This photo is notable for being the first time I’ve seen one of the rhinos down this end of the exhibit. The nyala herd are to the right (also a rare sighting down this end).
@Abbey Yes, it’s just the male/female difference. The bulls are a dark brown; the females and young are the auburn brown. Auckland Zoo has an all female herd on display. I believe they have at least one bull off display as they bred recently.
@Zoofan15 Thanks, that makes sense! I believe Sydney Zoo has a bachelor herd, as their animals (the only nyalas I've seen in person) are the dark brown colour you mention).