Between 2014 and 2016, conservationists moved 22 groups of Mount Kenya guerezas, a rare subspecies of colobus monkey, from a badly degraded riverine habitat to Karura Forest near Nairobi, Kenya. A new study titled ‘ Demography of reintroduced Mount Kenya guereza (Colobus guereza kikuyuensis) at Karura Forest, Kenya ’ in Primate Biology followed the 142 translocated monkeys for eight years, from 2016 to 2023, tracking births, deaths and how groups settled into their new home. By December 2023, researchers had recorded 109 births and watched the population grow from 22 groups to 31 through natal dispersal.
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How 22 groups of colobus monkeys were reintroduced to Karura Forest
The 142 Mount Kenya guerezas were released into Karura Forest, an upland forest four kilometres north of Nairobi, between May 2014 and March 2016. They had come from a highly degraded, fragmented riverine habitat in Nyandarua County, roughly 130 kilometres away. The groups, with a mean size of eight individuals, were named alphabetically from C to X in order of release and held in cages for three nights beforehand to help them acclimatise. Researchers from Jomo Kenyatta University of Agriculture and Technology then carried out monthly censuses over eight years, recording group size, composition, births, deaths and natal dispersal to judge how well the reintroduction had worked.
Where the reintroduced colobus monkeys settled in Karura Forest
Fourteen of the 22 released groups, almost two-thirds, settled along riverine zones near the Karura, Getathuru, Thigiri and Turaco rivers, at distances of 10 to 140 metres from the water. Riverine forest offers succulent young leaves, tall trees and continuous canopy cover, which suits colobus monkeys particularly well, since their diet depends heavily on the moisture content of their food. Two bachelor groups moved further afield, settling outside the reserve at City Park and Red Hill Estate instead. Neither the number of males in a group nor its overall size had a significant effect on how far it settled from its original release point.
Births and deaths among the colobus monkeys over eight years
Group size fell in the first six months after release, likely due to the stress of translocation, before climbing steadily over the following years. By December 2023, the average group size had grown from 6.24 at release to 9.51, and researchers recorded 109 births across the reintroduced population. Births rose in most years but dipped noticeably during a prolonged drought in 2019 and 2020, echoing similar reproductive declines seen in other primate species under drought stress elsewhere in Africa. Meanwhile, only 26 deaths were recorded over the full eight-year study, with tree falls, electrocution and disease among the leading recorded causes.
Group splits and the success of the Karura Forest reintroduction
No cases of group fusion were recorded during the eight-year study, but seven groups with more than two adult males began splitting from 2019 onwards, eventually raising the total number of groups from 22 to 31. In each split, one adult male typically departed with a handful of females and juveniles, while infants and their mothers always stayed together in the original group. Researchers recorded just one case of infanticide, following an attempted takeover by a rival male. The authors concluded that the reintroduction had succeeded in establishing a self-sustaining colobus population within Karura Forest, a promising sign for future primate translocations.
