Providing evidence to improve practice

Action: Allow primates to adapt to local habitat conditions for some time before introduction to the wild Primate Conservation

Key messages

  • Two studies in Brazil and Thailand found that reintroduced primate populations were smaller after 12-17 months and one study in Belize found primate populations increased five years after allowing individuals to adapt to local habitat conditions before introduction into the wild, alongside other interventions. One study found that a reintroduced population of black howler monkeys had a birth rate of 20% after they were allowed to adapt to local habitat conditions before introduction into the wild, along with other interventions.
  • Seven studies in Brazil, Madagascar, Malaysia, French Guiana, South Africa found that a minority of primates survived for at least 15 weeks to 12 years after allowing them to adapt to local habitat conditions before introduction into the wild, along with other interventions. Four studies in Belize, Brazil, Gabon, South Africa found that the majority of primates survived for at least four to 12 months. One study in Vietnam found that half of reintroduced pygmy slow lorises survived for at least two months.
  • Two before-and-after studies in Gabon and the Republic of Congo found that a majority of western lowland gorillas survived for nine months to four years after allowing them to adapt to local habitat conditions before introduction into the wild, along with other interventions.
  • Three studies in Liberia and the Congo found that a majority of chimpanzees survived for at least three to five years after allowing them to adapt to local habitat conditions before introduction into the wild, along with other interventions. One before-and-after study in Uganda found that a chimpanzee repeatedly returned to human settlements after allowing it to adapt to local habitat conditions before introduction into the wild, along with other interventions.
  • A study in Indonesia found that Sumatran orangutans that were allowed to adapt to local habitat conditions before introduction performed less well than individuals that were directly released into the forest, alongside other interventions.
  • One controlled study in Indonesia found that after being allowed to adapt to local habitat conditions a pair of introduced Bornean agile gibbons had a similar diet to wild gibbons.

Supporting evidence from individual studies

1 

A before-and-after trial in 1954-1985 in degraded rainforest in Poço das Antas Reserve, Brazil found that a translocated captive-born golden lion tamarin Leontopithecus rosalia population that was allowed to acclimatize to the local environment before release along with 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 were released as pairs one month later. Individuals spend an unknown amount of time in 15 x 4.5 x 3 m outside forest enclosures before release. They were habituated to humans and fostered to facilitate survival in the wild. The reserve included natural predators. Sick or injured tamarins were captured and treated. Reintroduced tamarins were supplied with food for ten months post-release. Artificial nesting boxes, which were hollow logs provided to them during training, were set up in the reserve. The study does not distinguish between the effects of the different interventions mentioned above.

2 

A before-and-after trial in 1984-1991 in coastal forest in Poço das Antas Reserve, Brazil found that the majority of reintroduced golden lion tamarins Leontopithecus rosalia, some of which were allowed to adapt to local habitat conditions before release along with 14 other interventions, did not survive over seven years. Fifty-eight (64%) out of 91 reintroduced tamarins did not survive in the wild. However, 57 infants were born (reproductive rate=63%) during the study period, of which 38 (67%) survived. Tamarin groups (families or pairs) were kept in large forest acclimatization cages at the release sites. Different groups of captive-bred or orphaned tamarins were introduced in different years into habitat already occupied by the species and predators. Groups were provided with supplementary food, water and nesting boxes. Tamarins were quarantined, underwent veterinary checks and were treated for parasites before release. Sick or injured animals were rescued, treated and re-released. The reserve became officially protected in 1983. The study does not distinguish between the effects of the different interventions mentioned above.

3 

A study, which was included in a review, in 1976-1977 in tropical forest in Sai Yok National Park, Thailand on captive lar gibbons Hylobates lar that were allowed to adapt to local conditions before they were released along with other interventions found that their population decreased by 6% and no infants were born in the first 17 months post-release. No statistical tests were carried out to determine whether this decrease was significant. Four gibbons joined wild groups. Anaesthetized gibbons were either kept in separate cages from which they could hear, but not see each other for 14 days before release, or laid out on the forest floor. Thirty-one gibbons were introduced as individuals, pairs, or family groups and into habitat with resident wild gibbons. Injured animals were recaptured and treated. In 1961, gibbons became officially protected in Thailand. Reserve staff was permanently present. The study does not distinguish between the effects of the different interventions mentioned above.

