This condition is common in captive Indian rhinos, If you look at the Indian rhino pic in the Whipsnade gallery from the 1970s it shows the same thing back then.
Its a form of stereotype- similar to head bobbing or weaving in elephants and polar bears.
I can remember the original male Mohan at Whipsnade rubbing his horn base back and forth along the railing. Mark's photos show what is probably his son 'Manik' with a similar worn down horn. More recently I watched another male at Whipsnade waiting to be let indoors- he walked to the gate and started this same stereotyped motion until it was opened.
This behaviour seems restricted to Indian rhinos only, I have never seen it in the other species.
