Paula Kahumbu, an ecologist, is quoted in a widely circulated article about the pair:
It is incredible. A-less-than-a-year-old hippo has adopted a male tortoise, about a century old, and the tortoise seems to be very happy with being a 'mother.' The hippo follows the tortoise exactly the way it follows its mother. If somebody approaches the tortoise, the hippo becomes aggressive, as if protecting its biological mother.
The hippo is a young baby, he was left at a very tender age and by nature, hippos are social animals that like to stay with their mothers for four years.