Ecological factors have a pervasive impact on animal population sizes and the structure of their social systems. In a number of ungulate species, predator pressure exerts a major influence on group size. Given that giraffe (Giraffa camelopardalis) live in an extremely flexible social system, and that breeding is non-seasonal, they are an ideal species for examining how ecological variables contribute to fluctuations in herd size. We present an analysis of 34 years of data on a population of Thornicroft’s giraffe