Release translocated/captive-bred mammals in larger unrelated groups
Overall effectiveness category Likely to be beneficial
Number of studies: 5
Background information and definitions
Mammals are sometimes wild-caught and translocated or bred in captivity and released to re-establish populations that have been lost, or to augment an existing population. This action includes studies comparing the effects of translocating or releasing mammals in larger, unrelated groups (i.e. not family or social groups), rather than in smaller groups (which might include as few as one animal). This may be done for a variety of reasons, such as increased protection against predators, greater access to potential mates or social groups and an increased chance of establishing self-sustaining breeding populations.
Studies of unrelated translocated mammals that were held together to form social groups prior to release, or unrelated captive-bred animals raised and released together are described in Release translocated/captive-bred mammals in family/social groups.
Studies of releases of unrelated animals that were not held together, and where the effect of group size was not tested, are described in Translocate to re-establish or boost population in native range and Release captive-bred individuals to re-establish or boost population in native range.
Supporting evidence from individual studies
A replicated, paired sites study in 1990–1991 in three grassland sites in Colorado, USA (Robinette et al. 1995) found that larger groups of translocated black‐tailed prairie dogs Cynomys ludovicianus attracted more immigrants and had higher reproductive success and initial daily survival rate than smaller groups. Over one year, prairie dogs translocated in groups of 59 individuals attracted more immigrants (13.7) than those translocated in groups of 30 (4.0) or 10–11 (1.5). Reproductive success was higher in prairie dogs translocated as groups of 59 individuals (0.79 pups/animal released) than groups of 10–11 (0.28 pups/animal released), but similar to those released as groups of 30 individuals (0.62 pups/animal released). Groups of 59 prairie dogs had higher daily survival rates in the first 23-51 days after release (99.1%) than groups of 30 (98.5%) or 10 prairie dogs (97.7%) but by the second monitoring period (139-142 days later) daily survival rates were the same for all three groups sizes (99.8%). Between July and October 1990, six groups of 10–11, three of 30 and three of 59 prairie dogs were released into three experimental blocks with four plots (2-6 ha depending on group size) in each (2 containing 10-11 prairie dog groups, 1x 30 prairie dog group and 1x 59 prairie dog group, randomly assigned), within a 69-km2 military area. Prairie dogs were trapped four times during one year post-release, using 1.5 traps/released individual, over four days.Study and other actions tested
A study in 1981–1998 in a savannah reserve in North West province, South Africa (Hofmeyr & van Dyk 1998) found that following the release of rehabilitated and captive-bred cheetahs Acinonyx jubatus in groups (unrelated and family) and as individuals, most adults survived at least one year and animals had reproduced in the wild. Most rehabilitated adult females (3 of 4) and all rehabilitated adult males (4 of 4) survived at least one year. Two rehabilitated adult females produced a second litter within two years of release. Three of 10 cubs released survived to independence, including a female who raised a litter of cubs to independence. The total population numbered 17 cheetahs one year after the end of a five year release program, compared to 18 animals released. An earlier release in the same National Park found that captive-bred cheetahs had bred successfully but most animals were subsequently removed to protect ungulate populations. Between 1995 and 1997, eighteen cheetahs (4 adult males, 4 adult females, 10 dependent cubs) were introduced to a National Park (55, 000 ha) from a rehabilitation facility (unknown if wild-born or captive-bred). Cheetahs were released in family groups (mothers with cubs), in unrelated groups (of males) or individually. In 1981-1982, seven cheetahs were released from a captive-breeding facility and after an unspecified period of time, seven cheetahs were removed leaving a group of three males. Individuals were monitored by radio-tracking.Study and other actions tested
A study in 2000–2003 in a mixed karoo grassland reserve in Northern Cape Province, South Africa (Venter 2004) found that a larger group of translocated Cape buffalo Syncerus caffer released into a fenced reserve (after being held in a holding pen) formed a single herd and stayed in the reserve and bred, whilst a smaller group scattered and escaped the reserve. A group of 10 translocated animals formed a single herd (with two previously released animals) and over 10 months all animals survived and remained in the reserve. A year after release, five calves were born. One month after release, a group of four buffalo had split into two solitary animals and a male-female pair. One of the solitary animals was not seen again, the second solitary male was located two years after release on a neighbouring farm and was released into the second group of translocated animals in May 2003. The pair escaped the reserve three times in 13 months. After the third escape, the male was moved to a different reserve and a new male introduced to form a herd with the remaining female. Four subadult buffalo (2 male, 2 female) were placed in a holding pen in July 2000 and released in August into a fenced 12,000-ha reserve. A second group of seven adult and three sub-adult animals (4 male, 6 female) was placed into a holding pen in August 2002 and released into a 200 ha area in September before being completely released in October 2002. Both groups were monitored weekly using radio-tracking until October 2003.Study and other actions tested
A study in 1981–2005 of 81 reserves across Namibia and South Africa (Linklater et al. 2011) found that releasing translocated black rhinoceros Diceros bicornis in larger groups did not affect survival in the first year post-release. Seventy-four of 682 translocated black rhinoceroses died during the first year post-release, but the number of individuals released together did not affect survival in the first year (data reported as statistical result). First-year post-release mortality was higher when animals were released into reserves occupied by other rhinoceroses (restocking, 13.4% mortality of 268 animals) than releases into new reserves (reintroduction, 7.9% mortality of 414 animals). At least 243 rhinoceroses survived at least 10 years after release. For restocking events, first-year post-release mortality was higher in rhinoceroses less than two years old (59%) than in all other age classes (9–20%), but there was no difference for reintroductions. Data on 89 reintroduction and 102 restocking events of black rhinoceroses into 81 reserves from 1981–2005 were compiled from the Namibia and South Africa Rhino Management Group reports. Animals were released in groups of one to 30 individuals, and reserves received up to five releases. Translocations were considered as different if the releases of individuals to the same reserve were more than 1 month apart. Deaths were detected by reserve staff. The location of reserves included in the study is not provided.Study and other actions tested
A controlled study in 2013 at a desert site in South Australia, Australia (Bannister et al. 2016) found that releasing translocated animals in a larger group, to swamp predator activities, did not promote population persistence of burrowing bettongs Bettongia lesueur. There was no significant difference in post-release persistence between a large release (bettongs last recorded 42 days after the final release) and three smaller releases (bettongs persisted 41–53 days after releases). A total of 1,492 bettongs were translocated between July and December 2013 and released into rabbit warrens. The large release was of 1,266 bettongs, released in July–October 2013 in a 250-ha unfenced area. Three smaller releases, of 48–56 bettongs, occurred in October 2013, at sites 4 km from the large release and from each other. Following no bettong records at two of these sites for ≥7 weeks, further releases of 29 and 39 animals were made in December 2013. From May–December 2003 feral cats Felis catus and foxes Vulpes vulpes were intensively controlled in a 500-km2 area by 428 hours of shooting patrols. Bettong persistence was monitored using track counts, camera trapping, warren monitoring and live-trapping.Study and other actions tested