Twin western lowland gorillas (Gorilla gorilla gorilla)—one female, one male—cling tightly to their mother’s chest as if keenly aware of how vulnerable they are in this rapidly changing world. N’Hasa, the female, and her brother, N’Kato, are precious cargo for their mother, N’Gayla, as well as for conservationists devoted to protecting this critically endangered species. Less than half of all gorillas born in the wild typically survive to adulthood. And newborns are few and far between, with adult females giving birth on average to a single baby just once every five years. Fortunately for these wide-eyed youngsters, they were born at the Royal Burgers’ Zoo in the Netherlands, which participates in a conservation breeding program designed to ensure the continued existence of the species. There, the availability of additional care and food resources will boost the odds that both babies survive.

Raising a single gorilla infant—let alone two—presents an armful of challenges no matter where a gorilla resides. Like their human counterparts, gorilla mothers give birth at the end of a nine-month pregnancy to babies that are almost completely helpless. For the next six months, a mother will cradle her infant in one arm as she goes about her day. Even after her baby has developed sufficient strength, balance, and dexterity to climb aboard and ride on her back, she alone will provide the baby’s sustenance, protection, and transportation until it’s at least a year old, refusing the assistance of family members to help care for her young. For obvious reasons, the challenges of keeping up with the group while toting two infants are significant, and they likely outweigh any reproductive benefit a mother would gain from having twins. This selective pressure is probably why gorilla twins are so rare.

For N’Gayla, the care and resources provided by zoo staff made the nearly impossible possible. Today, N’Hasa and N’Kato are fully weaned from their mother and are thriving in the safety of their threat-free habitat. Their wild counterparts, however, are not as fortunate. Western lowland gorillas are listed by the IUCN as Critically Endangered across their native range in western equatorial Africa. Poaching, infectious disease, and habitat destruction all threaten the species’ survival, while climate change is altering food availability and forest ecology in dramatic ways. Breeding programs like those at the Royal Burgers’ Zoo and around the globe are helping to ensure gorilla populations remain genetically diverse, increasing the resilience of the species in the face of such sweeping changes.