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Gradual release of rescued primates into the wild is highly effective

Each year, rescue centers release animals into the wild, assuming that they will thrive most in their natural habitat. However, such assumptions have not been tested scientifically with primates and investigations into the most effective methods of release have been scarce. 

Now, a team of scientists led by Durham University and the Jane Goodall Institute has found that a group of mandrills (Mandrillus sphinx) – a species of monkey native to west central Africa – who were returned to the wild using a carefully phased approach adapted well, exhibiting a lower stress response in the wild than in the sanctuary from which they were released.

“Many primate releases are conducted based on the assumption that it improves animal welfare, but very few studies have actually tested this,” said senior author Joanna Setchell, a zoologist at Durham. “Our work applies rigorous science to this assumption, providing evidence that a carefully planned release of mandrills can work well with the right design, monitoring, and evaluation.”

How the research was conducted

The International Union for the Conservation of Nature (IUCN) recommends a gradual release and post-release monitoring to ensure that, when animals are released into the wild, their stress responses are low, allowing them to survive and adapt to the new environment. However, this slow and steady process is rarely applied due to high costs and time restrictions.

For the current study, the experts transferred 15 mandrills from the Tchimpounga Sanctuary – which were orphaned by hunting and rescued by Congolese authorities – to a pre-release enclosure in the Conkouati-Douli National Park, and afterwards into the park itself. 

This process involved the mandrills spending time in a pre-release enclosure built in the forest, provisions of extra food, careful observations of the animals’ condition, and interventions to remove animals that were not doing well. Even though all of the 15 mandrills survived, three of them had to be returned to the sanctuary as their transfer was not successful.

Measuring the primates’ stress response 

At every stage of the transfer, the scientists collected fecal samples from the mandrills, which they then used to measure their stress response non-invasively. As expected, the animals’ stress response initially increased after their move from the sanctuary to the pre-release enclosure, most likely due to their being transported in crates by car and boat. 

However, in the pre-release enclosure, which provided the mandrills a safe place to adjust to their new environment, their stress response dropped again and did not increase after they were released in the forest. In fact, just one month after their release into the wild, their stress response dropped below the levels observed while they were in the sanctuary. 

Moreover, after one year spent in the wild, the response was nearly half what it had been in the sanctuary, suggesting that the mandrills were faring quite well in their natural habitat. The results are published in the journal Conservation Physiology.

“This project was ultimately successful because of years of pre-release preparation and extensive post-release support. We went to the extreme lengths to conduct this study because it’s important that all releases are learnt from and the results are reported,” said lead author Miles Woodruff, a primatologist at Durham.

“We are dealing with the life and death of critical species in sensitive environments and every individual matters. Our findings are very exciting because we now have physiological evidence to support the ‘why’ behind the IUCN’s suggestions to conduct soft releases of animals.”

More about mandrills 

Mandrills (Mandrillus sphinx) are a type of primate that are native to rainforests in equatorial Africa, specifically in countries like Cameroon, Gabon, Equatorial Guinea, and Congo. They’re a member of the Old World monkey family, Cercopithecidae, and are close relatives of baboons and drill monkeys.

Physically, mandrills are recognized for their vibrant, multicolored facial and rump markings. Males, in particular, have incredibly vivid markings, with blue and red skin on their faces and a yellow beard, while their rumps feature similar coloration. These striking colors become brighter when the animal is excited. 

Mandrills are sexually dimorphic, meaning males and females have different physical characteristics. Male mandrills are significantly larger and more colorful than females, with males weighing up to 54 kilograms (120 pounds) and females usually around 12 kilograms (26 pounds).

In terms of behavior, mandrills are social animals, living in large groups called hordes, which can number in the hundreds. These groups are typically dominated by a single, older male and consist of several females and their young. They are primarily terrestrial animals, spending much of their time on the ground, but they can climb trees and often sleep in them.

Mandrills are omnivorous and their diet consists of a mix of fruits, roots, and insects, but they can also eat small vertebrates. Their cheek pouches allow them to carry food to be eaten later. 


By Andrei Ionescu, Staff Writer

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