Northern white rhinoceros (Ceratotherium simum cottoni)

The male "Angalifu" of the remaining pair.

Why has the pair not bred?

They are past breeding age.

Is it a wall behind the rhino? Meaning, are they off exhibit now?

They're still in the South African savannah exhibit (which is also home to further non-South African species).

Northern White Rhinoceros

Can someone more knowledgeable than me, just talk me through how this can be identified as a Northern from this picture, please?

One I've seen mentioned (though I'm not sure how reliable it is) is to look for a hairy fringe on the ears for it to be a Northern - which is a bit vague. Basically they are very hard to tell apart.

Genetically-speaking (which generally carries more weight in modern taxonomy, rightly or wrongly) they are significantly different, but of course that can't be seen from looking! Colin Groves (a taxonomic splitter par excellence) has even proposed that be split into two species. (PLoS ONE: The Sixth Rhino: A Taxonomic Re-Assessment of the Critically Endangered Northern White Rhinoceros)

For the record, Groves used dentition and craniometry (i.e. skull measurements) to distinguish the two morphologically.

Northern White Rhinoceros

Many thanks Maguari; sounds like the Douroucouli [have I spelt that right?] all over again.

The 'new' douroucoulis are actually often quite physically distinct compared to some of the things Groves has split!

It is sad because all the remaining Northern White Rhinos are in captivity and past breeding age. I only saw the female but she really made my day when I went there this summer ( knowing that I saw a species that is on the brink of total extinction ) Sad isn't it. But still very lucky to see such a gem.