Action: Captive rear in large enclosures prior to release
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- Four studies evaluated the effects of captive rearing mammals in large enclosures prior to release. Two studies were in the USA, one was in Mexico and one was in Australia.
COMMUNITY RESPONSE (0 STUDIES)
POPULATION RESPONSE (3 STUDIES)
- Reproductive success (1 study): A study in Mexico found that peninsular pronghorn taken from the wild and kept in a large enclosure bred successfully and the population increased, providing stock suitable for reintroductions.
- Survival (2 studies): A replicated, controlled study in USA found that black-footed ferrets reared in outdoor pens had higher post-release survival rates than did ferrets raised indoors. A controlled study in Australia found that Tasmanian devils reared free-range in large enclosures did not have greater post-release survival rates than animals from intensively managed captive-rearing facilities.
- Condition (1 study): A controlled study in Australia found that Tasmanian devils reared free-range in large enclosures did not gain more body weight post-release compared to animals from intensively managed captive-rearing facilities.
BEHAVIOUR (1 STUDY)
- Behaviour change (1 study): A controlled study in USA found that captive-bred black-footed ferrets raised in large enclosures dispersed shorter distances post-release than did ferrets raised in small enclosures.
Captive-bred mammals may take time to adapt to conditions in the wild post-release, making them especially vulnerable to predation, starvation and disease. If they are reared in large enclosures, with habitat that resembles natural conditions, they may develop more natural behaviour and be better able to find food and shelter in the wild, compared to those animals reared in smaller pens.
Supporting evidence from individual studies
A replicated, controlled study in 1991–1996 at three grassland sites in South Dakota, Wyoming and Montana, USA (Biggins et al. 1998) found that black-footed ferrets Mustela nigripes reared in outdoor pens had a higher survival rate after release than did ferrets raised indoors. Nine months after release, a higher proportion of black-footed ferrets that were reared in outdoor pens were still alive (20%) than of animals reared in indoor cages (2%). In 1991–1995, one hundred and ninety-one ferrets were reared in indoor cages and 58 were raised in outdoor pens. Pens were 18–280 m2 and were stocked with white-tailed prairie dogs Cynomys ludovicianus (as food for ferrets and to dig burrows that were used by ferrets). Ferrets, implanted with Passive Integrated Transponder (PIT) tags, were released in August–November of 1991–1995 at three sites. In 1991–1996, each area was surveyed on at least three consecutive nights by 8–32 people, on foot or in vehicles. All ferrets located were individually identified using PIT tags.
A controlled study in 1992 in a grassland area in Wyoming, USA (Biggins et al. 1999) found that captive-bred black-footed ferrets Mustela nigripes raised in large enclosures dispersed smaller distances and moved less after release than did ferrets raised in small enclosures. Black-footed ferrets raised in large enclosures had a lower average maximum dispersal distance during the first three days post-release (1.7 km) and lower average cumulative movement over any three-day period post-release (8.2 km) than ferrets raised in small enclosures (maximum dispersal distance: 5.6 km; average cumulative movement: 21.1 km). Between September and October 1992, twenty-five 16.5–18-week-old captive-bred black-footed ferrets were radio-tagged and released into a 20,596-ha area. Eight ferrets were born in cages but raised in 80-m2 outdoor pens with prairie dog burrows and 17 were born and raised in indoor-1.5 m2 cages. All ferrets were fed live prairie dogs. Ferrets were followed in October–November 1992.
A study in 1998–2003 at a captive breeding facility in Baja California Sur, Mexico (Cancino et al. 2005) found that peninsular pronghorn Antilocapra americana peninsularis taken from the wild and kept in a large enclosure increased in number and provided a suitable resource for future reintroductions. Nine adult pronghorns and 16 fawns were captured in the wild, in 1998–2003, to establish the captive breeding herd. Births in captivity occurred from 2000, with 85 occurring up to 2003. There were 20 deaths. In 2003, the captive population stood at 90 animals. The captive breeding facility measured 1,400 × 1,850 m, with moveable internal divisions to manage animal separations where necessary. The founder animals were wild-caught. Fawns caught wild were bottle-fed until weaned. A different male was used for mating each year.
A controlled study in 2012–2015 on a forested island in Tasmania, Australia (Rogers et al. 2016) found that Tasmanian devils Sarcophilus harrisii reared free-range in large enclosures did not have greater post-release survival rates and body weight gains compared to animals from intensively managed captive-rearing facilities. Survival of animals reared in free-range enclosures (eight of nine animals survived ≥825 days after release) did not differ from that of those reared in intensive captive facilities (18 of 19 survived ≥825 days after release). Free-range enclosure animals did not gain more body weight than did intensive captive facility animals over 440 days post-release (average 14% gain across all animals). Twenty-eight adult (c.1 year old) Tasmanian devils (13 females, 15 males) were released. Nine had been reared in free-range enclosures (22-ha pens) and 19 in intensive captive rearing facilities (which included zoos and hand-rearing).
- Biggins E., Godbey J.L., Hanebury L.R., Luce B., Marinari P.B., Matchett M.R. & Vargas A. (1998) The effect of rearing methods on survival of reintroduced black-footed ferrets. The Journal of Wildlife Management, 62, 643–653
- Biggins D.E., Vargas A., Godbey J.L. & Anderson S.H. (1999) Influence of prerelease experience on reintroduced black-footed ferrets (Mustela nigripes). Biological Conservation, 89, 121-129
- Cancino J., Sanchez-Sotomayor V. & Castellanos R. (2005) From the field: capture, hand-raising, and captive management of peninsular pronghorn. Wildlife Society Bulletin, 33, 61-65
- Rogers T., Fox S., Pemberton D. & Wise P. (2016) Sympathy for the devil: Captive-management style did not influence survival, body-mass change or diet of Tasmanian devils 1 year after wild release. Wildlife Research, 43, 544-552