4 

A before-and-after trial in 1976-1988 in degraded tropical forest in Semenggoh Forest Reserve, Malaysia found that at least 77 of 87 (90%) reintroduced captive, wild-born Müller's Bornean gibbons Hylobates muelleri that were allowed to adapt to local habitat conditions for some time before reintroduction along with other interventions, did not survive after release. Confiscated gibbons had undergone veterinary checks and were placed in holding cages in a forest clearing for an unknown amount of time. Some individuals were released within days of being received at the sanctuary. When possible, males and females were paired in cages prior to release into habitat without resident wild gibbons. The species was fully protected in Sarawak. Surveys of direct sightings and gibbon calls along grid squares (500 x 500 m) covering a total of 9.5 km were conducted simultaneously by three or four observers on non-rainy days on eight mornings in February-March 1988. The study does not distinguish between the effects of the different interventions mentioned above.

5 

A replicated study in 1992–1993 in tropical forest at Cockscomb Basin Wildlife Sanctuary (CBWS), Belize found that the majority of reintroduced black howler monkeys Alouatta pigra that were allowed to adapt to local habitat conditions prior to release along with other interventions, survived for at least ten months and reproduced. Twelve (86%) out of 14 reintroduced monkeys survived for at least ten months post-release. One male and juvenile disappeared two months post-release. Two infants were born in two of the three release groups, 3-8 months post-release. Howlers were kept in an 8 x 12 x 10 m enclosure for two days to acclimatize. Wild howlers were captured at Community Baboon Sanctuary and were translocated to CBWS. Prior to release, monkeys underwent veterinary screens. Three groups were released into habitat without resident howlers. Six individuals were fitted with ball-chain radio-collars and six were implanted with radio-transmitters, but transmitter signals got lost six weeks post-release. The study does not distinguish between the effects of the different interventions mentioned above.

6 

A before-and-after trial in 1992-1994 in tropical forest in Cockscomb Basin Wildlife Sanctuary in Belize found that a reintroduced population of black howler monkeys Alouatta pigra that was allowed to adapt to local habitat conditions before release into the wild along with other interventions, increased in size over time. By 1997, the population had increased by 61% (62 to > 100 individuals). No statistical tests were carried out to determine whether this increase was significant. One-month-, 6-month-, 1-year, and 2-year survival rates for the different cohorts released in the dry seasons of 1992-1994, were 81-100%. Birth rate was 20% (N=12) and infant survival rate was 75% (N=9). Entire social groups were reintroduced at once, and ten of the 14 groups were held in cages for 1-3 days before release with a distance of 700-1000 m to the neighbouring troop. All individuals underwent veterinary screens, were permanently marked, and adults were radio-collared. Hunting was largely controlled and the local community was educated about black howler conservation. The study does not distinguish between the effects of the different interventions mentioned above.

7 

A before-and-after-trial in 1995 in tropical forest in Kibale National Park, Uganda found that a female captive, 4-6 year old wild-born chimpanzee Pan troglodytes schweinfurthii that was allowed to adapt to local habitat conditions for three weeks before reintroduction into a human-habituated community of wild chimpanzees along with other interventions, repeatedly returned to human settlements and was subsequently returned to captivity. Eight days post-release, she left the forest for the first time and was taken back into the forest. For the following ten days, she travelled, fed, nested and engaged in social activities with the wild community. She increased ranging distance to humans and use of height, and visually monitored humans less regularly. However, she increasingly spent time alone and was returned to captivity six weeks after post-release. Three weeks before her introduction, caretakers recorded her activity, height off the ground, distance from nearest human and diet. She underwent pre-release training, a tuberculosis test and was quarantined before reintroduction into habitat with a resident wild chimpanzee community. Ten members of the local human community worked on the project. The study does not distinguish between the effects of the different interventions mentioned above.

