Wihermanto S. & Nekaris K.A.I. (2014) Compassionate conservation, rehabilitation and translocation of Indonesian slow lorises. Endangered Species Research, 26, 93-102.