Black rhinos are solitary animals except for when the females are in heat.  A mating pair will

Black rhinos are solitary animals except for when the females are in heat.  A mating pair will stay together for days or weeks, mating several times a day.  And they have stamina - a single session of copulation will last over a half-hour.

Fifteen months later, the female gives birth to a single 80 - 110 pound calf.  These calves will nurse for up to two years and stay with their mothers for at least another year.  Female calves may stay with their mothers even longer, forming small groups of related females.