In response to these events, a spokesperson for the zoo told reporters, "We actively encourage natural animal behaviors and group dynamics. Our animals are also housed in natural enclosures, replicating environments they would be used to in the wild."
While this and other zoos make attempts to provide "natural" or "semi-natural" environments for their residents, it is impossible to replicate them in captive situations. And, it's well known that the behavior of animals can change when they are confined and housed near others who usually are not their nearest neighbors in the wild. They are incredibly sensitive to visual, auditory, and olfactory stimuli and it's extremely difficult, if not impossible, to know how "extraneous" factors outside of their cages influence their behavior.
While incidents like this are rare, it is essential that they be reported and remedied as long as animals are forced to live in captivity. Although the Bristol Zoo is considered to be a "good zoo," clearly it is not good enough to avoid these tragic events. I share the distress of zoo workers, and perhaps these tragic events will serve notice that zoos can and do change the behavior of their residents and there never can be too much surveillance of their activities.