They are largely solitary. A bull will hold a vast territory and tolerate younger males only at the edges. A cow with a calf at heel is the most common pairing you are likely to see, the calf following her closely for up to four years before being driven off as the next pregnancy approaches. Black rhino mothers are famously protective, and they will charge lions, hyenas and even elephants that approach the calf. Their eyesight is genuinely poor, but their hearing and sense of smell are extraordinary. A rhino will pivot its enormous head, ears swivelling like satellite dishes, and lock onto a sound from hundreds of metres away. The defensive charge is mostly a bluff, but the front horn can grow over 60 centimetres long and is made of keratin, the same material as your fingernails. They have a long, almost tender relationship with red billed oxpeckers, who pick parasites from their hides and act as an early warning system. Tanzania's black rhinos tell a story of near loss and slow recovery. The Ngorongoro population was reduced to a handful of survivors in the 1990s and has been rebuilt through round the clock protection by Tanzanian rangers and conservation NGOs. Each individual is named, tracked daily, and known by the rangers who guard them. To see one in the wild today is to witness an active ongoing conservation effort that has refused to fail.