For endangered species, conservation breeding in zoos and other institutes is of high importance with the goal to recreate a self-sustaining wild population. If suffering from poor welfare, the reproductive behaviour of the animals can be reduced resulting in a lower reproductive success. It is therefore important for endangered species in conservation breeding programmes, like the drill, to maintain a good welfare for the survival of their species.

There are several factors that can affect the welfare of zoo animals. One of those are visitors. How many they are and their sound level (visitor intensity) have shown to affect the behaviour of several animal species, including primates. These effects caused by visitors are usually seen as negative for the animals. Suggestions from previous research have been to install visual barriers as a method to diminish the visitor effect.

Another influencing factor is the environment. The zoo environment differs a lot from the wild. For instance, at zoos, animals are provided with food that has already been processed and placed out for them, giving the animals no or little opportunity to search, select or process the food themselves. This can lead to a major issue for primates that, in the wild, spend the majority of their day foraging. Enriching the environment for animals is one strategy commonly used at zoos, providing suitable environmental stimulation for the animals.

To have access to an outdoor enclosure is shown to have several positive effects on the behaviour and welfare of zoo animals. The outdoor environment provides more sensory stimulation (visual, olfactory and auditory) and environmental stimuli (e.g. grass and sunshine) for zoo animals than the indoor environment. Also, outdoor enclosures mostly have a higher environmental complexity than indoor enclosures and may therefore provide more opportunities for animals to engage in their species-specific behaviours. However, even though having an access to both an indoor enclosure and outdoor enclosure, some primate studies have shown that the animals do not use the outdoor enclosure as much as expected. Why they choose to be most of the time indoors when the outdoors provide them with so much more stimulation and complexity is still unknown.

This study included two projects conducted on the drills at Parken Zoo: 1) Visual barrier project and 2) Enrichment project.

Visual barrier project
With the conclusion from an earlier study that the drills at Parken Zoo are stressed by visitors, the aim in the first project was to investigate the effect of visual barriers on the behaviour of the drills and if the visual barriers can moderate the visitor effect on the drills.

Enrichment project
The aim in the second project was to investigate how outdoor-only access together with feeding enrichment affects the behaviour of the drills compared to their ordinary indoor/outdoor access without added feeding enrichment.