ive seen this before in other pictures and photos on the web not from here. my guess is this happens often yet i do not believe either has ever been hurt.
It might have looked like they were separate because the nyala have a small area where the hippos can't get to. This was the case at the start of the year (although the hippos were locked indoors because it was freezing cold).
Just checked my photos and appears PAT is right - they are not permanently separated but on my visit the nyala were all in the part of the enclosure that the hippos can't access and the hippos all in th other bit. Seems the nyala were just choosing not to mix on this occasion....