8 

A study in 1987-1988 on an island in tropical moist forest in Liberia found that the majority of reintroduced western chimpanzees Pan troglodytes verus that were allowed to adapt to the local environment before being released along with other interventions, survived for at least one year. Seven out of 30 released chimpanzees had difficulties to adjust to the new social environment and were brought back to captivity. On site, chimpanzees were allowed to adapt to the local habitat in enclosures for some time; younger and low-ranking individuals were released earlier to reduce stress. Chimpanzees were screened for diseases before release in groups and socialized in naturalistic enclosures where they were taught behaviour to facilitate their survival in the wild. Released chimpanzees were continuously provided with food. Sick and injured animals were temporarily removed and treated. The study does not distinguish between the effects of the different interventions mentioned above.

9 

A study in 1994-1995 in primary forest at Petit Saut dam, French Guiana found that less than half of the translocated and monitored red howler monkeys Alouatta seniculus that were allowed to adapt to local habitat conditions for some time before their release along with other interventions, survived over 18 months post-release. Of the 16 females monitored for 18 months with radio-tags, seven females survived, with an estimated survival rate of 44-63%. Deaths related to the translocation process included screwworm fly larvae infestations under radio-collars (N=2) and trauma (N=1). Three (19%) females gave birth post-release, but all infants disappeared and probably died. All females studied for longer than three months (50%) settled within the release area. Of the 122 captured and translocated howlers from 28 different troops, ten out of 11 (91%) documented troops broke apart after release. Howlers spent up to 24 hours together in one of three forest enclosures, 3 km from the release site. All animals underwent veterinary screens before release and were reintroduced in groups into habitat already occupied by the species. The study does not distinguish between the effects of the different interventions mentioned above.

10 

A study in 1999 in tropical forest of Morro do Diabo State Park, São Paulo, Brazil found that only some of the individuals in a group of reintroduced wild and captive-bred black lion tamarins Leontopithecus chrysopygus, that were allowed to adapt to the local habitat before their release along with other interventions, survived over four months. Four months after release of three individuals, one tamarin died. The group was held for three weeks in an enclosure to adapt to the local environment where predators occurred. The released group consisted of two wild females and one captive-born male bred in a free-ranging environment where he had been fostered natural behaviour to facilitate reintroduction. The male was treated when sick. Tamarins underwent veterinary screens prior to transport to the release site. Monkeys were fitted with radio-transmitters and supplemented with food until the end of the study. The study does not distinguish between the effects of the different interventions mentioned above.

11 

A before-and-after trial in 1996-1999 in tropical rainforest in Conkouati Reserve, Republic of Congo found that 14 of 20 reintroduced wild-born orphaned chimpanzees Pan troglodytes troglodytes that were allowed to acclimatize to the local environment before their release along with other interventions, survived over three and a half years. Estimated mortality was 10-30%. None of the adult females reproduced. Chimpanzees fed on 137 different plant species, a variety similar to wild chimpanzees, and had activity budgets that resembled those of wild chimpanzees. No statistical tests were carried out to determine whether differences were statistically insignificant. Before reintroduction in groups into habitat with low densities of wild chimpanzees, individuals spent 6-9 years on one of three forested islands in the region to acclimatize. Chimpanzees underwent veterinary screens, were treated for endoparasites, and vaccinated for poliomyelitis and tetanus. Orphan chimpanzees were rehabilitated and fostered at a nearby sanctuary. Researchers were permanently on-site and monitored chimpanzees with radio-collars. The study does not distinguish between the effects of the different interventions mentioned above.

12 

A before-and-after trial in 1998-1999 in tropical forest on an island in French Guiana found that a small number of reintroduced squirrel monkeys Saimiri sciureus that were allowed to adapt to local habitat conditions prior to release along with other interventions, survived over 15 weeks after reintroduction. Six (43%) out of 14 released monkeys survived for at least 15 weeks. Two individuals died in release cages, and one was apparently killed by resident wild squirrel monkey. One month post-release, five monkeys (36%) were rescued and brought back to captivity. The remaining reintroduced six monkeys were all wild-born. Animals were kept as one group in an isolated cage for three months where two females gave birth. After transfer to the release site, they were held in an enclosure 6 x 4 x 4 m in size for four months to adapt to local habitat conditions. The release site was already occupied by resident conspecifics. The study does not distinguish between the effects of the different interventions mentioned above.

