2011, Rwanda 

Portrait of Kabatwa, and her twins Isango Gakuru and Isango Gato. 

Twins are a very rare occurrence among great apes. The twin mountain gorilla babies captured in these photographs were born on February 3, 2011.

Out of all the great apes, gorillas are the most successful in raising twins to adulthood. While chimpanzee and orangutan twin births usually result in one baby dying within the first two weeks of life, gorilla twins usually survive.

The twins are a special occurrence, but also a great value to the species. Mountain gorillas are critically endangered and every birth counts. "Gorilla mothers usually have only one baby every four years or so - which is one reason why they are so vulnerable - so twins give a rare double gain in one birth," adds Says Ian Redmond, Chairman of the Ape Alliance.