Post-release survival rates and welfare of rehabilitated vervet monkeys in Malawi
Angley, Laura Patricia
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Rehabilitation-release is a form of species reintroduction where sick, injured, or rescued animals are rehabilitated before release back into the wild. Published research on rehabilitation releases of rehabilitant non-human primates is limited, and released troop mortality rates are generally high or difficult to determine. The objective of this study was to add to the limited scientific literature on primate rehabilitation and release by investigating factors affecting survival rates and welfare of a rehabilitant troop of vervet monkeys (Chlorocebus pygerythrus rufoviridis) released in Malawi in 2016, using pre-existing datasets from the Lilongwe Wildlife Trust. I hypothesized that 1) higher social rank, more complete forest strata use, close proximity to troop members, and frequent predator vigilance would be associated with greater survival, and 2) rank stability/ group cohesion will be strong post-release, activity budgets will show low levels of stress-related behaviors, and behavioral diversity will increase post-release, suggesting welfare improvements. The Lilongwe Wildlife Trust troop had a survival rate of 36%, which is comparable to other vervet releases. Using a combination of linear modeling, survival analysis, and preliminary social network analysis, I found that being a juvenile, being more highly ranked, and being in close proximity to others was significantly associated with lower risk of death – but these results were not consistent and should be considered with caution. Contrary to predictions, forest strata use did not differ greatly across individuals despite differences in survival. Interestingly, the troop’s mean hourly count of predator vigilance decreased post-release, but this did not influence individual survival. In support of my predictions, the troop’s dominance hierarchy appeared stable post-release, group cohesion was strong, and activity budgets showed low levels of stress-related behaviors. However, mean behavioral diversity across individuals decreased post-release, contrary to predictions. These findings suggest that vervet dominance hierarchy, age, and social proximity may influence post-release survival with higher ranking individuals, juveniles, and highly socially connected individuals more likely to survive. Juveniles may be more ecologically adaptable than adults and so better able to survive in a new habitat. Lower ranked individuals, as well as those with low social connectedness, may be more disconnected from the troop while traveling or foraging, placing them at a higher risk of predation but more research is needed to confirm this. Decreased behavioral diversity post-release may have been caused by an increase in foraging and troop movement and generalized behavior categorization may have limited the accuracy of behavioral diversity measurements. Future studies that wish to use behavioral diversity to assess welfare should use highly specific ethograms to capture unique behaviors. Release troops may also benefit from pre-release feeding regimes, such as platform feeders, that encourage more complete canopy use as well as more time at the release site prior to the start of the rainy season. Predator-awareness training is highly recommended to strengthen anti-predator behaviors, especially if the troop has any wild individuals. Finally, the Lilongwe Wildlife Trust’s extensive pre- and post-release monitoring provides vital insight into the troop’s social dynamics, behavioral repertories, and overall survival. Other rehabilitation centers should follow this strategy, since all newly monitored and reported releases will add valuable information to the development of the vervet monkey rehabilitation and release program.
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