The availability and complexity of escape routes are key aspects in modulating aggression. The effect of increased vertical space on the occurrence and severity of aggression was studied in a large group of chimpanzees (Pan troglodytes). Fifty wooden poles were added to the outdoor
enclosure at Chester Zoological Gardens, England, UK. Connected with ropes and nets to form a complex vertical structure, they substantially increased the potential for escape routes during aggression. This study is an investigation into whether the use of the new vertical structure reduced
the severity of aggression. Aggressive interactions among 29 chimpanzees (five adult males, 15 adult females, and nine immatures) were recorded whenever observed. We found that the proportion of total aggression involving the use of the vertical structure was much lower than expected based
on the time individuals spent on the structure. We also found that no severe aggression was initiated when individuals were on the vertical structure. Most importantly, the proportion of severe aggression was lower when recipients of aggression moved onto the vertical structure than when they
stayed on the ground. Furthermore, incidents of serious injuries were reduced after the vertical structure was added. The vertical structure appeared to function as a deterrent of aggression and an important escape route during aggressive interactions, which reduced the severity of aggression.
Our results suggest that complex vertical structures are highly beneficial for semi-arboreal species with the potential for severe aggression and should be strongly considered by zoos housing such species