That exhibit functions as the transfer chute for the nyalas to get to there nighthouse. The gazelles are in there most of the time except when the male nyala needs to be separated or the nyalas are entering/leaving the nighthouse.
Ah, that explains it. Thank you for the information, uszoo. And it was the male n'yala, in fact, that I specifically remember being in the exhibit on one of my visits.