13 

A before-and-after trial in 1996-2001 in tropical lowland forest in Conkouati-Douli National Park, Republic of Congo found that the majority of wild-born orphan chimpanzees Pan troglodytes troglodytes that were allowed to adapt to local habitat conditions for some time before reintroduction, along with other interventions, survived for over five years. Twenty-six of 36 released animals survived over five years and only three died. Seven chimpanzees disappeared, resulting in an estimated survival of 72-92%. An infant was born after five years.. Chimpanzees were rehabilitated on islands where they were provided with food before their reintroduction to the mainland. After release, individuals were radio-collared and followed to record female cycling status, interactions with wild chimpanzees and sexual behaviour. Analysis of hair-extracted DNA of all released chimpanzees and infants was used to determine parentage. Chimpanzees were released into a forest where wild chimpanzees and predators were known to exist. Injured chimpanzees were treated. The study does not distinguish between the effects of the different interventions mentioned above.

14 

A before-and-after trial in 1997-2002 in primary forest in Betampona Reserve, Madagascar found that less than half of all captive-bred, parent-reared, reintroduced black-and-white ruffed lemurs Varecia variegata variegata that were allowed to adapt to local habitat conditions before release along with ten other interventions, survived over five years. Five (38.5%) of 13 individuals survived in the wild and six individuals were born, of which four survived. One female and one male reproduced with wild lemurs and the male became fully integrated into the wild group. Lemurs were held in a timber and chain-link wire mesh cage at the release site for 3-14 days before release. Released animals were fitted with radio- collars for monitoring. Captive lemurs had limited semi-free-ranging experience, were quarantined and underwent veterinary screens before their reintroduction in groups into habitat with predators and resident wild lemurs. They were recaptured and treated when sick and provided with supplementary food and water for a certain period of time. Dead lemurs were examined to determine the cause of death. The study does not distinguish between the effects of the different interventions mentioned above.

15 

A controlled study in 2002-2003 in swamp forest in Mintin Island, Borneo, Indonesia found that a wild-born, captive-raised Bornean agile gibbon Hylobates albibarbis pair that was allowed to adapt to local habitat conditions before reintroduction 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, the latter spent more time singing and socializing and travelling, probably because the reintroduced gibbon pair split up almost immediately after their release. The two gibbons were quarantined for at least 12 months before reintroduction and underwent veterinary screens. They were kept in enclosures (3 x 3 x 3 m) to socialize and acclimatize and during this time, were supplemented with vitamins and leaves once a week. They were introduced as a pair and into habitat with resident wild gibbons. The behaviour of the reintroduced gibbon pair was compared to a pair of wild gibbons at another site. The study does not distinguish between the effects of the different interventions mentioned above.

16 

A study in 2002–2006 in tropical forest in Lékédi Park, Gabon found that one third of captive-bred mandrills Mandrillus sphinx that were allowed to adapt to local habitat conditions for some time before their reintroduction into the wild along with other interventions, died within the first year after release. During this year, mortality was 33% (12 out of 36 individuals), mostly affecting dependent infants. Fertility rate was 42% (5 of 12 females), where two of the five infants survived over six months. Mortality decreased to 4% in the second year and fertility rate remained at 42%, but all five infants survived for at least six months. Their range remained limited during the first two years after release. In 2006, the group numbered 22 individuals, including 12 of the mandrills originally released, all in good physical condition. To acclimatize, mandrills were placed in a small holding enclosure of 0.5 ha for 2-4 weeks before release. They were reintroduced as a group into habitat already occupied by the species and with predators. They were treated for endoparasites before release and supplemented with food until 2005. The study does not distinguish between the effects of the different interventions mentioned above.

