Population structure of giraffes is affected by management in the Great Rift Valley, Kenya

A protected giraffe population can have plenty of adults and almost no calves. In Kenya's Lake Nakuru National Park, adults dominated a giraffe population living behind a fence. Nearby Soysambu Conservancy, also fenced, held a much younger population. The contrast raised a question that a simple headcount could miss: were these populations equally capable of replacing their adults?

Zoe Muller photographed giraffes repeatedly, identifying individuals by their coat patterns and assigning age classes. She followed 77 animals at Soysambu and 89 at Lake Nakuru, spending nine months at each site in consecutive years with matching seasons. The reserves shared a broadly similar environment, but Lake Nakuru supported a high density of lions, whereas Soysambu had none.

Calves under a year old made up about 34% of Soysambu's identified giraffes, compared with only about 6% at Lake Nakuru. Older juveniles were also less common in the national park. Lion predation was a plausible explanation, not a demonstrated cause: the sites differed in other respects, and the study did not establish how individual calves died. The important warning is about population structure. Protecting adults inside a reserve does not by itself show that enough young giraffes are surviving to sustain their population.