17 

A controlled study in 2004-2005 in a mosaic of logged and secondary tropical forest in Bukit Tigapuluh National Park, Sumatra, Indonesia found that reintroduced Sumatran orangutans Pongo abelii that were directly released into the forest along with other interventions, performed better after release than individuals that were allowed to adapt to local habitat conditions for some time at the release site. The behaviour of the three orangutans that were released directly into the new habitat resembled that of wild conspecifics more than that of the five individuals that were allowed to adapt for 7-9 months prior to release to local habitat conditions to adjust and learn how to built nests, select food and use the canopy. In addition to the adaptation period on-site, the latter group was guided daily into the forest by rangers trying to foster natural behaviour. The group directly released into the forest spent more time interacting socially with previously released orangutans. The group directly released into the forest spent more time interacting socially with previously-released orangutans. The orangutans in this group were housed at a sanctuary for 6-month before release. The study does not distinguish between the effects of the different interventions mentioned above.

18 

A before-and-after trial in 2007-2008 in dry forest-grassland mosaic near Richmond, South Africa found that a small number of vervet monkeys Chlorocebus aethiops that were allowed to adapt to local habitat conditions along with other interventions, survived for at least ten months after reintroduction. Out of 35 monkeys released in troop one, only six (17%) survived for ten months post-release, after which monitoring ceased. Twenty-two (63%) vervets went missing and seven (20%) died. However, two infants were born 10-11 months after release. Out of 24 vervets released as troop two, 12 (50%) survived, seven (29%) went missing and five (21%) died. Monkeys underwent veterinary checks, and were released in groups into habitat already occupied by the species. They also received supplementary food and water after their release. The study does not distinguish between the effects of the different interventions mentioned above.

19 

A study in 2007-2010 in subtropical forest-shrubland mosaic in Mondi Forestry, South Africa found that a small number of the 31 rehabilitated and reintroduced vervet monkeys Chlorocebus aethiops that were allowed to adapt to the release site in enclosures along with other interventions, survived for at least 12 months. After 12 months of post-release monitoring, ten (32%) individuals had survived and 20 (65%) could not be tracked. One (3%) individual was euthanized three days post-release after raiding houses and acting aggressively towards people. Vervets were held in a 55 m2 and 2 m-high enclosure at the release site for four days before release. The release group included both wild captured- (61%) (due to injury) and hand-raised orphaned (39%) monkeys. Monkeys underwent veterinary screens, were released as a group and supplemented with food for eight weeks. Eleven individuals were fitted with radio collars that worked circa nine months after release. The study does not distinguish between the effects of the different interventions mentioned above.

20 

A before-and-after trial in 2008-2010 in forest-savanna mosaic in Mafou forest, Haut Niger National Park, Guinea found that the majority of wild-born orphaned western chimpanzees Pan troglodytes verus, some of which were allowed to acclimatize to local habitat conditions prior to release along with other interventions, survived reintroduction and remained free-living for at least 27 months. Only one of 12 released chimpanzees died after anaesthesia during a recovery mission. One female returned to the sanctuary voluntarily and one male was returned after suffering injuries. Five chimpanzees remained together at the release site and two females gave birth to an infant, both of which survived. Another female immigrated and integrated into a wild chimpanzee community and three chimpanzees moved to an area away from the release site. Five adult males were held in an enclosure (1.5 ha) with an annex cage (25 m2) for 1-4 months prior to release. All chimpanzees were screened for diseases before their collective release into habitat with wild chimpanzees and predators. Chimpanzees were initially supplemented with food on a daily-, and later on, a weekly basis. The study does not distinguish between the effects of the different interventions mentioned above.

21 

A before-and-after, site comparison study in 1996-2006 in tropical forests of Lesio-Louna Wildlife Reserve, Republic of Congo (Congo) and Batéké Plateau National Park, Gabon found that the majority of reintroduced western lowland gorillas Gorilla gorilla gorilla that were allowed to adapt to local environment before release along with 14 other interventions, survived at least four years. Twenty-one (84%) of 25 gorillas released in Congo and 22 (85%) of 26 gorillas released in Gabon survived over four years. Nine females gave birth to 11 infants, of which nine (82%) survived. In Gabon, gorillas were accompanied daily to the forest and spent the night in enclosures for an average of 15 months. In Congo, groups were either walked to the release site or brought there by vehicles and familiar staff. During quarantine, gorillas underwent disease screening and vaccinations. They were supplemented with food before release and released in groups into habitat with no resident wild gorillas. Released gorillas were treated for parasites and when sick. So-called ‘problem’-animals were removed and relocated and bodies of dead gorillas were examined. Forty-three individuals were rehabilitated wild-born orphaned gorillas and eight gorillas were ex-situ captive-born individuals. Both sites became protected areas before reintroduction procedures. The study does not distinguish between the effects of the different interventions mentioned above.

22 

A before-and-after trial in January-July 2008 in a coastal forest at Isishlengeni Game Farm, South Africa found that 62% of rehabilitated vervet monkeys Chlorocebus aethiops that were allowed to acclimatize to the new environment before being reintroduced into the wild along with other interventions, survived for at least six months. Five (17%) of 29 introduced individuals were reported dead. Of these, one was predated and four were killed by domestic hunting dogs Canis lupus familiaris. Six individuals (21%) went missing. No females reproduced. To acclimatize, monkeys spent two nights in a release enclosure, 49 m2 in size, 2 m in height, with a 60% shade cloth roof and natural enrichment and roosting places, before being released. Monkeys were introduced as one troop of 29 individuals into habitat already occupied by the species and with predators. Monkeys were provided daily supplementary food. Medical care was provided when necessary before release and while housed at the nearby rehabilitation centre. The study does not distinguish between the effects of the different interventions mentioned above.

23 

A site comparison in 2008-2012 in bamboo thicket-dominated forest at Dao Tien Island (DTI) and mixed forest in Dong Nai Biosphere Reserve (DNBR), Vietnam found that several reintroduced pygmy slow lorises Nycticebus pygmaeus that were allowed to acclimatize to local the environment before release along with eight other interventions, survived over two months. Four out of eight lorises survived for at least two months after release, whereas the remaining lorises either died or their radio-collar signal was lost soon after release. Lorises were kept in an in situ cage for <2 months and for two days, and were subsequently supplemented with food for 7-30 days in DTI and DNBR, respectively. Lorises were released during the wet season after a 6-week quarantine, veterinary screens and treatment for parasites. Both release sites were officially protected, no resident lorises occurred there, but predators were present. Bodies of dead lorises were examined. The study does not distinguish between the effects of the different interventions mentioned above.

24 

A before-and-after trial in 2006-2011 in tropical forest at Gunung Halimun Salak National Park and Batutegi Nature Reserve, Indonesia found that using a large habituation cage increased the probability of survival of translocated Javan slow lorises Nycticebus javanicus, but not of greater slow lorises Nycticebus coucang. The size of the cage influenced survival success, with longer survival for individuals that had access to larger habituation cages. Cage size was differentiated into ‘small’ and ‘large’ cages, where the latter consisted of 50 m perimeter open-top enclosures that were situated at the release site. Time allowed to acclimatize varied from four to 123 days and had no effect on survival.

25 

A before-and-after trial in 2008-2010 in a tropical forest-grassland mosaic at Batéké Plateau National Park, Gabon found that the majority of reintroduced western lowland gorillas Gorilla gorilla gorilla that were allowed to adapt to local environmental conditions at the release site along with ten other interventions, survived for at least nine months. Four (80%) out of five juvenile gorillas survived for at least nine months after release when they spent the nights in a 4 x 4 x 3 m3 wooden cage on-site. The enclosure was equipped with nesting platforms, nesting material, supplementary food and water. Gorillas were dewormed regularly on-site. Three captive-bred and two orphaned wild born individuals were reintroduced as a group into habitat with predators and without resident wild gorillas. Caretakers guided them into different forest patches on a daily basis. The study does not distinguish between the effects of the different interventions mentioned above.

Referenced papers

Please cite as:

Junker, J., Kühl, H.S., Orth, L., Smith, R.K., Petrovan, S.O. and Sutherland, W.J. (2017) Primate conservation: Global evidence for the effects of interventions. University of Cambridge, UK